Sonographic detection of Baker's cysts: comparison with MR imaging

The objective of this study was to assess the ability of sonography to reveal Baker's cysts using MR imaging as a gold standard.

The study group consisted of 36 consecutive knees in 36 patients evaluated with both MR imaging and sonography. Inclusion criteria included axial proton density-weighted or T2-weighted MR images, a sonography report that documented the evaluation of the popliteal region of the knee, and sonographic and MR images that were available for review. The MR images were retrospectively reviewed for the presence of Baker's cyst (fluid signal between the semimembranosus and medial gastrocnemius tendons). Sonography reports were compared with the MR imaging results. The sonographic images were also retrospectively reviewed to determine whether any characteristic findings on sonography were significantly associated with the presence of Baker's cyst on MR imaging.

Retrospective review of MR images revealed 21 Baker's cysts, one myxoid liposarcoma, one meniscal cyst, and 13 examinations with normal findings. The sonography reports revealed that the 21 Baker's cysts were correctly diagnosed, whereas the meniscal cyst and myxoid liposarcoma were misdiagnosed as Baker's cysts. Retrospective review of sonographic images showed a 100% sensitivity, specificity, positive predictive value, negative predictive value, and accuracy in the diagnosis of Baker's cyst when hypoechoic or anechoic fluid was present between the semimembranosus and medial gastrocnemius tendons. No other sonographic characteristics were significant.

Identification of fluid between the semimembranosus and medial gastrocnemius tendons in communication with a posterior knee cyst indicates Baker's cyst with 100% accuracy.