Psoas muscle landmark as a reliable tool for vertebral numbering on lumbar magnetic resonance imaging: a retrospective study using whole-spine magnetic resonance imaging

Background Accurate vertebral numbering on lumbar MRI is complicated by transitional vertebrae, risking wrong-level surgery when whole-spine imaging is unavailable. This study aimed to identify the most reliable anatomical landmark for vertebral numbering in lumbar MRI-only settings by comparing four references (last rib, upper psoas muscle, iliolumbar ligament, and first fused vertebra) across normal and transitional anatomies. Methods This retrospective study analyzed whole-spine MRI scans from 444 asymptomatic adults undergoing health screening. For each subject, the vertebral levels of the four landmarks were recorded on whole-spine MRI, and the most frequently observed vertebral level for each was defined as its standard reference position. These positions were then retrospectively applied to the lumbar-only portion of the same dataset, with cervical and thoracic images intentionally excluded, to assess their reliability in a simulated lumbar MRI-only scenario. Results The upper end of the psoas muscle exhibited the highest correct identification rate (95.2%), followed by the last rib (93.5%), iliolumbar ligament (92.6%), and first fused vertebra (90.4%). The non-overlapping 95% CIs indicated a significant difference only between the psoas and the first fused vertebra; no significant differences were observed among the other three landmarks due to overlapping CIs. Conclusions The upper psoas muscle is a reliable landmark for lumbar MRI vertebral numbering, showing comparable accuracy to the last rib and iliolumbar ligament. It can serve as a practical tool to support accurate numbering in routine clinical practice.