Pitfalls and Pearls in MRI of the Kneedocshare02.docshare.tips/files/31696/316962590.pdf · 2017. 1. 20. · 516 AJR:203, September 2014 Pitfalls and Pearls in MRI of the Knee Rakesh

516 AJR:203, September 2014

Pitfalls and Pearls in MRI of the Knee

Rakesh Mohankumar1,2 Lawrence M. White1,3 Ali Naraghi1,2

Mohankumar R, White LM, Naraghi A

1Joint Department of Medical Imaging, University Health Network, Mount Sinai Hospital and Women’s College Hospital, Toronto, ON, Canada.

2Department of Medical Imaging, University of Toronto, Toronto Western Hospital, 399 Bathurst St, Toronto, ON M5T 258, Canada. Address correspondence to A. Naraghi ([email protected]).

3Department of Medical Imaging, University of Toronto, Toronto General Hospital, Toronto, ON, Canada.

Musculoskeleta l Imaging • Review

AJR 2014; 203:516–530

0361–803X/14/2033–516

© American Roentgen Ray Society

Keywords: knee, ligaments, meniscus, MRI, postoperative MRI

DOI:10.2214/AJR.14.12969

Received April 4, 2014; accepted after revision May 7, 2014.

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interface of the junction of the anterior trans-verse intermeniscal ligament with the ante-rior horns of the menisci and the interface between the popliteus tendon and the later-al meniscus at the popliteal hiatus [7]. Small amounts of fluid may be seen along these interfaces; however, such signal intensity changes can be distinguished from menis-cal tears by careful delineation of the nor-mal anatomic structures on consecutive MR images and on orthogonal imaging planes. The normal anterior horn of the lateral me-niscus, close to its tibial root attachment, of-ten shows a speckled or striated appearance, particularly on short-TE sequences. This ap-pearance is believed to be related to the in-timate relationship of the insertions of the anterior root of the lateral meniscus and the tibial attachment of the anterior cruciate lig-ament (ACL), with the collagenous fibers of the ACL intertwining with the fibrocarti-lage of the anterior horn of the lateral me-niscus. The resultant signal intensity changes may contact the articular surface, simulating a meniscal tear [8]. However, isolated tears of the anterior horn of the lateral meniscus are relatively rare and account for only 16% of lateral meniscal tears [9]. Most of these tears occur more peripherally, adjacent to the junction of the anterior horn and body of the lateral meniscus. Circumferential longi-tudinal extension of signal intensity toward the body and possible associated parameni-scal cysts may be helpful indicators of true meniscal tears in this location. In contrast, the anterior root of the medial meniscus has a more homogeneous MRI appearance. The

The knee is the articulation most commonly assessed for internal derangement by MRI. A number of potential pitfalls and sources

of error related to the knee have been de-scribed in the MRI literature. Sources of such pitfalls include areas of normal anato-my, anatomic variants, and technique-related artifacts masquerading as abnormalities as well as commonly overlooked abnormalities. A thorough knowledge of such pitfalls is es-sential for the radiologist. This article will review the more commonplace sources of er-ror in MRI of the knee. We will address situ-ations in which normal anatomic variants can mimic abnormality and evaluate abnor-malities that can be overlooked.

MenisciMRI has sensitivity of 87–96% and speci-

ficity of 84–94% for medial meniscal tears and sensitivity of 70–92% with specificity of 91–98% for diagnosing tears of the later-al meniscus [1–5]. Identification of meniscal tears has long been based on two criteria: in-trameniscal signal intensity exiting the supe-rior or inferior articular surface of the me-niscus on short TE sequences and change in morphology of the meniscus [6, 7]. Evalua-tion of menisci on T1-weighted images may be misleading because it is difficult to distin-guish tears from areas of intrameniscal de-generation and the extent of a tear may be overestimated on T1-weighted imaging.

Normal anatomic interfaces may also mimic meniscal tears on orthogonal short-TE MRI acquisitions. Examples include the

OBJECTIVE. The purpose of this article is to review the common pitfalls in MRI of the knee and pearls on how to avoid them.

CONCLUSION. MRI of the knee is highly accurate in evaluation of internal derange-ments of the knee. However, a variety of potential pitfalls in interpretation of abnormalities related to the knee have been identified, particularly in evaluation of the menisci, ligaments, and articular cartilage.

Mohankumar et al.Pitfalls and Pearls in MRI of the Knee

Musculoskeletal ImagingReview

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AJR:203, September 2014 517


insertion point of the anterior root of the me-dial meniscus shows greater variability and can insert onto the anterior cortex of the tibia and be mistaken for anterior subluxation or hypermobility of the medial meniscus [10].

The insertion of the meniscofemoral lig-aments to the posterior horn of the lateral meniscus can be an area of diagnostic chal-lenge. True tears at the junction of the me-niscofemoral ligament and the posterior horn of the lateral meniscus, referred to as the “Wrisberg rip” [11], are typically associated with ACL tears and rotational biomechan-ics implicated in such injuries. Variability in insertion of the meniscofemoral ligament to the posterior horn of the lateral meniscus can lead to a false diagnosis of a vertical or oblique tear often referred to as the “Wris-berg pseudotear” [12, 13]. On average, the ligament of Wrisberg inserts onto the pos-terior horn of the lateral meniscus approxi-mately 14-mm lateral to the lateral edge of the posterior collateral ligament (PCL) [14], and extension of a cleft between the ligament of Wrisberg and the posterior horn of the lat-eral meniscus beyond this is highly suspi-cious for a tear (Fig. 1).

