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Magnetic resonance of the musculoskeletal system
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Magnetic resonance of the musculoskeletal system Jean Garcia M.D. Department of Radiology Hopital Cantonal Universitaire Geneva, Switzerland
Introduction The role of magnetic resonance imaging (MRI) for the evaluation of the musculoskeletal system is expanding rapidly. The most important limit for its growth is the access to a machine. Today, in our institution, a general hospital with 1400 beds, 30% of machine-time is devoted to the musculoskeletal system (and 50% to the central nervous system). About 5000 MRI systems are now installed; more than 50% of these units are installed in the USA with a majority of high performance systems of 1.0 and 1.5 Tesla field strength [1], but only 16% of the units are in Europe. The density situation in Europe reflects political influence regarding health care system restrictions. In addition to being noninvasive, MRI offers high-soft tissue contrast and multiplanar imaging. MRI has better soft tissue contrast than other imaging modalities [2]. Therefore, MRI has become an effective modality in evaluating numerous situations by permitting the right diagnosis. It offers early diagnosis in cases of stress fracture, avascular necrosis, transient osteoporosis, infections, metastases and also a good appreciation of the lesions in spinal malformations, soft tissue trauma, degenerative lesions, infections, tumours, etc. Quality criteria At the beginning of a revue such as this one, it is necessary to point out the different quality criteria: technical, clinical data, radiological synthesis, clinical radiological correlation and human quality. 1.
Technical Considerations
The field-strength question has divided the MRI community since the early 1980s. From analytical applications of MRI, it was known that the signal-ta-noise ratio increases when you increase the field strength. But higher fields also have technical disavantages. In the future, the new generation of buyers, the smaller hospitals and private practices, will prefer cost-efficient MRI systems which they
can use for most of the daily routine examinations. Bigger hospitals, and in particular those interested in spectroscopy research, will prefer high-field systems. In our own institution we only have one 1.5 T machine for clinical and research use and we are obliged to be very strict in our indications. MR imaging examinations must be conducted properly. Patient selection, positioning, coil selection and pulse sequences must all be considered to optimize information on the images [2]. Patient selection must take into consideration those with claustrophobia (2 - 3% of patients), patient size which is rarely a problem with body coils but more often with the surface coils. Patients with cardiac and respiratory monitoring must be excluded. Numerous heart valves have been studied; they create artifacts which are negligible and patients with prosthetic valves can safely be imaged. Patients with pacemakers or aneurysm clips are currently not examined. Orthopedic appliances are usually not ferromagnetic but may contain minimal quantities of iron impurities. Metal heating in patients should not be a problem even with large protheses [2]. Metallic materials cause less artifact on MR images than on CT images. The degree of artifact depends on the size, configuration of the metal and the sequence used (more artifact on field echo sequence than on T1 spin-echo sequence). Patient positioning considerations include size, body part, and expected examination time. The patient should be studied with the most closely coupled coil possible to achieve the maximum signal-to-noise ratio and best spatial resolution [2]. The spine is examined with rectangular surface coils (with a small curvature for the cervical level and a flat coil for the dorsal and lumbar levels). The hips and legs are examined in the body coil, the knees in a specially designed coil, the ankles and feet are comfortably examined in a head coil. The shoulder still remains a difficult area despite the use of a surface coil: the offcentre, i.e. the distance between the shoulder and the magnet isocentre, is quite large, and it is difficult to avoid respiratory motion artifacts. A small circular surface coil is usually utilized for the hand. As a rule, the depth of view of a flat coil is approximately one-half the diameter or width of the coil. In addition, the surface coils are most effective when centered in the magnetic field and the
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Garcia: MRI of the musculoskeletal system
signal-to-noise ratio may be increased four to six times compared to the body coils [2]. The MRI pulse sequences mostly used are the spin echo (SE) technique and the gradient echo technique. In spin echo technique the so-called T1 weighted images are obtained using short TR « 500 msec) and short TE (20 rnsec), and T2 weighted images with long 1R (>2000 msec) and long TE (>60 msec). In gradient echo technique the flip angle used is less than 9QO, 100 to 200 in our practice. The main advantage of gradient echo technique is short imaging times. MRI examination is indeed time consuming and MRI indications are so numerous that each examination should be carried out properly to save time without losing useful information. Multiplanar imaging is one of the principal advantages of MR imaging: transaxial, coronal, sagittal and even differen t oblique slices may be obtained. The plane selection depends on different factors: anatomy (for example, the an terior cruciate ligament is oblique and the MRI slices should be parallel), extent of the lesion (sagittal slices of the femoral head are very useful in avascular necrosis, a nd for spine imaging), etc. The field of view and the thickness of the slice must be adapted to the volume of interest: for example, 10 slices of 4 mm thickness for a femoral head, and 18 transaxial slices of 10 mm thickness for an osteosarcoma involving the distal femoral shaft. Usually only a small number of slices, about 10, can be obtained with a short TR (Tl weighted images); long TR (T2 we ighted images) allows a greater number of slices. With recent technological progress, the spatial resolution is good enough but still remains inferior to conventional radiography and therefore a subtle periosteal reaction is better seen on plain Xvrays, particularly since the calcium component has no signal. In recent years MRI has received new impulses from the clinical introduction of intravenous paramagnetic agents, e.g. gadolinium DIPA and gadolinium DOTA which have more or less the same qualities. Gadolinium is a paramagnetic agent that explores the vascular compartment and reduces the T1. Its usage is very wide; the main indications being tumours, infections, scar versus disc herniation, etc. It is a very accurate complement to the usual sequences. 2.
Clinical data
Although it may seem crucial to have clinical data before MRI examination, in daily practice this is not always the case. In practice, clinical data are not always available before MRl examination. Many clinicians indeed have no knowl edge of the limits of MRI and they do not give enough information to the radiologist. There are, for example, many ways of examining a knee: examination for cruciate ligaments or chondromalacia are quite different. The knee is a very good example in terms of management; MRI examination may show everything and, of course, everyone must shorten an examination when all useful information has been obtained.
3.
Radiological synthesis
All imaging techniques are characterized by a certain sensitivity and specificity. Most of them are complementary. For example, in acute osteomyelitis, bone scintigraphy is more sensitive than plain X-rays, although the latter offer us a good morphology of bony structures, particularly to appreciate the evolution. MRI is at least as sensitive as bone scintigraphy for an early diagnosis of acute osteomyelitis, but in cases of a blood disseminated infection, bone scintigraphy has the great advantage of exploring the whole skeleton. Everyone should know the technical limits to exploring a particular problem properly; do not hesitate to ask advice if in doubt. 4.
Correlation between clinical and radiological data
Magnetic resonance is a very advanced and sophisticated imaging technique, but it is not a "magic tool" which gives us all the answers. The basic principle of strict correlation between clinical and radiological data is still mandatory. MRI may give very subtle information, for example, the presence of oedema in bone marrow; but oedema is not specific. Is the oedema, in a particular case, due to transient osteoporosis, an early phase of avascular necrosis, a stress fracture or associated with an inflammatory or tumoral process? MRI findings must be correlated with clinical and other radiological data to improve its efficiency.
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Human quality
MRI is a new and very expansive imaging modality. Everybody is discussing the cost of machines and maintenance, especially administrators. However, the education of technicians and physicians should not be forgotten as it also needs time and money ! Trauma Musculoskeletal injuries are quite common. Bony lesions are generally diagnosed using routine radiography, conventional or computed tomography and bone scintigraphy. Evaluation of soft tissue injury is more difficult. The role of MRI is limited in acute skeletal injuries but more significan t in the evaluation of soft tissue trauma.
1. Fractures Fractures and fracture dislocations of the spine can be detected by radiography. CT is particularly useful in evaluating the position of bone fragments in the spinal canal and lesions affecting the neural arch. In acute injury, the ability of MRI to show antero-posterior displacement and possible cord compression in the sagittal plane is very impressive. In this situation the role of MRl is limited, especially if new neurological signs appear, MRl IS more useful, for example, in showing a syringomyelia. Skeletal traumata of the extremities are still accurately evaluated by conventional radiology. Complex pelvic fractures, especially those affecting the acetabulum and the posterior elements, are well evaluated by CT. Stress and
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pathological fractures are detected earlier by bone scintigraphy. MRI can be used to detect oedema of the bone marrow after acute trauma without any evidence of bone lesions in plain X-rays and also to detect stress and pathological fractures during the early phase (Fig. 1). MRI is at least as sensitive as bone scintigraphy and more specific, its use must be limited to particular conditions, especially when the differential diagnosis is difficult, for example, in the case of metastasis [3].
Fig. 2: MR!. Normal appearance of the tendon of the tibialis posterior (arrow) on a medial sagittal slice (T1). T = Tibia, N = Navicular bone, QT = quadratus plantae. Oedema is very well seen when it appears in the muscle, for example, during the early phase of myositis ossificans circumscripta (Fig 3); in this situation it is a very early sign which appears after two weeks of evolution, before the increased uptake seen on bone scintigraphy (at least four weeks) and soft tissue calcifications seen on plain Xrays (at least six weeks).
