Recent advances in musculoskeletal MRI

1 Imaging

Recent Advances in Musculoskeletal MRI

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E. G. McNally, D. J. Wilson __--_ __~ Introduction Magnetic resonance unaging (MRI) has now become well-established in current orthopaedic practice. The exquisite detail of both soft tissue and marrow have had a considerable impact in the management of many spinal. joint and soft tissue lesions. The purpose of this article is to review some of the more recent advances. both technical and clinical and discuss their impact on current orthopaedic practice. A knowledge of the basic applications of MRI is assumed.

Technical Advances Since its introduction into clinical practice. there have been steady advances in both MR technology and software developments. Lower-field and lower cost scanners have made MRI more widely available. Reduced maintenance costs (for example helium refills in superconducting magnets) and improved software have resulted in faster. cheaper imaging without loss of resolution. The net effect of these advances is that MRI has become more readily available at less cost. with an inevitable broadening of its applications and indications.

High speed imaging While conventional spin echo (CSE) Tl and T3 weighted sequences still form the mainstay of many imaging protocols. they have the disadvantage of relatively long imaging times. ~YJ -ially in the case of T2 weighted images. Typical j ‘an times for a Tl weighted image of the knee are 4.5 min. T2 weighted

F:. G. McNally FRCR, D. J. Wilson FRCR, Department of Radiology. Nuffield Orthopaedic Centre NHS Trust. Windmill Road, Headington. Oxford OX1 4LD. Correspondence snd Ijffprint requests to E G X4

X.5 min. while a gradient echo (GE) T2* FLASH image can be obtained in 5 min. GE sequences can acquire good Tl weighting with higher signal to noise ratios and shorter imaging times than CSE. Although true T2 weighting IS not seen M.ith GE sequences, T2 type information. called T?* (TZ Star) can be acquired with %hortet- imaging times than CSE sequences but with the disadvantage that local magnetic susceptibilitk efiects can suppress the signal from marrow and thus may mask pathology. GE sequences use a radiof‘requencb (RF) pulse that deflects the hydrogen protons by lesj than the 90” used in CSE. Examples include FLASH and FISP sequences in the Siemens machines and GRASS, SPOILED GRASS on GE scanners and FFE on Philips. Several ultrafast gradient echo sequences are also available including Turbo-FLASH and MP-RAGE. Using these technique>. image> can be obtained at the rate of I set per shce sacrificing some contrast and resolution but with the added advantage of edge enhancement which can occasionally be useful (Fig. 1). These superfast sequences can also be used to study enhancement characteristics in more detail during dynamic contrast administratlon. Mart: recenllq. the introduction of fast spin echo (FSE) sol&are has meant that Tl and true T? information can be acquired with imaging times less than GE sequence<. and approximately one fourth that o!‘CSE. bet with the same image resolution. The impact of faster imaging protocols on clinical practict has been substantial. Faster sequences reduce the amount of sc,mning time necessary for each patient Increased throughput reduces the overall cost per patlent. making MRI more widely available for a wider r.lngc of clinical applications. Faster scanning times irre als(i important for patients who have dlfficulrv keeping still. including children and patients in pain One ol‘ the problems associated with FSE,

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Fig. l-Coronal Turboflash of the retroperitoneum. One of a set of 20 slices acquired in 26 sec. Resolution is less than CSE seqLrences but sufficient to demonstrate the ileopsoas haematoma (arrow) in this haemophilic patient who had difficulty lying still due to pain.