The medial meniscus has a firmer broad-er peripheral capsular attachment than the lat-eral meniscus, typically with lack of fluid at the meniscocapsular junction. Meniscocapsu-lar separation often occurs in the setting of a rotational injury with associated cruciate lig-ament tears. The presence of peripheral per-imeniscal fluid and an irregular peripheral medial meniscal outline are indicators of me-niscocapsular injury [15]. Occasionally, small recesses may be present at the meniscocapsu-lar junction of the posterior horn of the medial meniscus and these may simulate a menisco-capsular tear. With a recess, such peripheral fluid signal intensity should not extend all the way superoinferiorly as opposed to complete meniscocapsular tears in which clefts of pe-ripheral juxtameniscal fluid signal intensity extends completely from the superior surface of the meniscocapsular junction to its inferi-or surface (Fig. 2). However, false-positive di-agnoses of meniscocapsular tears are not rare and are thought to be related to the propensity of these tears for healing [16].

The discoid meniscus is another menis-cal morphologic variant resulting in a thick-ened wafer-shaped meniscus with increased width and coverage of the articular surface of the joint. Discoid meniscus more com-monly involves the lateral meniscus, with a reported incidence ranging between 1.5%

and 4.6% compared with 0.3% for the me-dial meniscus [17]. A discoid meniscus is as-sociated with increased incidence of menis-cal tears secondary to increased mechanical stress and hypermobility [18]. Diagnosis of a tear of a discoid meniscus can occasional-ly be difficult. Linear increased signal inten-sity exiting the articular surface is diagnos-tic of a tear, whereas diffuse signal intensity to the surface is less predictive (60–80%) of a tear [7]. A rare variant of the discoid lat-eral meniscus is the “Wrisberg variant,” in which the meniscus lacks a posterior capsu-lar attachment to the tibia, with the Wrisberg meniscofemoral ligament as the sole sta-bilizer of the posterior horn [19]. On MRI, the meniscus lacks the normal fascicle at-tachments onto the posterior horn. These Wrisberg variant lateral discoid menisci are unstable and hypermobile, commonly asso-ciated with resultant mechanical symptoms in the articulation, and usually treated sur-gically. Vacuum phenomenon can simulate a discoid-type meniscus or a torn discoid me-niscus and is most commonly identified in the medial tibiofemoral joint compartment. This diagnostic pitfall should be considered in the presence of an unusually large menis-cus or meniscus fragments or a discoid me-dial meniscus [20].

The oblique meniscomeniscal ligament, which extends from the anterior horn of one meniscus to the posterior horn of the oth-er meniscus with a reported incidence of 1–4%, can mimic a bucket-handle tear [21] (Figs. 3A and 3B). Potential misdiagnosis of an oblique meniscomeniscal ligament for a bucket-handle meniscal tear may be avoided by following the ligament with confirmation of its classic anatomic orientation on con-secutive images. Another mimic of a bucket-handle tear is a rare congenital variant, most commonly involving the lateral meniscus, re-ferred to as a “ring meniscus.” A ring menis-cus variant is characterized by a ring shape in which the anterior and posterior horns are connected by an inner horn bridge and pres-ents on imaging as a complete ring of menis-cal tissue [22, 23]. The lack of meniscal tis-sue loss in the anatomic location of the horns and body of a ring meniscus and the well-defined smooth morphology and triangular shape of the central inner horn component should prompt this diagnosis [7] (Fig. 3C).

A meniscal flounce is an incidental redun-dancy or fold along the free edge of the me-niscus (Fig. 3D). A meniscal flounce is com-monly observed within the medial meniscus,

with an incidence of 0.2–5% on knee MRI examinations [24, 25]. As opposed to mor-phologic changes of a meniscal tear, a menis-cal flounce reflects a transient physiologic distortion of the meniscal inner margin, typ-ically seen when the knee is in a flexed posi-tion and typically disappears on full exten-sion of the joint [25]. Occasionally, meniscal tears can produce a flouncelike morphology, but these typically show other indicators of a meniscal tear [24].

MRI artifacts leading to pitfalls in menis-cal assessment include truncation [26] and motion artifacts [27]. Truncation or Gibbs artifact may be seen as a series of low- and high-signal-intensity linear artifacts run-ning parallel and adjacent to interfaces of abrupt signal intensity change. Such arti-facts can be superimposed on the meniscus as linear areas of high signal intensity sim-ulating the MRI appearance of a meniscal tear. However, such artifacts are manifest-ed by subtle signal intensity changes exactly paralleling the articular surface of the me-niscus and on careful inspection can extend beyond the boundaries of the meniscus it-self. Magic angle effect can also lead to ar-tifactual increased meniscal signal intensity on short-TE MRI acquisitions of the knee, particularly affecting the posterior horn of the menisci as they extend to their posteri-or root attachments. Magic angle phenome-non is seen within highly ordered collagen fibers, which are oriented at 55° relative to the main magnetic field on MRI, and can be seen clinically on MRI of obliquely oriented portions of the menisci where meniscal col-lagen fibers are oriented at 55° to the main magnetic field. It can potentially mimic sig-nal intensity changes of meniscal degenera-tion on short-TE imaging acquisitions [28].