Fig. 1: MRI. Pathological fracture of the right tibia in a 60 year old woman after 2 weeks evolution and not seen on plain X-rays. The fracture line (arrowhead) is surrounded by oedema (hypo intense in comparison with fat). The patient was treated one year previously for lung cancer. 2. Soft tissue injuries
Evaluation of soft tissue injury is quite difficult, particularly when localized deeply. Today, perhaps more in Europe than in the USA, sonography is extensively used for the diagnosis of large tendon and muscle traumata which are often related to sports. The excellent and unequalled soft tissue contrast provided by MRI allows the best differentiation of muscle, fat, nerves and blood vessels. The normal tendons and ligaments appear as bands of low intensity and homogeneous signal both on T1 and T2 weighted images (Fig. 2). This appearance is attributed to the fact that they are composed predominantlyof dense collagen. The intra-articular fibrocartilage, such as the menisci and the triangular fibra-cartilage of the wrist, also have the same appearance and appear as a black area. The signal of hyaline cartilage is slightly more intense. Muscle signal intensity is qualified as intermediate and the fat signal is very intense on both T1 and T2 weighted images. In SE technique, liquid is usually characterized by a low intensity on T1 and a very high intensity, higher than fat, on T2.
Muscle tears are, of great importance and common in trained and untrained athletes. Determination of the degree of injury is important for treatment and prognosis, especially in the professional category. Sonography is still very useful, but MRI became the technique of choice because it is not examiner-dependent, it allows a good evaluation of the extent of injury and may exclude other pathology such as infections, tumours or atrophy, etc.
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Garcia: MRI of the musculoskeletal system
Fig. 3: MR!. Myositis ossificans circumscripta of the right gluteus minimus after 2 weeks evolution. a)
Tl coronal slice (top), the lesion is very subtle (arrow). After i.v, injection of gadolinium-DTPA (bottom) there is a signal enhancement (arrowhead).
b)
T2 transaxial slices: the oedema involving a single musc:le is very intense (arrow head).
Any contour deformity and increased signal intensity of a tendon is usually considered pathologic and indicative of tendinous tear (Fig. 4), degeneration or inflammation. The large superficial tendon of the knee and the Achilles tendon are well appreciated by sonography and MRI. MRl is more efficient for deep tendons. Rotator cuff injury is usually considered to be chronic in aetiology [4]. Tears do not occur unless the tendon is weakened; Neer feels that rotator cuff injury represents a continuum of disease: from oedema and haemorrhage at the beginning to a frank tear at the end . On MR images the differential diagnosis between tendinitis, partial tear and tendon degeneration is very difficult A frank tear, tendon retraction and muscle atrophy (fatty involvement) are well seen and of great value (Fig. 5); in this situation, MRI is superior to arthrography which shows only the tear. The most significant early MRl finding in shoulder impingment syndrome is an increased signal in the supraspinatus tendon near its insertion, presumably due to inflammation, oedema,
haemorrhage or a combination of the three [5]. Nevertheless, the tendon orientation may affect tendon signal intensity and be misdiagnosed as a lesion [6].
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The normal and pathologic appearances described above for tendons are also valid for ligaments such as the cruciate ligaments. The anterior cruciate ligament is smaller and oblique and therefore more difficult to study than the posterior one. However, today MRI is the technique of choice for evaluation of the cruciate ligaments, showing not only frank tears, but also partial tears. Lateral ligaments of the knee and ankle are also well appreciated by MRI in the coronal plane.
Fig. 4: MRI. Complete tear of the tibialis posterior tendon (arrow): contour deformity and increased signal intensity compared to Fig. 2.
The scapholunate and lunotriquetral interosseous ligaments are not always well seen on coronal MR images and until now we used wrist arthrography for evaluating the tears of these ligaments, however, Reicher and Kellerhouse find MRI to be efficient [7]. It is much easier to obtain very good images of the triangular fibrocartilage complex in our practice with MR images [8]. MRI had a good accuracy of 90% in a personal series of 30 posttraumatic lesions (17 of them controlled by arthrography and surgical findings). Degeneration of the fibrocartilage can be visualized on MR images as a disruption of the normal radial attachments and an effusion is identified by its high signal intensity on T2 weighted images [91.
rig. :J: Uld tear ot the ngnt rotator curt. a)
MRI. Coronal slices of the right shoulder on T1 (top) and T2 (bottom). The suprasupinatus tendon is thin (small arrow), the tear is obvious (arrow head), and there is a fatty infiltration of muscle mass (large arrow).
b)
Arthrography (same patient): the contrast medium fills in the tear (arrowhead) and the gleno-humeral bursa (black arrow) communicates with the sub-acromial bursa (white arrow). Supra supinatus lesions are underestimated.
Garcia: MRI of the musculoskeletal system
Arthroscopy is still considered the best way to diagnose intra-articular knee problems but that may change since MRI shows all meniscal lesions very well; not only those seen on the surface but also associated lesions [10, 11, 12} such as discoid menisci (Fig. 6), meniscal degeneration, meniscal cysts (Fig. 7), etc. A normal meniscus is characterized by its shape, size and low signal intensity on all pulse sequences. MRI accuracy is very high according to numerous authors with a reported sensitivity and specificity of over 90% [10, 11]. A negative MRI in patients harboring a meniscal tear has a high predictive value. Meniscal lesions are graded: grade 1 and 2 are characterized by an abnormal intense signal which does not communicate with the surfaces (they are mucoid degenerative changes); grade 3 communicates with an articular surface and correlates with arthroscopically detectable tears [10]. Acute traumatic tears in young athletes can have a normal lack of internal signal within the meniscus, with abnormal morphology which is the sole indicator of a tear [10]. Sometimes it is very difficult to classify rneniscal lesions in grade 2 or 3, particularly on the posterior hom of both menisci; this ambiguous situation was encountered in 14% (20/142) of the MRI examinations reported by Kaplan [11].
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Degenerative lesions Degenerative disc disease is a very common cause of functional incapacitation and one of the most frequent indications for MRI, which is the most sensitive technique for evaluating the intervertebral disc and the vertebral endplates [13). Radiographs show only the advanced lesions by indirect signs (narrowing, subchondral sclerosis and osteocytes). In the early stages MRI may show very subtle lesions: a decreased signal in the disc on T1 and T2 weighted images is the expression of dehydratation which appears before indirect osseous signs. "Degeneration leads to greater water loss in the nucleus than in the anulus... The disc becomes more fibrous and disorganized, resulting in an amorphous fibrous cartilage... The degenerative cycle includes disc degenerative disease, facet arthritis, ligamentous and capsular hypertrophy, spinal instability, and stenosis" [13]. The contrast sensitivity and multiplanar approach to MRl provide a unique noninvasive method showing disc herniation, in comparison with cr which is more frequently available. At the cervical and dorsal levels MRI is much more efficient than cr for depicting small herniation and, therefore, MRI should be chosen. Canal stenosis, ligamentous hypertrophy and facet disease can
Fig. 6: MRI. Lateral discoid meniscus of the right knee in a 14 year old boy. On sagittal ~nd co~nal T1 slices th~ extent is well visible. Increased signal intensity (arrowhead) in comparison with the normal medial meruscus (small white arrow). The hyaline normal cartilage (small black arrow) has a normal signal intensity.
Fig. 7: MRI. Cyst of the lateral meniscus (left knee). On coronal and sagittal T1 slices, the mass (arrow) is well appreciated and has an intermediate signal in comparison with the high signal of the subcutaneous fat (white).
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all be depicted with CT or MRI with almost equal performance. The evaluation of the postoperative spine in patients with residual or recurrent low back pain has always been a challenge. MRI appears to be more accurate than myelography or CT in the differentiation between scar and disc herniation, esper.ially using sequences after i.v. injection of gadolinium which enhances the scar signal. MRl is also very sensitive to the presence of other postoperative complications such as haematoma, CSF collection, arachnoiditis and infection. In a personal series of patients explored for failed back surgery syndrome, we observed a group of 25 patients who on MRI examination presented an inflammatory process after discotomy which may be misdiagnosed as infection. We propose naming this entity "pseudo-spondylodiscitis" [14). Today MR imaging seems to be the best technique for providing information on cartilage abnormalities [15). Hayes studied 14 freshly disarticulated knee specimens using MR imaging [16). A comparison was made between MRI findings and sectionned specimens, side-by-side, using the Shahriaree system for staging [17]. Although contrast between joint fluid and cartilage was not as high as with II or T2 weighted images, the authors found that II weighted images allowed the discrimination of focal lesions (>3 to 4 mm in diameter) and diffuse thinning. The II weighted images were adequate for visualizing stage III to V lesions in specimens. MRI also demonstrates the early stages of chondromalacia very well (Fig. 8). We arc currently exploring the sensitivity of gradient echo sequences in comparison with spin echo II weighted images for all cartilaginous and subchondral bone lesions of the knee; the initial results are promising.
