Susceptibility-weighted MR imaging of radiation therapy-induced cerebral microbleeds in patients with glioma: a comparison between 3T and 7T

Cerebral microbleeds have been observed in normal-appearing brain tissue of patients with glioma years after receiving radiation therapy. The contrast of these paramagnetic lesions varies with field strength due to differences in the effects of susceptibility. The purpose of this study was to compare 3T and 7T MRI as platforms for detecting cerebral microbleeds in patients treated with radiotherapy using susceptibility-weighted imaging (SWI). SWI was performed with both 3T and 7T MR scanners on ten patients with glioma who had received prior radiotherapy. Imaging sequences were optimized to obtain data within a clinically acceptable scan time. Both T2*-weighted magnitude images and SWI data were reconstructed, minimum intensity projection was implemented, and microbleeds were manually identified. The number of microbleeds was counted and compared among datasets. Significantly more microbleeds were identified on SWI than magnitude images at both 7T (p = 0.002) and 3T (p = 0.023). Seven-tesla SWI detected significantly more microbleeds than 3T SWI for seven out of ten patients who had tumors located remote from deep brain regions (p = 0.016), but when the additional three patients with more inferior tumors were included, the difference was not significant. SWI is more sensitive for detecting microbleeds than magnitude images at both 3T and 7T. For areas without heightened susceptibility artifacts, 7T SWI is more sensitive to detecting radiation therapy-induced microbleeds than 3T SWI. Tumor location should be considered in conjunction with field strength when selecting the most appropriate strategy for imaging microbleeds.