Increased signal intensity can be seen in the absence of meniscal tears within the me-niscus in a variety of settings. Intrasubstance mucoid degeneration of the meniscus is iden-tified as linear or globular increased signal intensity within the meniscus and is often asymp to mat ic [29, 30]. In the context of trau-ma, meniscal contusions can also produce a similar appearance with an area of increased intrameniscal signal intensity change that is typically less discrete than either menis-cal tears or intrameniscal degeneration and is usually associated with adjacent marrow contusion [31]. In children, it is also com-mon to see peripheral high signal intensi-ty within the meniscus, which reduces with age and is thought to represent normal pe-

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Mohankumar et al.

ripheral meniscal vascularity [32]. Any of these causes of intrameniscal signal inten-sity change can contact the meniscal articu-lar surface, mimicking a meniscal tear. In-creased specificity for a meniscal tear can be achieved by identifying morphologic chang-es of the meniscus or visualization of intra-meniscal high signal intensity extending to the articular surface of the meniscus on at least two contiguous slices. A positive pre-dictive value of 94–96% has been reported in diagnosis of meniscal tear by using this “two-slice-touch” rule [33].

In certain situations, the accuracy of MRI assessment of meniscal tears may be dimin-ished. The positive predictive value of MRI for detection of longitudinal tears is signifi-cantly lower than other tear morphologies [16]. Tears located at the periphery, particularly at the meniscocapsular junction, and those that exit only the superior articular surface lead to false-positive diagnoses. Interval spontaneous healing of these meniscal tears is thought to result in the lower positive predictive value for longitudinal tears. In the presence of an ACL tear, it has also been shown that the sensitiv-ity for meniscal tears, particularly peripheral tears of the posterior horn of the lateral me-niscus, is also significantly lower. These tears may be subtle and require careful diagnostic scrutiny at MRI evaluation [34].

Displacement of meniscal tissue is an in-direct sign of a meniscal tear and can present with symptoms of joint locking and clicking. A bucket-handle tear is a classic displaced meniscal tear, and MRI has high accuracy for detection of such lesions. More common-ly, displaced meniscal fragments are identi-fied adjacent to the posterior root of the me-dial meniscus, posterior to the PCL, or in the medial and lateral gutters of the articula-tion [35]. Less commonly, unstable meniscal tear fragments or flap tears may flip under the meniscus itself. Identification and local-ization of flipped fragments are important as the fragments may be situated in potential blind spots on arthroscopy (Fig. 4).

Meniscal ossicles, most commonly seen within the posterior horn of the medial me-niscus, can rarely be mistaken for an intraar-ticular body [36]. The signal intensity that is characteristic of the ossicle typically par-allels that of marrow fat. Continuity of the ossicle with the adjacent meniscus aids in distinguishing a meniscal ossicle from an in-traarticular body.

Although MRI offers excellent evalua-tion of the native meniscus, evaluation of a

postsurgical meniscus is more challenging. Morphologic truncation of the meniscus and persistent surfacing signal intensity on short-TE pulse sequences, reflecting conventional criteria for diagnosis of preoperative menis-cal tears, have limited accuracy of 50–68% in the diagnosis of a postoperative meniscal tear [37, 38]. The presence of increased in-trameniscal signal intensity contacting the articular surface on short-TE sequences af-ter meniscal surgery can represent a healing tear, area of intrameniscal degeneration con-tacting the neoarticular surface after partial meniscectomy, or the residual stable com-ponent of a treated meniscal tear. Changes in meniscal morphology are also not spe-cific for recurrent tearing and can reflect changes after meniscectomy. Conventional MRI, MRI arthrography, and CT arthrog-raphy have been advocated for evaluation of the postoperative meniscus [37, 39–42]. Re-injury of the meniscus can be most reliably diagnosed by visualizing intrameniscal im-bibition of intraarticular gadolinium on T1-weighted images at MR arthrography or fluid on T2-weighted nonarthrographic imaging or by visualizing displaced meniscal frag-ments or meniscal fragmentation [43] (Fig. 5). Interval change in morphology of the me-niscus in comparison with previous postop-erative MRI, if available, is also indicative of a recurrent tear.

Cruciate LigamentsA variety of primary and secondary MRI

signs have been described in assessment of complete tears of the ACL. Primary signs in-clude discontinuity of the ACL, nonvisual-ization of the ACL, and replacement of the ACL with an ill-defined masslike area con-sisting of hemorrhage. These signs have high diagnostic accuracy in the evaluation of complete ACL disruption [44, 45].

Potential pitfalls in assessment of ACL in-juries may arise by reliance solely on sagittal imaging in evaluation of the ACL. Prescribed sagittal imaging planes may not adequately parallel the ACL, and depending on the im-aging plane orientation, slice thickness, and interslice gap used at imaging, volume aver-aging may be encountered between the prox-imal ACL and the lateral femoral condyle. This may result in erroneous observations of intrasubstance signal intensity changes and possibly incomplete visualization on sagit-tal imaging alone of contiguous fibers along the entire course of the ACL. Such pitfalls can be avoided by evaluating possible signal

intensity changes and ligamentous continu-ity on axial and coronal planes in addition to sagittal MRI acquisitions of the knee [46].

