The BOLD Sensitivity and Vessel Size Specificity along CPMG and GRASE Echo Trains

While the vessel size specificity of gradient and spin echoes has been extensively characterized in simulations and measurements, other sequence types that recently gain increasing attention in ultra-high field and high-resolution BOLD fMRI such as GRASE are yet not fully characterized. GRASE, that is based on a CPMG excitation scheme, samples a gradient echo train within consecutive (gradient and rf) refocusing pulses. Furthermore, CPMG or GRASE offers several sequence parameters such as variable refocusing flip angles, echo spacing and k-space reordering schemes along the gradient and spin echo train that might influence its sensitivity to certain vessel radii. In this project, we present an analysis of the vessel size specificity and the BOLD sensitivity along the echo train of CPMG and GRASE sequences across field strength, for different echo spacings, reduced and varying refocusing flip angles, and for acquisition time points before and after the spin echo to assess gradient echo related contributions. Results are based on Monte Carlo simulations of extravascular signals disturbed by randomly oriented cylinders with different radius as well as with measurements on microspheres.

Scheffler K, Engelmann J, Heule R:
BOLD sensitivity and vessel size specificity along CPMG and GRASE echo trains.

Magn Reson Med. Accepted May 2021

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