Clinical Cartilage Imaging of the Knee and Hip Joints

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1 Musculoskeletal Imaging Review Kijowski Imaging of the Knee and Hip Joints Musculoskeletal Imaging Review Richard Kijowski 1 Kijowski R FOCUS ON: Keywords: articular cartilage, hip, knee, MRI DOI: /JR Received March 22, 2010; accepted after revision pril 28, Department of Radiology, University of Wisconsin Clinical Science Center, E3/311, 600 Highland ve., Madison, WI ddress correspondence to R. Kijowski (rkijowski@uwhealth.org). JR 2010; 195: X/10/ merican Roentgen Ray Society Clinical Cartilage Imaging of the Knee and Hip Joints OJECTIVE. MRI is commonly used to evaluate the articular cartilage of the knee and hip joints in clinical practice. This article will discuss the advantages and limitations of currently available MRI techniques for evaluating articular cartilage. CONCLUSION. ecause of its high spatial resolution, multiplanar capability, and excellent tissue contrast, MRI is the imaging technique of choice for evaluating the articular cartilage of the knee and hip joints. O steoarthritis is one of the most prevalent chronic diseases in the United States and results in a significant socioeconomic burden on merican society [1, 2]. The knee and hip joints are the most common sites of debilitating osteoarthritis [3]. Until recently, patients with knee and hip osteoarthritis had to suffer from the disease until they were old enough to undergo total joint arthroplasty. However, new medical and surgical treatment options are now available for patients with osteoarthritis [4]. If they are to be successful, these treatment options must be performed during the early stages of the disease process before the development of advanced joint degeneration. MRI is one of the most commonly used imaging techniques to evaluate patients with joint pain. ccurate detection of cartilage lesions within the knee and hip joints in clinical practice is essential to help identify patients who may benefit from early medical and surgical intervention. This article will discuss the appearance of cartilage lesions on MRI and the advantages and limitations of currently available MRI techniques for evaluating articular cartilage. This article will also discuss the role of MR arthrography in clinical cartilage imaging. Cartilage Imaging of the Knee Joint Importance of Clinical Cartilage Imaging lthough a cartilage repair procedure is the indication for only 5% of knee arthroscopies performed in the United States [5], it is nevertheless important to achieve high diagnostic performance when evaluating the articular cartilage of the knee joint in clinical practice. The relationship between cartilage loss and perceived pain in patients with osteoarthritis and acute cartilage injury is poorly understood [6 8]. However, identifying cartilage loss can explain the cause of joint pain in many symptomatic patients with no evidence of internal derangement on MRI [9]. In addition, new treatment options are currently available for patients with osteoarthritis and acute cartilage injury. There has been much recent interest in the development of diseasemodifying osteoarthritis drugs (DMODs). lthough there are no DMODs currently approved by the United States Food and Drug dministration for use in patients with osteoarthritis, many promising pharmaceutical agents such as bisphosphonates, matrix-metalloproteinase and cytokine inhibitors, and calcitonin are presently being investigated in clinical trials [10, 11]. Surgical interventions such as mosaicplasty [12, 13] and autologous chondrocyte transplantation [14, 15] are also now available to treat patients with acute cartilage injury. Furthermore, identifying degenerative and posttraumatic cartilage loss within the knee joint is an important prognostic factor for determining the long-term success of anterior cruciate ligament reconstruction surgery [16] and partial meniscectomy [17]. ppearance of Cartilage Lesions on MRI The appearance of cartilage lesions on MRI depends on whether they are degenerative or posttraumatic in cause. Early degenerative cartilage lesions in patients with osteoarthritis appear as fibrillation, pitting, 618 JR:195, September 2010

