M u s c u l o s k e l e t a l I m a g i n g • R ev i ew Mohankumar et al. Pitfalls and Pearls in MRI of the Knee
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Musculoskeletal Imaging Review
Rakesh Mohankumar 1,2 Lawrence M. White1,3 Ali Naraghi1,2 Mohankumar R, White LM, Naraghi A
Pitfalls and Pearls in MRI of the Knee 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 derangements 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.
T
Keywords: knee, ligaments, meniscus, MRI, postoperative MRI DOI:10.2214/AJR.14.12969 Received April 4, 2014; accepted after revision May 7, 2014. 1
Joint Department of Medical Imaging, University Health Network, Mount Sinai Hospital and Women’s College Hospital, Toronto, ON, Canada.
2 Department of Medical Imaging, University of Toronto, Toronto Western Hospital, 399 Bathurst St, Toronto, ON M5T 258, Canada. Address correspondence to A. Naraghi (
[email protected]). 3 Department of Medical Imaging, University of Toronto, Toronto General Hospital, Toronto, ON, Canada.
AJR 2014; 203:516–530 0361–803X/14/2033–516 © American Roentgen Ray Society
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he 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 described in the MRI literature. Sources of such pitfalls include areas of normal anatomy, anatomic variants, and technique-related artifacts masquerading as abnormalities as well as commonly overlooked abnormalities. A thorough knowledge of such pitfalls is essential for the radiologist. This article will review the more commonplace sources of error in MRI of the knee. We will address situations in which normal anatomic variants can mimic abnormality and evaluate abnormalities that can be overlooked. Menisci MRI has sensitivity of 87–96% and specificity of 84–94% for medial meniscal tears and sensitivity of 70–92% with specificity of 91–98% for diagnosing tears of the lateral meniscus [1–5]. Identification of meniscal tears has long been based on two criteria: intrameniscal signal intensity exiting the superior or inferior articular surface of the meniscus on short TE sequences and change in morphology of the meniscus [6, 7]. Evaluation of menisci on T1-weighted images may be misleading because it is difficult to distinguish tears from areas of intrameniscal degeneration and the extent of a tear may be overestimated on T1-weighted imaging. Normal anatomic interfaces may also mimic meniscal tears on orthogonal shortTE MRI acquisitions. Examples include the
interface of the junction of the anterior transverse intermeniscal ligament with the anterior horns of the menisci and the interface between the popliteus tendon and the lateral 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 meniscal tears by careful delineation of the normal anatomic structures on consecutive MR images and on orthogonal imaging planes. The normal anterior horn of the lateral meniscus, close to its tibial root attachment, often shows a speckled or striated appearance, particularly on short-TE sequences. This appearance is believed to be related to the intimate relationship of the insertions of the anterior root of the lateral meniscus and the tibial attachment of the anterior cruciate ligament (ACL), with the collagenous fibers of the ACL intertwining with the fibrocartilage of the anterior horn of the lateral meniscus. 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 longitudinal extension of signal intensity toward the body and possible associated parameniscal 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
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Pitfalls and Pearls in MRI of the Knee insertion point of the anterior root of the medial 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 ligaments to the posterior horn of the lateral meniscus can be an area of diagnostic challenge. True tears at the junction of the meniscofemoral ligament and the posterior horn of the lateral meniscus, referred to as the “Wrisberg rip” [11], are typically associated with ACL tears and rotational biomechanics 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 “Wrisberg pseudotear” [12, 13]. On average, the ligament of Wrisberg inserts onto the posterior horn of the lateral meniscus approximately 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 lateral meniscus beyond this is highly suspicious for a tear (Fig. 1). The medial meniscus has a firmer broader peripheral capsular attachment than the lateral meniscus, typically with lack of fluid at the meniscocapsular junction. Meniscocapsular separation often occurs in the setting of a rotational injury with associated cruciate ligament tears. The presence of peripheral perimeniscal fluid and an irregular peripheral medial meniscal outline are indicators of meniscocapsular injury [15]. Occasionally, small recesses may be present at the meniscocapsular junction of the posterior horn of the medial meniscus and these may simulate a meniscocapsular 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 peripheral juxtameniscal fluid signal intensity extends completely from the superior surface of the meniscocapsular junction to its inferior surface (Fig. 2). However, false-positive diagnoses 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 meniscal morphologic variant resulting in a thickened wafer-shaped meniscus with increased width and coverage of the articular surface of the joint. Discoid meniscus more commonly involves the lateral meniscus, with a reported incidence ranging between 1.5%
