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The role of magnetic resonance imaging in the musculoskeletal system
The role of Magnetic Resonance Imaging in the Musculoskeletal System
J. H. Buckley, R. R. M a w h i n n e y , B. S. W o r t h i n g t o n , M. J. G i b s o n , B. J. P r e s t o n
Physical Principles of Magnetic Resonance Imaging
The tissue contrast seen with Computed Tomography (CT) depends on the differential attenuation of X-rays. At the energy levels of X-rays used in diagnostic imaging this is dependent on tissue density and on the atomic numbers of the constituent atoms in tissue. The resulting soft tissue contrast is relatively poor. The inherent soft tissue contrast on an N M R image is superior. Additionally, the contrast between different tissues can be manipulated by altering the respective density (T 1 and T2 weighting) seen on the image. Pathological changes can be highlighted in this way. The sequences used in MRI are Inversion Recovery, Spin Echo and Saturation Recovery. It is the time constants relating to the transmitted radio-waves within each of these sequences which are altered to produce an image which is weighted as outlined above.
MRI is based on the phenomenon "of Nuclear Magnetic Resonance (NMR) which was discovered simultaneously by Bloch and Purcell in 1946, and for which they were jointly awarded the 1952 Nobel Prize for Physics. They showed that certain atomic nuclei (those with an odd number of either protons or neutrons), when placed in a magnetic field and stimulated by radio-waves of a specific frequency, reemit some of the absorbed energy in the form of radiowaves. The radio-waves represent the N M R signal. From this work N M R spectroscopy was developed, and has been used since by physicists and chemists as a major tool in the elucidation of the molecular structure of matter. In 1972 Lauterbaur proposed the use of gradient coils operating along three axes (X, Y and Z), and it was this development which enabled spatial encoding to be applied to the N M R signals from matter, thus permitting two-dimensional imaging. Magnetic Resonance images reflect three main parameters: the distribution density of hydrogen nuclei, the TI, and the T2 relaxation times. These T~ and T 2 relaxation times relate to the motion of the hydrogen nuclei in tissue. Different sequences of transmitted radio-waves have been devised so that the resulting N M R signal is weighted to different degrees by the hydrogen proton density and the T1 and T2 relaxation times, e.g. the tissue contrast on a 'T~ weighted image' is mainly related to the variation ofT1 relaxation times within the different tissues.
Table l--Available MRI Sequences
Image Weighting
Spin Echo With short Repetition Time (TR) and short Echo Delay Time (TE)
TI
Spin Echo with long TR and long TE
T2
Partial Saturation (no Refocussing Echo)
Proton Density
Inversion Recovery with medium or long Inversion Time (TI)
TI
STIR Inversion Recovery with short TI
J. H. Buckley, R. R. Mawhinney, B. S. Worthington, B. J. Preston Department of Radiology, University Hospital, Nottingham M. J. Gibson Department of Orthopaedics, Newcastle Royal Infirmary Current Orthopaedics (1986)I, 101-113 © 1986LongmanGroupUKLtd
Sequence
10 1
Summated T t and T2 (Lesions with long TI and long T2 relaxation times give high signal)
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THE ROLE OF MAGNETIC RESONANCE IMAGING IN THE MUSCULOSKELETAL SYSTEM
Fig. 1 - - ( A ) Transverse Scan Spin Echo (TR 1200 TE 80). This cut at the level of the upper shaft of the fibula demonstrates the medial extension of the soft tissue mass to involve the neurovascular bundle. (B) Sagittal Scan Spin Echo (TR 1000 TE 80). Area of low signal in the head and upper shaft of the fibula due to replacement of marrow by tumour. The distal extension of the soft tissue tumour component is well seen in this plane (arrow).
These time constants are the Repetition Time (TR), the E c h o Delay Time (TE), and the Inversion Time (TI). In practice the sequences we use are the Spin Echo with short T R and short T E giving relatively Tl weighted images, the Spin E c h o with long T R and long T E producing T 2 weighted images, the Inversion Recovery sequence with long T R and m e d i u m or long T I which gives strongly T1 weighted images, and the Short T I Inversion Recovery (STIR) sequence recently devised at the Royal Postgraduate Medical School, L o n d o n , which produces images in which changes in T1 and T 2 relaxation times due to p a t h o l o g y are summated. As a result o f this s u m m a t i o n the S T I R sequence is particularly sensitive to t u m o u r and inflammation (Table 1).
General Considerations The m a j o r advantages o f M R I are the excellent contrast, and the multiplanar facility allowing scanning in the transverse, sagittal, coronal and oblique planes (Figs. 1A, B). Table 2 outlines the signal intensity seen with n o r m a l structures on each o f the most c o m m o n l y e m p l o y e d spin sequences. The high contrast obtained between muscle and fat permits a clear outline o f individual muscles, and also o f muscular components. The d y n a m i c flow characteristics o f tissue are also reflected in the Magnetic Resonance image. Rapidly flowing blood gives almost no signal, and static blood a
Table 2 SignalIntensities Seen with Different Tissues on the Major MRI Sequences Employed Signal Intensity Tissue
Fat Medullary Bone Cortical Bone Muscle Tendons Ligaments Static Blood Flowing Blood Articular Cartilage IV Disc Nucleus Pulposis IV Disc Annulus Fibrosis Meniscus (Knee) CSF Peripheral Nerves
T~ Weighted SE Sequence
T2 Weighted SE Sequence
IR Sequence
Partial Saturation Sequence
STIR Sequence
High High Low Medium Low Low High Very Low Low
High High Low Medium Low Low High Very Low Low
High High Low Medium Low Low High Very Low Low
High Low Low Medium Low Low High Variable High
Low High Low Low-Medium Low Low High Variable Low
Medium
High
Medium
Medium
High
Low Low Low Medium
Low Low Medium Medium
Low Low Low Medium
High Medium High Medium
Low Low High Low
CURRENT ORTHOPAEDICS 103 high signal. Arteries (very low signal) can therefore be distinguished from veins (medium signal) due to the different rate of blood flow within these vessels. Magnetic Resonance can additionally reliably differentiate between peripheral nerves, and ligaments and tend/ms. The illustrated images were produced on the Picker International Resistive machine operating at 0.15 Tesla field strength. Most of the patients were scanned using standard head or body coils. When surface coils are employed there is a striking improvement in image quality because (a) there is an approximate three fold increase in signal to noise ratio, and (b) the field size being interrogated is smaller with resulting improved spatial resolution when the Matrix size is constant. The design of surface coils is important. It is also complex and difficult. Continued refinement can be anticipated with resulting improvement in image quality. Musculoskeletal Tumours
Precise delineation of tumour extent is necessary for the planning of surgery and radiotherapy, particularly in view of the fact that current treatments favour reconstructive surgery to a much greater degree than previously. The information required is: extent of bone involvement; soft tissue extent; whether the lesion is intra- or extra-compartmental; whether the neurovascular bundle is involved; and if possible, tissue characterisation. (1) Soft Tissue Tumours The extent of these lesions is poorly displayed on
standard X-rays and radionuclide scans. CT is currently the major method for radiological staging of these lesions. The main deficiencies of CT are that the neurovascular bundle is sometimes poorly visualised, as are the peripheral limits of the tumour when adjacent muscle is compressed. Angiography is frequently required to assess whether there is involvement of the neurovascular bundle. Magnetic Resonance provides superior delineation of the extent of tumour in soft tissues in comparison with the other imaging methods (Fig. 2). This is due to the very high soft tissue contrast resolution of MRI. The tumours are well delineated from adjacent muscle and fat. Malignant tumours generally have prolonged T~ and T 2 relaxation times, and the lesions are therefore well demonstrated on either a heavily T~ weighted Inversion Recovery, or heavily T 2 weighted (long TR/long TE) Spin Echo sequence. We have found the STIR sequence, in which alterations in T1 and T 2 relaxation times are summated, also provides good outline of tumour extent (Fig. 3). The ability of MRI to accurately define extent applies both to 1° soft tissue tumours and to soft tissue involvement of 1° bone tumours. 'Additionally, assessment of involvement of the neurovascular bundle can be reliably made on MRI, and angiography is therefore unnecessary in most cases. 2 Additionally Pettersson has found that involvement of adjacent bone is better demonstrated on MRI than with CT. Calcification gives almost no signal on MRI, and is well demonstrated on CT, which is a weakness of MRI. Like CT the specificity of Magnetic Resonance is
Fig. 2--Pigmented Villo-Nodular Synovitis Right Knee, Transverse Scan. Spin Echo (TR 1000 TE 40). Abnormal signal area defines the extent of the lesion. The fat in the suprapatellar bursa has been replaced by the synovial proliferation. The linear high signal area (arrow) represents subperiosteal haematoma following biopsy. The normal left side is shown for comparison.
