Pseudoaneurysm Versus Chronic Expanding Hematoma on MRI: Hematoma-like Lesions with Distinct Therapeutic Strategies

<b>Background/Objectives:</b> Pseudoaneurysm and chronic expanding hematoma (CEH) are uncommon soft tissue lesions that can mimic hematoma or hemorrhagic tumors on magnetic resonance imaging (MRI). Because treatment strategies differ, accurate differentiation is important. This study aimed to compare MRI characteristics of pseudoaneurysm and CEH and identify distinguishing imaging features. <b>Methods:</b> We retrospectively reviewed 12 patients diagnosed between June 2010 and June 2023 with pseudoaneurysm (n = 6) or CEH (n = 6). Patient demographics, lesion depth, and size were compared. MRI features were evaluated for morphology, internal characteristics, pulsatile artifact, and involvement of adjacent structures. <b>Results:</b> Pseudoaneurysms were consistently located in the muscle layer, whereas CEHs were predominantly found in the subcutaneous fat layer (83.3%, <i>p</i> = 0.015). CEHs were significantly larger than pseudoaneurysms (13.5 ± 3.9 cm vs. 6.1 ± 3.3 cm, <i>p</i> = 0.005). Pseudoaneurysm more frequently exhibited ovoid morphology (100%), central flow void on T1WI and T2WI (100%), inner peripheral high SI on T1WI (83.3%), and neurovascular bundle involvement (100%) (all <i>p</i> < 0.05), while CEHs demonstrated multilobular morphology (100%) and internal septations (83.3%) (<i>p</i> < 0.05). <b>Conclusions:</b> Lesion location, size, morphology, central flow void, inner peripheral high T1 signal, septation, and neurovascular involvement enables reliable MRI differentiation between pseudoaneurysm and CEH, guiding accurate diagnosis and guiding appropriate management.