and 4.6% compared with 0.3% for the medial meniscus [17]. A discoid meniscus is associated with increased incidence of meniscal tears secondary to increased mechanical stress and hypermobility [18]. Diagnosis of a tear of a discoid meniscus can occasionally be difficult. Linear increased signal intensity exiting the articular surface is diagnostic 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 lateral meniscus is the “Wrisberg variant,” in which the meniscus lacks a posterior capsular attachment to the tibia, with the Wrisberg meniscofemoral ligament as the sole stabilizer of the posterior horn [19]. On MRI, the meniscus lacks the normal fascicle attachments onto the posterior horn. These Wrisberg variant lateral discoid menisci are unstable and hypermobile, commonly associated with resultant mechanical symptoms in the articulation, and usually treated surgically. Vacuum phenomenon can simulate a discoid-type meniscus or a torn discoid meniscus and is most commonly identified in the medial tibiofemoral joint compartment. This diagnostic pitfall should be considered in the presence of an unusually large meniscus or meniscus fragments or a discoid medial meniscus [20]. The oblique meniscomeniscal ligament, which extends from the anterior horn of one meniscus to the posterior horn of the other 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 consecutive images. Another mimic of a buckethandle tear is a rare congenital variant, most commonly involving the lateral meniscus, referred to as a “ring meniscus.” A ring meniscus variant is characterized by a ring shape in which the anterior and posterior horns are connected by an inner horn bridge and presents on imaging as a complete ring of meniscal tissue [22, 23]. The lack of meniscal tissue loss in the anatomic location of the horns and body of a ring meniscus and the welldefined smooth morphology and triangular shape of the central inner horn component should prompt this diagnosis [7] (Fig. 3C). A meniscal flounce is an incidental redundancy or fold along the free edge of the meniscus (Fig. 3D). A meniscal flounce is commonly observed within the medial meniscus,
with an incidence of 0.2–5% on knee MRI examinations [24, 25]. As opposed to morphologic changes of a meniscal tear, a meniscal flounce reflects a transient physiologic distortion of the meniscal inner margin, typically seen when the knee is in a flexed position and typically disappears on full extension 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 meniscal 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 running parallel and adjacent to interfaces of abrupt signal intensity change. Such artifacts can be superimposed on the meniscus as linear areas of high signal intensity simulating the MRI appearance of a meniscal tear. However, such artifacts are manifested by subtle signal intensity changes exactly paralleling the articular surface of the meniscus and on careful inspection can extend beyond the boundaries of the meniscus itself. Magic angle effect can also lead to artifactual 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 posterior root attachments. Magic angle phenomenon 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 collagen fibers are oriented at 55° to the main magnetic field. It can potentially mimic signal intensity changes of meniscal degeneration on short-TE imaging acquisitions [28]. Increased signal intensity can be seen in the absence of meniscal tears within the meniscus in a variety of settings. Intrasubstance mucoid degeneration of the meniscus is identified as linear or globular increased signal intensity within the meniscus and is often asymptomatic [29, 30]. In the context of trauma, meniscal contusions can also produce a similar appearance with an area of increased intrameniscal signal intensity change that is typically less discrete than either meniscal tears or intrameniscal degeneration and is usually associated with adjacent marrow contusion [31]. In children, it is also common to see peripheral high signal intensity 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 intensity change can contact the meniscal articular surface, mimicking a meniscal tear. Increased specificity for a meniscal tear can be achieved by identifying morphologic changes of the meniscus or visualization of intrameniscal high signal intensity extending to the articular surface of the meniscus on at least two contiguous slices. A positive predictive 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 diminished. The positive predictive value of MRI for detection of longitudinal tears is significantly 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 sensitivity for meniscal tears, particularly peripheral tears of the posterior horn of the lateral meniscus, is also significantly lower. These tears may be subtle and require careful diagnostic scrutiny at