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OF
MAGNETIC
RESONANCE
IMAGING
IN
THE
MUSCULOSKELETAL
SYSTEM
Fig. 3~Osteosarcoma Right Upper Femur. Coronal Scans (A) Spin Echo (TR 820 TE 40). Extensive loss of the normally high marrow signal in the right upper femur. Patchy loss of signal is also seen in the right innominate bone indicating tumour involvement at this site also. (B) STIR Sequence. The tumour is seen as high signal areas on this sequence. In addition to the bony abnormality, the extensive soft tissue component is best demonstrated on this sequence (arrows).
poor, although lipomas are characteristic--giving very high signal on both T 1 and T2 weighted sequences. Fibrosis due to scar tissue gives a low signal on both T1 and T 2 weighted images, and tumour with a high signal on T2 weighted images can therefore be well differentiated when this sequence is employed. (2) Bone Tumours Magnetic Resonance is the most sensitive method of assessing intramedullary tumour extent 1 (Figs. 4, 5). Additionally, Magnetic Resonance is a valuable method of assessing tumour breakthrough of cortical bone, and also the degree o f any articular surface involvement. As with primary soft tissue tumours, the technique is a specific marker of primary bone tumours. In consequence Magnetic Resonance is superior to all other modalities in the staging of bone tumours. Limitations still exist in specifying tissue composition, and in separating pathological tissue from reactive change. Magnetic Resonance has also been used in the investigation of bone marrow disease. Table 3~Relative
Fig. 4--Chondrosarcoma Right Upper Femur Coronal Scan. Spin Echo (TR 820 TE 40). The upper extent of the lesion at the intertrochanteric region is well seen due to tumour replacement of marrow fat with resulting decreased signal.
M e r i t s o f D i f f e r e n t I m a g i n g M o d a l i t i e s in A s s e s s m e n t o f M u s c u l o s k e l e t a l
Tumours
(a) S o f t T i s s u e T u m o u r s Tumour Extent Involvement of adjacent bone Calcification Neurovascular bundle involvement Nature of lesion Metastases
M R
CT
X-rays
Isotopes
Angiography
+ + + + + + + + +
+ + + + + + + +
+ +
+ + + + +
+ + + + +
M R
CT
X-rays
Isotopes
+ + + + + + + +
+ + + + + +
+ + + + + + + +
+ + + + + + + +
(b) B o n e T u m o u r s Intramedullary Extent Cortical Extent Soft Tissue Extent Calcification Nature of lesion Metastases
+ + + + + +
+ + + + -
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Fig. 5 ~ G i a n t Cell Tumour Lateral Condyle Right Tibia. (A) Spin Echo (TR 1000 TE 80). The lesion is not well seen on this T 2 weighted Spin Echo image. Note menisci contrasted with the synovial fluid. (B) Spin Echo (TR 500 TE 40). Clear definition of the tumour is obtained on the T 1 weighted Spin Echo image. Cystic lesions are often best seen on T 1 weighted sequences.
Cohen 3 and Olsen have found it to be sensitive in the detection of marrow infiltration with l e u k e m i a - - a signal reduction indicates tumour replacing fatty marrow. Additionally, Olsen found a distinctive appearance with lymphoma: areas of reduced signal and focal areas of markedly reduced signal. In our own experience neuroblastoma extent and metastatic infiltration are also well seen. The marrow changes of aplastic anaemia cannot, at present, be reliably demonstrated, the marrow signal being either normal or increased (due to the replacement of erythropoietic tissue by fat). Mancuso 4 and Cohen 3 have found a change in the signal intensity with both radiotherapy and chemotherapy. Magnetic Resonance therefore has considerable potential in the staging and follow-up of patients with bone marrow neoplasms. An additional benefit of Magnetic Resonance is that the image degradation due to metallic implants and prostheses is less marked than with C T - - g i v i n g Magnetic Resonance a further advantage over CT in the assessment of recurrence following surgery. Calculated T 1 and T2 Relaxation Times These provide a more precise definition of abnormal tissue characteristics than an image in which the arbitrary signal intensity is interpolated by the computer on to a grey scale to provide an image. This method of analysis is currently under investigation in the hope that the actual T1 and T 2 relaxation times will correlate with the histological diagnosis. Preliminary results are unpromising. Pettersson found no direct relationship between the calculated relaxation times
and the histological diagnosis in a study of 50 bone and soft tissue tumours.
Further Applications The Knee Normal Anatomy. High resolution Magnetic Resonance using surface coils provides much information on the anatomy of the knee. The menisci are readily distinguished as low signal areas on both coronal and sagittal views (Fig. 5). The collateral ligaments are best seen on coronal slices, and their continuity can be assessed. Articular cartilage is well seen on both T1 weighted, and on partial saturation images. The cartilage can be clearly distinguished from the menisci. Coronal and sagittal scans are again the planes in which this structure is optimally imaged. The sagittal view is the plane in which the continuity of the cruciate ligaments (Fig. 6) and patellar tendon can be viewed. The whole length of the anterior cruciate can most often be seen when the leg is externally rotated by 20°. 5 On the transverse axial view the muscles, tendons, nerves, arteries and veins are readily distinguished and differentiated. 5,6 Pathology. The first M R I demonstrations of ligamentous and cartilaginous injury of the knee were made by Li and by Turner. 7 Meniscal tears and the anatomy of cruciate and collateral ligaments were displayed on Magnetic Resonance images. Joint effusions were also demonstrated. A meniscal tear is seen as a focal high intensity
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Fig. 6-~Normal Left Knee. Sagittal Scan Spin Echo (TR 500 TE 40), The posterior cruciate ligament (arrow) is best seen in this plane.
Fig. 7--Normal Left Hip. Coronal Spin Echo (TR 820 TE 40). The central weight bearing trabeculae are seen as an area of reduced signal within the femoral head (arrow).
Hips (white) area within the normally low intensity (black) meniscus. The signal alteration is thought to be due to synovial fluid within the cleft created by the tear. Reicher 8 compared MRI with both arthroscopy and arthrography. The M R ! images were obtained using .a T~ weighted spin echo sequence (short TR, short TE), imaging initially in the sagittal plane and subsequently coronally if the diagnosis was unclear. Excellent correlation was found, and M R I did not give any false negative results. It shares with arthrography the advantage over arthroscopy that the posterior menisci can be well seen. This work suggests that MRI employing surface coils is likely to become the primary screening technique for meniscal tears, in view of the major advantage that it is non-invasive, and requires no manipulation of the knee. Cruciate ligament tears were also studied by Reicher, with comparative arthroscopic and surgical findings. The M R I was considered positive when the continuity of the ligament was lost. Again the correlation between the three techniques was good, with no false negative results on MRI. Disruption o f the patellar tendon is difficult to assess both clinically and arthroscopically. Arthrography is of no value. The continuity is well seen on sagittal Magnetic Resonance images. In addition to the discontinuity due to rupture, haemorrhage can be clearly distinguished due to the high signal of static blood. Steinbrich found that osteochondritis dissicans was well detected by M R ! with both cartilage abnormality and reduced subchondral intensity due to local bone marrow displacement being seen. The multiplanar imaging capability of M R I is likely to be of value in the assessment of osteochondral fragments--in order to clarify if such fragments are loose bodies, or are connected to other bony structures. 8
The normal anatomy demonstrated has been documented by Heller and by Totty. 9 The cortical outlines are well defined. The central weight bearing trabeculae are seen as a broad band of low signal intensity (Fig. 7). The joint capsule and the remnant of the growth plate are also demonstrated. Several studies have established M R I as a sensitive method of detecting avascular necrosis of the femoral head 9 (Figs. 8, 9). The consistent finding is that M R I is the most sensitive method of detecting avascular necrosis, superior to both X-rays and radionuclide scans. The change observed on the MRI scan is a variable decrease in the signal at the site of necrosis. It
Fig. 8~Avascular Necrosis. Right Femoral Head. Coronal Spin Echo (TR 795 TE 40). The necrosis results in reduced signal from the femoral head, and some irregularity at the articular surface. X-rays and isotope scan were normal.
CURRENT ORTHOPAEDICS 107 able, David found that MRI was able to demonstrate the changes of this disease at an earlier stage and with more detail than with X-rays. (iii) Haemophilic Arthropathy. In addition to evaluating synovial hypertrophy, cartilage irregularities and subchondral cysts, MRI is a sensitive indicator of intra-articular bleeding, and therefore is ideally suited for early evaluation and monitoring of haemophilic arthropathy.