A complete ACL tear may undergo scar-ring, and various scar patterns have been recognized [47]. These include scarring of torn ACL fibers to the PCL, roof of the in-tercondylar notch, or lateral femoral condyle. In such scenarios, although the knee may re-main clinically unstable, the MRI appear-ance of a scarred ACL can be erroneously mistaken for a contiguous intact or partial-ly torn ACL (Fig. 6). Scar formation at the site of a chronic complete ACL tear can lead to focal thickening, attenuated scar tissue, or focal angulation of the ligament. However, these features may also be identified to some degree in normal ligaments or with ACL partial tears [48]. Scarring onto a nonana-tomic point within the intercondylar notch or presence of prior imaging showing a com-plete tear are useful indicators of the severity of the original injury.

MRI evaluation of partial tears of the ACL is challenging [49], with relatively low diagnostic accuracy [50–52]. Partial tears may be mistaken for complete tears, mucoid degeneration, and normal ACL [50]. A par-tial tear of the ACL may show either focal or diffuse increased intrasubstance signal intensity as well as laxity or posteroinferior bowing of ligamentous fibers. Partial tears of the femoral origin of the ACL can be partic-ularly difficult to diagnose on sagittal imag-es. Similarly, isolated injuries of one bundle may be overlooked when the other bundle re-mains intact. Interrogation of axial images may be valuable in evaluation of the normal low-signal-intensity ACL at its femoral ori-gin as well as in assessing the degree of liga-ment fiber disruption in the setting of partial ACL injuries [45].

Imaging the postoperative reconstruct-ed ACL is a common indication for MRI. Complete review of the surgical techniques and imaging appearances of reconstruc-tion grafts is beyond the scope of this ar-ticle. During the first postoperative year, biologic graft constructs undergo “ligamen-tization” and neovascularization resulting in increased signal intensity of the graft on T1- and T2-weighted sequences, which may be mistaken for graft tear or graft impingement [53]. Graft signal intensity changes during the ligamentization process are not as high as fluid on T2-weighted images, and the ACL graft typically shows normal signal intensi-ty by 18–24 months [54, 55]. However, small

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areas of persistent striated or globular sig-nal intensity change may be evident within ACL reconstruction grafts for several years, even in the absence of symptoms [56]. These findings may simulate graft changes associ-ated with graft impingement or partial tears but should be interpreted with caution in as-ymptomatic individuals with normal graft positioning and a lack of graft discontinui-ty. Similarly, anterior translation of the tibia, which has high specificity for tearing of the native ACL [57], may be seen in the absence of anterior translational knee laxity and has low sensitivity and positive predictive value for anterior knee laxity postoperatively [58].

In comparison with ACL tears, MRI as-sessment of PCL tears can be more challeng-ing. In addition to focal ligamentous discon-tinuity, PCL tearing may be manifest simply by ligamentous thickening, which may be overlooked, especially if a relevant clinical history is not provided [59]. Distinguishing partial PCL tears from complete tears can be especially challenging, and there may be a discrepancy between clinical and MRI grad-ing of PCL injuries (Fig. 7). Both partial and complete tears can result in thickening of the ligament with ill-defined margins and in-creased signal intensity [60]. Complete PCL tears tend to show focal discontinuity more commonly than partial tears and are more frequently associated with other ligamentous or meniscal injuries of the joint.

Chronic PCL tears have a propensi-ty to heal and scar and can be easily over-looked on MRI [61]. In a study of 46 cases of PCL tears evaluated at a mean interval of 15 months after injury, 28% showed an al-most normal ligament with an additional 44% showing continuity of the ligament with variable deformity on MRI [62]. Such chron-ic PCL tears may heal in a stretched state, re-sulting in lengthening of the ligament, which may be difficult to assess on MRI despite clinical features of PCL insufficiency. The ratio of the lateral femoral condyle to PCL length has been used as a method to diagnose ligament lengthening in chronic tears, with a mean ratio of 1.96 in normal individuals and a decrease in the ratio in patients with chron-ic PCL tears [63].

Similar to the ACL, the PCL can under-go mucoid degeneration [64]. Distinguishing mucoid degeneration of the PCL from a PCL tear can prove a diagnostic challenge because thickening and increased signal intensity of the PCL can also be seen with longitudinal interstitial tears. The tram-track appearance

of the PCL, manifested as homogeneous lon-gitudinal increased intra substance signal in-tensity of the PCL bounded by well-defined intact rims of low signal intensity, has been described as a reliable MRI finding in me-chanically stable PCLs with mucoid degen-eration [65].

Collateral LigamentsThe medial collateral ligament (MCL) is

commonly injured in valgus injuries to the knee. Acute MCL injuries are invariably as-sociated with periligamentous edema. How-ever, fluid and edema superficial to the MCL is nonspecific and can be seen with medial meniscal tears as well as medial compart-mental osteoarthritis [66, 67]. These chang-es may mimic MRI findings of a partial low-grade tear of the MCL. More significant edematous changes may also be seen sur-rounding the medial restraints of the knee in the setting of subchondral insufficiency frac-tures (Fig. 8). The intense edema, both os-seous and soft-tissue that may be related to repetitive stress on the injured subchondral plate [68]; identification of subchondral lin-ear signal intensity changes with intense ede-ma; and commonly associated posterior root tears lead to the correct diagnosis and differ-entiation from MCL injuries.

