Figure 5. A 74-year-old female on long-term oral corticosteroid therapy, with documented osteoporosis, with rapid onset of severe low back pain was referred for lumbar spine MRI. (a) Axial T₁ weighted MR image (repetition time 700/echo time 20) through sacrum shows an area of low signal in the right sacral ala (arrow) that demonstrated enhancement after intravenous gadolinium chelate (b). There were similar less marked appearances in the left sacral ala. (c) CT scan through the sacrum confirmed right sacral fracture (arrow). (d) Posterior view of isotope bone scan shows uptake in the sacral ala (solid arrows) and across sacral body (open arrow) with the classic "H-sign" of bilateral sacral insufficiency fracture. There was also increased uptake throughout much of the lumbar spine, correlating with marked degenerative changes on plain film.