Common and unusual diseases involving the iliopsoas muscle compartment: spectrum of cross-sectional imaging findings

Although relatively uncommon, many different infectious, hemorrhagic and neoplastic disease processes may involve the iliac and psoas muscles and are increasingly diagnosed especially in referral hospitals. Furthermore, the iliopsoas compartment may become injured during trauma, percutaneous instrumentation, laparoscopic or open surgical procedures. State-of-the-art cross-sectional imaging including volumetric multidetector CT and multiplanar MRI acquisitions allows prompt detection, comprehensive visualization and confident characterization of most iliopsoas lesions, and the possibility to guide percutaneous biopsy and drainage. The pertinent regional anatomy is reviewed in correlation with disease pathways and imaging modalities. Neoplastic lesions, purulent and mycobacterial iliopsoas infections are discussed with examples. Imaging plays the key role in the differentiation of primary versus secondary abscesses due to intestinal, urinary and musculoskeletal infections, that determines medical therapy and surgical need. The iliopsoas compartment may become involved through direct extension by retroperitoneal, skeletal and pelvic tumors, and should be carefully scrutinized when reviewing oncologic imaging studies since it represents one of the preferred sites of skeletal muscle metastatization. Iliopsoas hemorrhages due to trauma, aortic aneurysms and anticoagulation are reviewed, with a special focus on determining whether the bleeding comes from aneurysmal rupture or from coagulopathy, a critical differentiation to decide about medical or surgical treatment. Postoperative complications involving the iliopsoas compartment are discussed with examples, including retroperitoneal bleeding, infections, urinary leaks and collections following various surgical or instrumentation procedures. Emphasis is placed on choosing the correct imaging modality and technique, particularly to detect active bleeding or urine leakage, and to reduce artifacts related to presence of metallic implants.