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Background
The pelvis is a compact space containing important organs of the genitourinary (bladder, urethra and prostate), gynecologic (uterus, cervix and vagina) and gastrointestinal (rectum and anus) systems, not to forget the supporting structures built up of a network of nerves, vasculature, lymphatics, as well as the ligaments and muscles that hold this anatomy together. This anatomy can be daunting to understand and imaging interpretation is potentially challenging. Pelvic masses can often be hard to figure out, since there is a plethora of complex masses that can reside within the pelvis with seemingly similar appearances. MRI is often more useful than Ultrasound and CT due to its intrinsic superior contrast detail and multi planar capability, which is why we will be focusing on this modality. Radiologists need to be cognizant of these differential diagnoses and MR imaging findings to effectively narrow the differential diagnosis and provide a potential roadmap for management. This exhibit targets the uncommon intraperitoneal and extraperitoneal pelvic conditions with various rare benign and malignant case-based images to highlight the pertinent imaging findings in each condition.

Educational Goals / Teaching Points
1. Exploring the utility of MRI in diagnosing complex pelvic masses. 2. Breaking down the intricate anatomy of the pelvis to aid in approaching pelvic masses through a compartment based approach. 3. Classifying common and uncommon pelvic tumors with special attention to rare pelvic masses to acquaint the reader with emphasis on the MR imaging appearances of these pathologies in a case based format.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
MR IMAGING TECHNIQUE: high resolution 3 plane T2, pre T1 axial - with fat sat, post T1 3 plane – with fat sat & DWI ORGANIZATIONAL ANATOMY: PELVIS —> Peritoneal Cavity (peritoneal space) & Subperitoneal Space SUBPERITONEAL SPACE —> INTRAPERITONEAL (small bowel, sigmoid colon, ovaries) & EXTRAPERITONEAL Space EXTRAPERITONEAL SPACE —> Anterior, Middle (in females) & Posterior Compartments + Extraperitoneal Mesenchymal Tissue (fat, muscle, nerve, lymphatics, vessels) (1) Anterior Compartment: bladder, urethra (& prostate, in males) Middle Compartment: uterus, cervix, vagina Posterior Compartment: rectum, anus CLASSIFICATION OF PATHOLOGIES: A. Peritoneal space B. Intraperitoneal (Ovaries and fallopian tube) C. Intraperitoneal (GI) D. Round and uterosacral ligaments E. Mesorectal space F. Retrorectal and presacral space G. Extraperitoneal Space Mesenchyme A few entities in each subgroup will be discussed with heavy pictorial emphasis.

Conclusion
A varied spectrum of pathologies can occur in the pelvis. Understanding the anatomy and localizing the mass to a particular space, followed by characterizing the imaging features unique to each mass is crucial to generate a narrow list of differentials, which can influence the next step in management - biopsy and the subsequent choice of treatment. The radiologist would benefit from having a list of ready differential diagnoses and by being aware of their imaging appearances on MRI to be sufficiently armed when confronted with these often confounding conundrums.