Mri Of Articular Cartilage Technical Considerations

Historically, clinical evaluation of articular cartilage has primarily relied on two acquisition techniques: (1) three dimensional (3D), fat-suppressed, Tl-weighted, spoiled gradient echo; and (2) 2D proton-density (PD)-weighted, fast-spin-echo (FSE) techniques. Each has relative advantages and disadvantages with respect to evaluation of articular cartilage and diagnosis of osteochondral injuries.

Initial MRI investigations of focal cartilage lesions used 3D, Tl-weighted, gradient-echo acquisitions to identify focal defects. As illustrated in Fig. 1, this technique provides high-resolution images with excellent differentiation of cartilage and underlying subchondral bone. The major advantage of this technique is high spatial resolution, which is particularly important in evaluation of small joints or curved articular cartilage surfaces such as the talar dome, where thin sections are needed to clearly delineate cartilage interfaces and minimize volume averaging [15]. Using this technique at 1.5 Tesla (T), it is possible to obtain images with a 1.0 to 2.0 mm section thickness and in-plane resolution of 200 to 350 microns per pixel. For comparison, 2D FSE techniques are generally limited to 3 to 4 mm section thickness and 300 to 500 micron in-plane resolution. Because of high spatial resolution, 3D, T1-weighted, gradient-echo acquisitions are becoming valuable tools in clinical research applications to quantitatively determine cartilage volume, thickness, and surface area. These tissue measures are being explored as possible endpoints in assessment of new chondroprotective therapies [16].

Several disadvantages limit routine clinical application of gradient-echo techniques in larger joints where spatial resolution is less of a premium. A practical limitation of the 3D acquisition is the relatively long imaging times, ranging from 6 to 10 minutes, needed for coverage of large joints such as the knee. This is less of a problem in evaluation of the foot and ankle, where fewer imaging sections are needed to cover anatomy in the sagittal plane. A second limitation of the gradient-echo technique is relatively poor image contrast, particularly at the articular surface. Although the technique produces reliable high-contrast images of cartilage and subchondral bone, contrast at the articular surface can vary, depending on protein content or blood degradation products in the synovial fluid. This can lower sensitivity for detection of superficial fibrillation or fissures occurring with cartilage injury [17]. In addition, the T1-weighted technique is relatively insensitive to signal alterations within the cartilage or subchondral bone marrow that can be important indicators of os-teochondral injury.

In the knee the T1-weighted, fat-suppressed, gradient-echo technique has a diagnostic accuracy of 65% to 95% for detection of focal cartilage defects [18-21]. Diagnostic sensitivity has generally been shown to be substantially lower for superficial cartilage lesions confined to the outer 50% of cartilage. The ability to characterize the size of the lesion can be helpful for preoperative planning [22]. In validation studies of focal cartilage defects, MRI has been shown to underestimate the depth of the articular defect [23]. Size of the lesion has been

Mri Right Ankle Talar Dome

Fig. 1. Seventeen-year-old female 3 weeks following ankle injury with persistent ankle pain. An unstable osteochondral fracture was found at surgery: (A) Anteroposterior (AP) radiograph demonstrates osteochondral fracture of the lateral talar dome. (B) Coronal PD-weighted FSE MRI confirms osteochondral fracture with subchondral marrow edema (arrow). High signal intensity consistent with fluid is present in the demarcation zone, indicative of an unstable fragment (arrowhead). (C) Coronal 3D fat-suppressed, Tl-weighted spoiled gradient echo sequence provides better demonstration of overlying articular cartilage, but is less sensitive for detection of marrow edema and fluid in the demarcation zone.

Fig. 1. Seventeen-year-old female 3 weeks following ankle injury with persistent ankle pain. An unstable osteochondral fracture was found at surgery: (A) Anteroposterior (AP) radiograph demonstrates osteochondral fracture of the lateral talar dome. (B) Coronal PD-weighted FSE MRI confirms osteochondral fracture with subchondral marrow edema (arrow). High signal intensity consistent with fluid is present in the demarcation zone, indicative of an unstable fragment (arrowhead). (C) Coronal 3D fat-suppressed, Tl-weighted spoiled gradient echo sequence provides better demonstration of overlying articular cartilage, but is less sensitive for detection of marrow edema and fluid in the demarcation zone.

shown to be important in accuracy of MRI. In a recent ex vivo validation study [24], MRI was shown to be accurate in estimating the size for 5 mm cartilage lesions, but overestimated the size of 3 mm lesions.

There have been relatively few studies validating these techniques in the ankle [25]. Given the relatively thin cartilage and complex curved surface of the tibiotalar joint, it is anticipated that accuracy and reproducibility would be lower for this joint. The 3D, spoiled gradient-echo technique has demonstrated the greatest accuracy in evaluating cartilage thickness of the talar dome [25]. In comparison studies, multidetector CT arthrography had greater accuracy than 3D gradient-echo MRI in determining the depth of cartilage lesions of the ankle [26], and clinical diagnosis of cartilage pathology of the ankle [27]. Other studies evaluating diagnostic efficacy for osteochondral lesions of the talus have found equivalent efficacy of helical CT and MRI [28].