2 Imaging of the Knee and Hip Joints and fissuring of the articular surface. s the disease progresses, a focal partial-thickness cartilage defect with obtuse margins develops in the area of cartilage degeneration (Fig. 1). In patients with advanced osteoarthritis, multiple partial-thickness and full-thickness cartilage defects, cartilage delamination, and diffuse cartilage thinning involving opposing articular surfaces of the knee joint are typically present [18, 19]. Degenerative cartilage lesions can be associated with changes in the underlying subchondral bone marrow. The bone marrow edema pattern, which consists of ill-defined areas of high T2 signal, is the most common finding and can be seen adjacent to 5 54% of cartilage lesions depending on their size and depth [20]. lthough the high T2 signal within the subchondral bone marrow was originally thought to represent marrow edema, studies have shown that these areas of signal abnormality actually correspond to areas of marrow necrosis, fibrosis, and trabecular disorganization on histologic analysis [21]. The bone marrow edema pattern may be a particularly important secondary finding in patients with degenerative cartilage lesions because its presence has been correlated with pain [22] and disease progression [23] in some research studies. In patients with advanced osteoarthritis, high T2-signal cysts or low T1- and T2-signal sclerosis can be seen within the subchondral bone marrow adjacent to deep partial-thickness and full-thickness cartilage defects. Posttraumatic cartilage lesions within the knee joint are the result of shearing, tangential, or rotational forces on the articular surface, and their appearance on MRI depends on the mechanism of injury. Shearing forces secondary to injuries, such as transient patellar dislocation, typically result in deep partial-thickness or full-thickness cartilage defects with acutely angulated margins that may also involve the underlying subchondral bone. Linear cartilage flap tears and delamination injuries at the junction between the articular cartilage and subchondral bone also can occur (Fig. 2). Cartilage shearing injuries are commonly unstable and result in partially or completely displaced cartilaginous or osteochondral fragments with associated intraarticular loose bodies [18, 19]. Tangential and rotational forces on articular cartilage secondary to injuries, such as anterior cruciate ligament tear, typically result in osteochondral impaction injuries. These impaction injuries manifest as areas of high T2-signal subchondral bone marrow edema within the anterior lateral femoral condyle and posterior tibial plateau with occasional depression of the articular surface and trabecular fracture lines (Fig. 3). The articular cartilage overlying areas of bone marrow edema usually appears normal on MRI and even at arthroscopy [18, 19]. However, evidence of acute cartilage injury is apparent on histologic analysis [24] and when using physiologic cartilage imaging techniques, such as delayed gadolinium-enhanced imaging [25] and T1 rho imaging [26]. Cartilage Imaging Sequences The most common sequences used in clinical practice to evaluate the articular cartilage of the knee joint are 2D fast spin-echo (FSE) sequences with T2-weighted and intermediateweighted contrast [27 32]. The advantages of FSE sequences include their high in-plane spatial resolution and their ability to evaluate the menisci, ligaments, and osseous structures in addition to the articular cartilage [33 35]. ecause of their multiple off-resonance pulses, FSE sequences also produce a magnetization transfer effect that can help distinguish between normal articular cartilage and areas of early cartilage degeneration [36, 37]. However, FSE sequences have relatively thick slices and gaps between slices that can obscure small cartilage lesions secondary to partial volume averaging. In addition, T2-weighted FSE sequences have poor contrast between articular cartilage and subchondral bone, which can Fig year-old male volunteer with knee pain., Sagittal fat-saturated FSE-Cube (GE Healthcare) image of knee joint shows superficial cartilage fissure (arrow) within medial femoral condyle., Sagittal fat-saturated FSE-Cube image of knee joint performed 4 months after shows progression of cartilage degeneration and formation of superficial partial-thickness cartilage defect (arrow) with obtuse margins on medial femoral condyle. Fig year-old woman with history of transient patellar dislocation., xial 2D fat-saturated T2-weighted fast spin-echo (FSE) image of knee joint shows large partial-thickness flap tear (arrows) of lateral patellar facet. lso note lateral tilt of patella (arrowhead)., Corresponding sagittal 2D fat-saturated T2-weighted FSE image of knee joint shows cartilage delamination of lateral femoral trochlea (arrow) with underlying subchondral bone marrow edema (small arrowhead). lso note patella alta (large arrowhead). JR:195, September