MRI evaluation [34]. Displacement of meniscal tissue is an indirect 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 commonly, displaced meniscal fragments are identified adjacent to the posterior root of the medial meniscus, posterior to the PCL, or in the medial and lateral gutters of the articulation [35]. Less commonly, unstable meniscal tear fragments or flap tears may flip under the meniscus itself. Identification and localization 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 meniscus, can rarely be mistaken for an intraarticular body [36]. The signal intensity that is characteristic of the ossicle typically parallels that of marrow fat. Continuity of the ossicle with the adjacent meniscus aids in distinguishing a meniscal ossicle from an intraarticular body. Although MRI offers excellent evaluation of the native meniscus, evaluation of a
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postsurgical meniscus is more challenging. Morphologic truncation of the meniscus and persistent surfacing signal intensity on shortTE pulse sequences, reflecting conventional criteria for diagnosis of preoperative meniscal tears, have limited accuracy of 50–68% in the diagnosis of a postoperative meniscal tear [37, 38]. The presence of increased intrameniscal signal intensity contacting the articular surface on short-TE sequences after meniscal surgery can represent a healing tear, area of intrameniscal degeneration contacting the neoarticular surface after partial meniscectomy, or the residual stable component of a treated meniscal tear. Changes in meniscal morphology are also not specific for recurrent tearing and can reflect changes after meniscectomy. Conventional MRI, MRI arthrography, and CT arthrography have been advocated for evaluation of the postoperative meniscus [37, 39–42]. Reinjury of the meniscus can be most reliably diagnosed by visualizing intrameniscal imbibition of intraarticular gadolinium on T1weighted images at MR arthrography or fluid on T2-weighted nonarthrographic imaging or by visualizing displaced meniscal fragments or meniscal fragmentation [43] (Fig. 5). Interval change in morphology of the meniscus in comparison with previous postoperative MRI, if available, is also indicative of a recurrent tear. Cruciate Ligaments A variety of primary and secondary MRI signs have been described in assessment of complete tears of the ACL. Primary signs include discontinuity of the ACL, nonvisualization of the ACL, and replacement of the ACL with an ill-defined masslike area consisting of hemorrhage. These signs have high diagnostic accuracy in the evaluation of complete ACL disruption [44, 45]. Potential pitfalls in assessment of ACL injuries 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 imaging plane orientation, slice thickness, and interslice gap used at imaging, volume averaging may be encountered between the proximal ACL and the lateral femoral condyle. This may result in erroneous observations of intrasubstance signal intensity changes and possibly incomplete visualization on sagittal 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 continuity on axial and coronal planes in addition to sagittal MRI acquisitions of the knee [46]. A complete ACL tear may undergo scarring, and various scar patterns have been recognized [47]. These include scarring of torn ACL fibers to the PCL, roof of the intercondylar notch, or lateral femoral condyle. In such scenarios, although the knee may remain clinically unstable, the MRI appearance of a scarred ACL can be erroneously mistaken for a contiguous intact or partially 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 nonanatomic point within the intercondylar notch or presence of prior imaging showing a complete 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 partial 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 particularly difficult to diagnose on sagittal images. Similarly, isolated injuries of one bundle may be overlooked when the other bundle remains intact. Interrogation of axial images may be valuable in evaluation of the normal low-signal-intensity ACL at its femoral origin as well as in assessing the degree of ligament fiber disruption in the setting of partial ACL injuries [45]. Imaging the postoperative reconstructed ACL is a common indication for MRI. Complete review of the surgical techniques and imaging appearances of reconstruction grafts is beyond the scope of this article. During the first postoperative year, biologic graft constructs undergo “ligamentization” and neovascularization resulting in increased signal intensity of the graft on T1and 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 intensity by 18–24 months [54, 55]. However, small