The posterolateral corner of the knee is composed of a complex set of structures. These include the fibular collateral liga-ment, popliteus tendon, popliteofibular liga-ment, arcuate ligament, fabellofibular liga-ment, and biceps tendon. Injuries to these structures are typically associated with cru-ciate ligament injuries or multiligamentous injuries. Although the fibular collateral lig-ament, popliteus tendon, and biceps femoris are consistently seen on MRI, identification of other structures is more variable [69]. Of the more inconsistently visualized ligamen-tous structures contributing to posterolateral corner stability, exclusion of injury and dis-ruption of the popliteofibular ligament is the most critically important for patient manage-ment. Isolated injuries of the popliteofibular ligament, however, are rare, and MRI identi-fication of injuries to the fibular collateral lig-ament and popliteus should raise concern for concomitant popliteofibular ligament tears and posterolateral corner instability [70] (Fig. 9). Similarly, although the arcuate liga-ment is typically not well seen on MRI, post-traumatic edema and hemorrhage along the posterolateral capsule and popliteal hiatus may be features reflective of an arcuate liga-

ment injury. In individuals with posterolateral corner instability and multiligamentous inju-ry, the neurovascular structures should also be scrutinized because they can be injured in approximately 15% of cases [71].

Extensor MechanismA bipartite patella in which secondary or

accessory ossification centers of the patella fail to unite with the main osseous body of the patella is a normal developmental variant seen in 2% of the population. The most com-mon type is a bipartite fragment involving the superolateral pole of the patella (75%). A bipartite patella can be distinguished on MRI from a fracture by the location of the bi-partite segment, presence of well-corticated margins to the accessory segment, and typ-ical integrity of underlying articular carti-lage of the patella overlying the incompletely united accessory ossification center. Marrow edema at the interface of the bipartite seg-ment is suggestive of micromotion at the syn-chondrosis, and defects in the normally intact articular cartilage may be features associat-ed with symptomatic anterior knee pain [72] (Fig. 10A). The dorsal defect of the patella is a further variant thought to be related to nor-mal enchondral ossification involving the su-perolateral patella, which is seen in up to 1% of individuals [73]. On MRI, a dorsal defect of the patella appears as a small symmetri-cally round subchondral bony defect with in-tact overlying articular cartilage within the superolateral patella in contrast with osteo-chondritis dissecans of the patella, which is more commonly central or superomedial in location and often variable in shape and mor-phology [74] (Fig. 10B).

Magic angle effect in the patellar tendon is common because of the orientation of highly ordered collagen fibers of the tendon. Such ar-tifacts can result in areas of increased signal intensity on short-TE pulse sequences, partic-ularly along the deep margin of the tendon, with decreasing prominence on T2-weighted acquisitions [75]. This is in contrast to signal intensity changes seen in the setting of patel-lar tendinopathy, which are observed on both short-TE and T2-weighted acquisitions.

Transient lateral patellar dislocation can be a difficult diagnosis clinically, and pa-tients are often referred for imaging for sus-pected meniscal or MCL injuries [76]. The MRI diagnosis of transient lateral patellar dislocation is classically characterized by contusive injury to the medial facet of the patella and the anterior aspect of the lateral

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femoral condyle. Identification of osteochon-dral lesions and the integrity of the medial patellofemoral ligament are critical factors to accurately assess on MRI and important features in clinical management of patients with confirmed transient lateral patellar dis-locations (Fig. 11). Patients with injuries to the femoral attachment of the medial patel-lofemoral ligament are more likely to have recurrent chronic instability and may be ap-propriate candidates for medial patellofem-oral ligament repair or reconstruction [77]. Differentiation of MRI findings of traumatic disruption of the medial patellofemoral liga-ment versus nonvisualization due to anatom-ic variation is characterized by findings of edema and hemorrhage at the expected fem-oral origin of the medial patellofemoral liga-ment and along the medial-inferior border of the vastus medialis obliquus as well as eleva-tion of the femoral attachment of the vastus medialis obliquus.

Signal intensity changes are commonly encountered in relation to the fat pads anteri-orly at the knee joint. Edema may be seen in-volving the superolateral aspect of the Hoffa fat pad in the setting of patellar tendon lat-eral femoral condyle friction syndrome or in relation to the quadriceps-suprapatellar fat (Fig. 12). These changes can be associated with symptoms of anterior knee pain and pa-tellar maltracking. Additional findings of pa-tella alta, lateral subluxation of the patella, and swelling of the suprapatellar fat pad can also be encountered.

