The BOLD sensitivity of rapid steady‐state sequences

Abstract

Purpose Relaxation and dephasing of water protons embedded in a vascular network is driven by local magnetic field inhomogeneities around deoxygenated blood vessels. These effects strongly depend on the relation between mean diffusion length and diameter of blood vessels, as well as on the chosen imaging sequence. In this work, the BOLD sensitivity of steady‐state sequences as a function of vessel size, field strength, and sequence parameters are analyzed. Methods Steady‐state magnetization within a network of artificial cylinders is simulated with Monte Carlo methods for different coherence pathways. In addition, measurements on microspheres were performed to confirm theoretical results. Results Simulations and phantom results demonstrate a vessel size‐dependent signal attenuation effect of all coherence pathways. Both the FID and ECHO pathways show a signal profile similar to spin echo sequences where in the static dephasing regime the effect of larger vessels is suppressed. Conclusion The BOLD effect measured in steady‐state sequences is most sensitive to microvessels and might therefore be closer to the underlying neuronal event compared to gradient echo sequences.

Layer-dependent multiplicative effects of spatial attention on contrast responses in human early visual cortex

“…The laminar profile of bSSFP fMRI signals ( Figure 6A) showed weaker bias toward the cortical surface compared to the response profile of GE BOLD signals ( Figure 4A, middle panel). This finding is consistent with previous layer-fMRI studies using SE sequences (Zhao, Wang et al 2006, Olman, Harel et al 2012, De Martino, Zimmermann et al 2013, thus supporting the theory and simulation that bSSFP fMRI signal at short TR primarily detects diffusion activity around small vessels (Scheffler, Heule et al 2018). Passband bSSFP fMRI signals also helped to reveal the neural effect of attention modulation, suggesting a closer relationship of T2 BOLD with underlying neural activity.…”

“…Attended Unattended ultrahigh field are strongly contributed by extravascular macrovessel activity, such as large draining veins (Kim 2018); while bSSFP fMRI at short TR mainly captures diffusion activity around small vessels in the gray matter (Lee, Dumoulin et al 2008, Miller 2012, Scheffler, Heule et al 2018). Thus we hypothesize that the discrepancy from the two pulse sequences might be related to the difference of attention modulation on the responses from large draining veins and microvessel activity from gray matter.…”

“…Due to extremely short readout time and rapid spin refocusing, bSSFP image has minimal distortion or blur. At short TR, the rapid refocusing mechanism of balanced SSFP suppresses large scale off-resonance effect, and is more sensitive to diffusion activity around small vessels (Lee, Dumoulin et al 2008, Miller 2012, Scheffler, Heule et al 2018). Thus the contrast mechanism of rapid bSSFP is similar to T2 weighted spin echo BOLD, and may provide high spatial specificity for lamina-resolution fMRI.…”

Intravascular BOLD signal characterization of balanced SSFP experiments in human blood at high to ultrahigh fields

“…All BOLDrelated signal differences were calculated in units of M 0, and therefore did not consider the increase of M 0 with the power of 1.5-1.7 as a function of B 0 . 34,35 The relative extravascular and intravascular bSSFP BOLD contributions of microvasculature and macrovasculature were analyzed for optimal flip angles according to Figure 4 in Scheffler et al 13 (ie, 30° at 3 T and 20° at 9.4 T/14.1 T).…”

Validating layer-specific VASO across species

“…While an extensive range of techniques exists (e.g. GRASE (Feinberg et al 2008;Oshio and Feinberg 1991), ASL (Ivanov et al 2016), SSFP (Goa et al 2014;Scheffler et al 2018), functional diffusion (Truong and Song 2009), phase-sensitive fMRI (Menon 2002; Vu and Gallant 2015) etc); one of the fastest growing approaches for layer-based fMRI utilises slice-saturation slab-inversion vascular space occupancy (SS-SI VASO) (Huber et al 2014b). The VASO contrast exploits the difference between longitudinal relaxation times (T 1 ) of tissue and blood (Lu et al 2003).…”