Intravascular BOLD signal characterization of bSSFP experiments in human blood at high to ultra-high fields
To fully understand the neurovascular bSSFP fingerprint observed in BOLD experiments, extravascular and intravascular contributions have to be identified separately. The extravascular BOLD component has been analyzed extensively dependent on vessel size, static magnetic field strength, and sequence parameters such as repetition time or flip angle by means of Monte Carlo methods. The intravascular contribution, on the other hand, has not yet been entirely explored since it requires knowledge about the apparent longitudinal (R1) and transverse (R2) relaxation rates of blood. We investigated the intravascular bSSFP BOLD signal changes in human blood samples, which are prepared at different oxygenation levels and measured at high to ultra-high fields (3 T, 9.4 T, and 14.1 T). The oxygen sensitivity of bSSFP depending on TR and flip angle is quantified by intrinsic R2 estimation from a series of phase-cycled bSSFP scans using motion-insensitive rapid configuration relaxometry (MIRACLE) as well as direct calculation of passband bSSFP signal differences between blood probes with oxygenation levels reflecting activated and resting state. In addition, R1 and R2 values are derived based on standard spin-echo techniques; i.e. inversion recovery spin-echo (IR-SE) with variable inversion times for R1 estimation and single-echo SE with variable echo times for R2 estimation. Results from Monte Carlo simulations are used to quantify the extravascular BOLD contribution of bSSFP and SE. The relative intravascular and extravascular contributions to the BOLD effect are derived separately for micro- and macrovascular regimes for both, bSSFP and SE, versus field strength and sequence parameters.