Fig. 9--Avascular Necrosis Left Femoral Head. Coronal Scan Spin Echo (TR 797 TE 40). Well defined loss of the normal marrow signal is seen in the upper part of the femoral head.
can be anticipated that aseptic necrosis at other sites will also be well demonstrated. Additionally, Scoles ~° has found MRI to be of value in the detection and monitoring of Legg-Calv6-Perthes disease of the hip.
(c) Rotator Cuff Abnormality The ligaments and tendons of the rotator cuff muscles are clearly demonstrated as low intensity structures. The articular capsule of the shoulder joint is also well defined.12 High resolution Magnetic Resonance with surface coils can demonstrate tears of both the supraspinatus and infraspinatus tendons. 1z Additionally, the degree of separation of the torn fragments can usually be assessed. Kneeland has concluded that MRI has a sensitivity in the detection of complete rotator cuff tears which approaches that of arthrography. (d) Achilles Tendon Rupture This condition has been clearly demonstrated on sagittal MRI, as an area of high signal (due to oedema and haematoma) between the separated low signals of the tendon.
Miscellaneous Conditions (a) Infection Infection with resulting inflammatory change results in lengthening of both the TI and T2 relaxation times. This condition is consequently well suited for assessment by MRI (Figs. 10, 11). Markiez found Magnetic Resonance to be the most sensitive imaging technique in cases of acute infection, by comparison with isotope, CT and plain X-ray studies. Chronic infective and inflammatory processes were well seen by all four methods. Our own experience is consistent with their findings. Additionally Magnetic Resonance can detect small effusions within joints 1~ and this capability might prove to be useful in the early detection of arthritis. (b) Arthritis (i) Rheumatoid Arthritis. MRI is likely to have a significant role in the assessment of this condition (Fig. 12). As stated above, joint effusion is readily seen, the degree of swelling due to synovial tissue hypertrophy can be assessed, and additionally the synovial inflammation results in alteration of the signal by comparison with normal. High resolution Magnetic Resonance with surface coils can demonstrate cartilage irregularity, although the ability of Magnetic Resonance to demonstrate small erosions is limited by the spatial resolution currently attainable. (ii) Osteoarthrosis. Although the sensitivity of MRI in the early detection of osteoarthrosis is again limited by the current slice thickness and spatial resolution attain-
(e) Tempero-Mandibular Joints High resolution MRI of the tempero-mandibular joints (TMJ's) has been studied by Katzberg and by Harms. The diseased meniscus is well demonstrated-giving a lower signal than normal, probably due to calcium deposition. Meniscal displacement is also visualised, and Katzberg found a good comparison
Fig. l O ~ L u m b a r Spine. Osteomyelitis. Sagittal Scan Spin Echo (TR 2000 TE 80). The increased signal (white area) at the vertebral borders of the L4/5 disc is clearly displayed on this T 2 weighted Spin Echo image. X-rays were normal.
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Fig. 11--Osteomyelitis Lumbar Spine. (A) Sagittal Scan. Spin Echo (TR 500 TE 40). The abnormality is not clearly seen on this T 1 weighted Spin Echo image. (B) Sagittal Scan, STIR Sequence. The increased signal involving part of the L3 body and the whole of the L4 body is present on the STIR sequence.
between CT, Arthography and MRI in assessment of this condition. In the light of the findings of these authors, it is possible that Magnetic Resonance will become the technique of choice in the primary diagnostic evaluation of abnormalities of the tempero-mandibular joint.
(f) Vascular Abnormalities Rapidly flowing blood gives no appreciable signal on Magnetic Resonance because the protons excited by the transmitted radiofrequency pulses move from the plane in which the signal is received before their N M R signal can be recorded (Fig. 13). At lower flow rates some signal is received, thus the signal from blood in normal veins is higher than from arterial blood. 13 Arteries and veins are clearly differentiated from each other, and from surrounding tissues. Rapoport has shown that a static column of blood can be readily distinguished from flowing blood. Clearly, therefore, there is a potential for MRI to be used in the diagnosis of deep venous thrombosis. Accurate assessment of flow rates would also be helpful in the management of peripheral arterial occlusive disease. MRI can also provide information of value regarding vascular malformations--the extent of the lesion, the origin of the feeding vessels, and which feeding vessels display rapid flow and would be amenable to embolisation.
The Spine
Fig. 12--Rheumatoid Arthritis. Cervical Spine. Sagittal Scan Spin Echo (TR 860 TE 40). Previous laminectomy at C2 level which has resulted in loss of the signal from the posterior epidural fat. There is posterior compression of the upper cervical cord due to fibrosis which gives a low signal (arrow). Marked atlanto-axial subluxation with wide separation between the anterior arch of the atlas (open arrow), and the anterior part of the dens. The focal area of high signal (curved arrow) is a surgical clip. It has not resulted in significant degradation in image quality. Note how well the cord contrasts with CSF.
Normal Anatomy The vertebral bodies are well outlined. As in the appendicular skeleton, the high signai from cancellous bone contrasts with the low signal from the surrounding cortical bone. The normal nucleus pulposis has a long T2 relaxation time and thus gives a high signal on T2 weighted images. This is believed to be due to the high water content of the nucleus. The annulus fibrosis has a shorter T 2 relaxation time, and therefore its low
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and STIR sequences (Fig. 14B). Surface coils produce high resolution images with slice thickness of 3M mm making it possible to visualise nerve roots on transverse axial scans.
Disease of the Spine Magnetic Resonance has already established a major role in the assessment of spinal disorders.
Fig. 13--Vascular Malformation Left Lower Leg. Coronal Scan. Spin Echo (TR 500 TE 40). Marked increase in subcutaneous fat on the left side. The aberrant vessels are seen as linear areas of reduced signal within this fat.
signal on the T2 weighted image contrasts with that of the nucleus. Lateral and posterior epidural fat is well seen on all sequences. The longitudinal ligaments (with low signal) cannot be distinguished from cortical bone. The spinal cord is well outlined on T~ weighted images. The cerebrospinal fluid, and the extent of the spinal theca, are best seen on T2 weighted (Fig. 14A)
Vertebral Column (a) Disc Disease. Prior to Magnetic Resonance, the only accurate method of assessing disc degeneration was by contrast discography, an unpleasant and invasive procedure. Magnetic Resonance produces an image of the nucleus which is comparable in shape with that produced by discography. Additionally, MRI is at least as accurate as, and probably more accurate than, discography in the assessment of this condition. 1~ The diagnosis of degeneration is made on two criteria: alteration from the normal nucleus shape, and reduction in the normally high signal--presumably due to loss of water in association with the degeneration. An additional advantage of MRI over discography is that all the discs in the lumbar spine can be assessed on one multislice sagittal scan (Fig. 15). MRI is consequently the investigation of choice in assessing whether disc degeneration is present. Diagnosis of disc herniation can be made on multislice sagittal and oblique/transverse scans (Fig. 16). The accuracy of the technique is improved when surface coils are employed enabling the selection of thinner tissue slices. Edelman 15 found Magnetic Resonance to be equal to CT in accuracy of diagnosis of disc herniation. (b) Infection and Inflammation. In another study,
Fig. l z I ~ ( A ) Lumbar Spine. Sagittal Scan. Spin Echo (TR 1000 TE 80). The longer repetition time and echo delay time result in a more T 2 weighted image. The normal nucleus pulposis gives a high signal, and the degenerate disc at L5/$1 is therefore more clearly demonstrated. CSF gives a positive signal in contrast to the low signal from the annulus fibrosis. (B) Lumbar Spine. Sagittal Scan. STI R Sequence. The cerebro-spinal fluid gives a high signal on this sequence allowing accurate assessment of the dimensions of the subarachnoid space.
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THE ROLE OF M A G N E T I C R E S O N A N C E I M A G I N G I N T H E M U S C U L O S K E L E T A L SYSTEM
major advantage is that intraspinal extension can be assessed without the intrathecal administration of contrast medium. As with CT, intra-abdominal and intrathoracic structures are simultaneously imaged, and thereby any tumour involvement of these structures can be seen. (e) Trauma. Compression and dislocation of the vertebral bodies is seen. Without the use of contrast, the spinal theca can be assessed for narrowing. The cord is directly delineated (Fig. 20) and it should be possible to visualise a haematoma. Additionally, in cases of cord damage, the complication of post traumatic syringomyelia can be detected on serial follow-up scans.
Spinal Canal
Fig. 15---Lumbar Disc Degeneration. Sagittal Scan. Spin Echo (TR 1000 TE 80). The L2/3 and L3/4 intervertebral discs are normal in both outline and signal intensity. Degenerative change at L4/5 and L5/$1 more marked at the lower level.
