BOLD sensitivity and vessel size specificity along CPMG and GRASE echo trains

Purpose To assess the vessel size specificity and sensitivity of rapid CPMG and GRASE for functional BOLD imaging for different echo train lengths, echo spacings, field strength, and refocusing flip angle schemes. In addition, the behavior of signals acquired before and after the refocusing time points is analyzed. Methods Evolution of magnetization within a network of artificial cylinders is simulated with Monte Carlo methods for all relevant coherence pathways. In addition, measurements on microspheres were performed to confirm some of the theoretical results. Results For reduced refocusing flip angles, the peak of the vessel size sensitivity curve is shifting toward larger radii with increasing echo time. Furthermore, the BOLD‐related signal change along the echo train depends on the chosen refocusing flip angle scheme and in general does not follow corresponding echo amplitudes. Conclusion CPMG or GRASE can be used with low refocusing flip angles without significant loss of sensitivity to BOLD. The evolution of BOLD signal changes along the echo train can be used to design optimal k‐space reordering schemes. Signals acquired before or after the spin echo time point show contributions from larger vessels similar to gradient echo sequences. Short echo spacing (time between refocusing pulses) suppresses gradient echo‐related contributions from larger vessels, whereas the spin echo‐related contribution from small vessels remains constant and is independent of the echo spacing.