3 Kijowski make it difficult to detect diffuse cartilage thinning and estimate the exact depth of cartilage lesions. Intermediate-weighted FSE sequences are also limited by image blurring secondary to acquisition of high spatial frequencies late in the echo-train and by poor contrast between articular cartilage and synovial fluid [38, 39]. However, image blurring can be reduced by minimizing echo-train length and using short interecho spacing, whereas contrast between articular cartilage and synovial fluid on intermediate-weighted FSE sequences can be improved with the use of fat suppression. In clinical practice, 3D sequences also have been used to evaluate the articular cartilage of Fig year-old man with history of anterior cruciate ligament tear., Sagittal 2D fat-saturated T2-weighted fast spin-echo (FSE) image of knee joint shows subchondral bone marrow edema within anterior lateral femoral condyle (arrow) and posterior lateral tibial plateau (small arrowhead). lso note articular surface depression of anterior lateral femoral condyle (large arrowhead)., Corresponding sagittal 2D fat-saturated T2-weighted FSE image of knee joint at more medial location shows subchondral bone marrow edema pattern within posterior medial tibial plateau (arrow) with associated low T2- signal trabecular fracture line (arrowhead). Fig year-old male volunteer with knee pain. and, Sagittal iterative decomposition of water and fat with echo asymmetry and least-squares estimates (IDEL) gradient-recalled echo acquired in the steady-state (GRSS) () and corresponding sagittal IDEL spoiled gradient-recalled echo (SPGR) () images of knee joint show superficial cartilage fissure (arrow) within medial femoral condyle. Note that superficial cartilage lesion is more conspicuous on bright-fluid IDEL GRSS image than dark-fluid IDEL SPGR image. the knee joint. Fat suppression is typically added to these sequences to reduce chemical-shift artifact and to optimize the overall dynamic contrast range of the image. Frequency-selective fat-saturation is the most commonly used method to suppress fat signal [9, 40 42]. However, 3D sequences with higher cartilage signal-to-noise ratio (SNR) and greater contrast between cartilage and adjacent joint structures can be obtained using recently developed fatsuppression techniques, such as water excitation [43, 44], linear combination [45, 46], and iterative decomposition of water and fat with echo asymmetry and least-squares estimates (IDEL) [47 49]. y acquiring thin, continuous slices through the knee joint, 3D cartilage imaging sequences can reduce partial volume averaging. The 3D sequences also can be used to create multiplanar reformat images that allow articular cartilage to be evaluated in any orientation after a single acquisition. The disadvantages of 3D cartilage imaging sequences include their long acquisition times; increased susceptibility to artifacts; and limited ability to evaluate the menisci, ligaments, and osseous structures of the knee joint when compared with 2D FSE sequences. The 3D cartilage imaging sequences can be broadly divided into dark-fluid sequences and bright-fluid sequences on the basis of the signal intensity of synovial fluid. Darkfluid sequences consist of T1-weighted gradient-recalled echo (GRE) sequences such as spoiled gradient recalled-echo (SPGR) and fast low-angle shot (FLSH). These sequences have been successfully used to evaluate articular cartilage in clinical practice [9, 40 42] and to perform cartilage volume measurements in osteoarthritis research studies [50, 51]. However, the main disadvantage of using SPGR and FLSH sequences for clinical cartilage imaging is the low signal intensity of synovial fluid, which may decrease the conspicuity of superficial cartilage lesions (Fig. 4). Dark-fluid sequences have lower contrast between articular cartilage and synovial fluid than bright-fluid sequences (Kijowski R, et al., presented at the 2008 annual meeting of the International Society of Magnetic Resonance in Medicine). In addition, the surface properties of degenerative cartilage may influence the ability of 3D sequences to detect superficial cartilage lesions. Superficial degeneration shortens the T2 relaxation time of cartilage. For darkfluid sequences, the T2 shortening of degenerative cartilage has no effect on its signal intensity and contrast relative to synovial fluid. However, for bright-fluid sequences, the effect of T2 shortening is to decrease the signal intensity of degenerative cartilage and thus increase its contrast relative to synovial fluid, which may result in greater conspicuity of superficial cartilage lesions [52]. Various 3D cartilage imaging sequences with bright synovial fluid have been used to evaluate the articular cartilage of the knee joint. These bright-fluid sequences include dual-echo in the steady-state (DESS); driven equilibrium Fourier transform (DEFT); and T2*-weighted gradient-echo sequences, such as gradient-recalled echo acquired in the steady-state (GRSS) and gradient-recalled 620 JR:195, September 2010