Modic compared Magnetic Resonance with X-rays and Isotopes in a series o f patients with a clinical diagnosis of vertebral osteomyelitis. 16 Histology and microbiology were the final arbiters. In terms of both sensitivity and diagnostic accuracy Magnetic Resonance was at least equal to radionuclide scanning. Both were superior to plain radiography. In our preliminary experience we have found that Magnetic Resonance using the recently available STIR sequence, is superior to both isotope scans and X-rays in the demonstration of both vertebral osteomyelitis and discitis (Fig. 17). (c) Chymopapain. The biochemical findings of Stern indicate that intradisc injection of chymopapain resulted in biochemical changes similar to premature degeneration of the nucleus pulposis. This is probably due to a breakdown of proteoglycans in the nucleus with resulting loss of water. Our own findings on serial MRI scanning are consistent with this hypothesis. In our study 17 chymopapain injection caused a gradual loss of signal from the nucleus progressing to complete loss of the signal, usually by 6 weeks (Fig. 18). Reduction of the disc space also occurred. The degree of disc protrusion previously observed was little altered by chemonucleoysis. In a number of cases a patchy loss of the signal from the vertebral end plates occurred-suggesting the presence of a chemical discitis. The STIR sequence has been employed recently and has demonstrated increased signal around the vertebral end plates adjacent to the injection site in cases where chemonucleoysis failed to alleviate symptoms (Fig. 19). This change, consistent with inflammation, suggests that a chemical discitis is one cause of failure of chymopapain therapy. (d) Tumours. As in other locations, these are well demonstrated due to replacement of cancellous bone by the tumour. Soft tissue extension is well defined. A
The ability of MRI to directly image the cord in transverse, sagittal and coronal planes without the use of contrast medium has resulted in its widespread use in the assessment of cord tumours, demyelination, trauma, atrophy, syringomyelia, dysraphysm (Fig. 21) and other congenital anomalies and disc herniation (Fig. 22). Rheumatoid arthritis in the cervical spine is currently assessed with plain radiography, myelography and CT myelography in order to assess atlanto-axial subluxation, other bone lesions, and any cord compression resulting from subluxation or pannus. In a study comparing these modalities with Magnetic Resonance, we found that Magnetic Resonance could assess atlanto-axial subluxation, odontoid erosion, soft tissue swelling, the dimensions of the spinal theca, and any anterior or posterior cord compression resulting from this disease. There is no need for intrathecal contrast medium. Lumbar spinal stenosis is readily diagnosed on
Fig. 16--Degenerative Disc Disease Lumbar Spine. Sagittal Scan. Spin Echo (TR 1000 TE 80). Disc degeneration at L3/4 and L5/$1. Central disc herniation at L5/$1. Note the break in the L5/$1 fat line due to the disc herniation.
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Fig. 1 7 - - L u m b a r Spine Discitis. Patient complained of severe, increasing lower back pain. X-rays were normal. Isotope scan showed an area of slightly increased intensity at L4/5. Subsequent microbiology was negative. (A) Sagittal Scan. Spin Echo (TR 1000 TE 80). Reduction in the signal from the L4/5 disc. Unimpressive increase in signal adjacent to this disc space. (B) Sagittal Scan. STIR Sequence. Striking increase in the signal extending from the L4/5 disc space into the L4 and L5 vertebral bodies.
transverse and oblique scans. If a T 2 weighted sequence is employed, the CSF is shown as a positive signal and the dimensions of the spinal theca can thereby be measured. The high signal from CSF on the STIR sequence is also helpful in assessing the AP dimensions of the spinal theca (Fig. 23).
The Future Magnetic Resonance is becoming the imaging investigation of choice in a wide variety of musculoskeletal lesions. In time its role will almost certainly expand further. The chief reason for its increased applicability in the last 18 months or so, is the improved spatial resolution
available when surface coils are used. Diagnostically useful imaging of knee, shoulder, tempero-mandibular joints and rotator cuff can now be performed. In view of its ability to provide information relating to these structures which is otherwise obtainable only with invasive procedures, it will probably become the primary screening technique of choice for disorders of menisci, ligaments and tendons at these sites. MRI may have an important role in the monitoring of treatment of malignant diseases, such as leukemia, lymphomas and neuroblastoma, by reading the serial changes in marrow signal intensities, or the changes in the calculated T~ and T2 values. This form of evaluation of treatment may also be applicable to infection.
Fig. 1 8 - - L u m b a r Spine. (A) Sagittal Scan Spin Echo (TR 2000 TE 40). Pre-chymopapain. Disc degeneration at L5/S1 with central disc protrusion. (B) Nine weeks after administration of chymopapain at L5/$1. There is now complete absence of signal at this level.
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Fig. 1 9 ~ L u m b a r Spine. Sagittal Scan. STIR Sequence. One week after the administration of chymopapain at L4/5 level the patient continued to complain of severe pain. The increased signal around the vertebral end pi,~tes at this level suggests a chemical discitis due to chymopapain.
Fig. 21--Spinal Dysraphism. Sagittal Scan. Spin Echo (TR 500 TE 40). Low cord extending to L5 level. The posterior position of the cord is due to tethering.
There is enormous potential in the assessment of inflammatory joint diseases such as rheumatoid arthritis and haemophilic arthropathy. This will be realised with further improvements in spatial resolution, permitting visualisation of early changes in articular cartilage in both large and small joints. Imaging of elements other than ~H, and a better understanding of both the chemical information in the Magnetic Resonance signal (chemical shift data), and alteration in TI and T2 values may lead to better diagnosis and better understanding of degenerative muscle disease. A number of important further developments can be anticipated: improved receiver coils, higher resolution
scans with 512 × 512 matrix or greater, shorter scanning times, the widespread use of paramagnetic contrast media such as Gadolinium DTPA and the utilisation of new sequences such as chemical shift and STIR will increase the sensitivity, specificity and therefore the applicability of MRI. A further interesting possibility is the use of conjugates of paramagnetic elements such as Gadolinium with monoclonal antibodies, in tumour detection and staging. 18 The first published images of pathology demonstrated by MRI appeared just 6 years ago. Since then, its impact on the imaging approach to musculoskeletal pathology already exceeds any other modality since Roentgen.
Fig. 2 0 - - L o w e r Cervical/Upper Thoracic Spine. Sagittal Scan. Spin Echo (TR 860 TE 40). Automobile accident. Crush fracture T4. The resulting compression of the thoracic cord is seen (arrow).
Fig. 22--Cervical Spine. Sagitta] Spin Echo (TR 1000 TE 40). Disc degeneration and herniation at C5/6. Local compression of the cord at this level is seen.
