Cerebral blood volume mapping using Fourier‐transform–based velocity‐selective saturation pulse trains

Abstract

Purpose Velocity‐selective saturation (VSS) pulse trains provide a viable alternative to the spatially selective methods for measuring cerebral blood volume (CBV) by reducing the sensitivity to arterial transit time. This study is to compare the Fourier‐transform–based velocity‐selective saturation (FT‐VSS) pulse trains with the conventional flow‐dephasing VSS techniques for CBV quantification. Methods The proposed FT‐VSS label and control modules were compared with VSS pulse trains utilizing double refocused hyperbolic tangent (DRHT) and 8‐segment B1‐insensitive rotation (BIR‐8). This was done using both numerical simulations and phantom studies to evaluate their sensitivities to gradient imperfections such as eddy currents. DRHT, BIR‐8, and FT‐VSS prepared CBV mapping was further compared for velocity‐encoding gradients along 3 orthogonal directions in healthy subjects at 3T. Results The phantom studies exhibited more consistent immunity to gradient imperfections for the utilized FT‐VSS pulse trains. Compared to DRHT and BIR‐8, FT‐VSS delivered more robust CBV results across the 3 VS encoding directions with significantly reduced artifacts along the superior‐inferior direction and improved temporal signal‐to‐noise ratio (SNR) values. Average CBV values obtained from FT‐VSS based sequences were 5.3 mL/100 g for gray matter and 2.3 mL/100 g for white matter, comparable to literature expectations. Conclusion Absolute CBV quantification utilizing advanced FT‐VSS pulse trains had several advantages over the existing approaches using flow‐dephasing VSS modules. A greater immunity to gradient imperfections and the concurrent tissue background suppression of FT‐VSS pulse trains enabled more robust CBV measurements and higher SNR than the conventional VSS pulse trains.

Spatial dependency and the role of local susceptibility for velocity selective arterial spin labeling (VS‐ASL) relative tagging efficiency using accelerated 3D radial sampling with a BIR‐8 preparation

“…Velocity selective arterial spin labeling (VS-ASL) is a promising technique for transit time insensitive, non-contrast perfusion and vascular imaging. [6][7][8] VS-ASL has been used to demonstrate post labeling delay insensitive visualization of cerebral blood flow (CBF) in cases of delayed filling 9,10 and has also been used to demonstrate agreement with 15 O-PETbased measures of CBF. 11 Although no defined tagging plane is required, VS-ASL tagging is still subject to other error sources that can result in global or regional changes in tagging efficiency including eddy currents, cardiac cycle-dependent flow, velocity directionality, and heterogeneity of B 0 and B 1 .…”

Improved velocity‐selective‐inversion arterial spin labeling for cerebral blood flow mapping with 3D acquisition

“…In that paper, block pulses were recommended over composite pulses for refocusing, due to the adequate immunity of Mz‐velocity response to B 0 /B 1 field inhomogeneities with shorter pulse durations and less SAR constraints. For VSASL, when only correct scales were evaluated with respect to gradient imperfections, block refocusing pulses were shown sufficient to suppress false signal from static tissue 21,24 . In this current work, when poor B 1 conditions were examined as large spatial coverage would incur, simulations of Mz‐position responses (Figure 2), as well as phantom (Figure 3) and human (Supporting Information Figure ) results all demonstrated that composite refocusing pulses were more robust over block refocusing to minimize both eddy‐current and B 1 related artifacts.…”

“…For VSASL, when only correct B + 1 scales were evaluated with respect to gradient imperfections, block refocusing pulses were shown sufficient to suppress false signal from static tissue. 21,24 In this current work, when poor B 1 conditions were examined as large spatial coverage would incur, simulations of Mzposition responses (Figure 2), as well as phantom ( Figure 3) and human (Supporting Information Figure S2) results all demonstrated that composite refocusing pulses were more robust over block refocusing to minimize both eddy-current and B 1 related artifacts. The erroneous phases from eddy-currents associated with gradient lobes surrounding the refocusing pulses might be better canceled, when B 1 is with correct setting (middle column of Figures 2 and 3), or refocusing pulses are less sensitive to B 1 offset (second row of Figures 2 and 3).…”

“…The effect of ECs on the static spins after applying FT‐VSI pulse trains with block and composite refocusing pulses (90° x 180° y 90° x ) were examined. Subtracted responses of the longitudinal magnetizations (Mz) following both the velocity‐sensitive label and velocity‐compensated control (unipolar gradient lobes) pulse trains were calculated with EC amplitude of 0.025% and time constants of 10 −4 ‐1 s for static spins at distances from −23 cm to 24 cm as described by Qin et al, 21,24 with an interval of 0.1 mm. Their sensitivity to various scales, which are the ratio of actual flip angles to nominal input flip angles, from 0.7 to 1.3, were evaluated.…”

Ensuring both velocity and spatial responses robust to field inhomogeneities for velocity‐selective arterial spin labeling through dynamic phase‐cycling

“…However, with the 2 cm/s cutoff velocity used here, the b‐values for label and control were 0.5 and 0.0 s/mm 2 with scheme 1 and 3, versus 0.5 and 0.2 s/mm 2 with scheme 2. While this effect was negligible in this VSASL study, it remains an issue in the measurement of blood volume using velocity‐selective pulse trains, 20,21 by which much lower cutoff velocity is pursued. It is also worth emphasizing that, although the dynamic phase‐cycling removes subtraction errors of static spins and preserves the velocity‐selective profiles, the labeling efficiency is still scaled by incorrect setting (Figure 1A) because the inversion degree is determined by the flip angles of the hard pulses used at the beginning of each velocity‐encoding step 8 .…”