however, is the relatively higher T2 signal from fat compared with CSE. Perception of marrow abnormalities, including infiltration by tumour, infection or marrow oedema due to microfracture, depends on good contrast between the high T2 signal of the pathological process and the normal marrow signal. Increased marrow signal such as occurs in FSE is an obvious disadvantage. It can, however, be combined with a fat suppression sequence, where, by various means, the signal from fat is removed from the image. While the addition of fat suppression adds to the scanning time, the net advantage over CSE is still considerable. The two main methods of suppressing the signal from fat are spin echo and inversion recovery (STIR) sequences and by RF saturation. The STIR sequence, and the more recent faster version, Turbo-STIR, result in a uniform suppression of signal from fat over the whole image. High signal from abnormal tissue is therefore readily visible (Fig. 2). The disadvantage of the STIR sequence is that signal from Gadolinium-DTPA is also suppressed. RF fat suppression, or ‘fat sat’ gives a less uniform fat suppression over the whole image. It can, however, be used in conjunction with Gadolinium-DTPA enhancement on a T 1 weighted image. In musculoskeletal imaging this has advantages in defining the extent of tumoural oedema and in the study of abnormal bone perfusion, differentiating dead avascular bone from vascular reparative tissue. Blitz scanning

Increased patient throughput can be achieved in other ways and some centres set aside full days for scanning one particular body part. In our institution, whole

Fig. 2-Sagittal Fast STIR sequence suppressing the relatively high fat signal from marrow and increasing conspicuity of the bone oedema (white arrows) in this athlete with stress response of the Talus.

days will be devoted to knee or lumbar spine MRI. By focusing attention on a single body part with a simple protocol, the number of patients that can be scanned per unit time increases. Active intervention by the radiologist does not occur during blitz days. In our centre, 25 patients can be scanned during an 8 h knee

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Tl weighted ‘body shot’ of C2 to L4 with in posterior elements of T6 and T7.

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blitz day. Patients with difficult or complex problems or patients who may require Gadolinium-DTPA enhancement would not be appropriate for this type of booking.’

The other major area for technical development are improvements in receiver coils. Coils are essentially the antennae that detect the signal emitted from the patient. They come in various shapes and sizes, large flat coils for lumbar spine imaging down to tiny surface coils designed for accurate imaging of the temporomandibular joint (TMJ). Transrectal and transvaginal coils have been developed with applications in the early detection and accurate staging of pelvic disease. In spinal imaging a particularly useful development is the whole body coil combined with 512 matrix imaging. A single Tl sequence typically acquired in IO min in conjunction with high resolution 512 matrix imapinp can demonstrate the entire vertebral column from atlas to sacrum in children and 75 ‘)/oof it in the adult (Fig. 3,. This is especially useful rn acute cord compression where the site or sites of disease cL[n be quickly and accurately located. Imaging rhe whole spine IS particularly important where multiple levels of disease are suspected or indeed where the exact level of disease is difficult to determine clinically. Because a large area of the spine is shown. (“2 or the lumbosacral junction can be included on

Fig. 4--(A) Normal anterior cruclate ligament on a T2’ gradient echo FLASH image. The anterior fibres of the normal ligament are seer] as a hypo-Intense line on all sequences (black arrows) A large effuston (long white arrows) and prepatellar bursttrs (short white arrows) are also seen in this patient (B) Complete tear of the anterior cruclate ligament with a large effusion, the hypo-Intense line IS 10 longer vlsrble

thoracx spine scans to enable accurate iocalisation n!‘ the disease process to a named vertebra. In the pas1 these Ltr_ge body coils suffered from problems 01‘poolsignal to noise ratio but technical developments have resulted in newer coils with excellent resolution despitl their large silt.

Some \cannert allow data acquisition from ;I i,olumc of tissue rather than the usual slice bq slice technique normally used with CSE or CT. By using .!D displa) softwar-e. images can be displayed in an! plane Lvithout 1~~s~oi‘ resolution. This is partlculari\ useful

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Fig. 5-There is thinning of the cartilage of the medial retropatellar normal cartilage of the lateral facet.

in the knee for examining the cruciate and collateral ligaments, which are not oriented in orthogonal planes. A single block of data acquired typically in 4-6 min can be reconstructed to produce sagittal, axial and coronal images. With the addition of a 6 min T2* weighted GE image, MRI provides an extremely useful examination of patients with suspected internal derangement in under 10 min acquisition time, equivalent to about 20 min of machine time. Studies have shown an accuracy over 90% compared to arthroscopy for the diagnosis of meniscal tears and approaching 100% for assessment of the cruciate ligament?,” (Fig. 4). Volume acquisition is also useful in the ankle for assessment of the collateral ligaments and occasionally in patients with severe scoliosis, where a plane of reconstruction can be chosen so as to ‘unwind’ the curves.