CURRENT ORTHOPAEDICS
Fig. 23--Lumbar Spine. Sagittal STIR Sequence. Spinal stenosis. Generalised narrowing of the canal is seen. Marked narrowing at L3/4 due to herniation of the degenerate disc at this level. ReUcI"CHCCS
1. Zimmer W D, Berquist T H, McLeod R A, Sire F H, et al 1985 Bone Tumours: Magnetic Resonance Imaging Versus Computed Tomography. Radiology. 155:709-718 2. Pettersson H, Hamlin D J, Mancuso A, Scott K N 1985 Magnetic Resonance Imaging of the Musculoskeletal System. Acta Radiologica; May~une: 225-234 3. Cohen M D, Klatte E C, Baehmer R, Smith J A, et a11984 Magnetic Resonance Imaging of Bone Marrow Disease in Children. Radiology. 151:715-718 4. Mancuso A, Fitzsimmons J, Mareci T, Million R, Cassisi N 1984 Magnetic Resonance Imaging of the Upper Pharynx and Neck. Variations of Normal and Possible Applications in Detecting and Staging Malignant Tumours. Part II: Pathology. Radiology. 153:140 5. Reicher M A, Rauschning W, Gold R H, Bassett L W, et a11985 High-Resolution Magnetic Resonance Imaging of the Knee
1 13
Joint: Normal Anatomy. American Journal of Roentgcnology. 145:895-902 6. Kean D M, Worthington B S, Preston B J, Roebuck E J, et al 1983 Nuclear Magnetic Resonance Imaging of the Knee: Examples of Normal Anatomy and Pathology. British Journal of Radiology. 56:355-364 7. Turner D A, Prodromos C C, Petasnick J P, et al 1985 Acute Injury of the Ligaments of the Knee: Magnetic Resonance Evaluation. Radiology. 154:717-722 8. Reicher M A, Bassett L W, Gold R H 1985 High-Resolution Magnetic Resonance Imaging of the Knee Joint: Pathologic Correlations. American Journal of Roentgenology. 145: 903-909 9. Totty W G, Murphy W A, Ganz W 1, Kumar B, et a11984 Magnetic Resonance Imaging of the Normal and Ischaemic Femoral Head. American Journal of Roentgenology. 143:1273 10. Scoles P V, Yoon Y S, Makley J T, Kalamchi A 1984 Nuclear Magnetic Resonance Imaging in Leg~ Calv~ Perthes Disease. Journal of Bone and Joint Surgery. 66A: 1357 11. Beltram J, Noto A M, Herman L J, Mosure J C, et al 1986 Joint Effusions: MR Imaging. Radiology. 158:133-137 12. Huber D J, Sauter R S, Mueller E, Requardt H, Weber H 1986 Magnetic Resonance Imaging of the Normal Shoulder. Radiology. 158:405-408 13. Bradley W G, Waluch V 1985 Bloodflow: Magnetic Resonance Imaging. Radiology. 154:443-450 14. Gibson M J, Buckley J H, Mawhinney R R, Worthington B S, Mulholland R C A 1986 Comparison of Magnetic Resonance Imaging and Discography in the Assessment of Intervertebral Disc Degeneration. Journal of Bone and Joint Surgery 68B : 369-373 15. Edelman R R, Shonkinas G M, Stark D D, Davis K R, New P F J, Siani S, Rosenthal D I, Wismer G L, Brady T J 1985 High Resolution Surface Coil Imaging of Lumbar Disc Disease. American Journal of Roentgenology. 144:1123-1130. 16. Modic M T, Feiglin D H, Piraino D W, Boumphrey F, Weinstein M A, Duchesneau P M, Rehm S 1985 Vertebral Osteomyelitis: Assessment using Magnetic Resonance. Radiology. 157:157-166 17. Gibson M J, Buckley J H, Mulholland R C, Worthington B S The Changes in the lntervertebral Disc after Chemonucleolysis Demonstrated by MRI. Journal of Bone and Joint Surgery (November 1986, in press) 18. Scott J A, Rosenthal D I, Brady T J December 1984 The Evaluation of Musculoskeletal Disease with Magnetic Resonance Imaging. Radiologic Clinics of North America. 22:917-924
J. H. Buckley, R. R. M a w h i n n e y , B. S. W o r t h i n g t o n , M. J. G i b s o n , B. J. P r e s t o n
Physical Principles of Magnetic Resonance Imaging
The tissue contrast seen with Computed Tomography (CT) depends on the differential attenuation of X-rays. At the energy levels of X-rays used in diagnostic imaging this is dependent on tissue density and on the atomic numbers of the constituent atoms in tissue. The resulting soft tissue contrast is relatively poor. The inherent soft tissue contrast on an N M R image is superior. Additionally, the contrast between different tissues can be manipulated by altering the respective density (T 1 and T2 weighting) seen on the image. Pathological changes can be highlighted in this way. The sequences used in MRI are Inversion Recovery, Spin Echo and Saturation Recovery. It is the time constants relating to the transmitted radio-waves within each of these sequences which are altered to produce an image which is weighted as outlined above.
MRI is based on the phenomenon "of Nuclear Magnetic Resonance (NMR) which was discovered simultaneously by Bloch and Purcell in 1946, and for which they were jointly awarded the 1952 Nobel Prize for Physics. They showed that certain atomic nuclei (those with an odd number of either protons or neutrons), when placed in a magnetic field and stimulated by radio-waves of a specific frequency, reemit some of the absorbed energy in the form of radiowaves. The radio-waves represent the N M R signal. From this work N M R spectroscopy was developed, and has been used since by physicists and chemists as a major tool in the elucidation of the molecular structure of matter. In 1972 Lauterbaur proposed the use of gradient coils operating along three axes (X, Y and Z), and it was this development which enabled spatial encoding to be applied to the N M R signals from matter, thus permitting two-dimensional imaging. Magnetic Resonance images reflect three main parameters: the distribution density of hydrogen nuclei, the TI, and the T2 relaxation times. These T~ and T 2 relaxation times relate to the motion of the hydrogen nuclei in tissue. Different sequences of transmitted radio-waves have been devised so that the resulting N M R signal is weighted to different degrees by the hydrogen proton density and the T1 and T2 relaxation times, e.g. the tissue contrast on a 'T~ weighted image' is mainly related to the variation ofT1 relaxation times within the different tissues.
Table l--Available MRI Sequences
Image Weighting
Spin Echo With short Repetition Time (TR) and short Echo Delay Time (TE)
TI
Spin Echo with long TR and long TE
T2
Partial Saturation (no Refocussing Echo)
Proton Density
Inversion Recovery with medium or long Inversion Time (TI)
TI
STIR Inversion Recovery with short TI
J. H. Buckley, R. R. Mawhinney, B. S. Worthington, B. J. Preston Department of Radiology, University Hospital, Nottingham M. J. Gibson Department of Orthopaedics, Newcastle Royal Infirmary Current Orthopaedics (1986)I, 101-113 © 1986LongmanGroupUKLtd
Sequence
10 1
Summated T t and T2 (Lesions with long TI and long T2 relaxation times give high signal)
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THE ROLE OF MAGNETIC RESONANCE IMAGING IN THE MUSCULOSKELETAL SYSTEM
Fig. 1 - - ( A ) Transverse Scan Spin Echo (TR 1200 TE 80). This cut at the level of the upper shaft of the fibula demonstrates the medial extension of the soft tissue mass to involve the neurovascular bundle. (B) Sagittal Scan Spin Echo (TR 1000 TE 80). Area of low signal in the head and upper shaft of the fibula due to replacement of marrow by tumour. The distal extension of the soft tissue tumour component is well seen in this plane (arrow).
These time constants are the Repetition Time (TR), the E c h o Delay Time (TE), and the Inversion Time (TI). In practice the sequences we use are the Spin Echo with short T R and short T E giving relatively Tl weighted images, the Spin E c h o with long T R and long T E producing T 2 weighted images, the Inversion Recovery sequence with long T R and m e d i u m or long T I which gives strongly T1 weighted images, and the Short T I Inversion Recovery (STIR) sequence recently devised at the Royal Postgraduate Medical School, L o n d o n , which produces images in which changes in T1 and T 2 relaxation times due to p a t h o l o g y are summated. As a result o f this s u m m a t i o n the S T I R sequence is particularly sensitive to t u m o u r and inflammation (Table 1).
General Considerations The m a j o r advantages o f M R I are the excellent contrast, and the multiplanar facility allowing scanning in the transverse, sagittal, coronal and oblique planes (Figs. 1A, B). Table 2 outlines the signal intensity seen with n o r m a l structures on each o f the most c o m m o n l y e m p l o y e d spin sequences. The high contrast obtained between muscle and fat permits a clear outline o f individual muscles, and also o f muscular components. The d y n a m i c flow characteristics o f tissue are also reflected in the Magnetic Resonance image. Rapidly flowing blood gives almost no signal, and static blood a
Table 2 SignalIntensities Seen with Different Tissues on the Major MRI Sequences Employed Signal Intensity Tissue
Fat Medullary Bone Cortical Bone Muscle Tendons Ligaments Static Blood Flowing Blood Articular Cartilage IV Disc Nucleus Pulposis IV Disc Annulus Fibrosis Meniscus (Knee) CSF Peripheral Nerves
T~ Weighted SE Sequence
T2 Weighted SE Sequence
IR Sequence
Partial Saturation Sequence
STIR Sequence
High High Low Medium Low Low High Very Low Low
High High Low Medium Low Low High Very Low Low
High High Low Medium Low Low High Very Low Low
High Low Low Medium Low Low High Variable High
Low High Low Low-Medium Low Low High Variable Low
Medium
High
Medium
Medium
High
Low Low Low Medium
Low Low Medium Medium
Low Low Low Medium
High Medium High Medium
Low Low High Low
CURRENT ORTHOPAEDICS 103 high signal. Arteries (very low signal) can therefore be distinguished from veins (medium signal) due to the different rate of blood flow within these vessels. Magnetic Resonance can additionally reliably differentiate between peripheral nerves, and ligaments and tend/ms. The illustrated images were produced on the Picker International Resistive machine operating at 0.15 Tesla field strength. Most of the patients were scanned using standard head or body coils. When surface coils are employed there is a striking improvement in image quality because (a) there is an approximate three fold increase in signal to noise ratio, and (b) the field size being interrogated is smaller with resulting improved spatial resolution when the Matrix size is constant. The design of surface coils is important. It is also complex and difficult. Continued refinement can be anticipated with resulting improvement in image quality. Musculoskeletal Tumours
Precise delineation of tumour extent is necessary for the planning of surgery and radiotherapy, particularly in view of the fact that current treatments favour reconstructive surgery to a much greater degree than previously. The information required is: extent of bone involvement; soft tissue extent; whether the lesion is intra- or extra-compartmental; whether the neurovascular bundle is involved; and if possible, tissue characterisation. (1) Soft Tissue Tumours The extent of these lesions is poorly displayed on
standard X-rays and radionuclide scans. CT is currently the major method for radiological staging of these lesions. The main deficiencies of CT are that the neurovascular bundle is sometimes poorly visualised, as are the peripheral limits of the tumour when adjacent muscle is compressed. Angiography is frequently required to assess whether there is involvement of the neurovascular bundle. Magnetic Resonance provides superior delineation of the extent of tumour in soft tissues in comparison with the other imaging methods (Fig. 2). This is due to the very high soft tissue contrast resolution of MRI. The tumours are well delineated from adjacent muscle and fat. Malignant tumours generally have prolonged T~ and T 2 relaxation times, and the lesions are therefore well demonstrated on either a heavily T~ weighted Inversion Recovery, or heavily T 2 weighted (long TR/long TE) Spin Echo sequence. We have found the STIR sequence, in which alterations in T1 and T 2 relaxation times are summated, also provides good outline of tumour extent (Fig. 3). The ability of MRI to accurately define extent applies both to 1° soft tissue tumours and to soft tissue involvement of 1° bone tumours. 'Additionally, assessment of involvement of the neurovascular bundle can be reliably made on MRI, and angiography is therefore unnecessary in most cases. 2 Additionally Pettersson has found that involvement of adjacent bone is better demonstrated on MRI than with CT. Calcification gives almost no signal on MRI, and is well demonstrated on CT, which is a weakness of MRI. Like CT the specificity of Magnetic Resonance is
Fig. 2--Pigmented Villo-Nodular Synovitis Right Knee, Transverse Scan. Spin Echo (TR 1000 TE 40). Abnormal signal area defines the extent of the lesion. The fat in the suprapatellar bursa has been replaced by the synovial proliferation. The linear high signal area (arrow) represents subperiosteal haematoma following biopsy. The normal left side is shown for comparison.
