Simplified assessment for chemical exchanged saturation transfer (CEST) imaging: local offset frequency and CEST effect.

The aim of this study is to develop a novel phantom for the evaluation of clinical CEST imaging settings, e.g., B₀ and B₁ field inhomogeneities, CEST contrast, and post-processing. We made a phantom composed of two slice sections: a grid section for local offset frequency evaluation and a sample section for CEST effect evaluation using different concentrations of an egg white albumin solution. On a 3 Tesla MR scanner, a phantom study was performed using CEST imaging; the mean B₁ amplitudes were set at 1.2 and 1.9 µT, and CEST images with and without B₀ corrections were acquired. Next, region of interest (ROI) analysis was performed for each slice. Then, CEST images with and without B₀ corrections were compared at each B₁ amplitude. The B₀ corrected Z-spectrums at each local region in the grid section showed a shifting of the curve bottom to 0 ppm. Z-spectrum at B₁ = 1.9 µT showed a broader curve shape than that at 1.2 µT. Moreover, MTR_asym values at 3.5 ppm for each albumin sample at B₁ = 1.9 µT were about two times higher than those at 1.2 µT. Our phantom enabled us to evaluate and optimize B₀ inhomogeneity and the CEST effect at the B₁ amplitude.