Correction of B1‐inhomogeneities for relaxation‐compensated CEST imaging at 7 T

Abstract

Chemical exchange saturation transfer (CEST) imaging of endogenous agents in vivo is influenced by direct water proton saturation (spillover) and semi-solid macromolecular magnetization transfer (MT). Lorentzian fit isolation and application of the inverse metric yields the pure CEST contrast AREX, which is less affected by these processes, but still depends on the measurement technique, in particular on the irradiation amplitude B1 of the saturation pulses. This study focuses on two well-known CEST effects in the slow exchange regime originating from amide and aliphatic protons resonating at 3.5 ppm or -3.5 ppm from water protons, respectively. A B1-correction of CEST contrasts is crucial for the evaluation of data obtained in clinical studies at high field strengths with strong B1-inhomogeneities. Herein two approaches for B1-inhomogeneity correction, based on either CEST contrasts or Z-spectra, are investigated. Both rely on multiple acquisitions with different B1-values. One volunteer was examined with eight different B1-values to optimize the saturation field strength and the correction algorithm. Histogram evaluation allowed quantification of the quality of the B1-correction. Finally, the correction was applied to CEST images of a patient with oligodendroglioma WHO grade 2, and showed improvement of the image quality compared with the non-corrected CEST images, especially in the tumor region.

Relaxation‐compensated CEST‐MRI at 7 T for mapping of creatine content and pH – preliminary application in human muscle tissue in vivo

“…To correct for inhomogeneities, Z-spectra at different reference amplitudes were acquired and corrected using the Z-spectrum correction approach of Windschuh et al (18). …”

An overview of CEST MRI for non-MR physicists

“…The susceptibility-related voxel-wise magnetic field shift is proportionally increased with field strength and hence places a more stringent requirement on the B0 correction. RF homogeneity deteriorates severely at 7 T and, if not properly calibrated, a nonuniform saturation profile across the field-of-view may result [27, 72]. The SAR level also becomes a hazardous issue at ultra-high field.…”

Proton Chemical Exchange Saturation Transfer (CEST) MRS and MRI

“…To correct for B 1 inhomogeneity, it is useful to measure B 1 maps, 133 from which Z-values for different B 1 values can be interpolated for each pixel, and an MTR asym map generated. In a related method, referred to as the 'n-point Z-B 1 correction' and the 'n-point contrast B 1 correction', 134 the MTR asym calculation is performed after a Lorentzian fit is applied prior to the interpolation of contrast for every B 1 . Inhomogeneity in B 1 is typically not an issue in clinical scanners at 1.5 T and is a relatively minor problem with dual-channel transmit instruments at 3 T. It is an issue at 7 T, but that should ameliorate with new eight-channel transmit hardware becoming available.…”