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Pitfalls and Pearls in MRI of the Knee areas of persistent striated or globular signal 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 associated with graft impingement or partial tears but should be interpreted with caution in asymptomatic individuals with normal graft positioning and a lack of graft discontinuity. 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 assessment of PCL tears can be more challenging. In addition to focal ligamentous discontinuity, 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 grading of PCL injuries (Fig. 7). Both partial and complete tears can result in thickening of the ligament with ill-defined margins and increased 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 propensity to heal and scar and can be easily overlooked 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 almost normal ligament with an additional 44% showing continuity of the ligament with variable deformity on MRI [62]. Such chronic PCL tears may heal in a stretched state, resulting 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 chronic PCL tears [63]. Similar to the ACL, the PCL can undergo 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 longitudinal increased intrasubstance signal intensity of the PCL bounded by well-defined intact rims of low signal intensity, has been described as a reliable MRI finding in mechanically stable PCLs with mucoid degeneration [65]. Collateral Ligaments The medial collateral ligament (MCL) is commonly injured in valgus injuries to the knee. Acute MCL injuries are invariably associated with periligamentous edema. However, fluid and edema superficial to the MCL is nonspecific and can be seen with medial meniscal tears as well as medial compartmental osteoarthritis [66, 67]. These changes may mimic MRI findings of a partial lowgrade tear of the MCL. More significant edematous changes may also be seen surrounding the medial restraints of the knee in the setting of subchondral insufficiency fractures (Fig. 8). The intense edema, both osseous and soft-tissue that may be related to repetitive stress on the injured subchondral plate [68]; identification of subchondral linear signal intensity changes with intense edema; and commonly associated posterior root tears lead to the correct diagnosis and differentiation from MCL injuries. The posterolateral corner of the knee is composed of a complex set of structures. These include the fibular collateral ligament, popliteus tendon, popliteofibular ligament, arcuate ligament, fabellofibular ligament, and biceps tendon. Injuries to these structures are typically associated with cruciate ligament injuries or multiligamentous injuries. Although the fibular collateral ligament, popliteus tendon, and biceps femoris are consistently seen on MRI, identification of other structures is more variable [69]. Of the more inconsistently visualized ligamentous structures contributing to posterolateral corner stability, exclusion of injury and disruption of the popliteofibular ligament is the most critically important for patient management. Isolated injuries of the popliteofibular ligament, however, are rare, and MRI identification of injuries to the fibular collateral ligament and popliteus should raise concern for concomitant popliteofibular ligament tears and posterolateral corner instability [70] (Fig. 9). Similarly, although the arcuate ligament is typically not well seen on MRI, posttraumatic 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 injury, the neurovascular structures should also be scrutinized because they can be injured in approximately 15% of cases [71]. Extensor Mechanism A 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 common 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 bipartite segment, presence of well-corticated margins to the accessory segment, and typical integrity of underlying articular cartilage of the patella overlying the incompletely united accessory ossification center. Marrow edema at the interface of the bipartite segment is suggestive of micromotion at the synchondrosis, and defects in the normally intact articular cartilage may be features associated with symptomatic anterior knee pain [72] (Fig. 10A). The dorsal defect of the patella is a further variant thought to be related to normal enchondral ossification involving the superolateral patella, which is seen in up to 1% of individuals [73]. On MRI, a dorsal defect of the patella appears as a small symmetrically round subchondral bony defect with intact overlying articular cartilage within the superolateral patella in contrast with osteochondritis dissecans of the patella, which is more commonly central or superomedial in location and often variable in shape and morphology [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 artifacts can result in areas of increased signal intensity on short-TE pulse sequences, particularly 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 patellar tendinopathy, which are observed on both short-TE and T2-weighted acquisitions. Transient lateral patellar dislocation can be a difficult diagnosis clinically, and patients are often referred for imaging for suspected 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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Mohankumar et al. femoral condyle. Identification of osteochondral 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 dislocations (Fig. 11). Patients with injuries to the femoral attachment of the medial patellofemoral ligament are more likely to have recurrent chronic instability and may be appropriate