The BOLD Sensitivity of Rapid Steady State Sequences
The BOLD sensitivity of different sequences to the underlying neurovascular vessel size is a key feature to characterize the measured BOLD signal changes. The vessel size dependence of the BOLD effect is often divided into two regimes: the diffusion narrowing regime for small vessel sizes of up to about 5-10 mm radius, and the static dephasing regime for larger vessels. In the static dephasing regime, dephasing effects around vessels are almost completely refocused for SE, but remain for GE and thus making them sensitive to larger vessels. Therefore, it is generally accepted that SE is more sensitive to oxygenation changes located within micro vessels and thus might be closer to the neuronal event compared to GE. Steady state sequences such as balanced SSFP (bSSFP) and non-balanced (gradient-spoiled) SSFP such as S2-SSFP generate echoes that are composed of contributions from different coherent pathways or gradient and spin echo configurations. Each configuration exhibits a different sensitivity to oxygenation changes and additionally might show different sensitivity to the vessel size. Until now, a throughout investigation of the vessel sensitivity of SSFP pathways has not been reported, except for the PSIF path and for bSSFP. We present an analysis of the BOLD sensitivity of non-balanced and balanced SSFP across field strength as a function of vessel size for several different coherence pathways, including S1-SSFP, the S2-echo, the balanced echo, and higher order coherences. All calculations are based on Monte Carlo simulations of randomly oriented cylinders across different vessels diameters, repetition times, flip angles, and field strength.