Sonography is useful for the detection of ankle joint effusions. Intra-articular fluid is demonstrated as anechoic or hypoechoic distension of the superior and inferior compartments of the anterior recess (Fig. 28) . As the size of the effusion increases, the intracapsular fat pad is displaced anteriorly . Sonography can reproducibly detect intra-articular fluid volumes of 2 mL, whereas MRI can detect 1 mL of fluid . It is unclear if the greater sensitivity of MRI is clinically advantageous. An advantage of sonography is that it can provide guidance for aspiration of ankle fluid.
Sonography can aid in the diagnosis of synovitis, which is demonstrated as hypoechoic thickening of the synovium of the anterior joint capsule . Increased flow on power Doppler imaging can be demonstrated with synovitis, and can be helpful distinguishing between synovitis and joint effusion, a distinction that requires intravenous gadolinium during MRI. Synovitis may present without increase in Doppler flow, however . At this time, MRI with
intravenous gadolinium contrast should be considered the best imaging modality of choice for evaluating the patient who has synovitis.
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