# Human liver CEST imaging at 7 T: Impact of B1+ shimming

## TL;DR

This study shows that CEST imaging in the human liver at 7T is feasible using B1+ shimming to improve accuracy and reduce signal dropouts.

## Contribution

The first demonstration of reliable relaxation-compensated CEST contrast quantification in the human liver at ultrahigh field using B1+ shimming.

## Key findings

- Static pTx (B1+ shimming) effectively eliminates B1+ dropouts in liver CEST imaging.
- MTRRex values for amide, nuclear Overhauser effect, and guanidino protons showed low inter-subject variation.
- Proposed B1+ mapping technique improved MTRRex correction accuracy in abdominal imaging.

## Abstract

To explore the feasibility of CEST imaging in the human liver at 7 T with B1+ shimming.

CEST MRI was performed on a 7 T whole‐body scanner with a parallel transmission (pTx) system in five healthy volunteers. Static pTx (B1+ shimming) was applied to locally maximize the B1+ magnitude per input power within a given region of interest (ROI) of approximately 30 mm diameter (ROIshim). Relaxation‐compensated inverse magnetization transfer ratio (MTRRex) values were quantified for amide protons, guanidino protons, and relayed nuclear Overhauser effect signals based on five‐pool Lorentzian fit analysis. MTRRex values were corrected for B1 inhomogeneities using an absolute, accurate MR fingerprinting–based B1+ mapping technique.

Within the ROIshim, reliable MTRRex values could be calculated for an average of 85% of voxels. The mean MTRRex values and corresponding coefficient of variations across the group are: 0.113 ± 0.009, 8.8% for amide; 0.167 ± 0.010, 6.3% for nuclear Overhauser effect; and 0.079 ± 0.010, 12.9% for guanidino. MTRRex values exhibit low variation between subjects, as reflected by low coefficient of variations.

In this study, we have demonstrated for the first time the feasibility of acquiring and quantifying relaxation‐compensated CEST contrasts in the human liver at ultrahigh field. The application of static pTx effectively eliminates B1+ dropouts and allows for accurate CEST contrast quantification within the selected ROI. In addition, the proposed B1+ mapping technique shows efficacy for enhanced MTRRex B1 corrections in the abdomen.