Functional MRI Using Spin Lock Editing Preparation Pulses

Abstract

A novel approach for detecting blood oxygenation level-dependent (BOLD) signals in the brain is investigated using spin locking (SL) pulses to selectively edit the effects of extravascular diffusion in field gradients from different sized vascular structures. We show that BOLD effects from diffusion amongst susceptibility gradients will contribute significantly not only to transverse relaxation rates (R2* and R2) but also to R1ρ, the rate of longitudinal relaxation in the rotating frame. Similar to the ability of 180-degree pulses to refocus static dephasing effects in a spin echo, moderately strong SL pulses can also reduce contributions of diffusion in large-scale gradients and the choice of SL amplitude can be used to selectively emphasize smaller scale inhomogeneities (such as microvasculature) and to drastically reduce the influence of larger structures (such as veins). Moreover, measurements over a range of locking fields can be used to derive estimates of the spatial scales of intrinsic gradients. The method was used to detect BOLD activation in human visual cortex. Eight healthy young adults were imaged at 3T using a single-slice, SL-prepped turbo spin echo (TSE) sequence with spin-lock amplitudes ω1=80Hz and 400Hz, along with conventional T2*-weighted and T2-prepped sequences. The BOLD signal varied from 1.1±0.4 % (ω1=80Hz) to 0.7±0.2 % (at 400Hz), whereas the T2-weighted sequence measured 1.3±0.3 % and the T2* sequence measured 1.9±0.3 %. This new R1ρ functional contrast can be made selectively sensitive to intrinsic gradients of different spatial scales, thereby increasing the spatial specificity of the evoked response.