Advances in Clinical Applications Imaging of articular cartilage Chotldrotnalacia patellae. MRI of the articular cartilage is possible owing to the contrast provided by different signal intensities of joint fluid and subchondral bone. Currently the greatest clinical impact is in the knee where MRI is used to diagnose chondromalacia and other abnormalities of articular cartilage. Chondromalacia, is, of course. a common clinical problem and currently there is no satisfactory non-invasive method of investigation. Despite being a source of chronic disability there is little in the way of active intervention at present. Improvements in MRI, particularly with regard to small field of view surface coils may be the key to more accurate diagnosis and classification. Better disease definition may lead to improved treatment protocols and prognosis. In the retropatellar cartilage, three zones are identified on

facet (arrows)

indicating

chondromalacia.

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MR images. Zone 1 is a low-intensity zone near the articular surface, which corresponds to dense, tangentially oriented layers of collagen on histologic section. Zone 2 is of higher signal intensity and correlates with cartilage in the transitional zone. The innermost zone is also of low intensity and corresponds to a combination of deep radiate and calcified cartilage and cortical bone.3 A variety of sequences have been suggested to detect articular cartilage lesions including T 1, volume acquisition (FISP and Spoiled GRASS) and T2* weighted images (FLASH and GRASS). STIR techniques with suppression of the signal from fat have also been used.5 In many cases routine knee protocols may underestimate chondromalacia unless small field of view, high resolution dedicated studies are obtained. Even with high resolution studies, lesions classified as Stage 1 (cartilage oedema and blistering) cannot be detected and stage 2 lesions (crabmeat fibrillation of the surface) are frequently missed6 (Fig. 5). The sensitivity of MRI to these smaller lesions can be increased by using intra-articular contrast, either GadoliniumDTPA in very small dose, intra-articular air or intravenous Gadolinium-DTPA.’ The former two convert the examination to a more invasive procedure albeit a relatively minor one. The diagnosis of post traumatic cartilage lesions depends on whether they are full or partial thickness. With experience the MRI sensitivity is 80% for full thickness and approximately 50% for partial thickness lesions8 Other cartilage lesions such as those related to osteo-arthritis are also detected and changes on MRI correlate well with histological change.” Occult bone and cartilage injuries. Several occult post

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bone and cartilage disorders are readily

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of an ill-defined hypo-intense area on TI images which shows increased signal on T?. If there is a significant component of haemosiderin deposition. low T? signal is seen. Bone bruises are self-limiting and benign and they are not identified al arthroscopy. Stress and microfractures appear similar but contain irregular linear dark lines on Tl represenring the fracture lines. Osteochondral fractures or the more subtle chondral bruise or fissure can also be identified.” The latter are easy to overlook if there 1s no ,1ssociateJ bony abnormality to point to the lesion

Fig. &-Bone bruise. Sagittal showing ill defined increased condyle (arrow)

Fig. 7-Sagittal Osteochondritis

GE volume dissecans

T2’ weighted signal within

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FLASH Image the medial femoral

FISP sequence

diagnosed by MRI. Bone bruises or contusions are the most common lesion. and are particularly common in association with tears of the anterior cruciate ligament when they are seen in the posterior lateral tibia1 condyle and the overlying femoral condyle in a high percentage of cases I” (Fig. 6). Typical appearances are