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THE
ROLE
OF
MAGNETIC
RESONANCE
IMAGING
IN
THE
MUSCULOSKELETAL
SYSTEM
Fig. 3~Osteosarcoma Right Upper Femur. Coronal Scans (A) Spin Echo (TR 820 TE 40). Extensive loss of the normally high marrow signal in the right upper femur. Patchy loss of signal is also seen in the right innominate bone indicating tumour involvement at this site also. (B) STIR Sequence. The tumour is seen as high signal areas on this sequence. In addition to the bony abnormality, the extensive soft tissue component is best demonstrated on this sequence (arrows).
poor, although lipomas are characteristic--giving very high signal on both T 1 and T2 weighted sequences. Fibrosis due to scar tissue gives a low signal on both T1 and T 2 weighted images, and tumour with a high signal on T2 weighted images can therefore be well differentiated when this sequence is employed. (2) Bone Tumours Magnetic Resonance is the most sensitive method of assessing intramedullary tumour extent 1 (Figs. 4, 5). Additionally, Magnetic Resonance is a valuable method of assessing tumour breakthrough of cortical bone, and also the degree o f any articular surface involvement. As with primary soft tissue tumours, the technique is a specific marker of primary bone tumours. In consequence Magnetic Resonance is superior to all other modalities in the staging of bone tumours. Limitations still exist in specifying tissue composition, and in separating pathological tissue from reactive change. Magnetic Resonance has also been used in the investigation of bone marrow disease. Table 3~Relative
Fig. 4--Chondrosarcoma Right Upper Femur Coronal Scan. Spin Echo (TR 820 TE 40). The upper extent of the lesion at the intertrochanteric region is well seen due to tumour replacement of marrow fat with resulting decreased signal.
M e r i t s o f D i f f e r e n t I m a g i n g M o d a l i t i e s in A s s e s s m e n t o f M u s c u l o s k e l e t a l
Tumours
(a) S o f t T i s s u e T u m o u r s Tumour Extent Involvement of adjacent bone Calcification Neurovascular bundle involvement Nature of lesion Metastases
M R
CT
X-rays
Isotopes
Angiography
+ + + + + + + + +
+ + + + + + + +
+ +
+ + + + +
+ + + + +
M R
CT
X-rays
Isotopes
+ + + + + + + +
+ + + + + +
+ + + + + + + +
+ + + + + + + +
(b) B o n e T u m o u r s Intramedullary Extent Cortical Extent Soft Tissue Extent Calcification Nature of lesion Metastases
+ + + + + +
+ + + + -
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105
Fig. 5 ~ G i a n t Cell Tumour Lateral Condyle Right Tibia. (A) Spin Echo (TR 1000 TE 80). The lesion is not well seen on this T 2 weighted Spin Echo image. Note menisci contrasted with the synovial fluid. (B) Spin Echo (TR 500 TE 40). Clear definition of the tumour is obtained on the T 1 weighted Spin Echo image. Cystic lesions are often best seen on T 1 weighted sequences.
Cohen 3 and Olsen have found it to be sensitive in the detection of marrow infiltration with l e u k e m i a - - a signal reduction indicates tumour replacing fatty marrow. Additionally, Olsen found a distinctive appearance with lymphoma: areas of reduced signal and focal areas of markedly reduced signal. In our own experience neuroblastoma extent and metastatic infiltration are also well seen. The marrow changes of aplastic anaemia cannot, at present, be reliably demonstrated, the marrow signal being either normal or increased (due to the replacement of erythropoietic tissue by fat). Mancuso 4 and Cohen 3 have found a change in the signal intensity with both radiotherapy and chemotherapy. Magnetic Resonance therefore has considerable potential in the staging and follow-up of patients with bone marrow neoplasms. An additional benefit of Magnetic Resonance is that the image degradation due to metallic implants and prostheses is less marked than with C T - - g i v i n g Magnetic Resonance a further advantage over CT in the assessment of recurrence following surgery. Calculated T 1 and T2 Relaxation Times These provide a more precise definition of abnormal tissue characteristics than an image in which the arbitrary signal intensity is interpolated by the computer on to a grey scale to provide an image. This method of analysis is currently under investigation in the hope that the actual T1 and T 2 relaxation times will correlate with the histological diagnosis. Preliminary results are unpromising. Pettersson found no direct relationship between the calculated relaxation times
and the histological diagnosis in a study of 50 bone and soft tissue tumours.
Further Applications The Knee Normal Anatomy. High resolution Magnetic Resonance using surface coils provides much information on the anatomy of the knee. The menisci are readily distinguished as low signal areas on both coronal and sagittal views (Fig. 5). The collateral ligaments are best seen on coronal slices, and their continuity can be assessed. Articular cartilage is well seen on both T1 weighted, and on partial saturation images. The cartilage can be clearly distinguished from the menisci. Coronal and sagittal scans are again the planes in which this structure is optimally imaged. The sagittal view is the plane in which the continuity of the cruciate ligaments (Fig. 6) and patellar tendon can be viewed. The whole length of the anterior cruciate can most often be seen when the leg is externally rotated by 20°. 5 On the transverse axial view the muscles, tendons, nerves, arteries and veins are readily distinguished and differentiated. 5,6 Pathology. The first M R I demonstrations of ligamentous and cartilaginous injury of the knee were made by Li and by Turner. 7 Meniscal tears and the anatomy of cruciate and collateral ligaments were displayed on Magnetic Resonance images. Joint effusions were also demonstrated. A meniscal tear is seen as a focal high intensity
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THE ROLE OF M A G N E T I C R E S O N A N C E I M A G I N G I N T H E M U S C U L O S K E L E T A L SYSTEM
Fig. 6-~Normal Left Knee. Sagittal Scan Spin Echo (TR 500 TE 40), The posterior cruciate ligament (arrow) is best seen in this plane.
Fig. 7--Normal Left Hip. Coronal Spin Echo (TR 820 TE 40). The central weight bearing trabeculae are seen as an area of reduced signal within the femoral head (arrow).