Articular CartilageThe excellent spatial resolution, tissue con-

trast, and multiplanar capability make MRI an excellent tool for assessment of articular carti-lage [78]. MRI of articular cartilage is suscep-tible to MR artifacts, including magic angle, partial volume averaging, chemical shift, and susceptibility artifacts. The collagen fibers in articular cartilage are highly organized. This can lead to magic angle effect result-ing in focal increased signal intensity with-in the articular cartilage [79, 80]. Increasing the TE will reduce this effect. At bone-car-tilage interfaces, on non–fat-suppressed im-aging, chemical shift artifacts related to the marrow fat may be encountered, leading to misregistration artifacts in the frequency en-coding direction superimposed over areas of articular cartilage, mimicking the appearance of focal chondral lesions. Such artifacts can be reduced by increasing the bandwidth, ana-tomically directing artifacts elsewhere in the

image by swapping the frequency and phase encoding directions, or they can be eliminat-ed by application of fat suppression [81]. Pul-sation artifacts from the popliteal artery as well as patient motion artifacts can also lead to focal areas of linear or rounded signal in-tensity change that may mimic focal areas of chondrosis or flap tears. Pulsation artifacts are seen in the phase encoding direction and are typically identified by a repeating pattern of pulsation extending beyond the margins of the articular cartilage. Truncation artifacts may also be encountered in imaging of the articu-lar cartilage, leading to a laminar appearance within the articular cartilage [82, 83].

MRI is useful in assessment of osteochon-dral abnormalities within the knee. A partic-ular imaging pitfall of note is femoral condy-lar ossification irregularities that can mimic osteochondritis dissecans in the pediatric population. These are particularly common in the posterior aspect of the lateral femo-ral condyle and likely reflect a developmen-tal variant related to the enchondral ossifi-cation of secondary ossification centers [84]. The posterior location, presence of intact overlying normal articular cartilage, lack of associated marrow edema, and presence of multiple ossification centers are helpful in distinguishing these developmental variants from osteochondral lesions [85] (Fig. 13).

Bone and Soft TissuesHematopoietic marrow or red marrow

conversion can be a prominent finding in the knee and can raise concern for a neoplas-tic marrow infiltrative process. Red marrow conversion may be seen in a variety of set-tings, including anemia, smoking, and high athletic activity [86]. On MRI, involved ar-eas of hematopoietic marrow conversion show intermediate-to-high signal intensity on fluid-sensitive sequences and low-to-in-termediate signal intensity on T1-weight-ed imaging. Typically, there is metaphyse-al involvement, and several patterns have been identified, including uniform conflu-ent areas, punctate areas of signal intensity change, and focal masslike areas of marrow signal intensity change [87]. On T1-weighted images, areas of red marrow are typically of higher signal intensity than adjacent muscles [88]. Sparing of the epiphyseal regions of the distal femur and proximal tibia is a useful identifier of red marrow (Fig. 14). On in- and out-of-phase imaging, signal intensity drop-out can be seen on out-of-phase images in the setting of red marrow [89].

A cortical desmoid, or distal femoral corti-cal irregularity, is seen at the posterior aspect of medial supracondylar femur in adolescents, which is thought to be a tug lesion involv-ing the adductor magnus insertion or medial gastrocnemius head origin. The location and knowledge of imaging appearance are impor-tant to distinguish a cortical desmoid from other abnormalities, such as fibrous cortical defect or parosteal osteosarcoma [90].

Synovial plicae are embryologic rem-nants of synovial membrane of the knee. Three synovial plicae are commonly en-countered at arthroscopy and imaging: the suprapatellar, infrapatellar, and medial pli-cae, with a lateral plica less common. These are most commonly asymptomatic but can occasionally result in symptoms. The in-frapatellar plica, anterior to the ACL and extending through the Hoffa fat pad, may be thickened and mistaken for focal synovitis or part of the ACL. Medial plica syndrome occurs in adolescents because of thickening and inflammation of the medial plica [91]. A thickened medial plica extending into the patellofemoral joint, with associated chon-dral changes of the patella or medial femo-ral condyle, in the absence of other causes of symptoms, suggests a diagnosis of plica syndrome [92] (Fig. 15).

Osseous and subchondral marrow edema can often provide valuable insights to the mechanism of traumatic injury sustained in the knee, such as the edema pattern in piv-ot-shift injury, transient patellar dislocation, and hyperextension injury.

Surrounding soft tissues can also reveal anatomic variants. Muscle variants can eas-ily be overlooked. These include accessory heads of gastrocnemius muscle and an acces-sory popliteus or a double configuration to biceps femoris [93–96]. These can be associ-ated with popliteal artery entrapment, a pal-pable mass, or common peroneal nerve com-pression, respectively.

Finally, vascular variants, such as popli-teal artery entrapment, cystic adventitial dis-ease, or deep vein thrombosis, may cause symptoms around the knee and can poten-tially be overlooked on MRI of the knee un-less specifically reviewed (Fig. 16).

SummaryIn this article, we have reviewed the com-

mon pitfalls that are encountered in MRI of the knee, knowledge of which is useful for providing an accurate diagnosis when evalu-ating the images.

