fMRI at 1.5, 3 and 7 T: Characterising BOLD signal changes

Blood oxygenation level dependent (BOLD) signal changes occurring during execution of a simple motor task were measured at field strengths of 1.5, 3 and 7 T using multi-slice, single-shot, gradient echo EPI at a resolution of 1×1×3 mm3, to quantify the benefits offered by ultra-high magnetic field for functional MRI. Using four different echo times at each field strength allowed quantification of the relaxation rate, R2⁎ and the change in relaxation rate on activation, ΔR2⁎. This work adds to previous studies of the field strength dependence of BOLD signal characteristics, through its: (i) focus on motor rather than visual cortex; (ii) use of single-shot, multi-slice, gradient echo EPI for data acquisition; (iii) co-registration of images acquired at different field strengths to allow assessment of the BOLD signal changes in the same region at each field strength. ΔR2⁎ was found to increase linearly with field strength (0.51±0.06 s−1 at 1.5 T; 0.98±0.08 s−1 at 3 T; 2.55±0.22 s−1 at 7 T), while the ratio of ΔR2⁎/R2, which dictates the accessible BOLD contrast was also found to increase (0.042±0.002 at 1.5 T; 0.054±0.002 at 3 T; 0.084±0.003 at 7 T). The number of pixels classified as active, the t-value calculated over a common region of interest and the percentage signal change in the same region were all found to peak at TE∼T2⁎ and increase significantly with field strength. An earlier onset of the haemodynamic response at higher field provides some evidence for a reduced venous contribution to the BOLD signal at 7 T.