Hips (white) area within the normally low intensity (black) meniscus. The signal alteration is thought to be due to synovial fluid within the cleft created by the tear. Reicher 8 compared MRI with both arthroscopy and arthrography. The M R ! images were obtained using .a T~ weighted spin echo sequence (short TR, short TE), imaging initially in the sagittal plane and subsequently coronally if the diagnosis was unclear. Excellent correlation was found, and M R I did not give any false negative results. It shares with arthrography the advantage over arthroscopy that the posterior menisci can be well seen. This work suggests that MRI employing surface coils is likely to become the primary screening technique for meniscal tears, in view of the major advantage that it is non-invasive, and requires no manipulation of the knee. Cruciate ligament tears were also studied by Reicher, with comparative arthroscopic and surgical findings. The M R I was considered positive when the continuity of the ligament was lost. Again the correlation between the three techniques was good, with no false negative results on MRI. Disruption o f the patellar tendon is difficult to assess both clinically and arthroscopically. Arthrography is of no value. The continuity is well seen on sagittal Magnetic Resonance images. In addition to the discontinuity due to rupture, haemorrhage can be clearly distinguished due to the high signal of static blood. Steinbrich found that osteochondritis dissicans was well detected by M R ! with both cartilage abnormality and reduced subchondral intensity due to local bone marrow displacement being seen. The multiplanar imaging capability of M R I is likely to be of value in the assessment of osteochondral fragments--in order to clarify if such fragments are loose bodies, or are connected to other bony structures. 8
The normal anatomy demonstrated has been documented by Heller and by Totty. 9 The cortical outlines are well defined. The central weight bearing trabeculae are seen as a broad band of low signal intensity (Fig. 7). The joint capsule and the remnant of the growth plate are also demonstrated. Several studies have established M R I as a sensitive method of detecting avascular necrosis of the femoral head 9 (Figs. 8, 9). The consistent finding is that M R I is the most sensitive method of detecting avascular necrosis, superior to both X-rays and radionuclide scans. The change observed on the MRI scan is a variable decrease in the signal at the site of necrosis. It
Fig. 8~Avascular Necrosis. Right Femoral Head. Coronal Spin Echo (TR 795 TE 40). The necrosis results in reduced signal from the femoral head, and some irregularity at the articular surface. X-rays and isotope scan were normal.
CURRENT ORTHOPAEDICS 107 able, David found that MRI was able to demonstrate the changes of this disease at an earlier stage and with more detail than with X-rays. (iii) Haemophilic Arthropathy. In addition to evaluating synovial hypertrophy, cartilage irregularities and subchondral cysts, MRI is a sensitive indicator of intra-articular bleeding, and therefore is ideally suited for early evaluation and monitoring of haemophilic arthropathy.
Fig. 9--Avascular Necrosis Left Femoral Head. Coronal Scan Spin Echo (TR 797 TE 40). Well defined loss of the normal marrow signal is seen in the upper part of the femoral head.
can be anticipated that aseptic necrosis at other sites will also be well demonstrated. Additionally, Scoles ~° has found MRI to be of value in the detection and monitoring of Legg-Calv6-Perthes disease of the hip.
(c) Rotator Cuff Abnormality The ligaments and tendons of the rotator cuff muscles are clearly demonstrated as low intensity structures. The articular capsule of the shoulder joint is also well defined.12 High resolution Magnetic Resonance with surface coils can demonstrate tears of both the supraspinatus and infraspinatus tendons. 1z Additionally, the degree of separation of the torn fragments can usually be assessed. Kneeland has concluded that MRI has a sensitivity in the detection of complete rotator cuff tears which approaches that of arthrography. (d) Achilles Tendon Rupture This condition has been clearly demonstrated on sagittal MRI, as an area of high signal (due to oedema and haematoma) between the separated low signals of the tendon.
Miscellaneous Conditions (a) Infection Infection with resulting inflammatory change results in lengthening of both the TI and T2 relaxation times. This condition is consequently well suited for assessment by MRI (Figs. 10, 11). Markiez found Magnetic Resonance to be the most sensitive imaging technique in cases of acute infection, by comparison with isotope, CT and plain X-ray studies. Chronic infective and inflammatory processes were well seen by all four methods. Our own experience is consistent with their findings. Additionally Magnetic Resonance can detect small effusions within joints 1~ and this capability might prove to be useful in the early detection of arthritis. (b) Arthritis (i) Rheumatoid Arthritis. MRI is likely to have a significant role in the assessment of this condition (Fig. 12). As stated above, joint effusion is readily seen, the degree of swelling due to synovial tissue hypertrophy can be assessed, and additionally the synovial inflammation results in alteration of the signal by comparison with normal. High resolution Magnetic Resonance with surface coils can demonstrate cartilage irregularity, although the ability of Magnetic Resonance to demonstrate small erosions is limited by the spatial resolution currently attainable. (ii) Osteoarthrosis. Although the sensitivity of MRI in the early detection of osteoarthrosis is again limited by the current slice thickness and spatial resolution attain-
(e) Tempero-Mandibular Joints High resolution MRI of the tempero-mandibular joints (TMJ's) has been studied by Katzberg and by Harms. The diseased meniscus is well demonstrated-giving a lower signal than normal, probably due to calcium deposition. Meniscal displacement is also visualised, and Katzberg found a good comparison
Fig. l O ~ L u m b a r Spine. Osteomyelitis. Sagittal Scan Spin Echo (TR 2000 TE 80). The increased signal (white area) at the vertebral borders of the L4/5 disc is clearly displayed on this T 2 weighted Spin Echo image. X-rays were normal.
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THE ROLE OF M A G N E T I C R E S O N A N C E I M A G I N G IN T H E M U S C U L O S K E L E T A L SYSTEM
Fig. 11--Osteomyelitis Lumbar Spine. (A) Sagittal Scan. Spin Echo (TR 500 TE 40). The abnormality is not clearly seen on this T 1 weighted Spin Echo image. (B) Sagittal Scan, STIR Sequence. The increased signal involving part of the L3 body and the whole of the L4 body is present on the STIR sequence.
between CT, Arthography and MRI in assessment of this condition. In the light of the findings of these authors, it is possible that Magnetic Resonance will become the technique of choice in the primary diagnostic evaluation of abnormalities of the tempero-mandibular joint.
(f) Vascular Abnormalities Rapidly flowing blood gives no appreciable signal on Magnetic Resonance because the protons excited by the transmitted radiofrequency pulses move from the plane in which the signal is received before their N M R signal can be recorded (Fig. 13). At lower flow rates some signal is received, thus the signal from blood in normal veins is higher than from arterial blood. 13 Arteries and veins are clearly differentiated from each other, and from surrounding tissues. Rapoport has shown that a static column of blood can be readily distinguished from flowing blood. Clearly, therefore, there is a potential for MRI to be used in the diagnosis of deep venous thrombosis. Accurate assessment of flow rates would also be helpful in the management of peripheral arterial occlusive disease. MRI can also provide information of value regarding vascular malformations--the extent of the lesion, the origin of the feeding vessels, and which feeding vessels display rapid flow and would be amenable to embolisation.
The Spine
Fig. 12--Rheumatoid Arthritis. Cervical Spine. Sagittal Scan Spin Echo (TR 860 TE 40). Previous laminectomy at C2 level which has resulted in loss of the signal from the posterior epidural fat. There is posterior compression of the upper cervical cord due to fibrosis which gives a low signal (arrow). Marked atlanto-axial subluxation with wide separation between the anterior arch of the atlas (open arrow), and the anterior part of the dens. The focal area of high signal (curved arrow) is a surgical clip. It has not resulted in significant degradation in image quality. Note how well the cord contrasts with CSF.
Normal Anatomy The vertebral bodies are well outlined. As in the appendicular skeleton, the high signai from cancellous bone contrasts with the low signal from the surrounding cortical bone. The normal nucleus pulposis has a long T2 relaxation time and thus gives a high signal on T2 weighted images. This is believed to be due to the high water content of the nucleus. The annulus fibrosis has a shorter T 2 relaxation time, and therefore its low
CURRENT ORTHOPAEDICS
109
and STIR sequences (Fig. 14B). Surface coils produce high resolution images with slice thickness of 3M mm making it possible to visualise nerve roots on transverse axial scans.
Disease of the Spine Magnetic Resonance has already established a major role in the assessment of spinal disorders.
Fig. 13--Vascular Malformation Left Lower Leg. Coronal Scan. Spin Echo (TR 500 TE 40). Marked increase in subcutaneous fat on the left side. The aberrant vessels are seen as linear areas of reduced signal within this fat.
signal on the T2 weighted image contrasts with that of the nucleus. Lateral and posterior epidural fat is well seen on all sequences. The longitudinal ligaments (with low signal) cannot be distinguished from cortical bone. The spinal cord is well outlined on T~ weighted images. The cerebrospinal fluid, and the extent of the spinal theca, are best seen on T2 weighted (Fig. 14A)
Vertebral Column (a) Disc Disease. Prior to Magnetic Resonance, the only accurate method of assessing disc degeneration was by contrast discography, an unpleasant and invasive procedure. Magnetic Resonance produces an image of the nucleus which is comparable in shape with that produced by discography. Additionally, MRI is at least as accurate as, and probably more accurate than, discography in the assessment of this condition. 1~ The diagnosis of degeneration is made on two criteria: alteration from the normal nucleus shape, and reduction in the normally high signal--presumably due to loss of water in association with the degeneration. An additional advantage of MRI over discography is that all the discs in the lumbar spine can be assessed on one multislice sagittal scan (Fig. 15). MRI is consequently the investigation of choice in assessing whether disc degeneration is present. Diagnosis of disc herniation can be made on multislice sagittal and oblique/transverse scans (Fig. 16). The accuracy of the technique is improved when surface coils are employed enabling the selection of thinner tissue slices. Edelman 15 found Magnetic Resonance to be equal to CT in accuracy of diagnosis of disc herniation. (b) Infection and Inflammation. In another study,
Fig. l z I ~ ( A ) Lumbar Spine. Sagittal Scan. Spin Echo (TR 1000 TE 80). The longer repetition time and echo delay time result in a more T 2 weighted image. The normal nucleus pulposis gives a high signal, and the degenerate disc at L5/$1 is therefore more clearly demonstrated. CSF gives a positive signal in contrast to the low signal from the annulus fibrosis. (B) Lumbar Spine. Sagittal Scan. STI R Sequence. The cerebro-spinal fluid gives a high signal on this sequence allowing accurate assessment of the dimensions of the subarachnoid space.
