Improving B0 Correction for pH-Weighted Amine Proton Chemical Exchange Saturation Transfer (CEST) Imaging by Use of k-Means Clustering and Lorentzian Estimation

Amine chemical exchange saturation transfer (CEST) echoplanar imaging (EPI) provides unique pH and ammo acid MRI contrast, enabling sensitive detection of altered micro-environment properties in various diseases. However, CEST contrast is sensitive to static magnetic field (B-0) inhomogeneities. Here we propose 2 new B-0 correction algorithms for use in correcting pH-weighted amine CEST EPI based on k-means clustering and Lorentzian fitting of CEST data: the iterative downsampling estimation using Lorentzian fitting and the 2-stage Lorentzian estimation with 40 polynomial fitting. Higher quality images of asymmetric magnetization transfer ratio (MTRasym) at 3.0 ppm could be obtained with the proposed algorithms than with the existing B-0 correction methods. In particular, the proposed methods are shown to improve the intertissue consistency, interpatient consistency, and tumor region signal-to-noise ratio of MTRasym at 3.0 ppm images, with nonexcessive computation time.