Subcutaneous epidermal inclusion cysts: Ultrasound (US) and MR imaging findings

To describe the characteristic US and MR findings of subcutaneous epidermal inclusion cysts.

Seventy-nine patients with subcutaneous epidermal inclusion cysts underwent US (n = 70), MR (n = 7), or both (n = 2). On US, the margin, shape, echogenicity, through-transmission, wall, internal debris and vascularity were evaluated. On MR, the shape, wall, signal intensity, internal debris, and enhancement pattern were evaluated.

On US, characteristic findings were well circumscribed (n = 69, 96%), ovoid-shaped (n = 56, 78%), heterogeneously and mildly echogenic (n = 66, 92%), increased through-transmission (n = 66, 92%) and low echoic rim (n = 48, 67%). Internal debris was seen in 31 cases (43%) and often contained linear echogenic reflections (n = 12, 17%), dark clefts (n = 13, 18%), or a mixture (n = 5, 7%). Most masses showed no Doppler flow (n = 70, 97%). On MR, all cases demonstrated a well-demarcated oval-shaped mass with a surrounding rim. On T1-weighted image (WI), the mass showed slightly high T1 signal in 4/9 (44%) and iso-signal in 5/9 (56%). On T2WI, the mass showed high signal in 6/9 (67%), intermediate in 2/9 (22%), and a target appearance in 1/9 (11%). Internal linear dark T2 signal debris was observed in 4/9 (44%). All lesions showed peripheral rim enhancement without central enhancement.

On US, subcutaneous epidermal inclusion cysts are usually well-circumscribed, oval-shaped, mildly echogenic masses with occasional linear anechoic and/or echogenic reflections, increased through-transmission, hypoechoic rim and no Doppler flow. On MR, an intermediate to high T2 signal mass with occasional low signal debris and no central enhancement can strengthen the diagnosis.