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

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 . PUSHUP homogenizes the saturation by using multiple saturation pulses with alternating -shims. Using a database of 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 -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 -correction, PUSHUP CEST maps were artifact-free throughout the whole brain. A 1-point retrospective -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 -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.