Axial T1WI with fat suppression and contrast showing enhancement in the periphery of the thecal sac due to leptomeningeal spread.
This is commonly seen in the lumbosacral spine when cancer cells, directed by gravity, cling to and coat the nerve roots and the cauda equina (Figures 11-5, 11-6, and 11-7). Blastic tumors appear hypoin-tense on T1- and T2-weighted images (T1WI and T2WI). Extensively involved vertebral bodies may seem darker than the adjacent discs on T1WI. Hypointense sclerotic bone metastases may appear somewhat brighter then the normal bone following contrast administration, especially when a fat suppression sequence is utilized.
The signal characteristics of lytic metastases are similar to those of blastic metastases on T1WI: hypointense compared with unin-volved, normal bone marrow. Following contrast administration, the involved vertebral segments will enhance usually and will become more conspicuous when fat suppression is applied (Figure 11-8). T2WI will show either hypointense or hyperintense segments or a combination of both; they have a heterogeneous appearance. Occasionally, areas of hyperintensity surrounding a region that is hypointense—the target sign—can be seen. Extensive infiltration may weaken the vertebral body and lead to vertebral collapse with retropulsed segments that may compromise the neural elements. This can be clearly captured on both sagittal and axial T1WI and T2WI (Figure 11-9).
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