ARRS 2022 Abstracts

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E1444. Fluid in the Fossa on Venous Ultrasound: It’s Not Always a Popliteal Cyst!
Authors
  1. Neema Patel; Mayo Clinic
  2. Madhura Desai; Mayo Clinic
Background
Venous ultrasound (US) is the study of choice when deep venous thrombus (DVT) is suspected in patients with unilateral leg pain and swelling. Popliteal cysts (also known as Baker cysts) can present similarly, particularly in the event of acute rupture. Popliteal cysts, while not true cysts, are often regularly encountered, with a prevalence of 40% on US for DVT, and can be confidently diagnosed in most cases. However, imagers should be aware of atypical imaging features and pathology that can arise in the popliteal fossa and mimic simple or complicated popliteal cysts.

Educational Goals / Teaching Points
The goals of this exhibit are as follows. Identify the classic imaging features of a popliteal cyst as well as features seen with common complications (acute rupture, hemorrhage, and infection). Review benign and malignant lesions that mimic popliteal cysts. These include other cystic lesions that can arise in the popliteal fossa (cystic or necrotic neoplasms, vascular abnormalities, and periarticular collections) and solid lesions that frequently simulate cysts on ultrasound (nerve sheath tumors, lymphadenopathy, myxoid neoplasms, and synovial neoplasms). Discuss how to approach and manage indeterminate lesions that may require further evaluation with MRI or tissue sampling (biopsy).

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Classically, US imaging of a popliteal cyst demonstrates an anechoic cyst with an imperceptible wall, posterior acoustic enhancement, and absent color Doppler flow. The cyst is shaped like a speech bubble, with a thin neck extending between the semimembranosus tendon and medial head of the gastrocnemius tendon. At times, atypical imaging features within a popliteal cyst may result in difficult diagnosis. There are key sonographic features that should raise suspicion for a popliteal cyst mimic and warrant further investigation (often with contrast-enhanced MRI). These include lack of a “communicating” neck, presence of internal vascularity on color flow Doppler US, solid components, associated calcification, or poor sonographic visualization due to size or suboptimal penetration.

Conclusion
Abdominal radiologists will frequently encounter incidental or symptomatic popliteal cysts on venous US exams. As such, they should identify imaging features that can allow for confident diagnosis of popliteal cysts while being mindful of benign and malignant popliteal cyst mimics (including those of musculoskeletal and vascular etiologies) that may confound diagnosis.