B 1 + $$ {B}_1^{+} $$ -correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

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 saturationB 1 rms $$ {\mathrm{B}}_1^{\mathrm{rms}} $$ . PUSHUP homogenizes the saturationB 1 rms $$ {\mathrm{B}}_1^{\mathrm{rms}} $$ by using multiple saturation pulses with alternatingB 1 $$ {\mathrm{B}}_1 $$ -shims. Using a database ofB 1 $$ {\mathrm{B}}_1 $$ 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 differentB 1 $$ {\mathrm{B}}_1 $$ -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 extensiveB 1 $$ {\mathrm{B}}_1 $$ -correction, PUSHUP CEST maps were artifact-free throughout the whole brain. A 1-point retrospectiveB 1 $$ {\mathrm{B}}_1 $$ -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-basedB 1 $$ {\mathrm{B}}_1 $$ -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.

Brain structure and function: a multidisciplinary pipeline to study hominoid brain evolution

To decipher the evolution of the hominoid brain and its functions, it is essential to conduct comparative studies in primates, including our closest living relatives. However, strong ethical concerns preclude in vivo neuroimaging of great apes. We propose a responsible and multidisciplinary alternative approach that links behavior to brain anatomy in non-human primates from diverse ecological backgrounds. The brains of primates observed in the wild or in captivity are extracted and fixed shortly after natural death, and then studied using advanced MRI neuroimaging and histology to reveal macro- and microstructures. By linking detailed neuroanatomy with observed behavior within and across primate species, our approach provides new perspectives on brain evolution. Combined with endocranial brain imprints extracted from computed tomographic scans of the skulls these data provide a framework for decoding evolutionary changes in hominin fossils. This approach is poised to become a key resource for investigating the evolution and functional differentiation of hominoid brains.