Osteochordritis disswat~s. Knowledge 0t‘ Iragmenl stability and the presence of an urticulx cartilage defect can be helpful in the management elf osteochondritis dissecans (Fig. 7). The ;Ipparcnt hoie may represent a bony defect or it ma!’ be filled with fibrous tissue or fibrocartilage. Bone fragments may be partially or firmly attached, the former being prone to displacement. A high-signal intervxc on T7 weighted sequences is indicative of fund her\+een the lesion and underlying bone. This is evidence of lesion and correlates welt with cjperative instability Increased conspicuity of‘ thix line of findings.” separation can be achieved by injecting tiadotiniumDTPA into the joint and in certain instances by intravenous Gadolinium-DTPA.” Other c,artilage lesions. MRI has also been used to assess articular cartilage and fibrocartilage in the hip. TMJ and the wrist. The acetabular and epiphyseal cartilage of the hip affected by Perthes disease can be imaged accurately. allowing assessment of femoral head containment. congruity of the acetabular and femoral articular surfaces. and intracapsular softtissue irregularities.‘” Labral tears can be difficult to diagnose and occasionally need Gadolinium-DTPA arthrography for their demonstration (Fig. 8). Tears of the triangular cartilage are well-demonstrated (Fig 9) and correlate well with surgical and arthroscopic findings.‘“, I4 The TMJ meniscus can be accurate11 demonstrated on open and closed mouth images.” GE fast scan images can also be obtained at various degrees of mouth opening and the serial images displayed in a simulated video mode to provide additional information about meniscal movement during opening and closing.‘” Anterior dislocation and the presence or absence of reduction can be demonstrated on sagittal oblique images. Demonstration of pure medial or lateral displacement is best achieved with images in the coronal plane. Results correlate lvell with arthrography and surger! .” Occasionally. dynamic subluxation ma! Ihi1 10 show abnormality on ;I series of essentially ctalic MR images. Arthrography still has a role in rhe patient uith a normal MRI where there is a strong clinical suspicion of meniscal dysfunction. MRI can be of further assistance in the postoperative patient to demonstrate meniscal position post mrniscoplasty or replacement.” ”

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Fig. 9-Tl weighted coronal image of the right hip following intra-articular Gadolinium-DTPA administration.Note the high signal from the synovial fluid which is normally intermediate signal on a Tl weighted image. The labral tear indicated by high signal fluid within the labrum (arrow) was not visible on conventional sequences.

Fig. 9-Coronal T2” FLASH image of the wrist in a patient with a tear of the triangular cartilage. High signal fluid is seen within the cartilage (small arrow) indicating a tear with fluid communicating with the distal radio-ulnar joint (large arrow).

result of oedema. The normal high signal of marrow on Tl weighting, which results from its fat content, is replaced by signal of intermediate intensity indicating oedema. Corresponding increased signal is seen on T2 images, especially when fat suppression techniques are used. Because cortical bone produces signal void, small steps due to fragment displacement are better demonstrated by plain films, plain tomography or CT. MRI can provide earlier detection of dead bone whether due to massive trauma or avascular necrosis. Information about viability may be useful in surgical planning. MRI can predict the vascularity of the ununited scaphoid accurately (Fig. 10) and may demonstrate adjacent ligament tears in patients with snuff-box tenderness and normal plain films.” Earlier diagnosis of non-union speeds definitive surgery reducing the length of time spent in plaster. Osteonecrosis of the hip from any cause can be detected by MRI often well in advance of other techniques. Following Gadolinium-DTPA administration, both enhancing and unenhancing areas can be detected in the femoral head. Histologically, enhancing and nonenhancing areas correspond to viable and necrotic tissue respectively.“l MRI, therefore, has an important role to play in the early detection of osteonecrosis, but also has a role to play in staging and follow-up after treatment. Soft tissue traunza

MRI in Trauma Fractures

MRI is excellent at detecting subtle fractures principally due to the signal changes within marrow as a

MRI provides an ideal means, unmatched by other imaging techniques with the possible exception of ultrasound, of examining tendons and ligaments. Small field of view coils have markedly improved the resolution of these structures. In addition, the abun-