Fig. 8: MRI. Chondromalacia patellae of the right knee (examined at 25° flexion). On transaxial slices the femoropatellar joint is well appreciated. A linear abnormal signal is obvious (arrow) and corresponds to a stage II lesion (verified by arthroscopy). On the T2 image the articular fluid (arrowhead) has a signal which is more intense on T2 (right) than on the subcutaneous fat and on proton density images (left). Inflammatory lesions Rheumatoid arthritis (RA) is a synovial membrane disease in its early stage. MRI has proven to be very sensitive in demonstrating exudative synovitis. The fibrous pannus has low signal intensity on T2 weighted images and contrasts
well with joint effusion. The hypervascular pannus is hyperintense and cannot be differentiated from fluid, cysts or zones of oedema. After application of gadolinium-DTPA the differentiation of hyperintense lesions is easier. The enhancement factor after contrast application is very high, according to Koning [18): 85% to 220% for hypervascular pannus, 10% for cartilage and bone marrow, 16% for muscle, and no enhancement of joint effusion. After the hands and feet, the cervical spine is the most common location of rheumatoid arthritis and a multitude of pathologic findings are usually associated with RA of this spinal segment. The involvement of CI-C2 causes subluxation which may have various neurologic complications and require surgical stabilization. Einig studied the role of MRI in a series of 60 patients with cervical RA (19J. 17% of the patients showed spinal cord compression and 20% showed an obliteration of the subarachnoid space. A dorsal erosion of the odontoid process was often seen in sagittal slices. The danger of cord compression is directly related to the degree of CI-C2 subluxation. MRI docs seem to be indicated for subluxation over 9 mm; subluxations between 6 and 9 mm were associated with an increased number of subarachnoid obliterations. MRI findings were also associated with an RA duration of more than 20 years with 25% cervical cord compression, 31% obliteration of subarachnoid space and 44% normal cord configuration. In ankylosing spondylitis (AS), Andersson described discovertebral destruction which mimics infective spondylitis. CT and MRI are helpful in distinguishing these lesions: Andersson lesions are characterized by the absence of the soft-tissue lesions around the vertebral
bodies frequently observed in infection [20). The temporomandibular joint may be involved in various rheumatic diseases, especially RA, AS and psoriatic arthropathy. MRI may demonstrate both bone and soft tissue lesions of various degrees, including fibrous pannus, disc destruction, joint fluid and bone destruction [21]. MRI has great potential for demonstrating temporomandibular joint abnormalities.
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Garcia: MRI of the musculoskeletal system
Osteonecrosis and transient osteoporosis 1. Avascular necrosis of the hip
Numerous studies have already shown that the early diagnosis of avascular necrosis (AVN) of the femoral head is easier today because of MRI which has the double advantage of being more sensitive and more specific than bone scintigraphy [22, 23]. The MRl aspects of AVN vary accord ing to the degree of evolution (Fig. 9). Mitchell classified them into four classes based on the intensity of the signals on T1 and T2 weighted images [22, 23].
Stage 0:
Normal radiography, abnormal MRl images
nannaI radionucl ide,
Stage 1:
Normal radiography, abnormal radionuclid e
Stage 2:
Sclerotic and/or lucent (cystic) areas without evidence of subchondral or cortical fracture
Stage 3:
Subchondral fracture , depicted as subcortical lucency (crescent sign)
Stage 4:
Collapse of the femoral articular cortex, depicted as flattening
Stage 5:
Superimposed degenerative hip disease
b Fig. 9: MRI. Bilateral avascular necrosis. a)
Coronal and sagittal slices of the right femoral head on T1 images: A characteristic "geographic map" appearance (arrow), class A of Mitchell on a normal plain X-ray (stage 1).
b)
Coronal and sagittal slices of the left side: a more advanced lesion with flattening (arrow) and a good correlation with the plain X-ray (stage 4).
The comparison of radiologic, bone scintigraphy and MRI findings is interesting and the author has proposed a classification in five stages that includes all information obtained from these three methcxls; this new classification is adapted from Ficat and Arlet:
2 Immature bone infarcts Bone infarcts habitually develop in the metaphysio -diaphyseal region of long bones, especially in the knee. In advanced forms the radiologic appearance is characteristic and includes a calcified enchondroma in the differential
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diagnosis. In the early stages, the diagnosis on plain Xrays is almost impossible because the lesion manifests itself only by a zone of subtle metaphyseal radiolucency which is not always outlined by peripheral sclerosis. MRI permitted a more precise diagnosis in four patients reported by Munk (24). On the Tl weighted images, the centre of the infarct has an intermediate signal which contrasts with the elevated signal of the surrounding fat. Its limit is sometimes marked by a fine band of low signal which corresponds to the beginning of the sclerosis. On T2 weighted images the central necrotic zone has a high signal. The extension of the lesion can be evaluated very well.
3. Transient osteoporosis In transient osteoporosis plain X-rays show focal osteopenia after a few weeks of evolution (usually six); this osteopenia is a late sign and non-specific [25]. Bone scintigraphy is much more sensitive and is characterized by an increased uptake after 4 weeks of evolution; bone scintigraphy is also non-specific. cr is not very helpful in showing focal osteopenia, neither are radiographs. MRI seems to be a good method for an early and correct diagnosis by showing bone marrow oedema [26, 27), joint effusion and sometimes a subcutaneous network of dilated veins. Bone marrow oedema appears as an area of low
Fig. 10: MRI. Transient osteoporosis of the right hip. a)
Coronal slices on T1 images: oedema involving the proximal femur (arrow) with enhancement of signal intensity after i.v. injection of gadclinium-DO'I'A (bottom).
b)
Transaxial slice on T2: the femoral head is heterogeneous because of the high intensity signal due to oedema (arrow).
Garcia: MRI of the musculoskeletal system
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Fig. 11: Staphylococcus aureus sub-acute osteomyelitis of the right tibia.
a) Radiography showing only a 2 ern lacuna (arrow). b) MRI showing all tissue components allowing a correct diagnosis. On the coronal T1 slice (left): oedema zone (large black arrow) centered by a cavity containing fluid (small white arrow) which is hypointense. After i.v. injection of gadolinium (middle) the pseudo-capsule is seen better (arrowhead) and the signal intensity is enhanced in the oedema zone. On the T2 image (right), fluid in the cavity has a high intensity signal (small black arrow).
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signal intensity on Tl weighted images, enhanced after gadolinium i.v. injection (Fig. 10) and high signal intensity on T2 weighted images. When transient osteoporosis is present on the hip, oedema is seen from the femoral head to the trochanter. During the evolution, oedema gradually disappears after the third month [28]. Turner described the MRI signs of bone marrow oedema [29J in five patients suffering from AVN (avascular necrosis) who presented early as being very similar to those of transient osteoporosis. Osteonecrosis was shown to be present in all patients by core biopsy and subsequent development of focal MRI abnormalities were reported to be highly specific for AVN [29]. Vascular congestion and oedema of the bone marrow are thought to occur early in the course of AVN and transient osteoporosis, as a non-specific response to an injury [29], and is probably more severe in cases of AVN. Infections Infections of the musculoskeletal system include a large variety of clinical situations with different patterns and always constitute a challenge for an early and correct diagnosis [30]. If diagnosed early, infection can be cured with antibiotic therapy and, sometimes, operative drainage and debridment. Complications of delayed or inadequate treatment are severe, especially in spondylodiscitis or arthritis.
1. Acute osteomyelitis In acute haematogenous osteomyelitis bone scintigraphy is still the best technique of choice because of its high sensitivity and to show the whole skeleton. Radiographs must be performed when the symptoms first appear even if they are normal because they are necessary in following up the evolution. In a few particular cases CT may be helpful in demonstrating an enhancement of the medullary content or, exceptionally, the presence of gas [30]. MRl shows very subtle bone marrow abnormalities which consist of a moderate to marked decrease of signal intensity compared to a strong signal in normal marrow on Tl weighted images, enhancement after i.v. injection of gadolinium, and increase of signal intensity on T2 weighted images. These modifications are mainly due to oedema. The associated soft tissue lesions are also well identified and their limits well defined [31]. The extent of the lesion is much better appreciated than in bone scintigraphy, which is notorious for its tendency to overestimate the extent. The previous remarks are valid for all locations, including the spine and extremities. According to Unger [32], MRI compared to bone scintigraphy is more sensitive (92% vs 82%) and more specific (96% vs 65 %). When local factors induce osteomyelitis, such as in diabetic foot, the problem is different For early diagnosis bone scintigraphy, radiographs and CT have the limitations mentioned above. Only few reports have been published concerning the role of MRI in diabetic foot but the preliminary results are encouraging: MRI provides accurate information concerning the presence and extent of infection and differentiating neuropathic joint from osteomyelitis [33]. We are presently evaluating this common problem at our institution.
2. Subacute osteomyelitis This form of infection is the so-called Brodie's abscess. MRI is definitely superior to other imaging modalities in showing all tissue components: liquid in the cavity, a wellvascularized pseudo-capsule and oedema around the focus of infection (Fig. 11).
3. Chronic and recurrent osteomyelitis Chronic and recurrent osteomyelitis is a much more difficult situation: clinical and biological information is often ambiguous. Imaging modalities are only helpful in showing a sequestrum, soft tissue lesions, and the extent. CT has been the technique of choice for many years for greater accuracy [30]. MRI has not been evaluated in a good prospective study but it seems to have the same possibilities as CT even though the calcified component could be underestimated.
4. Arthritis When clinical and biological data lead to the suspicion of arthritis a puncture must be carried out. It is the sole procedure for a correct diagnosis. In particular cases, when the patient status is confusing, bone scintigraphy and MRI may allow an early diagnosis. MRI may be more accurate in showing bone marrow oedema around the articular surfaces after two weeks of evolution, and fluid in the joint [30].
5. Soft tissue infections Whether or not associated with bone lesions, soft tissue infections are well appreciated by CT and MRI in every location. MRI is more accurate than CT in depicting epidural abscesses [13]. Tumours 1. Primary bone tumours
a) Benign In benign tumours the role of MRI is very limited despite a multiplanar approach and very good tissue differentiation. All imaging techniques are efficient for different reasons, especially plain X-rays and CT for evaluating the extent, relationships and morphology before a possible biopsy; a good example is osteoid osteoma. MRI may provide good information especially for non-calcified components but in general this technique is not necessary. b) Malignant The situation is quite different in the case of suspicion of malignancy on plain X-rays. Bone scintigraphy must be performed to estimate the number of lesions. MRI is not very helpful for diagnosis even if almost all large tumours are heterogeneous and the different tissue components well appreciated (Fig. 12). The local staging examination is essentially based on MRI because it allows the best estimation of intramedullary extension, intraarticular extension,
Garcia: MRJ of the musculoskeletal system
soft tissue extension [2, 34, 35]. During examination the field of view (zone of interest) must be large in order not to miss skip-lesions (small metastases separated from the main tumour by healthy tissue). In more recent reports, MRI appears to be superior to Cf for local staging [34, 35, 36]. Like Cf, MRI has limitations: inability to detect very small lesions and overestimation of the tumour volume on T2 weighted images because of peritumoral oedema [37].
