Fast 3D chemical exchange saturation transfer (CEST) imaging of the human brain

Chemical exchange saturation transfer magnetic resonance imaging can detect low-concentration compounds with exchangeable protons through saturation transfer to water. This technique is generally slow, as it requires acquisition of saturation images at multiple frequencies. In addition, multi-slice imaging is complicated by saturation effects differing from slice to slice because of relaxation losses. In this study, a fast three-dimensional chemical exchange saturation transfer imaging sequence is presented that allows whole-brain coverage for a frequency-dependent saturation spectrum (z-spectrum, 26 frequencies) in less than 10 min. The approach employs a three-dimensional gradient- and spin-echo readout using a prototype 32-channel phased-array coil, combined with two-dimensional sensitivity encoding accelerations. Results from a homogenous protein-containing phantom at 3T show that the sequence produced a uniform contrast across all slices. To show translational feasibility, scans were also performed on five healthy human subjects. Results for chemical exchange saturation transfer images at 3.5 ppm downfield of the water resonance, so-called amide proton transfer images, show that lipid signals are sufficiently suppressed and artifacts caused by B₀ inhomogeneity can be removed in postprocessing. The scan time and image quality of these in vivo results show that three-dimensional chemical exchange saturation trasfer MRI using gradient- and spin-echo acquisition is feasible for whole-brain chemical exchange saturation transfer studies at 3T in a clinical time frame.