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Fig. lO-Coronal Tl weighted image of the wrist.The normal high marrow signal is lost within the proximal scaphold (arrow) Indicating avascular necrosis

dance of adjacenl fat provides a superb contrast background. Findings in muscle tears include alterations in muscle shape and the presence of abnormal signal within the injured muscle. IntramuscuIar haematomas show in rhs subacute stage as areas of high signal on both TI and T2 weighted scans which, over a period of 23 weeks, becomes low on T2 due to the presence

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of haemosiderin”’ (Fig. 11). The presence of haemosiderin is particularly useful in helping to distinguish haematoma from tumour in patients where the history of trauma is minimal or lacking. MRI can also be used to study myalgia. strains. delayed-onset muscle soreness. chronic muscle overuse syndromes, muscle contracture. and sequellae of muscle injuries such as myositis nssificans and compartment syndrome. MRI documents ~hr distribution of affected muscles. the presence 4 focal haematoma. fascial herniation. and subsequent healing, fibrosis, or fatty infiltration. MRI \viil demonstrate acute and delayed exertional muxcle injuries.‘” This may be important in athletes though more work is necessary to relate the tindlngs to prognosis and return to training schedules. The ability of MRI to diagnose cruclate and collateral ligament tears in the knee I> ~11 known. and its place in ligament rupture around Ihe ankle joint is also well-documented. In the upper limb, MRI can be helpful in soft tissue trauma of the urist. elbou and shoulder. In dynamic carpal instabrlitk. plain radiographs Mith motion views can he useful, although direct observation of Lvrist motion (XII video. fluorohcopy ~+ith or without arthrograph) i\ oftera necessxy for diagnosis. M RI motion studiex pravidta better soft tissue definition and ma) sho\h subtle changes in the triangular-fibrocartil‘tge-abhociated distal radio-ulnar instability, as well as pet-i-art&Jar tendon subluxation about the mri51 ” There is. howevt;r. still a place for wrist arrhrography in thi> area a> not ali tears are well seen on MRI Complete rotator cuff tears are eah~ly diagnosed with MRI if high signal interrupts thrs normal Ion

Fig. 11-Intramuscular haematoma. The dark ring (arrows) on this T2 welghted The presence of dark slgnal on T2 IS helpful in distinguishing haematoma from can also demonstrate low T2 slgnal. aggl ‘esslve flbtomatosis

axial ttJmOU1.

image of the thigh IS due to haemosrderln though very fibrotic turnours. such iis

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Fig. 12.-Rotator cuff tear. High signal area (curved arrows) on this 12 weighted coronal oblique image

interrupts

signal supraspinatus tendon on T2 images (Fig. 12). Corroborative evidence is also obtained from the presence of high fluid signal within the sub-acromial bursa or in long standing cases from supraspinatus contraction and fat atrophy. If the rotator cuff shows homogenous low signal on all sequences then it can reliably be called normal. Difficulties can be encountered when there is inhomogenous intermediate signal present as distinguishing partial tears from so called tendonitis, which may in fact represent no more than innocent mucoid degeneration, is not always possible. Indeed some normal tendons can show signal heterogeneity related to interdigitating muscle and ligament bundles. In inconclusive cases arthrography may be necessary to confirm the presence of a partial or complete tear. Abnormalities of the biceps tendon commonly accompany other lesions about the shoulder, especially rotator cuff rupture, and are a frequent cause of a painful shoulder. Two types are recognised, the more common defect in the subscapularis apparatus with intra-articular entrapment of the biceps tendon and the less common incomplete dislocation, with the biceps tendon lying between a partially disrupted subscapularis tendonz5 There are pitfalls for the radiologist in the assessment of tendon injuries. Apparently abnormal signal can be detected with a normal tendon depending on its orientation within the magnetic field. Markedly increased intratendinous signal intensity was observed at an angle of 55”. the so called ‘magic angle’. The phenomenon was only observed when a short echo time was used. Increased signal intensity due to the magic angle effect may be misdiagnosed as tendinous degeneration, tendonitis, or frank tear.‘16

