Lipoid Pneumonia: HRCT and MRI Spectrum, Diagnostic Pitfalls, and Imaging-Based Diagnostic Workflow

Background/Objectives: Lipoid pneumonia (LP) is a rare and frequently underdiagnosed pulmonary condition with a broad spectrum of radiological manifestations that can closely mimic infectious, inflammatory, and neoplastic lung diseases. Despite its clinical relevance, no standardized imaging-based diagnostic pathway exists. For this reason, this pictorial narrative review aims to provide a structured, imaging-centred synthesis of LP, to characterise the full spectrum of high-resolution CT (HRCT) and magnetic resonance imaging (MRI) findings, and to propose a pragmatic diagnostic workflow. Methods: A systematic literature search was performed in PubMed, MEDLINE, Embase, and the Cochrane Library from January 1950 to February 2025. Search terms combined "lipoid pneumonia" with imaging-related keywords including "HRCT," "computed tomography," "MRI," and "fat attenuation." After screening 891 deduplicated records, 60 studies were included in the narrative synthesis. Eight illustrative institutional cases with imaging-pathology correlation were additionally selected to demonstrate key imaging phenotypes. Results: HRCT is the cornerstone modality, demonstrating intralesional fat attenuation (typically -30 to -150 HU) in 40-80% of cases depending on series and disease chronicity. Additional patterns include ground-glass opacity, crazy paving, centrilobular nodules, and mass-like consolidation mimicking malignancy. Fat attenuation is absent in up to 60% of cases when inflammatory exudate or fibrosis masks lipid content. MRI, particularly chemical shift imaging, serves as a problem-solving adjunct in pseudotumoral or densitometrically equivocal presentations. A pragmatic diagnostic workflow is proposed, integrating HRCT findings, exposure history, fat-sensitive MRI in selected cases, BAL cytology, and histopathological confirmation when required. Conclusions: A pattern-based radiological approach, anchored on HRCT and integrated with clinical exposure history, BAL cytology, and selective use of fat-sensitive MRI, enables accurate diagnosis of LP in most cases and can prevent unnecessary invasive procedures including surgical resection performed under suspicion of malignancy.