candidates for medial patellofemoral ligament repair or reconstruction [77]. Differentiation of MRI findings of traumatic disruption of the medial patellofemoral ligament versus nonvisualization due to anatomic variation is characterized by findings of edema and hemorrhage at the expected femoral origin of the medial patellofemoral ligament and along the medial-inferior border of the vastus medialis obliquus as well as elevation of the femoral attachment of the vastus medialis obliquus. Signal intensity changes are commonly encountered in relation to the fat pads anteriorly at the knee joint. Edema may be seen involving the superolateral aspect of the Hoffa fat pad in the setting of patellar tendon lateral 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 patellar maltracking. Additional findings of patella alta, lateral subluxation of the patella, and swelling of the suprapatellar fat pad can also be encountered. Articular Cartilage The excellent spatial resolution, tissue contrast, and multiplanar capability make MRI an excellent tool for assessment of articular cartilage [78]. MRI of articular cartilage is susceptible 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 resulting in focal increased signal intensity within the articular cartilage [79, 80]. Increasing the TE will reduce this effect. At bone-cartilage interfaces, on non–fat-suppressed imaging, chemical shift artifacts related to the marrow fat may be encountered, leading to misregistration artifacts in the frequency encoding direction superimposed over areas of articular cartilage, mimicking the appearance of focal chondral lesions. Such artifacts can be reduced by increasing the bandwidth, anatomically directing artifacts elsewhere in the
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image by swapping the frequency and phase encoding directions, or they can be eliminated by application of fat suppression [81]. Pulsation artifacts from the popliteal artery as well as patient motion artifacts can also lead to focal areas of linear or rounded signal intensity 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 articular cartilage, leading to a laminar appearance within the articular cartilage [82, 83]. MRI is useful in assessment of osteochondral abnormalities within the knee. A particular imaging pitfall of note is femoral condylar ossification irregularities that can mimic osteochondritis dissecans in the pediatric population. These are particularly common in the posterior aspect of the lateral femoral condyle and likely reflect a developmental variant related to the enchondral ossification 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 Tissues Hematopoietic marrow or red marrow conversion can be a prominent finding in the knee and can raise concern for a neoplastic marrow infiltrative process. Red marrow conversion may be seen in a variety of settings, including anemia, smoking, and high athletic activity [86]. On MRI, involved areas of hematopoietic marrow conversion show intermediate-to-high signal intensity on fluid-sensitive sequences and low-to-intermediate signal intensity on T1-weighted imaging. Typically, there is metaphyseal involvement, and several patterns have been identified, including uniform confluent 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 dropout can be seen on out-of-phase images in the setting of red marrow [89].
A cortical desmoid, or distal femoral cortical irregularity, is seen at the posterior aspect of medial supracondylar femur in adolescents, which is thought to be a tug lesion involving the adductor magnus insertion or medial gastrocnemius head origin. The location and knowledge of imaging appearance are important to distinguish a cortical desmoid from other abnormalities, such as fibrous cortical defect or parosteal osteosarcoma [90]. Synovial plicae are embryologic remnants of synovial membrane of the knee. Three synovial plicae are commonly encountered at arthroscopy and imaging: the suprapatellar, infrapatellar, and medial plicae, with a lateral plica less common. These are most commonly asymptomatic but can occasionally result in symptoms. The infrapatellar 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 chondral changes of the patella or medial femoral 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 pivot-shift injury, transient patellar dislocation, and hyperextension injury. Surrounding soft tissues can also reveal anatomic variants. Muscle variants can easily be overlooked. These include accessory heads of gastrocnemius muscle and an accessory popliteus or a double configuration to biceps femoris [93–96]. These can be associated with popliteal artery entrapment, a palpable mass, or common peroneal nerve compression, respectively. Finally, vascular variants, such as popliteal artery entrapment, cystic adventitial disease, or deep vein thrombosis, may cause symptoms around the knee and can potentially be overlooked on MRI of the knee unless specifically reviewed (Fig. 16). Summary In this article, we have reviewed the common pitfalls that are encountered in MRI of the knee, knowledge of which is useful for providing an accurate diagnosis when evaluating the images.