the normal low signal supraspinatus

tendon

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MRI in Atraumatic Muscle Disease MRI can be used in the assessment of primary skeletal muscle disease; muscular dystrophies, tong’ vital myopathies and polymyositis. The intermusctilar distribution of abnormal signal intensity, the grade of involvement of individual muscles and muscle groups can be determined. This allows staging of severity and type of muscle disease. In muscular dystrophy and polymyositis the overall involvement is more severe than in patients with congenital myopathy.” MRI is the most useful technique for showing the local extent of both bone and soft tissue tumours. Invasion of adjacent structures, including the neurovascular bundle, by tumour or peritumoural oedema i- important in presurgical planning. The presence of peritumoural oedema, however, does not necessarily indicate malignancy as a high proportion of benign lesions may demonstrate this finding. Massive peritumoural oedema, however, is an ominous finding with a high association with malignant tumours, a poorer response to initial chemotherapy, and a higher frequency of metastases at diagnosiszx

MRI in the Postsurgical Patient The presence of metal within the patient is not necessarily a contra-indication to MRI as long as it is firmly fixed and outside the brain. Artefact is inevitable in the immediate vicinity of the metal but the effect on image quality is difficult to predict and in many cases diagnostic scans can be obtained. GE sequences should be avoided in favour of CSE because the former tend to be associated with more artefact. The use of MRI in postoperative TMJ assessment

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has already been mentioned. Useful information can also be obtained in the AC1 reconstructed knee”‘.“” and Achilles tendon repairs”’ on the viability of vascularised”” and non-vascularised bone grafts. Bone grafts adJacent to metallic implants such as in acetabular augmentation in total hip replacements can often he imaged successfully despite the presence of arteLicl:‘.’ The ability of MR to distinguish scar tissue from recurrent or residual disc material in the postoperative spine is well described. Pitfalls exist however. ;IS bell-established fibrosis ma! fail to enhance and residual disc material can become invaded bk \ascuIar~sed granulation tissue and can cnhancc folloL\:inp intravenous Gadolinium-DTP.4.

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26. Erickson S J. iox I H, Hyde J S. Carrera G f-. Strandt J A. E>th~~w~1\11. D. Erect 01‘ tendon orientation on MR !maging slgnxl mIensIt: : 3 manifestation of the “magic angles phen.x~~enon. Radiology- 1991 : 1x113): !XY- 302 27. Lainimnen 4 !I. Magnetic rrzonance imaging of pr~n~.t~~> \I\elc!al tnuxlc dlseaser: patterna of’distrlbutlon :~nd .\cverlt> of in.,ol\emenr. Br J Radio1 1990: 63(756): Y16~Y5ll 7x Han!!:: S L. Fletcher B D. Parham D M. Bug M F \lu\clr ccielti~l 111InuaLuloakcletal tumors- MR imaging char:lcterlstxs ,ind clinical signifcance J Magn R<>,,i Ima@rn,o 1491 113,: 441449 x Rah K hl. Glliogly S D. Schaefer R A. I’akrb W il. I ilJedahl R R Anterior cruciate ligament reconstructioncvaiu,itlon with %lR imaging Radiolog! 1991: 17X(3) 551 i-h .%I. .4ut/ G. Good\+in c‘. Singson R D Magnet!c rcs,~n.~r:cr e\alu.itlon of anterior cruciate ligament repau us~np the patellx tendondouble bone block techmquc Sheier.li Radial I’rY:, 2(1(X)5x5 5% 31 LI~III M D. Le,a P. W’r!lda K. Higer H I’. Kilter (1. Bone ;r.ittlng 111 total nip replacement. Prrlimlnar!, ~rcsultb wlrh MRI I,ur J R;id\< I 19X9: 98 I /: 11 Ih