4 Imaging of the Knee and Hip Joints Fig year-old male volunteer with knee pain. Sagittal balanced steady-state free precession image of knee joint (0.3-mm isotropic resolution and 8-minute scanning time) with vastly undersampled isotropic projection radial k-space trajectory and alternating TR fat water separation shows superficial partial-thickness cartilage defect (arrow) on lateral femoral trochlea. echo (GRE). The DESS sequence combines two gradient echoes separated by a refocusing pulse into a single image that increases the signal intensity of both articular cartilage and synovial fluid [53]. The DEFT sequence uses a 90 pulse to return transverse magnetization to the z-axis, which increases the signal intensity of synovial fluid and other tissues with long T1 relaxation times [54 56]. GRSS and GRE sequences produce images with high-signal-intensity synovial fluid because of coherence of transverse magnetization with secondary T2* weighting [48, 57]. The bright synovial fluid on DESS, DEFT, and T2*-weighted gradient-echo sequences creates an arthrogram-like effect within the knee joint that may increase the conspicuity of superficial cartilage lesions. alanced steady-state free precession (SSFP) sequences are additional 3D sequences that have been used to evaluate the articular cartilage of the knee joint. alanced SSFP sequences include commercially available sequences such as fast imaging employing steady-state acquisition (FIEST) and true fast imaging with steady-state precession (true FISP) and variants such as fluctuating equilibrium MR (FEMR) [58] and vastly undersampled isotropic-projection steady-state free precession (VIPR-SSFP) [46]. These sequences have high SNR efficiency and produce images of the knee joint with T2-/T1-weighted contrast and bright synovial fluid. alanced SSFP sequences and their variants have higher cartilage SNR and greater contrast between cartilage and adjacent joint structures than 2D FSE and fat-saturated SPGR sequences [39, 49, 59]. When combined with VIPR radial k-space trajectory and alternating TR fat water separation, balanced SSFP images of the knee joint with 0.3-mm isotropic resolution can be obtained in as little as 8 minutes (Klaers JK, et al., presented at the 2010 annual meeting of the Society of Magnetic Resonance in Medicine) (Fig. 5). Recently, 3D FSE sequences, such as FSE- Cube (GE Healthcare) [60, 61] and sampling perfection with application oriented contrast using different flip angle evolutions (SPCE, Siemens Healthcare) [62], have been used to evaluate the articular cartilage of the knee joint. These sequences use variable flip angle modulation to constrain T2 decay over an extended echo-train, which allows intermediate-weighted images of the knee joint with bright synovial fluid to be acquired with minimal blurring. FSE-Cube and SPCE sequences have higher cartilage SNR but lower contrast between cartilage and synovial fluid TLE 1: Diagnostic Performance of Cartilage Imaging Sequences for Detecting Cartilage Lesions With the Knee Joint in Studies With Surgical Correlation Sequence [Reference] Scanner No. of Patients Voxel Size (mm) Sensitivity (%) Specificity (%) 2D-FSE [27] 1.5 T D-FSE [28] 1.5 T a a 2D-FSE [29] 1.5 T D-FSE [30] 1.5 T D-FSE [31] 1.5 T a a 2D-FSE [32] 1.5 T a a Fat-saturated SPGR [9] 1.5 T a a Water-excitation FLSH [30] 1.5 T Water-excitation SPGR [64] 1.5 T a a DESS [65] 1.5 T Water-excitation DESS [64] 1.5 T a a DEFT [55] 1.5 T Water-excitation true FISP [64] 1.5 T a a Water-excitation true FISP [66] 1.5 T a a 2D-FSE [31] 3 T a a 2D-FSE [32] 3 T a a IDEL GRSS [63] 3 T a a Fat-saturated FSE-Cube b [61] 3 T Note FSE = fast spin-echo, SPGR = spoiled gradient-recalled echo, FLSH = fast low-angle shot, DESS = dual-echo in the steady-state, DEFT = driven equilibrium Fourier transform, true FISP = true fast imaging with steady-state precession, IDEL = iterative decomposition of water and fat with echo asymmetry and least-squares estimates, GRSS = gradient-recalled echo acquired in the steady-state. a Individual values from multiple readers. b GE Healthcare. JR:195, September