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tumour volume [34, 36]. In large cancer centres investigators are working to define follow-up criteria based on MRI examination [36, 38]. After placement of a non-paramagnetic metal prosthesis, MRI allows evaluation of periprothetic soft tissues without artifacts in contrast to Cf images in which it may be degraded [34].
Fig. 12: Osteosarcoma of the right femur. a)
Radiography showing a very subtle osteopenia in the shaft (black arrow head) with a Cadman's triangle (white arrowhead) and very fine cortex destruction (small white arrow).
b)
MRI. Coronal T1 slice: excellent demonstration of extension into the medullary cavity (black arrowhead) and huge involvement of the soft tissue (white arrowhead) with a different signal intensity. Some oedema between the tumour distal limit (white arrow) and the cartilage line (black arrow).
c)
MRI. Transaxial T2 slice: in the soft tissue mass (white arrowhead), numerous cavities with septation and fluid-fluid levels (small black arrow). Femoral vessels are closed (large black arrow). The whole shaft is involved (black arrowhead).
Detection of pulmonary metastases is based on plain Xrays and chest Cf; detection of other bone lesions is based on bone scintigraphy. Most primary malignant tumours are treated with chemotherapy and irradiation (Fig. 13) MRI appears to be a very accurate technique for studying necrosis and
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When symptoms are not explained and scintigraphy is normal; case reports of metastases seen on MRI and not on scintigraphy have been published. To differentiate an osteoporotic vertebral collapse from metastases, Bone metastases are disseminated haematogenously and develop in the well-vascularized cancellous bone. This explains the high sensitivity of MRJ in the detection of small metastatic foci, even before bone scintigraphy. In the presence of vertebral collapse we must differentiate a benign osteoporotic lesion from a malignant one. When bone scintigraphy and plain X-rays are insufficient to define the aetiology, MRI may be helpful. In old benign collapse (more than 3 months old), the bone marrow .ign als are similar to those of adjacent vertebrae. In acut e benign osteoporotic collapse there is a decreased signal on T1 weighted images and a moderate increased signal on T2 weighted images. In malignant collapse the sign al enhancement on T2 is increased far more, there is a high enhancement after i.v, injection of gadolinium and sometimes a soft tissue extension into the spinal canal or around the vertebral body. These findings are not always sufficient for correct diagnosis but, nevertheless, MRI is more accurate than other imaging modalities. 3. Soft tissue tumours
Despite the advantages of sonography, cr has been the choice technique for evaluating soft tissue masses for many years [2, 40]. Recent studies, including our own experience during the last three years [40), indi cate that MRJ is generally superior to cr in lesion detection and in characterizing the extent of involvement of adjacent bone and soft tissue structures.
Fig. 13: MRI. Ewing sarcoma of the right ilium in an 8 year old girl, 2 1/2 years after chemotherapy and irradiation . Tl coronal and T2 transaxial slices show a high intensity signal in the ilium and the sacrum (irradiated area, arrowhead). No signs of recurrence. High intensity sig na l due to fatty infiltration. 2. Bone metastases Bone metastases are a very common problem. When the primary tumour is known bone scintigraphy is still the choice technique for surveillance. MRI may be useful in some situations as listed below: Whcn there is a discrepancy between an increased uptake on bone scintigraphy and a normal appearance on plain X-rays (Fig. 14). When it is necessary to appreciate the extent and soft tissue component, for example, in the case of cord compression. When a recurrence is suspected after chemotherapy or irradiation .
MRJ is characterized by a great potential in differentiating tissues and offers a multiplanar approach. Better tissue differentiation is particularly useful in regions where there is only a small amount of fat such as in the distal extremities. As in CT, adipose tissue is characteristic in MRJ; therefore, lipomas (Fig. 15) and liposarcomas are easy to identify. Cysts are well marginated, adjacent to joints and show very characteristic signals (40) . Vascular tumours, various types of haemangiomas, are better appreciated by MRJ (41). Desmoid tumours are often infiltrating and tend to recur following resection and pres ent a heterogeneous appearance on MRI which may be misdiagnosed as a malignancy. Most malignant tumours are heterogeneous. This appearance may be secondary to tumour necrosis, calcifications or the type of tumour matrix. This heterogenicity is much more apparent after i.v. injection of gadolinium. Tumoral calcifications are underestimated by MRI (Fig. 16) and better seen on plain X-rays and CT. The local staging of soft tissue sarcomas is mainly performed with MRI examination as well as for bone sarcomas. The local surveillance after treatment is also based on MRI which can detect even small recurrences [421.
Garcia: MRI of the musculoskeletal system
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Fig. 14: MRI. TH. 12 vertebral body metastases (adenocarcinoma of the duodenum). Sagittal slices: tumoral involvement hypointense on T1 (white arrowhead, left), hyperintense on 1'2 (black arrowhead, right) in comparison with normal ad jacent vertebrae. A focal uptake was seen on scintigraphy; radiographs and conventional tomographs were normal.
Fig. 15: MRI. Lipoma of r.e right buttock The tumour (white arrowhead) has a high intensity signal, is homogeneous and well-delineated. Its 'relations are well appreciated in 3 planes. The tumour compresses the sciatic nerve (small black arrow). A large uterus is seen (black arrowhead, 3rd month of pregnancy).
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Fig. 16: MRl. Primary soft tissue chondrosarcoma of the right thigh. a) Sagittal slices: a soft tissue mass (arrowhead) is seen on the anterior region with an intermediate signal intensity on T1 (left) and an intense enhancement after i.v. gadolinium-DOTA (right) which also emphasizes the heterogeneity (small arrow). b) Transaxial slices: the tumour matrix (arrowhead) is much more intense than normal adjacent muscles on TR 2000 TE 30 (left) and TR 2000 TE 60 (right). It is a myxoid component. Small calcifications (small arrow) were underestimated (but well seen on plain X-rdys). Conclusion
References
MRl is a new and advanced imaging modality. Its role has grown very rapidly in studying the musculoskeletal system during the last five years. MRI permits an early diagnosis in stress and pathological fractures, avascular necrosis, transient osteoporosis, infection and metastases. It is also a very accurate method for diagnosing soft tissue injury, degenerative disc disease (especially at cervical and dorsal level and after surgery at the lumbar level), involvement of C1-C2 level during the late stages of rheumatoid arthritis, and for the local staging of malignant primary bone tumours and soft tissue tumours.
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Koob L, Simm C (1991) Ten years of clinical resonance: Today's state-of-the-art system. Eur. Radiol 1:95-107
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Girard 1M, Garcia 1 (1991) lmagerie par resonance magnetique dans les fractures de fatigue ct par insuffisance. 1. Radiol. 72:79-89
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In the future, MRI developments will focus on two major requirements: more versatile and flexible application capabilities and general system efficiency - MRI in medicine has a bright future [1].
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Erickson SJ, Cox IH, Hyde JS, Carrera GF, Strandt JA, Estkowski LD (1991) Effect of tendon orientation on MR imaging signal intensity: A manifestation of the "magic angle" phenomenon. Radiology 181:389-392
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Reicher MA, Kellerhouse LE (ed) (1990) MRI of the wrist and hand. Raven Press, New York
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Garcia J (1990) IRM de la main et du poignet, Chap. Vi, 67-84. In: Morvan G, Godefroy D (ed) IRM osteoarticulaire. Sauramps Medical, Paris
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Kursunoglu-Brahme S, Gundry CR, Resnick D (1990) Advanced imaging of the wrist Radiol Cl. N. America 28:307-320
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Crues 1lI JV, Ryu R, Morgan FW (1990) Meniscal Pathology. The expanding role of magnetic resonance imaging. Cl Orthop 252:80-87
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imaging and tomographic manisfestations. Radiology 175:527-531 22. Mitchell DG, Vigay MR, Dalinka MI<, Spritzer CE, Alavi A, Steinberg ME, Fallon M, Kressel HY (1987) Femoral head avascular necrosis: Correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings . Radiology 162:709-715
23. Mitchell DG, Steinberg ME, Dalinka MK, Rao VM, Fallon M, Kressel HY (1989) Magnetic resonanc e imaging of the ischemic hip; alterations within the osteonecrotic, viable and reactive zones. Clin. Orthop, 224:60-77 24. Munk PL, Helms CA, Holt RG (1989) Immature bone infarcts: Findings on plain radiographs and MR scans. AJR 152:547-549 25.
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Higer HP, Grimm J, Pedrosa P, Apel R, Bandilla K (1989) Transitorische Osteoporose oder Femurkopfnecrose ? Friihdiagnose mit der MRT. Fortschr. Rontgenstr 150:407-412
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29. Turner DA, Templeton AC, Selzer PM, Rosenberg AG, Petasnick JP (1989) Femoral capital osteonecrosis: MR finding of diffuse marrow abnormalities without focal lesions. Radiology 171:135-140
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Garcia J (1990) lmagerie des infections de l'appareil locomoteur. ]. Radiol. 71:641-655
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Dalinka MK, Zlatkin MB, Chao P, Kricun ME, Kressel HY (1990) The use of magnetic resonance imaging in the evaluation of bone and soft tissue tumors. Radiol. Cl. N. America 28:461-470
Chondromalacia. Contemp.
