Chemical exchange saturation transfer weighted diffusion MRI at a high magnetic field

Abstract

Chemical exchange saturation transfer (CEST) MRI is a molecular imaging technique proven to be useful in imaging brain tumors or stroke. It has high sensitivity but lacks physical information about the cellular environment in biological tissues. Meanwhile, diffusion-weighted imaging (DWI) allows estimation of apparent diffusion coefficient (ADC) and provides information about the structure of tissues. In previous studies, it has been discussed that diffusion coefficients of brain metabolites can offer information on intracellular metabolites and their cellular structure. Nevertheless, little is known about the contrast map of metabolite ADC throughout the brain. The existing diffusion-weighted MR spectroscopy technique allows observation of metabolite ADC but the mapping is limited by the long scan time. However, CEST takes a shorter scan time and can easily be mapped. In that sense, we propose a CEST-weighted diffusion technique to map ADC weighted on different CEST agents. In this study, we aim to observe unique CEST-weighted ADC contrast on different frequency offsets and to examine the validity of the technique. The resulting CEST-w ADC values at amide proton transfer (APT) and nuclear Overhauser effect (NOE) peaks were different from one another and were smaller than traditional ADC in water protons in phantom and in vivo. The maps showed distinctive contrast, unlike CEST or ADC. The CEST-w ADC parameters were compared in different tissue types including cortical gray matter, white matter tracks, and CSF. The results suggest the technique can provide a novel contrast. We expect the proposed technique to allow us to understand brain metabolites in a new perspective, regarding the structure of intracellular space of CEST agents. Further study remains to understand the origin of the signal and to verify the role of the proposed contrast as a biomarker.