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A

Fig. 1—“Wrisberg rip” and pseudotear.A–C, 26-year-old man with Wrisberg rip of posterior horn of lateral meniscus. Sagittal fat-suppressed T2-weighted MR image (A) (TR/TE, 3450/65) centrally shows contusive injury consistent with pivot shift injury. There is fluid cleft between meniscofemoral ligament of Wrisberg (arrow) and posterior horn of lateral meniscus (arrowhead). Sagittal fat-suppressed T2-weighted MR image (B) (TR/TE, 3450/65) through midlateral compartment shows further lateral extension of fluid cleft (arrowhead) consistent with tear. Coronal intermediate-weighted MR image (C) (TR/TE, 3420/38) shows location of sagittal slices (lines) in A and B. D and E, 33-year-old man with Wrisberg pseudotear of posterior horn of lateral meniscus. Sagittal fat-suppressed T2-weighted MR image (D) (TR/TE, 4060/67) through medial aspect of lateral compartment shows cleft between meniscofemoral ligament of Wrisberg (arrow) and posterior horn of lateral meniscus (arrowhead). Sagittal fat-suppressed T2-weighted MR image (E) (TR/TE, 4060/67) through more lateral aspect of lateral compartment does not show lateral extension of cleft in keeping with normal interface.

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A

Fig. 2—Meniscocapsular tear and meniscocapsular recess.A, 30-year-old man with meniscocapsular tear. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3600/68) through medial compartment after acute injury shows irregular vertical cleft extending all way through meniscocapsular junction (arrowhead) as well as periphery of medial meniscus. Adjacent contusive injury to posterior medial tibial plateau is also noted (asterisk).B, 25-year-old woman with normal meniscocapsular recess. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3540/65) through medial compartment shows smooth fluid-filled recesses at meniscocapsular junction superiorly and inferiorly (arrowheads) with central band at meniscocapsular junction (arrow), which inhibits extension of fluid all way superoinferiorly.

B

A

Fig. 3—Meniscal variants.A and B, 54-year-old man with oblique meniscomeniscal ligament. Midsagittal fat-suppressed T2-weighted MR image (A) (TR/TE, 3750/68) shows linear low-signal-intensity structure (arrowhead ) within intercondylar notch mimicking displaced meniscal fragment. Axial fat-suppressed T2-weighted MR image (B) (TR/TE, 3660/65) through joint shows low-signal-intensity structure (arrowheads) extending obliquely from posterior horn of lateral meniscus to anterior horn of medial meniscus, consistent with oblique meniscomeniscal ligament.

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Fig. 3 (continued)—Meniscal variants. C, 23-year-old woman with ring meniscus. Coronal intermediate-weighted MR image (TR/TE, 3300/36) shows central triangular low-signal-intensity structure mimicking bucket-handle tear (arrowhead). Structure has smooth triangular appearance and remainder of lateral meniscus was normal without evidence of tear or loss of meniscal volume.D, 28-year-old man with meniscal flounce. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3350/72) through medial compartment shows focal waviness to inner border of body of medial meniscus (arrowhead) without signal intensity change or focal clefts, consistent with incidental meniscal flounce.

D

AFig. 4—Flipped meniscal fragments.A and B, 48-year-old woman with multiple flipped meniscal fragments. Coronal intermediate-weighted MR image (A) (TR/TE, 3100/35) shows flipped meniscal fragment (arrowhead) inferiorly into medial gutter deep to medial collateral ligament (arrow). Coronal intermediate-weighted MR image (B) (TR/TE, 3100/35) through posterior aspect of joint shows further flipped meniscal fragment (arrow) superior to posterior root of medial meniscus.C, 36-year-old woman with displaced meniscal fragment in posterior horn of lateral meniscus. Sagittal proton density MR image (TR/TE, 2150/15) through lateral compartment shows bulky posterior horn with increased tissue inferior to posterior horn (arrowheads) as result of flipped meniscal fragment.

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AFig. 5—Postoperative menisci.A, 32-year-old man with recurrent-residual meniscal tear. Coronal fat-suppressed T2-weighted MR image (TR/TE, 3800/70) 1 year after anterior cruciate ligament reconstruction and partial meniscectomy of medial meniscus shows diminutive body of medial meniscus with vertical high-signal-intensity cleft through body (arrowhead), consistent with recurrent or residual tear confirmed surgically.B and C, 45-year-old man with prior partial meniscectomy and postsurgical changes without recurrent tear. Sagittal proton density MR image (B) (TR/TE, 2230/16) 3 years after partial meniscectomy shows resection of part of inferior leaflet of horizontal cleavage tear with residual cleft visible on short TE image (arrowhead). Corresponding sagittal fat-suppressed T2-weighted MR image (C) (TR/TE, 3750/70) does not show imbibition of fluid into cleft (arrowhead). No tear was identified at arthroscopy.

CB

AFig. 6—25-year-old man with anterior cruciate ligament (ACL) rupture and recurrent scarring.A, Sagittal fat-suppressed T2-weighted MR image of knee (TR/TE, 3400/66) shows complete discontinuity and rupture of midsubstance of ACL (arrowhead).B, Sagittal fat-suppressed T2-weighted MR image of knee (TR/TE, 3500/68) obtained 3 years after initial injury shows low-signal-intensity fibers at site of prior rupture (arrow). Proximal fibers are not well visualized because of partial volume averaging.C, Axial fat-suppressed T2-weighted MR image of knee (TR/TE, 3600/66) through proximal ACL obtained 3 years after initial injury shows attachment of proximal ACL (arrow) at lateral femoral condyle.