Koning H, Sieper J, Wolf KJ (1990) Rheumatoid arthritis : Evaluation of hypervascular and fibrous pannus with dynamic MR imaging enhanced with GD-DTPA. Radiology 176:473.477 Einig M, Higer HP, Meairs S, Faust-Tinnefeldt G, Kapp H (1990) Magnetic resonance imaging of the cranio-cervical junction in rheumatoid arthritis: Value, limitations, indications. Skeletal Radiol 19:341346 Kenny JB, Hugues AL, Whitehouse GH (1990) Discovertebral destruction in ankylosing spondylitis: The role of computed tomography and magnetic resonance imaging. Br. J. Radiol. 63:448-455 Larheim TA, Smith H}, Aspestrand F (1990) Rheumatic disease of tempora-mandibular joint: MR
36. Seeger LL, Eckardt H, Bassett LW (1989) Crosssectional imaging in the evaluation of osteogenic sarcoma: MRI and cr. Sem. Roentgenology 24 (3):174-184
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Beltran J, Simon DC, Katz W, Weis LD (1987) Increased MR signal intensity in skeletal muscle adjacent to malignant tumors: Pathologic correlation and clinical relevance. Radiology 162:251-255
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Lemmi MA, Fletcher BD, Marina NM, Slade W, Parham DM, Jenkins 11, Meyer WH (1990) Use of MR imaging to assess results of chemotherapy for Ewing sarcoma. AJR 155:343-346
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Vanel D, Brossel R, Bonnerot V, Coffre C, Contesso G (1990) MRl in the follow-up of primary musculoskeletal tumors, 391-399. In: Bydder G et al (ed) Contrast Media in MRl. International Workshop, Medicom, Bussum
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Garcia J (1991) lmagerie des masses des tissus mous des membres. In: COUlS de perfectionnement, Societe Francaise De Radiologie, Paris
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Hawnaur JM, Whitehouse RW, Jenkins JPR, Isherwood I (1990) Musculoskeletal haemangiomas: Comparison of MRl with CT. Skeletal Radiol 19:251258
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Reuther G, Mutscher W (1990) Detection of local recurrent disease in musculoskeletal tumors: Magnetic resonance imaging versus computed tomography. Skeletal Radiol. 19:85-90
Magnetic resonance of the musculoskeletal system Jean Garcia M.D. Department of Radiology Hopital Cantonal Universitaire Geneva, Switzerland
Introduction The role of magnetic resonance imaging (MRI) for the evaluation of the musculoskeletal system is expanding rapidly. The most important limit for its growth is the access to a machine. Today, in our institution, a general hospital with 1400 beds, 30% of machine-time is devoted to the musculoskeletal system (and 50% to the central nervous system). About 5000 MRI systems are now installed; more than 50% of these units are installed in the USA with a majority of high performance systems of 1.0 and 1.5 Tesla field strength [1], but only 16% of the units are in Europe. The density situation in Europe reflects political influence regarding health care system restrictions. In addition to being noninvasive, MRI offers high-soft tissue contrast and multiplanar imaging. MRI has better soft tissue contrast than other imaging modalities [2]. Therefore, MRI has become an effective modality in evaluating numerous situations by permitting the right diagnosis. It offers early diagnosis in cases of stress fracture, avascular necrosis, transient osteoporosis, infections, metastases and also a good appreciation of the lesions in spinal malformations, soft tissue trauma, degenerative lesions, infections, tumours, etc. Quality criteria At the beginning of a revue such as this one, it is necessary to point out the different quality criteria: technical, clinical data, radiological synthesis, clinical radiological correlation and human quality. 1.
Technical Considerations
The field-strength question has divided the MRI community since the early 1980s. From analytical applications of MRI, it was known that the signal-ta-noise ratio increases when you increase the field strength. But higher fields also have technical disavantages. In the future, the new generation of buyers, the smaller hospitals and private practices, will prefer cost-efficient MRI systems which they
can use for most of the daily routine examinations. Bigger hospitals, and in particular those interested in spectroscopy research, will prefer high-field systems. In our own institution we only have one 1.5 T machine for clinical and research use and we are obliged to be very strict in our indications. MR imaging examinations must be conducted properly. Patient selection, positioning, coil selection and pulse sequences must all be considered to optimize information on the images [2]. Patient selection must take into consideration those with claustrophobia (2 - 3% of patients), patient size which is rarely a problem with body coils but more often with the surface coils. Patients with cardiac and respiratory monitoring must be excluded. Numerous heart valves have been studied; they create artifacts which are negligible and patients with prosthetic valves can safely be imaged. Patients with pacemakers or aneurysm clips are currently not examined. Orthopedic appliances are usually not ferromagnetic but may contain minimal quantities of iron impurities. Metal heating in patients should not be a problem even with large protheses [2]. Metallic materials cause less artifact on MR images than on CT images. The degree of artifact depends on the size, configuration of the metal and the sequence used (more artifact on field echo sequence than on T1 spin-echo sequence). Patient positioning considerations include size, body part, and expected examination time. The patient should be studied with the most closely coupled coil possible to achieve the maximum signal-to-noise ratio and best spatial resolution [2]. The spine is examined with rectangular surface coils (with a small curvature for the cervical level and a flat coil for the dorsal and lumbar levels). The hips and legs are examined in the body coil, the knees in a specially designed coil, the ankles and feet are comfortably examined in a head coil. The shoulder still remains a difficult area despite the use of a surface coil: the offcentre, i.e. the distance between the shoulder and the magnet isocentre, is quite large, and it is difficult to avoid respiratory motion artifacts. A small circular surface coil is usually utilized for the hand. As a rule, the depth of view of a flat coil is approximately one-half the diameter or width of the coil. In addition, the surface coils are most effective when centered in the magnetic field and the
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signal-to-noise ratio may be increased four to six times compared to the body coils [2]. The MRI pulse sequences mostly used are the spin echo (SE) technique and the gradient echo technique. In spin echo technique the so-called T1 weighted images are obtained using short TR « 500 msec) and short TE (20 rnsec), and T2 weighted images with long 1R (>2000 msec) and long TE (>60 msec). In gradient echo technique the flip angle used is less than 9QO, 100 to 200 in our practice. The main advantage of gradient echo technique is short imaging times. MRI examination is indeed time consuming and MRI indications are so numerous that each examination should be carried out properly to save time without losing useful information. Multiplanar imaging is one of the principal advantages of MR imaging: transaxial, coronal, sagittal and even differen t oblique slices may be obtained. The plane selection depends on different factors: anatomy (for example, the an terior cruciate ligament is oblique and the MRI slices should be parallel), extent of the lesion (sagittal slices of the femoral head are very useful in avascular necrosis, a nd for spine imaging), etc. The field of view and the thickness of the slice must be adapted to the volume of interest: for example, 10 slices of 4 mm thickness for a femoral head, and 18 transaxial slices of 10 mm thickness for an osteosarcoma involving the distal femoral shaft. Usually only a small number of slices, about 10, can be obtained with a short TR (Tl weighted images); long TR (T2 we ighted images) allows a greater number of slices. With recent technological progress, the spatial resolution is good enough but still remains inferior to conventional radiography and therefore a subtle periosteal reaction is better seen on plain Xvrays, particularly since the calcium component has no signal. In recent years MRI has received new impulses from the clinical introduction of intravenous paramagnetic agents, e.g. gadolinium DIPA and gadolinium DOTA which have more or less the same qualities. Gadolinium is a paramagnetic agent that explores the vascular compartment and reduces the T1. Its usage is very wide; the main indications being tumours, infections, scar versus disc herniation, etc. It is a very accurate complement to the usual sequences. 2.
Clinical data
Although it may seem crucial to have clinical data before MRI examination, in daily practice this is not always the case. In practice, clinical data are not always available before MRl examination. Many clinicians indeed have no knowl edge of the limits of MRI and they do not give enough information to the radiologist. There are, for example, many ways of examining a knee: examination for cruciate ligaments or chondromalacia are quite different. The knee is a very good example in terms of management; MRI examination may show everything and, of course, everyone must shorten an examination when all useful information has been obtained.
3.
Radiological synthesis
All imaging techniques are characterized by a certain sensitivity and specificity. Most of them are complementary. For example, in acute osteomyelitis, bone scintigraphy is more sensitive than plain X-rays, although the latter offer us a good morphology of bony structures, particularly to appreciate the evolution. MRI is at least as sensitive as bone scintigraphy for an early diagnosis of acute osteomyelitis, but in cases of a blood disseminated infection, bone scintigraphy has the great advantage of exploring the whole skeleton. Everyone should know the technical limits to exploring a particular problem properly; do not hesitate to ask advice if in doubt. 4.
Correlation between clinical and radiological data
Magnetic resonance is a very advanced and sophisticated imaging technique, but it is not a "magic tool" which gives us all the answers. The basic principle of strict correlation between clinical and radiological data is still mandatory. MRI may give very subtle information, for example, the presence of oedema in bone marrow; but oedema is not specific. Is the oedema, in a particular case, due to transient osteoporosis, an early phase of avascular necrosis, a stress fracture or associated with an inflammatory or tumoral process? MRI findings must be correlated with clinical and other radiological data to improve its efficiency.
