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.

The three rows depict BOLD-related signal changes along CPMG or GRASE echo trains for echo spacings ES of 38 ms (top row, for k2 ES=36 ms), 24 ms (middle row), and 15 ms (bottom row) simulated for 7T. The left and middle column shows the mean signal change DMmean (mean signal change along vessel size radius from 0.5 to 300 mm indicating the mean overall BOLD response) along the echo train (in blue) and echo amplitudes M (in red) for different schemes of refocusing flip angles, all in magnetization units of M0=1. In the left column a=20°/40° denotes constant refocusing flip angles of 20° and 40°, 90°+a/2 indicates constant refocusing flip angles with 20°/40° plus initial 100°/110° preparation (23), and bold curves are constant 180° refocusing pulses. In the middle column v1, v4 and v7 refers to variable refocusing flip angles as used in (24) and k2 as used in (25), and 0.05-0.07M0 indicates a refocusing flip angle scheme that produces constant echo amplitudes of 0.05-0.07 M0 (bottom middle). The right column shows BOLD signal change as a function of vessel size calculated in time steps of 0.4 ms along the multi-echo gradient echo and spin echo train with perfect 180° refocusing pulses and echo spacings of 38, 24 and 15 ms. A typical gradient echo behavior (i.e. constant and high sensitivity to larger vessels) can be observed just before the first refocusing pulse and typical spin echo behavior (although with increasing sensitivity to small vessels) at multiples of the echo spacing. The gradient echo-train profiles immediately before and after later refocusing pulses show increasingly less sensitivity to larger vessels.

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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