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University of California San Francisco San Francisco, CA
94117
University of Kiel 24105 Kiel, Germany
The article by Sonin et al. [1] in the November 2002 issue of the American Journal of Roentgenology described fast spin-echo proton density–weighted MR imaging without fat suppression for the evaluation of the articular cartilage of the knee using arthroscopy as a gold standard.
We concur with the authors that proton density–weighted MR imaging sequences are well-suited for the visualization of hyaline cartilage. Additionally, internal structures of the knee joint such as synovium, menisci, ligaments, and tendons can be excellently depicted with this technique [2–6]. We would like to comment on the advantages of additional fat suppression, which increases the value of proton density–weighted sequences for joint evaluation.
Two MR imaging techniques are used most commonly for visualizing the articular cartilage of the knee. T1-weighted fat-suppressed three-dimensional spoiled gradient-echo MR imaging has a relatively long imaging time and susceptibility to metallic artifacts. T2-weighted fast spin-echo MR imaging is susceptible to chemical shift artifacts (if performed without fat suppression) and the magic angle effect.
Although they are rarely investigated, proton density–weighted techniques in combination with fat saturation are highly water-sensitive and have proven to be equal or superior to other water-sensitive sequences for the detection of bone marrow edema or ligament injuries [7, 8]. The arthrographic effect of the joint fluid assists in the visualization of chondral surface irregularities, which may obviate MR arthrography (Fig. 3). Fat saturation furthermore increases the relevant dynamic range and improves the contrast between cartilage and joint fluid, bone, and fat. Our personal experience with a fat-saturated proton density–weighted sequence showed a superior performance compared with a three-dimensional spoiled gradient-echo sequence because of its water sensitivity—especially for low-grade lesions with cartilage softening or edema (Fig. 3).
In summary, the combination of fat saturation with proton density–weighted sequences is highly sensitive to cartilage and intramedullary osseous edematous alterations. Therefore, it has the potential to replace the combination of two sequences (e.g., three-dimensional spoiled gradient-echo and short tau inversion recovery) for the separate evaluation of cartilage and bone marrow changes, especially in trauma-related cases. We have found this technique to provide an accurate, time-saving, and cost-effective one-step investigation.
References
Radiology Imaging Associates Englewood, CO 80111
My colleagues and I are grateful for the interest shown by Mohr et al. in our recent article, "Grading Articular Cartilage of the Knee Using Fast Spin-echo Proton Density–Weighted MR Imaging Without Fat Suppression" [1]. The improved contrast-to-noise ratio and decreased chemical shift artifact afforded by fat suppressed fast spin-echo proton density–weighted MR imaging compared with similar fast spin-echo proton density–weighted sequences without fat suppression are well recognized and were alluded to in our article. As we stated in that work, the chemical shift artifact evident in spurious widening or narrowing of the cortical bone adjacent to the hyaline cartilage did not appear to affect our evaluation of the cartilage itself. Furthermore, because of limitations described by previous authors, our study did not focus on grade 1 cartilage lesions (i.e., those only visualized by signal changes in the cartilage on MR images), but rather on lesions of grade 2 or higher (those that constitute morphologic changes in the articular surface visible at arthroscopy).
At the time our study was performed, we felt that fast spin-echo proton density–weighted images obtained without fat suppression provided the best combination of attributes for our needs (visualization of menisci, hyaline cartilage, ligaments, and tendons). In my current practice, we use a combination of fat-suppressed and non–fat-suppressed fast spin-echo proton density–weighted sequences for knee MR imaging. We use the fat-suppressed version in the axial plane and the non–fat suppressed version in the sagittal and coronal planes, the latter for optimal evaluation of the menisci. We use a more heavily T2-weighted fat-suppressed image set in the coronal plane to assess the bone marrow.
An article by Rose et al. [2] reported use of a fat-suppressed fast spin-echo double-echo sequence in the evaluation of patellar cartilage. Those authors found that short-TE sequences with fat suppression provided the most satisfactory images. A similar technique using a more T2-weighted sequence is also described by Bredella et al. [3]. It is clear from the literature and communications with many colleagues that several effective techniques are available in the modern MR imaging armamentarium with which to evaluate articular cartilage. Yet none has proven so superior in terms of ease of use, clinical accuracy, and availability that it clearly renders the others obsolete; radiologists use whatever they find most appealing and practical for their needs.
Reliance on fat-suppressed images for the evaluation of articular cartilage raises issues with regard to the evaluation of other structures in the knee. Although some authorities have anecdotally advocated the use of fat-suppressed fast spin-echo proton density–weighted images for the evaluation of the menisci, I am aware of no published studies validating that technique. Regarding marrow imaging, the article by Lal et al. [4] cited by Mohr et al. compares fat-suppressed proton density–weighted MR imaging with unsaturated proton density–weighted and unsaturated T2-weighted fast spin-echo imaging for the evaluation of marrow abnormalities. It makes no comparison with fat-suppressed T2-weighted sequences for such abnormalities. Fat-suppressed proton density–weighted imaging is more sensitive to marrow abnormalities and soft-tissue edema than unsaturated images with similar parameters, but a more heavily T2-weighted sequence is preferred by most radiologists.
I agree with Mohr et al. that a greater dynamic range for cartilage visualization can be achieved by adding fat suppression to fast spin-echo proton density–weighted MR imaging. Such a technique could be useful, particularly in discerning subtle or low-grade cartilage lesions in which signal change in the cartilage is the primary imaging abnormality. I encourage the authors to publish their results using fat-suppressed fast spin-echo MR imaging for articular cartilage, thereby validating another useful cartilage imaging strategy in the existing body of knowledge.
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