5.
Human quality
MRI is a new and very expansive imaging modality. Everybody is discussing the cost of machines and maintenance, especially administrators. However, the education of technicians and physicians should not be forgotten as it also needs time and money ! Trauma Musculoskeletal injuries are quite common. Bony lesions are generally diagnosed using routine radiography, conventional or computed tomography and bone scintigraphy. Evaluation of soft tissue injury is more difficult. The role of MRI is limited in acute skeletal injuries but more significan t in the evaluation of soft tissue trauma.
1. Fractures Fractures and fracture dislocations of the spine can be detected by radiography. CT is particularly useful in evaluating the position of bone fragments in the spinal canal and lesions affecting the neural arch. In acute injury, the ability of MRI to show antero-posterior displacement and possible cord compression in the sagittal plane is very impressive. In this situation the role of MRl is limited, especially if new neurological signs appear, MRl IS more useful, for example, in showing a syringomyelia. Skeletal traumata of the extremities are still accurately evaluated by conventional radiology. Complex pelvic fractures, especially those affecting the acetabulum and the posterior elements, are well evaluated by CT. Stress and
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pathological fractures are detected earlier by bone scintigraphy. MRI can be used to detect oedema of the bone marrow after acute trauma without any evidence of bone lesions in plain X-rays and also to detect stress and pathological fractures during the early phase (Fig. 1). MRI is at least as sensitive as bone scintigraphy and more specific, its use must be limited to particular conditions, especially when the differential diagnosis is difficult, for example, in the case of metastasis [3].
Fig. 2: MR!. Normal appearance of the tendon of the tibialis posterior (arrow) on a medial sagittal slice (T1). T = Tibia, N = Navicular bone, QT = quadratus plantae. Oedema is very well seen when it appears in the muscle, for example, during the early phase of myositis ossificans circumscripta (Fig 3); in this situation it is a very early sign which appears after two weeks of evolution, before the increased uptake seen on bone scintigraphy (at least four weeks) and soft tissue calcifications seen on plain Xrays (at least six weeks).
Fig. 1: MRI. Pathological fracture of the right tibia in a 60 year old woman after 2 weeks evolution and not seen on plain X-rays. The fracture line (arrowhead) is surrounded by oedema (hypo intense in comparison with fat). The patient was treated one year previously for lung cancer. 2. Soft tissue injuries
Evaluation of soft tissue injury is quite difficult, particularly when localized deeply. Today, perhaps more in Europe than in the USA, sonography is extensively used for the diagnosis of large tendon and muscle traumata which are often related to sports. The excellent and unequalled soft tissue contrast provided by MRI allows the best differentiation of muscle, fat, nerves and blood vessels. The normal tendons and ligaments appear as bands of low intensity and homogeneous signal both on T1 and T2 weighted images (Fig. 2). This appearance is attributed to the fact that they are composed predominantlyof dense collagen. The intra-articular fibrocartilage, such as the menisci and the triangular fibra-cartilage of the wrist, also have the same appearance and appear as a black area. The signal of hyaline cartilage is slightly more intense. Muscle signal intensity is qualified as intermediate and the fat signal is very intense on both T1 and T2 weighted images. In SE technique, liquid is usually characterized by a low intensity on T1 and a very high intensity, higher than fat, on T2.
Muscle tears are, of great importance and common in trained and untrained athletes. Determination of the degree of injury is important for treatment and prognosis, especially in the professional category. Sonography is still very useful, but MRI became the technique of choice because it is not examiner-dependent, it allows a good evaluation of the extent of injury and may exclude other pathology such as infections, tumours or atrophy, etc.
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Fig. 3: MR!. Myositis ossificans circumscripta of the right gluteus minimus after 2 weeks evolution. a)
Tl coronal slice (top), the lesion is very subtle (arrow). After i.v, injection of gadolinium-DTPA (bottom) there is a signal enhancement (arrowhead).
b)
T2 transaxial slices: the oedema involving a single musc:le is very intense (arrow head).
Any contour deformity and increased signal intensity of a tendon is usually considered pathologic and indicative of tendinous tear (Fig. 4), degeneration or inflammation. The large superficial tendon of the knee and the Achilles tendon are well appreciated by sonography and MRI. MRl is more efficient for deep tendons. Rotator cuff injury is usually considered to be chronic in aetiology [4]. Tears do not occur unless the tendon is weakened; Neer feels that rotator cuff injury represents a continuum of disease: from oedema and haemorrhage at the beginning to a frank tear at the end . On MR images the differential diagnosis between tendinitis, partial tear and tendon degeneration is very difficult A frank tear, tendon retraction and muscle atrophy (fatty involvement) are well seen and of great value (Fig. 5); in this situation, MRI is superior to arthrography which shows only the tear. The most significant early MRl finding in shoulder impingment syndrome is an increased signal in the supraspinatus tendon near its insertion, presumably due to inflammation, oedema,
haemorrhage or a combination of the three [5]. Nevertheless, the tendon orientation may affect tendon signal intensity and be misdiagnosed as a lesion [6].
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The normal and pathologic appearances described above for tendons are also valid for ligaments such as the cruciate ligaments. The anterior cruciate ligament is smaller and oblique and therefore more difficult to study than the posterior one. However, today MRI is the technique of choice for evaluation of the cruciate ligaments, showing not only frank tears, but also partial tears. Lateral ligaments of the knee and ankle are also well appreciated by MRI in the coronal plane.
Fig. 4: MRI. Complete tear of the tibialis posterior tendon (arrow): contour deformity and increased signal intensity compared to Fig. 2.
The scapholunate and lunotriquetral interosseous ligaments are not always well seen on coronal MR images and until now we used wrist arthrography for evaluating the tears of these ligaments, however, Reicher and Kellerhouse find MRI to be efficient [7]. It is much easier to obtain very good images of the triangular fibrocartilage complex in our practice with MR images [8]. MRI had a good accuracy of 90% in a personal series of 30 posttraumatic lesions (17 of them controlled by arthrography and surgical findings). Degeneration of the fibrocartilage can be visualized on MR images as a disruption of the normal radial attachments and an effusion is identified by its high signal intensity on T2 weighted images [91.
rig. :J: Uld tear ot the ngnt rotator curt. a)
MRI. Coronal slices of the right shoulder on T1 (top) and T2 (bottom). The suprasupinatus tendon is thin (small arrow), the tear is obvious (arrow head), and there is a fatty infiltration of muscle mass (large arrow).
b)
Arthrography (same patient): the contrast medium fills in the tear (arrowhead) and the gleno-humeral bursa (black arrow) communicates with the sub-acromial bursa (white arrow). Supra supinatus lesions are underestimated.
Garcia: MRI of the musculoskeletal system
Arthroscopy is still considered the best way to diagnose intra-articular knee problems but that may change since MRI shows all meniscal lesions very well; not only those seen on the surface but also associated lesions [10, 11, 12} such as discoid menisci (Fig. 6), meniscal degeneration, meniscal cysts (Fig. 7), etc. A normal meniscus is characterized by its shape, size and low signal intensity on all pulse sequences. MRI accuracy is very high according to numerous authors with a reported sensitivity and specificity of over 90% [10, 11]. A negative MRI in patients harboring a meniscal tear has a high predictive value. Meniscal lesions are graded: grade 1 and 2 are characterized by an abnormal intense signal which does not communicate with the surfaces (they are mucoid degenerative changes); grade 3 communicates with an articular surface and correlates with arthroscopically detectable tears [10]. Acute traumatic tears in young athletes can have a normal lack of internal signal within the meniscus, with abnormal morphology which is the sole indicator of a tear [10]. Sometimes it is very difficult to classify rneniscal lesions in grade 2 or 3, particularly on the posterior hom of both menisci; this ambiguous situation was encountered in 14% (20/142) of the MRI examinations reported by Kaplan [11].
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Degenerative lesions Degenerative disc disease is a very common cause of functional incapacitation and one of the most frequent indications for MRI, which is the most sensitive technique for evaluating the intervertebral disc and the vertebral endplates [13). Radiographs show only the advanced lesions by indirect signs (narrowing, subchondral sclerosis and osteocytes). In the early stages MRI may show very subtle lesions: a decreased signal in the disc on T1 and T2 weighted images is the expression of dehydratation which appears before indirect osseous signs. "Degeneration leads to greater water loss in the nucleus than in the anulus... The disc becomes more fibrous and disorganized, resulting in an amorphous fibrous cartilage... The degenerative cycle includes disc degenerative disease, facet arthritis, ligamentous and capsular hypertrophy, spinal instability, and stenosis" [13]. The contrast sensitivity and multiplanar approach to MRl provide a unique noninvasive method showing disc herniation, in comparison with cr which is more frequently available. At the cervical and dorsal levels MRI is much more efficient than cr for depicting small herniation and, therefore, MRI should be chosen. Canal stenosis, ligamentous hypertrophy and facet disease can
Fig. 6: MRI. Lateral discoid meniscus of the right knee in a 14 year old boy. On sagittal ~nd co~nal T1 slices th~ extent is well visible. Increased signal intensity (arrowhead) in comparison with the normal medial meruscus (small white arrow). The hyaline normal cartilage (small black arrow) has a normal signal intensity.
Fig. 7: MRI. Cyst of the lateral meniscus (left knee). On coronal and sagittal T1 slices, the mass (arrow) is well appreciated and has an intermediate signal in comparison with the high signal of the subcutaneous fat (white).
