Simultaneous spin-echo and gradient-echo BOLD measurements by dynamic MRS

This study aimed to dissociate the intravascular and extravascular contributions to spin-echo (SE) and gradient-echo (GE) blood oxygenation level-dependent (BOLD) signals at 7 T, using dynamic diffusion-weighted MRS. We simultaneously acquired SE and GE data using a point-resolved spectroscopy sequence with diffusion weightings of 0, 600, and 1200 s/mm² . The BOLD signals were quantified by fitting the free induction decays starting from the SE center to a mono-exponential decay function. Without diffusion weighting, BOLD signals measured with SE and GE increased by 1.6 ± 0.5% (TE_SE  = 40 ms) and 5.2 ± 1.4% (nominal TE_GE  = 40 ms) during stimulation, respectively. With diffusion weighting, the BOLD increase during stimulation measured with SE decreased from 1.6 ± 0.5% to 1.3 ± 0.4% (P < 0.001), whereas that measured by GE was unaffected (P > 0.05); the post-stimulation undershoots in the BOLD signal time courses were largely preserved in both SE and GE measurements. These results demonstrated the feasiblity of simultaneous SE and GE measurements of BOLD signals with and without interleaved diffusion weighting. The results also indicated a predominant extravascular contribution to the BOLD signal time courses, including post-stimulation undershoots in both SE and GE measurements at 7 T.