About one in ten Australian patients with lumbar radiographs has a lumbosacral transitional vertebra. Radiologists report fewer than a third of them. On lumbar MRI without a whole-spine localiser, more than a third are read as normal segmentation, and among those recognised the numbering is wrong in more than half. Most wrong-level spine surgery occurs in patients with variant anatomy of this kind. In a minority of these patients the transitional segment causes persistent low back, buttock or hip pain that is treated for years as non-specific strain.

This guide sets out what a reporting radiologist needs to identify the anatomy and describe it in a form the referrer can act on. It covers the Castellvi and Jenkins classifications and the measurements to record on CT, vertebral enumeration and the methods with published support, the findings on radiographs, CT, MRI and scintigraphy that bear on whether the segment is symptomatic, the associated findings at the level above, a report structure and checklist, the diagnostic injection with its targets, technique and Medicare items, and the pitfalls that recur in the literature. Two further chapters set out what patients report about their imaging and what each referrer needs from the report.

Every clinical figure is drawn from the published evidence, and the notes at the end say what could and could not be verified.