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all be depicted with CT or MRI with almost equal performance. The evaluation of the postoperative spine in patients with residual or recurrent low back pain has always been a challenge. MRI appears to be more accurate than myelography or CT in the differentiation between scar and disc herniation, esper.ially using sequences after i.v. injection of gadolinium which enhances the scar signal. MRl is also very sensitive to the presence of other postoperative complications such as haematoma, CSF collection, arachnoiditis and infection. In a personal series of patients explored for failed back surgery syndrome, we observed a group of 25 patients who on MRI examination presented an inflammatory process after discotomy which may be misdiagnosed as infection. We propose naming this entity "pseudo-spondylodiscitis" [14). Today MR imaging seems to be the best technique for providing information on cartilage abnormalities [15). Hayes studied 14 freshly disarticulated knee specimens using MR imaging [16). A comparison was made between MRI findings and sectionned specimens, side-by-side, using the Shahriaree system for staging [17]. Although contrast between joint fluid and cartilage was not as high as with II or T2 weighted images, the authors found that II weighted images allowed the discrimination of focal lesions (>3 to 4 mm in diameter) and diffuse thinning. The II weighted images were adequate for visualizing stage III to V lesions in specimens. MRI also demonstrates the early stages of chondromalacia very well (Fig. 8). We arc currently exploring the sensitivity of gradient echo sequences in comparison with spin echo II weighted images for all cartilaginous and subchondral bone lesions of the knee; the initial results are promising.
Fig. 8: MRI. Chondromalacia patellae of the right knee (examined at 25° flexion). On transaxial slices the femoropatellar joint is well appreciated. A linear abnormal signal is obvious (arrow) and corresponds to a stage II lesion (verified by arthroscopy). On the T2 image the articular fluid (arrowhead) has a signal which is more intense on T2 (right) than on the subcutaneous fat and on proton density images (left). Inflammatory lesions Rheumatoid arthritis (RA) is a synovial membrane disease in its early stage. MRI has proven to be very sensitive in demonstrating exudative synovitis. The fibrous pannus has low signal intensity on T2 weighted images and contrasts
well with joint effusion. The hypervascular pannus is hyperintense and cannot be differentiated from fluid, cysts or zones of oedema. After application of gadolinium-DTPA the differentiation of hyperintense lesions is easier. The enhancement factor after contrast application is very high, according to Koning [18): 85% to 220% for hypervascular pannus, 10% for cartilage and bone marrow, 16% for muscle, and no enhancement of joint effusion. After the hands and feet, the cervical spine is the most common location of rheumatoid arthritis and a multitude of pathologic findings are usually associated with RA of this spinal segment. The involvement of CI-C2 causes subluxation which may have various neurologic complications and require surgical stabilization. Einig studied the role of MRI in a series of 60 patients with cervical RA (19J. 17% of the patients showed spinal cord compression and 20% showed an obliteration of the subarachnoid space. A dorsal erosion of the odontoid process was often seen in sagittal slices. The danger of cord compression is directly related to the degree of CI-C2 subluxation. MRI docs seem to be indicated for subluxation over 9 mm; subluxations between 6 and 9 mm were associated with an increased number of subarachnoid obliterations. MRI findings were also associated with an RA duration of more than 20 years with 25% cervical cord compression, 31% obliteration of subarachnoid space and 44% normal cord configuration. In ankylosing spondylitis (AS), Andersson described discovertebral destruction which mimics infective spondylitis. CT and MRI are helpful in distinguishing these lesions: Andersson lesions are characterized by the absence of the soft-tissue lesions around the vertebral
bodies frequently observed in infection [20). The temporomandibular joint may be involved in various rheumatic diseases, especially RA, AS and psoriatic arthropathy. MRI may demonstrate both bone and soft tissue lesions of various degrees, including fibrous pannus, disc destruction, joint fluid and bone destruction [21]. MRI has great potential for demonstrating temporomandibular joint abnormalities.
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Garcia: MRI of the musculoskeletal system
Osteonecrosis and transient osteoporosis 1. Avascular necrosis of the hip
Numerous studies have already shown that the early diagnosis of avascular necrosis (AVN) of the femoral head is easier today because of MRI which has the double advantage of being more sensitive and more specific than bone scintigraphy [22, 23]. The MRl aspects of AVN vary accord ing to the degree of evolution (Fig. 9). Mitchell classified them into four classes based on the intensity of the signals on T1 and T2 weighted images [22, 23].
Stage 0:
Normal radiography, abnormal MRl images
nannaI radionucl ide,
Stage 1:
Normal radiography, abnormal radionuclid e
Stage 2:
Sclerotic and/or lucent (cystic) areas without evidence of subchondral or cortical fracture
Stage 3:
Subchondral fracture , depicted as subcortical lucency (crescent sign)
Stage 4:
Collapse of the femoral articular cortex, depicted as flattening
Stage 5:
Superimposed degenerative hip disease
b Fig. 9: MRI. Bilateral avascular necrosis. a)
Coronal and sagittal slices of the right femoral head on T1 images: A characteristic "geographic map" appearance (arrow), class A of Mitchell on a normal plain X-ray (stage 1).
b)
Coronal and sagittal slices of the left side: a more advanced lesion with flattening (arrow) and a good correlation with the plain X-ray (stage 4).
The comparison of radiologic, bone scintigraphy and MRI findings is interesting and the author has proposed a classification in five stages that includes all information obtained from these three methcxls; this new classification is adapted from Ficat and Arlet:
2 Immature bone infarcts Bone infarcts habitually develop in the metaphysio -diaphyseal region of long bones, especially in the knee. In advanced forms the radiologic appearance is characteristic and includes a calcified enchondroma in the differential
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diagnosis. In the early stages, the diagnosis on plain Xrays is almost impossible because the lesion manifests itself only by a zone of subtle metaphyseal radiolucency which is not always outlined by peripheral sclerosis. MRI permitted a more precise diagnosis in four patients reported by Munk (24). On the Tl weighted images, the centre of the infarct has an intermediate signal which contrasts with the elevated signal of the surrounding fat. Its limit is sometimes marked by a fine band of low signal which corresponds to the beginning of the sclerosis. On T2 weighted images the central necrotic zone has a high signal. The extension of the lesion can be evaluated very well.
3. Transient osteoporosis In transient osteoporosis plain X-rays show focal osteopenia after a few weeks of evolution (usually six); this osteopenia is a late sign and non-specific [25]. Bone scintigraphy is much more sensitive and is characterized by an increased uptake after 4 weeks of evolution; bone scintigraphy is also non-specific. cr is not very helpful in showing focal osteopenia, neither are radiographs. MRI seems to be a good method for an early and correct diagnosis by showing bone marrow oedema [26, 27), joint effusion and sometimes a subcutaneous network of dilated veins. Bone marrow oedema appears as an area of low
Fig. 10: MRI. Transient osteoporosis of the right hip. a)
Coronal slices on T1 images: oedema involving the proximal femur (arrow) with enhancement of signal intensity after i.v. injection of gadclinium-DO'I'A (bottom).
b)
Transaxial slice on T2: the femoral head is heterogeneous because of the high intensity signal due to oedema (arrow).
Garcia: MRI of the musculoskeletal system
539
Fig. 11: Staphylococcus aureus sub-acute osteomyelitis of the right tibia.
a) Radiography showing only a 2 ern lacuna (arrow). b) MRI showing all tissue components allowing a correct diagnosis. On the coronal T1 slice (left): oedema zone (large black arrow) centered by a cavity containing fluid (small white arrow) which is hypointense. After i.v. injection of gadolinium (middle) the pseudo-capsule is seen better (arrowhead) and the signal intensity is enhanced in the oedema zone. On the T2 image (right), fluid in the cavity has a high intensity signal (small black arrow).
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signal intensity on Tl weighted images, enhanced after gadolinium i.v. injection (Fig. 10) and high signal intensity on T2 weighted images. When transient osteoporosis is present on the hip, oedema is seen from the femoral head to the trochanter. During the evolution, oedema gradually disappears after the third month [28]. Turner described the MRI signs of bone marrow oedema [29J in five patients suffering from AVN (avascular necrosis) who presented early as being very similar to those of transient osteoporosis. Osteonecrosis was shown to be present in all patients by core biopsy and subsequent development of focal MRI abnormalities were reported to be highly specific for AVN [29]. Vascular congestion and oedema of the bone marrow are thought to occur early in the course of AVN and transient osteoporosis, as a non-specific response to an injury [29], and is probably more severe in cases of AVN. Infections Infections of the musculoskeletal system include a large variety of clinical situations with different patterns and always constitute a challenge for an early and correct diagnosis [30]. If diagnosed early, infection can be cured with antibiotic therapy and, sometimes, operative drainage and debridment. Complications of delayed or inadequate treatment are severe, especially in spondylodiscitis or arthritis.