I10
THE ROLE OF M A G N E T I C R E S O N A N C E I M A G I N G I N T H E M U S C U L O S K E L E T A L SYSTEM
major advantage is that intraspinal extension can be assessed without the intrathecal administration of contrast medium. As with CT, intra-abdominal and intrathoracic structures are simultaneously imaged, and thereby any tumour involvement of these structures can be seen. (e) Trauma. Compression and dislocation of the vertebral bodies is seen. Without the use of contrast, the spinal theca can be assessed for narrowing. The cord is directly delineated (Fig. 20) and it should be possible to visualise a haematoma. Additionally, in cases of cord damage, the complication of post traumatic syringomyelia can be detected on serial follow-up scans.
Spinal Canal
Fig. 15---Lumbar Disc Degeneration. Sagittal Scan. Spin Echo (TR 1000 TE 80). The L2/3 and L3/4 intervertebral discs are normal in both outline and signal intensity. Degenerative change at L4/5 and L5/$1 more marked at the lower level.
Modic compared Magnetic Resonance with X-rays and Isotopes in a series o f patients with a clinical diagnosis of vertebral osteomyelitis. 16 Histology and microbiology were the final arbiters. In terms of both sensitivity and diagnostic accuracy Magnetic Resonance was at least equal to radionuclide scanning. Both were superior to plain radiography. In our preliminary experience we have found that Magnetic Resonance using the recently available STIR sequence, is superior to both isotope scans and X-rays in the demonstration of both vertebral osteomyelitis and discitis (Fig. 17). (c) Chymopapain. The biochemical findings of Stern indicate that intradisc injection of chymopapain resulted in biochemical changes similar to premature degeneration of the nucleus pulposis. This is probably due to a breakdown of proteoglycans in the nucleus with resulting loss of water. Our own findings on serial MRI scanning are consistent with this hypothesis. In our study 17 chymopapain injection caused a gradual loss of signal from the nucleus progressing to complete loss of the signal, usually by 6 weeks (Fig. 18). Reduction of the disc space also occurred. The degree of disc protrusion previously observed was little altered by chemonucleoysis. In a number of cases a patchy loss of the signal from the vertebral end plates occurred-suggesting the presence of a chemical discitis. The STIR sequence has been employed recently and has demonstrated increased signal around the vertebral end plates adjacent to the injection site in cases where chemonucleoysis failed to alleviate symptoms (Fig. 19). This change, consistent with inflammation, suggests that a chemical discitis is one cause of failure of chymopapain therapy. (d) Tumours. As in other locations, these are well demonstrated due to replacement of cancellous bone by the tumour. Soft tissue extension is well defined. A
The ability of MRI to directly image the cord in transverse, sagittal and coronal planes without the use of contrast medium has resulted in its widespread use in the assessment of cord tumours, demyelination, trauma, atrophy, syringomyelia, dysraphysm (Fig. 21) and other congenital anomalies and disc herniation (Fig. 22). Rheumatoid arthritis in the cervical spine is currently assessed with plain radiography, myelography and CT myelography in order to assess atlanto-axial subluxation, other bone lesions, and any cord compression resulting from subluxation or pannus. In a study comparing these modalities with Magnetic Resonance, we found that Magnetic Resonance could assess atlanto-axial subluxation, odontoid erosion, soft tissue swelling, the dimensions of the spinal theca, and any anterior or posterior cord compression resulting from this disease. There is no need for intrathecal contrast medium. Lumbar spinal stenosis is readily diagnosed on
Fig. 16--Degenerative Disc Disease Lumbar Spine. Sagittal Scan. Spin Echo (TR 1000 TE 80). Disc degeneration at L3/4 and L5/$1. Central disc herniation at L5/$1. Note the break in the L5/$1 fat line due to the disc herniation.
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Fig. 1 7 - - L u m b a r Spine Discitis. Patient complained of severe, increasing lower back pain. X-rays were normal. Isotope scan showed an area of slightly increased intensity at L4/5. Subsequent microbiology was negative. (A) Sagittal Scan. Spin Echo (TR 1000 TE 80). Reduction in the signal from the L4/5 disc. Unimpressive increase in signal adjacent to this disc space. (B) Sagittal Scan. STIR Sequence. Striking increase in the signal extending from the L4/5 disc space into the L4 and L5 vertebral bodies.
transverse and oblique scans. If a T 2 weighted sequence is employed, the CSF is shown as a positive signal and the dimensions of the spinal theca can thereby be measured. The high signal from CSF on the STIR sequence is also helpful in assessing the AP dimensions of the spinal theca (Fig. 23).
The Future Magnetic Resonance is becoming the imaging investigation of choice in a wide variety of musculoskeletal lesions. In time its role will almost certainly expand further. The chief reason for its increased applicability in the last 18 months or so, is the improved spatial resolution
available when surface coils are used. Diagnostically useful imaging of knee, shoulder, tempero-mandibular joints and rotator cuff can now be performed. In view of its ability to provide information relating to these structures which is otherwise obtainable only with invasive procedures, it will probably become the primary screening technique of choice for disorders of menisci, ligaments and tendons at these sites. MRI may have an important role in the monitoring of treatment of malignant diseases, such as leukemia, lymphomas and neuroblastoma, by reading the serial changes in marrow signal intensities, or the changes in the calculated T~ and T2 values. This form of evaluation of treatment may also be applicable to infection.
Fig. 1 8 - - L u m b a r Spine. (A) Sagittal Scan Spin Echo (TR 2000 TE 40). Pre-chymopapain. Disc degeneration at L5/S1 with central disc protrusion. (B) Nine weeks after administration of chymopapain at L5/$1. There is now complete absence of signal at this level.
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Fig. 1 9 ~ L u m b a r Spine. Sagittal Scan. STIR Sequence. One week after the administration of chymopapain at L4/5 level the patient continued to complain of severe pain. The increased signal around the vertebral end pi,~tes at this level suggests a chemical discitis due to chymopapain.
Fig. 21--Spinal Dysraphism. Sagittal Scan. Spin Echo (TR 500 TE 40). Low cord extending to L5 level. The posterior position of the cord is due to tethering.
There is enormous potential in the assessment of inflammatory joint diseases such as rheumatoid arthritis and haemophilic arthropathy. This will be realised with further improvements in spatial resolution, permitting visualisation of early changes in articular cartilage in both large and small joints. Imaging of elements other than ~H, and a better understanding of both the chemical information in the Magnetic Resonance signal (chemical shift data), and alteration in TI and T2 values may lead to better diagnosis and better understanding of degenerative muscle disease. A number of important further developments can be anticipated: improved receiver coils, higher resolution
scans with 512 × 512 matrix or greater, shorter scanning times, the widespread use of paramagnetic contrast media such as Gadolinium DTPA and the utilisation of new sequences such as chemical shift and STIR will increase the sensitivity, specificity and therefore the applicability of MRI. A further interesting possibility is the use of conjugates of paramagnetic elements such as Gadolinium with monoclonal antibodies, in tumour detection and staging. 18 The first published images of pathology demonstrated by MRI appeared just 6 years ago. Since then, its impact on the imaging approach to musculoskeletal pathology already exceeds any other modality since Roentgen.
Fig. 2 0 - - L o w e r Cervical/Upper Thoracic Spine. Sagittal Scan. Spin Echo (TR 860 TE 40). Automobile accident. Crush fracture T4. The resulting compression of the thoracic cord is seen (arrow).
Fig. 22--Cervical Spine. Sagitta] Spin Echo (TR 1000 TE 40). Disc degeneration and herniation at C5/6. Local compression of the cord at this level is seen.
CURRENT ORTHOPAEDICS
Fig. 23--Lumbar Spine. Sagittal STIR Sequence. Spinal stenosis. Generalised narrowing of the canal is seen. Marked narrowing at L3/4 due to herniation of the degenerate disc at this level. ReUcI"CHCCS
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