On the transmit field inhomogeneity correction of relaxation‐compensated amide and NOE CEST effects at 7 T

Abstract

High field MRI is beneficial for chemical exchange saturation transfer (CEST) in terms of high SNR, CNR, and chemical shift dispersion. These advantages may, however, be counter‐balanced by the increased transmit field inhomogeneity normally associated with high field MRI. The relatively high sensitivity of the CEST contrast to B 1 inhomogeneity necessitates the development of correction methods, which is essential for the clinical translation of CEST. In this work, two B 1 correction algorithms for the most studied CEST effects, amide‐CEST and nuclear Overhauser enhancement (NOE), were analyzed. Both methods rely on fitting the multi‐pool Bloch‐McConnell equations to the densely sampled CEST spectra. In the first method, the correction is achieved by using a linear B 1 correction of the calculated amide and NOE CEST effects. The second method uses the Bloch‐McConnell fit parameters and the desired B 1 amplitude to recalculate the CEST spectra, followed by the calculation of B 1‐corrected amide and NOE CEST effects. Both algorithms were systematically studied in Bloch‐McConnell equations and in human data, and compared with the earlier proposed ideal interpolation‐based B 1 correction method. In the low B 1 regime of 0.15–0.50 μT (average power), a simple linear model was sufficient to mitigate B 1 inhomogeneity effects on a par with the interpolation B 1 correction, as demonstrated by a reduced correlation of the CEST contrast with B 1 in both the simulations and the experiments.

Multiple interleaved mode saturation (MIMOSA) for B1+ inhomogeneity mitigation in chemical exchange saturation transfer

“…Notable in the phantom measurements is that the current correction methods do not remove the inhomogeneity in the central part of the phantom. This might be caused by a much too high B 1 value in this area, which is outside of the correctable B 1 regime where a linear interpolation is possible . In contrast, MIMOSA yields a MTR Rex value in the central part of the phantom that is slightly lower than the mean MTR Rex value in the whole phantom.…”

“…For example, the local effective value influences the magnitude of CEST contrast . Several approaches for correction were presented, including linear as well as nonlinear correction methods. Most of these methods utilize an interpolation based on acquired data and correct the CEST contrast with the use of an interpolated function.…”

Establishing upper limits on neuronal activity–evoked pH changes with APT‐CEST MRI at 7 T

“…Because of a low B 1 , no fast exchanging protons have been included in the simulations. The currently available B 1 correction methods would have increased the scan time dramatically, and therefore have not been used. Although there could be a spatial B 1 pattern, its temporal pattern remains unchanged.…”

A new approach to Z‐spectrum acquisition: prospective baseline enhancement (PROBE) for CEST/Nuclear Overhauser Effect

“…Disentangling such contributions is a challenging task. Potential solutions may be classified as (1) methods based on mathematical analysis and (2) methods based on sequence parameter variation or “editing.” The first category aims at extracting information by introducing new metrics or fitting model parameters to the data . For improved performance, a variation of multiple experimental parameters is desirable; however, this is often limited by the available scan time.…”