1. Acute osteomyelitis In acute haematogenous osteomyelitis bone scintigraphy is still the best technique of choice because of its high sensitivity and to show the whole skeleton. Radiographs must be performed when the symptoms first appear even if they are normal because they are necessary in following up the evolution. In a few particular cases CT may be helpful in demonstrating an enhancement of the medullary content or, exceptionally, the presence of gas [30]. MRl shows very subtle bone marrow abnormalities which consist of a moderate to marked decrease of signal intensity compared to a strong signal in normal marrow on Tl weighted images, enhancement after i.v. injection of gadolinium, and increase of signal intensity on T2 weighted images. These modifications are mainly due to oedema. The associated soft tissue lesions are also well identified and their limits well defined [31]. The extent of the lesion is much better appreciated than in bone scintigraphy, which is notorious for its tendency to overestimate the extent. The previous remarks are valid for all locations, including the spine and extremities. According to Unger [32], MRI compared to bone scintigraphy is more sensitive (92% vs 82%) and more specific (96% vs 65 %). When local factors induce osteomyelitis, such as in diabetic foot, the problem is different For early diagnosis bone scintigraphy, radiographs and CT have the limitations mentioned above. Only few reports have been published concerning the role of MRI in diabetic foot but the preliminary results are encouraging: MRI provides accurate information concerning the presence and extent of infection and differentiating neuropathic joint from osteomyelitis [33]. We are presently evaluating this common problem at our institution.
2. Subacute osteomyelitis This form of infection is the so-called Brodie's abscess. MRI is definitely superior to other imaging modalities in showing all tissue components: liquid in the cavity, a wellvascularized pseudo-capsule and oedema around the focus of infection (Fig. 11).
3. Chronic and recurrent osteomyelitis Chronic and recurrent osteomyelitis is a much more difficult situation: clinical and biological information is often ambiguous. Imaging modalities are only helpful in showing a sequestrum, soft tissue lesions, and the extent. CT has been the technique of choice for many years for greater accuracy [30]. MRI has not been evaluated in a good prospective study but it seems to have the same possibilities as CT even though the calcified component could be underestimated.
4. Arthritis When clinical and biological data lead to the suspicion of arthritis a puncture must be carried out. It is the sole procedure for a correct diagnosis. In particular cases, when the patient status is confusing, bone scintigraphy and MRI may allow an early diagnosis. MRI may be more accurate in showing bone marrow oedema around the articular surfaces after two weeks of evolution, and fluid in the joint [30].
5. Soft tissue infections Whether or not associated with bone lesions, soft tissue infections are well appreciated by CT and MRI in every location. MRI is more accurate than CT in depicting epidural abscesses [13]. Tumours 1. Primary bone tumours
a) Benign In benign tumours the role of MRI is very limited despite a multiplanar approach and very good tissue differentiation. All imaging techniques are efficient for different reasons, especially plain X-rays and CT for evaluating the extent, relationships and morphology before a possible biopsy; a good example is osteoid osteoma. MRI may provide good information especially for non-calcified components but in general this technique is not necessary. b) Malignant The situation is quite different in the case of suspicion of malignancy on plain X-rays. Bone scintigraphy must be performed to estimate the number of lesions. MRI is not very helpful for diagnosis even if almost all large tumours are heterogeneous and the different tissue components well appreciated (Fig. 12). The local staging examination is essentially based on MRI because it allows the best estimation of intramedullary extension, intraarticular extension,
Garcia: MRJ of the musculoskeletal system
soft tissue extension [2, 34, 35]. During examination the field of view (zone of interest) must be large in order not to miss skip-lesions (small metastases separated from the main tumour by healthy tissue). In more recent reports, MRI appears to be superior to Cf for local staging [34, 35, 36]. Like Cf, MRI has limitations: inability to detect very small lesions and overestimation of the tumour volume on T2 weighted images because of peritumoral oedema [37].
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tumour volume [34, 36]. In large cancer centres investigators are working to define follow-up criteria based on MRI examination [36, 38]. After placement of a non-paramagnetic metal prosthesis, MRI allows evaluation of periprothetic soft tissues without artifacts in contrast to Cf images in which it may be degraded [34].
Fig. 12: Osteosarcoma of the right femur. a)
Radiography showing a very subtle osteopenia in the shaft (black arrow head) with a Cadman's triangle (white arrowhead) and very fine cortex destruction (small white arrow).
b)
MRI. Coronal T1 slice: excellent demonstration of extension into the medullary cavity (black arrowhead) and huge involvement of the soft tissue (white arrowhead) with a different signal intensity. Some oedema between the tumour distal limit (white arrow) and the cartilage line (black arrow).
c)
MRI. Transaxial T2 slice: in the soft tissue mass (white arrowhead), numerous cavities with septation and fluid-fluid levels (small black arrow). Femoral vessels are closed (large black arrow). The whole shaft is involved (black arrowhead).
Detection of pulmonary metastases is based on plain Xrays and chest Cf; detection of other bone lesions is based on bone scintigraphy. Most primary malignant tumours are treated with chemotherapy and irradiation (Fig. 13) MRI appears to be a very accurate technique for studying necrosis and
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When symptoms are not explained and scintigraphy is normal; case reports of metastases seen on MRI and not on scintigraphy have been published. To differentiate an osteoporotic vertebral collapse from metastases, Bone metastases are disseminated haematogenously and develop in the well-vascularized cancellous bone. This explains the high sensitivity of MRJ in the detection of small metastatic foci, even before bone scintigraphy. In the presence of vertebral collapse we must differentiate a benign osteoporotic lesion from a malignant one. When bone scintigraphy and plain X-rays are insufficient to define the aetiology, MRI may be helpful. In old benign collapse (more than 3 months old), the bone marrow .ign als are similar to those of adjacent vertebrae. In acut e benign osteoporotic collapse there is a decreased signal on T1 weighted images and a moderate increased signal on T2 weighted images. In malignant collapse the sign al enhancement on T2 is increased far more, there is a high enhancement after i.v, injection of gadolinium and sometimes a soft tissue extension into the spinal canal or around the vertebral body. These findings are not always sufficient for correct diagnosis but, nevertheless, MRI is more accurate than other imaging modalities. 3. Soft tissue tumours
Despite the advantages of sonography, cr has been the choice technique for evaluating soft tissue masses for many years [2, 40]. Recent studies, including our own experience during the last three years [40), indi cate that MRJ is generally superior to cr in lesion detection and in characterizing the extent of involvement of adjacent bone and soft tissue structures.
Fig. 13: MRI. Ewing sarcoma of the right ilium in an 8 year old girl, 2 1/2 years after chemotherapy and irradiation . Tl coronal and T2 transaxial slices show a high intensity signal in the ilium and the sacrum (irradiated area, arrowhead). No signs of recurrence. High intensity sig na l due to fatty infiltration. 2. Bone metastases Bone metastases are a very common problem. When the primary tumour is known bone scintigraphy is still the choice technique for surveillance. MRI may be useful in some situations as listed below: Whcn there is a discrepancy between an increased uptake on bone scintigraphy and a normal appearance on plain X-rays (Fig. 14). When it is necessary to appreciate the extent and soft tissue component, for example, in the case of cord compression. When a recurrence is suspected after chemotherapy or irradiation .
MRJ is characterized by a great potential in differentiating tissues and offers a multiplanar approach. Better tissue differentiation is particularly useful in regions where there is only a small amount of fat such as in the distal extremities. As in CT, adipose tissue is characteristic in MRJ; therefore, lipomas (Fig. 15) and liposarcomas are easy to identify. Cysts are well marginated, adjacent to joints and show very characteristic signals (40) . Vascular tumours, various types of haemangiomas, are better appreciated by MRJ (41). Desmoid tumours are often infiltrating and tend to recur following resection and pres ent a heterogeneous appearance on MRI which may be misdiagnosed as a malignancy. Most malignant tumours are heterogeneous. This appearance may be secondary to tumour necrosis, calcifications or the type of tumour matrix. This heterogenicity is much more apparent after i.v. injection of gadolinium. Tumoral calcifications are underestimated by MRI (Fig. 16) and better seen on plain X-rays and CT. The local staging of soft tissue sarcomas is mainly performed with MRI examination as well as for bone sarcomas. The local surveillance after treatment is also based on MRI which can detect even small recurrences [421.
Garcia: MRI of the musculoskeletal system
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Fig. 14: MRI. TH. 12 vertebral body metastases (adenocarcinoma of the duodenum). Sagittal slices: tumoral involvement hypointense on T1 (white arrowhead, left), hyperintense on 1'2 (black arrowhead, right) in comparison with normal ad jacent vertebrae. A focal uptake was seen on scintigraphy; radiographs and conventional tomographs were normal.
Fig. 15: MRI. Lipoma of r.e right buttock The tumour (white arrowhead) has a high intensity signal, is homogeneous and well-delineated. Its 'relations are well appreciated in 3 planes. The tumour compresses the sciatic nerve (small black arrow). A large uterus is seen (black arrowhead, 3rd month of pregnancy).
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Fig. 16: MRl. Primary soft tissue chondrosarcoma of the right thigh. a) Sagittal slices: a soft tissue mass (arrowhead) is seen on the anterior region with an intermediate signal intensity on T1 (left) and an intense enhancement after i.v. gadolinium-DOTA (right) which also emphasizes the heterogeneity (small arrow). b) Transaxial slices: the tumour matrix (arrowhead) is much more intense than normal adjacent muscles on TR 2000 TE 30 (left) and TR 2000 TE 60 (right). It is a myxoid component. Small calcifications (small arrow) were underestimated (but well seen on plain X-rdys). Conclusion
References
MRl is a new and advanced imaging modality. Its role has grown very rapidly in studying the musculoskeletal system during the last five years. MRI permits an early diagnosis in stress and pathological fractures, avascular necrosis, transient osteoporosis, infection and metastases. It is also a very accurate method for diagnosing soft tissue injury, degenerative disc disease (especially at cervical and dorsal level and after surgery at the lumbar level), involvement of C1-C2 level during the late stages of rheumatoid arthritis, and for the local staging of malignant primary bone tumours and soft tissue tumours.
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