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Pitfalls and Pearls in MRI of the Knee
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sessed with 3.0 T MRI. Skeletal Radiol 2011; 40:1017–1023 31. Cothran RL, Major NM, Helms CA, Higgins LD. MR imaging of meniscal contusion in the knee. AJR 2001; 177:1189–1192 32. Takeda Y, Ikata T, Yoshida S, Takai H, Kashiwaguchi S. MRI high-signal intensity in the menisci of asymptomatic children. J Bone Joint Surg Br 1998; 80:463–467 33. De Smet AA, Tuite MJ. Use of the “two-slicetouch” rule for the MRI diagnosis of meniscal tears. AJR 2006; 187:911–914 34. De Smet AA, Graf BK. Meniscal tears missed on MR imaging: relationship to meniscal tear patterns and anterior cruciate ligament tears. AJR 1994; 162:905–911 35. McKnight A, Southgate J, Price A, Ostlere S. Meniscal tears with displaced fragments: common patterns on magnetic resonance imaging. Skeletal Radiol 2010; 39:279–283 36. Tuite MJ, Smet AA, Swan JS, Keene JS. MR imaging of a meniscal ossicle. Skeletal Radiol 1995; 24:543–545 37. Applegate GR, Flannigan BD, Tolin BS, Fox JM, Del Pizzo W. MR diagnosis of recurrent tears in the knee: value of intraarticular contrast material. AJR 1993; 161:821–825 38. Smith DK, Totty WG. The knee after partial meniscectomy: MR imaging features. Radiology 1990; 176:141–144 39. Gopez AG, Kavanagh EC. MR imaging of the postoperative meniscus: repair, resection, and replacement. Semin Musculoskelet Radiol 2006; 10:229–240 40. Sciulli RL, Boutin RD, Brown RR, et al. Evaluation of the postoperative meniscus of the knee: a study comparing conventional arthrography, conventional MR imaging, MR arthrography with iodinated contrast material, and MR arthrography with gadolinium-based contrast material. Skeletal Radiol 1999; 28:508–514 41. Magee T, Shapiro M, Rodriguez J, Williams D. MR arthrography of postoperative knee: for which patients is it useful? Radiology 2003; 229:159–163 42. White LM, Schweitzer ME, Weishaupt D, Kramer J, Davis A, Marks PH. Diagnosis of recurrent meniscal tears: prospective evaluation of conventional MR imaging, indirect MR arthrography, and direct MR arthrography. Radiology 2002; 222:421–429 43. Cardello P, Gigli C, Ricci A, Chiatti L, Voglino N, Pofi E. Retears of postoperative knee meniscus: findings on magnetic resonance imaging (MRI) and magnetic resonance arthrography (MRA) by using low and high field magnets. Skeletal Radiol 2009; 38:149–156 44. Tung GA, Davis LM, Wiggins ME, Fadale PD.
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Mohankumar et al. Tears of the anterior cruciate ligament: primary and secondary signs at MR imaging. Radiology 1993; 188:661–667 45. Lee JK, Yao L, Phelps CT, Wirth CR, Czajka J, Lozman J. Anterior cruciate ligament tears: MR imaging compared with arthroscopy and clinical tests. Radiology 1988; 166:861–864 46. Fitzgerald SW, Remer EM, Friedman H, Rogers LF, Hendrix RW, Schafer MF. MR evaluation of the anterior cruciate ligament: value of supplementing sagittal images with coronal and axial images. AJR 1993; 160:1233–1237 47. Crain EH, Fithian DC, Paxton EW, Luetzow WF. Variation in anterior cruciate ligament scar pattern: does the scar pattern affect anterior laxity in anterior cruciate ligament-deficient knees? Arthroscopy 2005; 21:19–24 48. Vahey TN, Broome DR, Kayes KJ, Shelbourne KD. Acute and chronic tears of the anterior cruciate ligament: differential features at MR imaging. Radiology 1991; 181:251–253 49. Van Dyck P, Vanhoenacker FM, Gielen JL, et al. Three tesla magnetic resonance imaging of the anterior cruciate ligament of the knee: can we differentiate complete from partial tears? Skeletal Radiol 2011; 40:701–707 50. Van Dyck P, De Smet E, Veryser J, et al. Partial tear of the anterior cruciate ligament of the knee: injury patterns on MR imaging. Knee Surg Sports Traumatol Arthrosc 2012; 20:256–261 51. Umans H, Wimpfheimer O, Haramati N, Applbaum YH, Adler M, Bosco J. Diagnosis of partial tears of the anterior cruciate ligament of the knee: value of MR imaging. AJR 1995; 165:893–897 52. Roychowdhury S, Fitzgerald SW, Sonin AH, Peduto AJ, Miller FH, Hoff FL. Using MR imaging to diagnose partial tears of the anterior cruciate ligament: value of axial images. AJR 1997; 168:1487–1491 53. Marumo K, Saito M, Yamagishi T, Fujii K. The “ligamentization” process in human anterior cruciate ligament reconstruction with autogenous patellar and hamstring tendons: a biochemical study. Am J Sports Med 2005; 33:1166–1173 54. Trattnig S, Rand T, Czerny C, et al. Magnetic resonance imaging of the postoperative knee. Top Magn Reson Imaging 1999; 10:221–236 55. Ntoulia A, Papadopoulou F, Zampeli F, Ristanis S, Argyropoulou M, Georgoulis A. Evaluation with contrast-enhanced magnetic resonance imaging of the anterior cruciate ligament graft during its healing process: a two-year prospective study. Skeletal Radiol 2013; 42:541–552 56. Saupe N, White LM, Chiavaras MM, et al. Anterior cruciate ligament reconstruction grafts: MR imaging features at long-term follow-up—correlation with functional and clinical evaluation. Radiology 2008; 249:581–590
