Calibration-free whole-brain CEST imaging at 7T with parallel transmit pulse design for saturation homogeneity utilizing universal pulses (PUSHUP)

Abstract

Purpose: Chemical exchange saturation transfer (CEST) measurements at ultra-high field (UHF) suffer from strong saturation inhomogeneity. Retrospective correction of this inhomogeneity is possible to some extent, but requires a time-consuming repetition of the measurement. Here, we propose a calibration-free parallel transmit (pTx)-based saturation scheme that homogenizes the saturation over the imaging volume, which we call PUlse design for Saturation Homogeneity utilizing Universal Pulses (PUSHUP).

Theory: Magnetization transfer effects depend on the saturation $B_1^{\text{rms}}$ . PUSHUP homogenizes the saturation $B_1^{\text{rms}}$ by using multiple saturation pulses with alternating $B_1$ -shims. Using a database of $B_1$ maps, universal pulses are calculated that remove the necessity of time-consuming, subject-based pulse calculation during the measurement.

Methods: PUSHUP was combined with a whole-brain three-dimensional-echo planar imaging (3D-EPI) readout. Two PUSHUP saturation modules were calculated by either applying whole-brain or cerebellum masks to the database maps. The saturation homogeneity and the group mean CEST amplitudes were calculated for different $B_1$ -correction methods and were compared to circular polarized (CP) saturation in five healthy volunteers using an eight-channel transmit coil at 7 Tesla.

Results: In contrast to CP saturation, where accurate CEST maps were impossible to obtain in the cerebellum, even with extensive $B_1$ -correction, PUSHUP CEST maps were artifact-free throughout the whole brain. A 1-point retrospective $B_1$ -correction, that does not need repeated measurements, sufficiently removed the effect of residual saturation inhomogeneity.

Conclusion: The presented method allows for homogeneous whole-brain CEST imaging at 7 Tesla without the need of a repetition-based $B_1$ -correction or online pulse calculation. With the fast 3D-EPI readout, whole-brain CEST imaging with 45 saturation offsets is possible at 1.6 mm resolution in under 4 min.

Keywords: CEST; magnetization transfer; parallel transmit; ultra‐high field; universal pulses.