Sacral insufficiency fracture is more common than stress fracture. It mostly occurs in middle-aged and elderly patients with weakened bone due to e.g., osteoporosis or radiotherapy. The changes by MRI usually consist of pronounced osseous edema, often bilateral with vertical fracture lines, best visualized on T1-weighted images. There may be concomitant transverse sacral fracture, best visualized on sagittal images as showed below; doi:10.1186/S12905-022-01758-2. In doubtful cases, CT can confirm the presence of fracture lines.
MRI in a woman previously receiving radiotherapy due to malignancy, semi-coronal STIR and T1 image with supplementary CT reconstructions. There is pronounced edema in the sacrum on the STIR image with dark intraosseous streaks on the T1-weighted image compatible with insufficiency fractures (arrows). Supplementary CT (images on the right and at the bottom) confirmed the sacral fracture lines (arrows).