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57. Vahey TN, Hunt JE, Shelbourne KD. Anterior translocation of the tibia at MR imaging: a secondary sign of anterior cruciate ligament tear. Radiology 1993; 187:817–819 58. Naraghi AM, Gupta S, Jacks LM, Essue J, Marks P, White LM. Anterior cruciate ligament reconstruction: MR imaging signs of anterior knee laxity in the presence of an intact graft. Radiology 2012; 263:802–810 59. Rodriguez W, Vinson EN, Helms CA, Toth AP. MRI appearance of posterior cruciate ligament tears. AJR 2008; 191:[web]W155–W159 60. Patten RM, Richardson ML, Zink-Brody G, Rolfe BA. Complete vs partial-thickness tears of the posterior cruciate ligament: MR findings. J Comput Assist Tomogr 1994; 18:793–799 61. Mariani PP, Margheritini F, Christel P, Bellelli A. Evaluation of posterior cruciate ligament healing: a study using magnetic resonance imaging and stress radiography. Arthroscopy 2005; 21:1354–1361 62. Jung YB, Jung HJ, Yang JJ, et al. Characterization of spontaneous healing of chronic posterior cruciate ligament injury: analysis of instability and magnetic resonance imaging. J Magn Reson Imaging 2008; 27:1336–1340 63. Orakzai SH, Egan CM, Eustace S, Kenny P, O’Flanagan SJ, Keogh P. Correlation of intra-articular osseous measurements with posterior cruciate ligament length on MRI scans. Br J Radiol 2010; 83:23–27 64. Viana SL, Fernandes JL, Mendonça JL, Freitas FM. Diffuse intrasubstance signal abnormalities of the posterior cruciate ligament: the counterpart of the mucoid degeneration of the anterior cruciate ligament? A case series. JBR-BTR 2008; 91:245–248 65. McMonagle JS, Helms CA, Garrett WE, Vinson EN. Tram-track appearance of the posterior cruciate ligament (PCL): correlations with mucoid degeneration, ligamentous stability, and differentiation from PCL tears. AJR 2013; 201:394–399 66. De Maeseneer M, Shahabpour M, Pouders C. MRI spectrum of medial collateral ligament injuries and pitfalls in diagnosis. JBR-BTR 2010; 93:97–103 67. Bergin D, Hochberg H, Zoga AC, Qazi N, Parker L, Morrison WB. Indirect soft-tissue and osseous signs on knee MRI of surgically proven meniscal tears. AJR 2008; 191:86–92 68. Kattapuram TM, Kattapuram SV. Spontaneous osteonecrosis of the knee. Eur J Radiol 2008; 67:42–48 69. Bolog N, Hodler J. MR imaging of the posterolateral corner of the knee. Skeletal Radiol 2007; 36:715–728 70. Vinson EN, Major NM, Helms CA. The posterolateral corner of the knee. AJR 2008; 190:449–458 71. Twaddle BC, Bidwell TA, Chapman JR. Knee dis-
locations: where are the lesions? A prospective evaluation of surgical findings in 63 cases. J Orthop Trauma 2003; 17:198–202 72. Elias DA, White LM. Imaging of patellofemoral disorders. Clin Radiol 2004; 59:543–557 73. Johnson JF, Brogdon BG. Dorsal effect of the patella: incidence and distribution. AJR 1982; 139:339–340 74. Ho VB, Kransdorf MJ, Jelinek JS, Kim CK. Dorsal defect of the patella: MR features. J Comput Assist Tomogr 1991; 15:474–476 75. Sonin AH, Fitzgerald SW, Bresler ME, Kirsch MD, Hoff FL, Friedman H. MR imaging appearance of the extensor mechanism of the knee: functional anatomy and injury patterns. RadioGraphics 1995; 15:367–382 76. Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. Radiology 2002; 225:736–743 77. Sillanpää PJ, Peltola E, Mattila VM, Kiuru M, Visuri T, Pihlajamäki H. Femoral avulsion of the medial patellofemoral ligament after primary traumatic patellar dislocation predicts subsequent instability in men: a mean 7-year nonoperative follow-up study. Am J Sports Med 2009; 37:1513–1521 78. Kijowski R. Clinical cartilage imaging of the knee and hip joints. AJR 2010; 195:618–628 79. Xia Y. Magic-angle effect in magnetic resonance imaging of articular cartilage: a review. Invest Radiol 2000; 35:602–621 80. Rubenstein JD, Kim JK, Morova-Protzner I, Stanchev PL, Henkelman RM. Effects of collagen orientation on MR imaging characteristics of bovine articular cartilage. Radiology 1993; 188:219–226 81. Disler DG, Peters TL, Muscoreil