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Fig. 8—62-year-old woman with subchondral insufficiency fracturing medial femoral condyle and secondary medial collateral ligament (MCL) changes.A, Sagittal proton density MR image (TR/TE, 2200/15) shows linear subchondral low-signal-intensity region (arrowheads), in keeping with subchondral insufficiency fracture. B, Axial fat-suppressed T2-weighted MR image of knee (TR/TE, 3450/58) shows thickening of MCL with adjacent soft-tissue edema (arrow) mimicking MCL partial tear. Diffuse edema of medial femoral condyle is also noted (asterisk).

BAFig. 7—28-year-old man with discrepancy between clinical and MRI grading of posterior cruciate ligament (PCL) tear. Sagittal proton density MR image (TR/TE, 2140/19) of knee shows diffuse thickening and increased signal intensity of PCL (arrowhead) with intact fibers visualized. This was interpreted as partial tear. Clinically, patient had grade III posterior draw sign.

A

Fig. 9—23-year-old man with anterior cruciate ligament tear (not shown) and posterolateral corner injury.A, Coronal intermediate-weighted MR image (TR/TE, 3150/32) shows focal edema involving styloid process of fibula (arrow), consistent with undisplaced arcuate fracture at attachment of popliteofibular ligament (arrowhead).B, Coronal intermediate-weighted MR image (TR/TE, 3150/32) anterior to A shows associated tear of fibular collateral ligament (arrow).

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Fig. 10—Patellar variants.A, 41-year-old man with bipartite patella. Axial fat-suppressed T2-weighted MR image (TR/TE, 3500/70) shows osseous fragment (arrowhead) involving superolateral patella with low-signal-intensity interface with patella. There is osseous edema on both sides of interface. Overlying articular cartilage is intact but shows focal signal change.B, 31-year-old man with dorsal defect of patella. Axial fat-suppressed T2-weighted MR image (TR/TE, 3350/60) shows focal osseous defect (arrowhead) involving lateral facet of patella. Overlying cartilage is intact.

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A

Fig. 11—Patellar dislocation.A, 23-year-old woman with acute transient patellar dislocation. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3530/65) through lateral compartment shows extensive bone marrow edema involving lateral femoral condyle (asterisk) and large hemarthrosis. There is focal chondral defect involving anterior aspect of lateral femoral condyle (arrowheads).B, 25-year-old man with acute transient patellar dislocation. Axial fat-suppressed T2-weighted MR image (TR/TE, 3600/70) shows bone marrow edema of medial patella (asterisk) as well as lateral femoral condyle. There is extensive edema and hemorrhage along course of medial patellofemoral ligament with nonvisualization of femoral attachment (arrow), consistent with complete tear.

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AFig. 12—Fat pad edema.A and B, 32-year-old woman with anterior knee pain. Sagittal fat-suppressed T2-weighted MR image (A) (TR/TE, 3500/60) shows increased signal intensity and swelling of suprapatellar fat pad (arrowhead). Corresponding proton density image (B) (TR/TE, 2180/14) shows low signal intensity involving suprapatellar fat pad (arrowhead).C, 41-year-old man with anterior knee pain. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3580/64) shows patella alta and focal area of edema involving supralateral aspect of Hoffa fat pad (arrowhead).

CB

Fig. 14—45-year-old woman with hematopoietic marrow involvement of distal femur.A, Proton density image (TR/TE, 2300/15) shows heterogeneous marrow signal intensity changes involving distal femoral diametaphysis (arrowheads). Signal intensity changes do not cross physeal scar, and there are areas of interspersed fat within involved area. Patient had no other medical history. Addition of T1-weighted imaging may be useful in atypical cases.B, Axial fat-suppressed T2-weighted MR image (TR/TE, 3550/70) shows mild patchy hyperintensity of distal femoral marrow (arrowheads).

BAFig. 13—10-year-old boy with distal femoral ossification irregularity. Sagittal fat-suppressed intermediate-weighted MR image (TR/TE, 3000/38) through lateral compartment of knee shows area of subchondral linear signal intensity change involving posterior aspect of lateral femoral condyle (arrowheads). There is no significant edema and overlying articular cartilage is intact.

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Fig. 16—Incidental vascular findings.A, 42-year-old woman with popliteal deep venous thrombosis. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3200/70) in this patient who was referred for assessment of internal derangement of knee shows heterogeneous signal intensity and expansion of popliteal vein (arrowheads) with surrounding soft-tissue edema, consistent with deep venous thrombosis, which was confirmed by sonography.B, 43-year-old man with cystic adventitial disease of popliteal artery. Sagittal fat-suppressed T2-weighted MR image of knee (TR/TE, 3980/62) after acute knee injury shows incidental extensive cystic changes in relation to popliteal artery (arrowheads).

BA

Fig. 15—28-year-old man with anterior knee pain and clicking secondary to medial plica syndrome. Axial fat-suppressed T2-weighted MR image (TR/TE, 3700/66) shows thickened medial plica (arrowhead) extending into patellofemoral joint with adjacent synovitis (asterisk). Focal chondral changes are present involving medial facet of patella (arrow).

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Pitfalls and Pearls in MRI of the Kneedocshare02.docshare.tips/files/31696/316962590.pdf · 2017. 1. 20. · 516 AJR:203, September 2014 Pitfalls and Pearls in MRI of the Knee Rakesh