Calibration-free whole-brain CEST imaging at 7T with parallel transmit PUlse design for Satura'on Homogeneity u'lizing Universal Pulses (PUSHUP)

Purpose: CEST measurements at UHF su↵er 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 PUSHUP.Theory : Magnetization transfer effects depend on the saturation Brms1 . PUSHUP homogenizes the saturation Brms1 by using multiple saturation pulses with alternating B1-shims. Using a database of B1 maps, universal pulses are calculated that remove the necessity of time-consuming, subjectbased pulse calculation during the measurement.Methods: PUSHUP was combined with a whole-brain 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 di↵erent B1-correction methods and were compared to circular polarized (CP) saturation in 5 healthy volunteers using an 8-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 B1-correction, PUSHUP CEST maps were artifact-free throughout the whole brain. A 1-point retrospective B1-correction, that does not need repeated measurements, suciently 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 B1-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 minutes.