SJ, et al. Fatsuppressed spoiled GRASS imaging of knee hyaline cartilage: technique optimization and comparison with conventional MR imaging. AJR 1994; 163:887–892 82. Frank LR, Brossmann J, Buxton RB, Resnick D. MR imaging truncation artifacts can create a false laminar appearance in cartilage. AJR 1997; 168:547–554 83. Erickson SJ, Waldschmidt JG, Czervionke LF, Prost RW. Hyaline cartilage: truncation artifact as a cause of trilaminar appearance with fat-suppressed three-dimensional spoiled gradient-recalled sequences. Radiology 1996; 201:260–264 84. Nawata K, Teshima R, Morio Y, Hagino H. Anomalies of ossification in the posterolateral femoral condyle: assessment by MRI. Pediatr Radiol 1999; 29:781–784 85. Gebarski K, Hernandez RJ. Stage-I osteochondritis dissecans versus normal variants of ossification in the knee in children. Pediatr Radiol
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Pitfalls and Pearls in MRI of the Knee 2005; 35:880–886 86. Shellock FG, Morris E, Deutsch AL, Mink JH, Kerr R, Boden SD. Hematopoietic bone marrow hyperplasia: high prevalence on MR images of the knee in asymptomatic marathon runners. AJR 1992; 158:335–338 87. Swartz PG, Roberts CC. Radiological reasoning: bone marrow changes on MRI. AJR 2009; 193(3 suppl):S1–S9 88. Kung JW, Yablon CM, Eisenberg RL. Bone marrow signal alteration in the extremities. AJR 2012; 196:[web]W492–W510 89. Lang P, Fritz R, Majumdar S, Vahlensieck M, Peterfy
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symptoms, and radiologic evaluation. RadioGraphics 2008; 28:481–499 94. Macedo TA, Johnson CM, Hallett JW, Breen JF. Popliteal artery entrapment syndrome: role of imaging in the diagnosis. AJR 2003; 181:1259–1265 95. Kim HK, Shin MJ, Kim SM, Lee SH, Hong HJ. Popliteal artery entrapment syndrome: morphological classification utilizing MR imaging. Skeletal Radiol 2006; 35:648–658 96. Elias DA, White LM, Rubenstein JD, Christakis M, Merchant N. Clinical evaluation and MR imaging features of popliteal artery entrapment and cystic adventitial disease. AJR 2003; 180:627–632
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Fig. 1—“Wrisberg rip” and pseudotear. A–C, 26-year-old man with Wrisberg rip of posterior horn of lateral meniscus. Sagittal fat-suppressed T2weighted 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 fatsuppressed 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 fatsuppressed 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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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.
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Fig. 3—Meniscal variants. A and B, 54-year-old man with oblique meniscomeniscal ligament. Midsagittal fatsuppressed T2-weighted MR image (A) (TR/ TE, 3750/68) shows linear low-signal-intensity structure (arrowhead) within intercondylar notch mimicking displaced meniscal fragment. Axial fatsuppressed 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. (Fig. 3 continues on next page)
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Pitfalls and Pearls in MRI of the Knee 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.
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Fig. 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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Fig. 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.
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Fig. 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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Pitfalls and Pearls in MRI of the Knee
A Fig. 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.
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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).
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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 fatsuppressed 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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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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Pitfalls and Pearls in MRI of the Knee
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Fig. 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).
A Fig. 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. 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).
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A 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).
530
B
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).
AJR:203, September 2014