Routine clinical evaluation of articular cartilage, particularly in the knee, relies heavily on PD-weighted FSE images, either with or without fat suppression [29,30]. The primary advantage of this technique is excellent soft-tissue contrast with relatively modest image acquisition times of 3 to 4 minutes. As illustrated in Fig. 2, the fat-suppressed, PD-weighted FSE technique demonstrates heterogeneity of the cartilage signal resulting from regional and zonal differences in composition and structure of the extracellular cartilage matrix. This sensitivity to internal cartilage damage is particularly important for identifying injuries of the bone/cartilage tidemark zone that may not be associated with a visible cartilage surface defect, but can have long-term consequences for tissue integrity. Also, with the addition of chemical-shift fat suppression, the technique is sensitive to elevated T2-weighted signal in the subchondral bone marrow that is frequently associated with overlying cartilage injury [31]. The technique also provides clinically useful information regarding other articular tissues such as menisci and ligaments, making it particularly useful in the clinical setting where it is necessary to evaluate the entire joint. The primary disadvantage of the technique is lower spatial resolution. This is a particular problem in the ankle, where visualization of the thin cartilage covering the curved talar dome requires high spatial resolution.

Initial studies by Potter and colleagues [29] report an accuracy of 92% for diagnosis of focal cartilage lesions in the knee using the PD-weighted FSE technique. Similar accuracy has been identified in subsequent studies [30,32] for full-thickness defects, and partial-thickness defects involving greater than 50% cartilage thickness. As with gradient-echo techniques, sensitivity is generally less than 50% for diagnosis of superficial fibrillation and erosion.

New techniques based on the steady-state, free precession, gradient-cho sequences [33-36] and multi-echo T2*-weighted sequences [37] have been proposed for cartilage imaging. These techniques provide high-resolution images of cartilage, with image contrast similar to that obtained with FSE techniques. Although preliminary results are promising, these techniques are not widely available, and have undergone limited validation for routine clinical use.

Evaluation of osteochondral injury requires high-contrast resolution and places a premium on a high signal-to-noise ratio (SNR) of the image [38]. The SNR of the MR image increases with magnetic field strength. Although low-field open configuration or dedicated extremity magnets have demonstrated accuracy comparable with 1.5 T scanners in diagnosis of meniscal or ACL tears [39], accuracy in diagnosis of cartilage injury is substantially lower on low field scanners [40-43], particularly partial-thickness cartilage lesions [44]. Although clinical experience with 3.0 T in cartilage is limited, preliminary results suggests that the

Articular Cartilage

Fig. 2. Effect of collagen fiber orientation on cartilage signal intensity. (A) Coronal fat-suppressed, PD-weighted FSE image of the femoral tibial compartment. The signal intensity of articular cartilage decreases as a function of depth from the articular surface. In addition, signal intensity is higher near the periphery of the articular surface. This zonal and regional heterogeneity in the T2-weighted MRI signal is a result of differences in organization of the collagen matrix. (B) Magnified view of the lateral compartment with preferential collagen fiber orientation of tibial cartilage schematically illustrated in the tibia [57]. In the central portion of the tibia, the Type II collagen fibers are preferentially arranged perpendicular to the bone in the deep zone. This highly anisotropic orientation of collagen leads to low signal intensity of cartilage. Near the articular surface and near the periphery of the joint, the tangential alignment of fibers and lower degree of anisotropy result in higher signal intensity.

Fig. 2. Effect of collagen fiber orientation on cartilage signal intensity. (A) Coronal fat-suppressed, PD-weighted FSE image of the femoral tibial compartment. The signal intensity of articular cartilage decreases as a function of depth from the articular surface. In addition, signal intensity is higher near the periphery of the articular surface. This zonal and regional heterogeneity in the T2-weighted MRI signal is a result of differences in organization of the collagen matrix. (B) Magnified view of the lateral compartment with preferential collagen fiber orientation of tibial cartilage schematically illustrated in the tibia [57]. In the central portion of the tibia, the Type II collagen fibers are preferentially arranged perpendicular to the bone in the deep zone. This highly anisotropic orientation of collagen leads to low signal intensity of cartilage. Near the articular surface and near the periphery of the joint, the tangential alignment of fibers and lower degree of anisotropy result in higher signal intensity.

higher field strength provides greater diagnostic accuracy in detection of focal defects in an animal model [45,46], the knee [47], and ankle [48]. For quantitative determination of cartilage morphology, preliminary findings indicate that higher spatial resolution images available at 3.0 T improve reproducibility [49], but have not been shown to improve accuracy in defining the size of focal defects [50].

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  • petra
    What is mri with cartilage technology?
    8 years ago

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