5 Kijowski when compared with 2D FSE sequences [60, 62]. The 3D FSE sequences acquire volumetric data sets with isotropic resolution that allow high-quality multiplanar reformat images to be obtained in any orientation after a single acquisition. However, these sequences have lower in-plane spatial resolution when compared with other 3D cartilage imaging sequences with similar acquisition times, which may reduce the conspicuity of superficial cartilage lesions. Diagnostic Performance of Cartilage Imaging Sequences Multiple studies with surgical correlation have evaluated the diagnostic performance of 2D and 3D sequences for detecting cartilage lesions within the knee joint [9, 30 32, 55, 61, 63 67]. The results of these studies are summarized in Table 1. In most studies, diagnostic performance for detecting cartilage lesions is highest on the thick articular surface of the patella and lowest on the lateral tibial plateau where the curved articular surface is more prone to partial volume averaging and imaging artifacts. On 1.5-T imaging systems, cartilage imaging sequences have sensitivity values ranging between 45% for water-excitation true FISP and 94% for 2D FSE, with specificity values ranging between 70% and 99%. On 3-T imaging systems, these sequences have sensitivity values ranging between 66% for IDEL GRSS to 80% for 2D FSE, with specificity values ranging between 80% and 97%. It is impossible to compare the sensitivity and specificity values of the cartilage imaging sequences used in previously published studies because of differences in MR hardware, imaging parameters, patient populations, and reader experience. Few previous studies have directly compared various 2D and 3D sequences for evaluating the articular cartilage of the knee joint. Duc and associates [64] found similar sensitivity and specificity values for water-excitation FLSH, water-excitation DESS, 2D fatsaturated intermediate-weighted FSE, and water-excitation true FISP sequences for detecting surgically confirmed cartilage lesions in 30 patients at 1.5 T. Mohr [30] found that a 2D fat-saturated intermediate-weighted FSE sequence had significantly higher (p < 0.05) sensitivity and similar specificity as a water-excitation FLSH sequence for detecting surgically confirmed cartilage lesions in 26 patients at 1.5 T. In two separate studies performed at 3 T in 100 patients with surgical correlation, Kijowski and associates found that fat-saturated FSE-Cube [61] and IDEL GRSS [63] sequences had similar sensitivity and specificity for detecting cartilage lesions as a routine MRI protocol consisting of multiplanar 2D fat-saturated FSE sequences. dditional studies are needed to directly compare various 2D and 3D cartilage imaging sequences to determine which sequence is best suited for evaluating the articular cartilage of the knee joint in clinical practice. Factors Influencing the Detection of Cartilage Lesions on MRI The main limitation of currently available sequences for evaluating the articular cartilage of the knee joint is their relatively low sensitivity for detecting superficial cartilage lesions. For those studies that report sensitivity values for individual grades of cartilage lesions, the sensitivity for detecting grade 2 lesions which compose less than 50% of the articular surface range between 44% and 75% [29, 31, 55, 61, 63]. The relatively low sensitivity for detecting superficial cartilage lesions is mainly attributed to suboptimal spatial resolution. Superficial morphologic changes of articular cartilage such as fibrillation and pitting can only be distinguished from the smooth surface of normal articular cartilage when using an in-plane spatial resolution of 0.3 mm, which is beyond the spatial resolution of most cartilage imaging sequences used in clinical practice [68]. Occasionally, these superficial cartilage lesions can be identified on T2-weighted FSE sequences as areas of increased signal intensity within articular cartilage [69, 70] (Fig. 6). The increased signal intensity is presumably secondary to increased water content and decreased collagen content of degenerative cartilage, which can be detected using the high fluid sensitivity and magnetization transfer contrast of T2-weighted FSE sequences [36, 37]. However, signal intensity changes within articular cartilage are nonspecific findings and also may be the result of imaging artifacts such as truncation [71] and magic angle effect [72]. dditional factors that contribute to the relatively low sensitivity of currently available sequences for detecting superficial cartilage lesions include suboptimal tissue contrast [52], partial volume averaging [38], and inability to evaluate cartilage in multiple planes [29] (Fig. 7). Cartilage Imaging at 1.5 and 3 T The use of 3-T imaging systems is becoming widespread in clinical practice and has the potential to improve clinical cartilage imaging. The 3-T systems can produce images of articular cartilage of the knee joint with higher spatial resolution and decreased slice thickness than 1.5-T systems without sacrificing SNR or prolonging acquisition time. Using optimized protocols, 3-T systems also can acquire images with greater contrast between