The presence of joint fluid is easily recognized by sonography, but may require optimal patient positioning . A joint effusion can present on ultrasound as anechoic fluid or as hypoechoic fluid, possibly containing debris (Fig. 13). Pan-nus or thickened synovium appears more hypoechoic on sonography than the adjacent soft tissues . Power Doppler can also demonstrate hyperemia in acutely inflamed synovium, which is helpful in distinguishing an acute from a chronic process. Sonography has the added value in providing guidance for joint aspiration and synovial biopsy; however, MRI provides a better thorough evaluation of effusion and synovitis when performed without and then after intravenous injection with gadolinium contrast. MRI also allows better evaluation of intra-articular erosions that can be assessed in only a limited manner using ultrasound. For this reason, the authors prefer MRI of the knee when synovitis is suspected.
On sonography, articular cartilage has a homogeneous hypoechoic appearance, a smooth contour, and a parallel echogenic interface with bone . Unfortunately, in knee imaging, not all the articular surfaces can be evaluated. Therefore, MRI is the best imaging modality for cartilage assessment.
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