E1544. Water on the Knee: The Magnetic Resonance Imaging Appearance of Common and Uncommon Fluid Collections Around the Knee
  1. Jens Verhey; Mayo Clinic Alix School of Medicine
  2. Christian Rosenow; Mayo Clinic Alix School of Medicine
  3. Erik Verhey; University of Notre Dame, College of Science
  4. Jeremiah Long; Mayo Clinic Department of Musculoskeletal Radiology
Magnetic resonance imaging (MRI) of the knee is a common examination in clinical practice. Familiarity with various fluid collections around the knee is vital for the practicing radiologist. The purpose of this educational exhibit is to review the MRI appearance of common and uncommon fluid collections around the knee.

Educational Goals / Teaching Points
After reviewing this presentation, the learner will be able to identify the MRI appearance of several bursae around the knee, recognize parameniscal cysts and their relationship to meniscal pathology, and describe the MRI characteristics of ganglia and Morel-Lavellée lesions when encountered at the knee.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Focal fluid collections around the knee are most often benign findings related to bursae or cysts. These collections are either present under normal conditions or are the result of inflammation or mechanical stress. Several bursae exist around the knee to reduce friction between moving structures, most notably the prepatellar and medial bursae. Distinct from bursae, common types of cysts around the knee include the popliteal cyst, parameniscal cysts, and ganglion cysts. As some neoplastic processes, vascular structures, hematomas, and abscesses may also present as T2 hyperintense lesions at the knee, it is particularly important to be familiar with the normal bursae and common fluid collections for accurate diagnosis. The MRI appearance of the following entities will be presented in case-based format: commonly inflamed bursae around the knee, popliteal fossa cysts, parameniscal cysts, ganglion cysts and Morel-Lavellée lesions. When relevant, the typical clinical presentation and treatment options will be included.

Fluid collections are often encountered as both a primary and incidental finding on knee MRI. Properly identifying these fluid collections is important for radiologists to maximize the diagnostic capabilities of knee MRI.