cartilage and adjacent joint structures [73, 74]. Two previous studies have compared the diagnostic performance of a routine MRI Fig year-old man with surgically confirmed superficial cartilage fibrillation on medial femoral condyle. (Reprinted with permission from [67]), Sagittal 2D fat-saturated T2-weighted fast spin-echo (FSE) image of knee joint shows two areas of high signal intensity within articular cartilage (arrows) of medial femoral condyle., Corresponding sagittal vastly undersampled isotropic projection (VIPR) with steady-state free precession (SSFP) image of knee joint shows normal-appearing cartilage (arrows) on medial femoral condyle. Superficial cartilage fibrillation could not be visualized on VIPR SSFP image because of decreased fluid sensitivity and lack of magnetization transfer effect when compared with T2-weighted FSE image. 622 JR:195, September 2010

6 Imaging of the Knee and Hip Joints protocol consisting of multiplanar 2D fat-saturated FSE sequences performed at 1.5 and 3 T for detecting cartilage lesions within the knee joint (Fig. 8). Kijowski and associates [31] compared the diagnostic performance of a routine MRI protocol performed at 1.5 and 3 T, with surgical correlation, on two different patient populations each consisting of 100 individuals. The routine MRI protocol had significantly higher (p < 0.05) specificity and accuracy and similar sensitivity for detecting cartilage lesions at 3 T than at 1.5 T. In a second study performed on 26 patients with surgical correlation who were evaluated on both 1.5-T and 3-T imaging systems, Wong and associates [32] found that a routine MRI protocol had significantly higher (p < 0.05) sensitivity and similar specificity and accuracy for detecting cartilage lesions at 3 T. In both of these studies, the routine MRI protocol had significantly higher accuracy for grading cartilage lesions at 3 T than at 1.5 T [31, 32]. Diagnostic performance for detecting cartilage lesions within the knee joint can be further improved at 3 T if 3D cartilage imaging sequences are used along with the 2D FSE sequences in the routine MRI protocol to evaluate articular cartilage. In a study performed on 200 patients with surgical correlation, Kijowski and associates presented at the 2009 annual meeting of the Society of Skeletal Radiology) compared the diagnostic performance of a routine MRI protocol for detecting cartilage lesions within the knee joint when used alone and when used along with either an Fig year-old man with surgically confirmed deep partial-thickness cartilage lesion on femoral trochlea. (Reprinted with permission from [61]), Sagittal 2D intermediate-weighted fast spin-echo (FSE) image of knee joint shows normal-appearing cartilage (arrow) on femoral trochlea., Corresponding sagittal 2D fat-saturated T2-weighted FSE image of knee joint shows poorly visualized deep partial-thickness cartilage defect (arrow) on femoral trochlea. C, Corresponding sagittal fat-saturated FSE-Cube (GE Healthcare) image of knee joint shows well-visualized deep partial-thickness cartilage defect (arrow) on femoral trochlea. Cartilage lesion could not be visualized on intermediate-weighted FSE image because of suboptimal tissue contrast and was poorly visualized on T2-weighted FSE image because of partial volume averaging with adjacent normal articular cartilage. IDEL SPGR or IDEL GRSS sequence. The routine MRI protocol had significantly higher (p < 0.05) specificity and accuracy for detecting cartilage lesions and significantly higher (p < 0.05) accuracy for grading cartilage lesions when used along with the IDEL SPGR or IDEL GRSS sequence than when used alone. Furthermore, the sensitivity for detecting cartilage lesions was significantly higher (p < 0.05) when the routine MRI protocol was used along with the bright-fluid IDEL GRSS sequence but not the dark-fluid IDEL SPGR sequence (Kijowski et al., presented at the 2009 annual meeting of the Society of Skeletal Radiology). MR rthrography MR arthrography also has been used to evaluate the articular cartilage of the knee joint in clinical practice. However, MR arthrography requires the injection of intraarticular contrast material, which increases the cost and invasiveness of the imaging procedure. The main advantage of MR arthrography is the presence of contrast material outlining the articular surface of the knee joint, which may increase the conspicuity of superficial cartilage lesions (Fig. 9). MRI sequences with bright synovial fluid can create a similar arthrogram-like effect, but this requires the presence of synovial fluid adjacent to the articular surface, which may not occur in all patients and at all joint locations. Previous studies have shown