Methods for an improved detection of the MRI‐CEST effect

Abstract

CEST imaging is a recently introduced MRI contrast modality based on the use of endogenous or exogenous molecules whose exchangeable proton pools transfer saturated magnetization to bulk water, thus creating negative contrast. One of the critical issues for further development of these agents is represented by their limited sensitivity in vivo. The aim of this work is to improve the detection of CEST agents by exploring new approaches through which the saturation transfer (ST) effect can be enhanced. The performance of the proposed methods has been tested in vitro and in vivo using highly sensitive and highly shifted lipoCEST agents, and the results were compared with the standard ST evaluation mode. The acquired Z-spectra were interpolated locally and voxel-by-voxel by smoothing splines. Besides expressing the ST in the standard mode, we explore two methods, enhanced and integral ST, which better exploit all the information contained in the Z-spectrum. By combining different modes for ST assessment a significant improvement in the detection of the lipoCEST agents, both in vitro and in vivo, has been found. The results obtained from the application of the proposed methods outline the importance of post-processing analysis for highlighting the CEST-MRI contrast.

In Vitro and In Vivo Assessment of Nonionic Iodinated Radiographic Molecules as Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Tumor Perfusion Agents

“…The Z-spectra were interpolated, on a voxel-by-voxel basis, by smoothing splines, B 0 -shift corrected, and saturation transfer efficiency (ST%) was measured by punctual analysis. 27 For in vivo images, difference contrast maps (ΔST%) were calculated by subtracting the ST contrast after iodinated contrast media injection from the ST contrast before the injection on a per voxel basis to reduce the confounding effect of the endogenous contributions. A threshold value of 2% was set, based on the ΔST variations between multiple precontrast ST maps (ca.…”

Magnetization Transfer Prepared Gradient Echo MRI for CEST Imaging

“…Z-spectra were calculated from the normalized images for ROI outlined in each phantom compartment. The scatters were drawn with mean signals as ordinate and RF offset as abscissas, smoothed using a spline function [27]. CEST images were calculated using the conventional MTR asymmetry analysis of three images: label (I label ) and reference (I ref ) images with RF saturation applied at the labile proton frequency (1.87 and 3.5 ppm for creatine and in vivo CEST MRI, respectively) and reference frequency (−1.87 and −3.5 ppm, respectively), in addition to a control scan without RF irradiation (I 0 ).…”

Improved pH measurements with a single PARACEST MRI contrast agent

“…This other method assumes that the CEST spectrum is symmetric about the water’s chemical shift in the absence of an exchanging chemical group. Other methods have also been described that characterize the CEST effect, but these other methods also rely on the symmetry assumption (47). In the case of Yb–DO3A–oAA, this assumption is not valid because each CEST effect at ω 0 is close to -ω 0 for the other CEST effect.…”