The BOLD sensitivity of rapid steady-state sequences

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 sequence