that MR arthrography has a significantly higher (p < 0.05) diagnostic performance than MRI for detecting surgically confirmed cartilage lesions within the knee joint [75, 76]. However, these comparison studies were performed at 1.5 T using sequences with relatively low in-plane spatial resolution, thick slices, and suboptimal tissue contrast when compared with cartilage imaging sequences currently available on 3-T imaging systems. lthough MR arthrography is the imaging technique of choice for evaluating postoperative menisci [77 80], additional studies are needed to document the advantages of MR arthrography over MRI for evaluating the articular cartilage of the knee joint in clinical practice. Cartilage Imaging of the Hip Joint Importance of Clinical Cartilage Imaging ccurate evaluation of the articular cartilage of the hip joint in patients undergoing MRI is clinically significant. During the past decade, conditions such as femoroacetabular impingement [81 83] and acetabular dysplasia [84, 85] have been gaining increased attention as causes of premature osteoarthritis of the hip joint in young patients. Surgical interventions such as labral repair or débridement [86 88] along with femoral and acetabular osteochondroplasty [86, 87] and acetabular osteotomy [88 90] are currently available to treat these patients. However, performing these procedures before the development of advanced joint degeneration is essential for their long-term success [91, 92]. Thus, early detection of cartilage degeneration with MRI C JR:195, September

7 Kijowski can help identify patients with hip pain who may benefit from early surgical intervention. Fig year-old man with surgically confirmed superficial partial-thickness cartilage lesion on medial femoral condyle., Sagittal 2D fat-saturated T2-weighted fast spinecho (FSE) image of knee joint from MRI examination shows subtle irregularity of articular cartilage (arrow) on medial femoral condyle., Corresponding sagittal 2D fat-saturated T1-weighted FSE image of knee joint from MR arthrography examination performed 6 weeks after shows superficial partial-thickness cartilage defect (arrow) on medial femoral condyle. C Cartilage Imaging Sequences and Their Diagnostic Performance oth 2D FSE [93] and 3D SPGR [94] sequences have been used to evaluate the articular cartilage of the hip joint in clinical practice. However, few previous studies with surgical correlation have documented the diagnostic performance of MRI for detecting cartilage lesions within the hip joint. Mintz and associates [93] found that multiplanar 2D FSE sequences with mm in-plane spatial resolution and 5.0-mm slice thickness had sensitivity between 88% and 93% and specificity between 78% and 87% for detecting surgically confirmed cartilage lesions in 92 patients. However, a study performed by Nishii and associates [94] in 20 patients with surgical correlation found that D Fig year-old man with surgically confirmed superficial partial-thickness cartilage lesion on medial femoral condyle. and, Sagittal 2D fat-saturated T2-weighted fast spin-echo (FSE) () and corresponding coronal 2D fat-saturated intermediate-weighted FSE () images of knee joint from 1.5-T MRI examination show subtle irregularity of articular cartilage (arrow) on medial femoral condyle. C and D, Sagittal 2D fat-saturated T2-weighted FSE (C) and corresponding coronal 2D fat-saturated intermediate-weighted FSE (D) images of knee joint from 3-T MRI examination performed 3 weeks after and show superficial partial-thickness cartilage defect (arrow) on medial femoral condyle. a fat-saturated SPGR sequence with mm in-plane spatial resolution and 1.5-mm slice thickness had sensitivity between 49% and 67% and specificity between 76% and 89% for detecting cartilage lesions. The conflicting results of these studies may be due to differences in reader experience and sequence selection. Evaluation of the articular cartilage of the hip joint with MRI is extremely challenging because of the thin articular cartilage and spherical surface geometry of the femoral head and acetabulum. Furthermore, the need for a large field of view and the absence of specialized coils for evaluating the hip joint results in images with relatively low spatial resolution. The use of sequences with higher in-plane spatial resolution and decreased slice thickness may improve the detection of cartilage lesions [95]. lternatively, the use of physiologic cartilage imaging sequences, such as delayed gadolinium-enhanced imaging [96 98], T1 rho imaging [95], and T2 mapping [99], may allow better detection of early cartilage degeneration within the hip joint. lthough high-resolution morphologic and physiologic cartilage imaging sequences may be well suited for evaluating articular cartilage in osteoarthritis research studies, 624 JR:195, September 2010

8 Imaging of the Knee and Hip Joints these sequences have long acquisition times, which limit their use in clinical practice. MR rthrography Most patients with hip pain in clinical practice are evaluated with MR arthrography using 2D T1-weighted FSE sequences. MR arthrography has higher diagnostic performance than MRI for detecting labral tears [ ]. However, MR arthrography has relatively low diagnostic performance for evaluating the articular cartilage of the hip joint. The sensitivity of MR arthrography for detecting surgically confirmed cartilage lesions ranges between 41% and 79%, with specificity values ranging between 77% and 100% [101, 103]. Furthermore, the sensitivity for detecting cartilage delamination in patients with femoroacetabular impingement is as low as 22% [104]. The 3D sequences can potentially improve the evaluation of the articular cartilage of the hip joint during MR arthrography [ ] (Fig. 10). Two previous studies have compared 2D and 3D sequences for cartilage assessment. Knuesel and associates [108] compared a sagittal water-excitation DESS sequence and a sagittal 2D fat-saturated T1-weighted FSE sequence for detecting surgically confirmed cartilage lesions in 21 patients at 1.5 T. oth sequences had similar sensitivity and specificity for detecting cartilage lesions, but waterexcitation DESS had significantly greater (p < 0.05) lesion conspicuity. In a larger study performed at 3 T, Ullrick and associates (presented at the 2009 annual meeting of the Fig year-old woman with surgically confirmed deep partial-thickness cartilage lesion on posterior superior femoral head., Sagittal 2D fat-saturated intermediate-weighted fast spin-echo (FSE) image of hip joint from MRI examination shows normal-appearing articular cartilage (arrow) on posterior superior femoral head. and C, Corresponding sagittal 2D fat-saturated T1-weighted FSE () and sagittal iterative decomposition of water and fat with echo asymmetry and least-squares estimates (IDEL) spoiled gradient-recalled echo (SPGR) (C) images of hip joint from MR arthrogram examination performed 2 weeks after show small contrast-filled deep partial-thickness cartilage defect (arrow) on posterior superior femoral head. Note that cartilage lesion is much more conspicuous on thinner IDEL SPGR image because of reduced partial volume averaging. merican Roentgen Ray Society) compared an IDEL SPGR sequence and multiplanar 2D fat-saturated T1-weighted FSE sequences for detecting surgically confirmed cartilage lesions in 80 patients. IDEL SPGR with multiplanar reformats had significantly higher (p < 0.05) sensitivity but significantly lower (p < 0.05) specificity and accuracy than the T1-weighted FSE sequences for detecting cartilage lesions. The sensitivity for detecting superficial cartilage lesions was extremely low, with only 22% and 32% of grade 2 cartilage lesions identified using the 2D and 3D sequences respectively (Ullrick SR, et al., presented at the 2009 annual meeting of the merican Roentgen Ray Society). dditional studies are needed to directly compare MRI and MR arthrography using both 2D and 3D sequences to determine which imaging technique and which sequence are best suited for evaluating the articular cartilage of the hip joint in clinical practice. Conclusions MRI and MR arthrography are the most commonly used imaging techniques for evaluating the articular cartilage of the knee and hip joints in clinical practice. Despite significant improvements in MRI technology over the past decade, a major limitation of currently available sequences is their inability to consistently detect superficial degenerative and posttraumatic cartilage lesions that may pro gress to more advanced osteoarthritis. The relatively low sensitivity of these sequences is mainly attributed to their suboptimal spatial resolution. However, additional factors, such as partial volume averaging, suboptimal tissue contrast, and inability to evaluate articular cartilage in multiple planes, also play important roles in limiting the sensitivity. In the future, the use of highfield-strength scanners, multichannel coils, and more SNR-efficient sequences may allow images of the knee and hip joint to be obtained with even higher spatial resolution and greater tissue contrast in clinically feasible scanning times. These sequences may further improve clinical cartilage imaging and will allow better identification of patients with knee and hip pain who may benefit from early medical and surgical intervention. References 1. [No authors listed]. 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