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

Abstract

Purpose To mitigate B1+ inhomogeneity in quantitative CEST MRI at ultra‐high magnetic field strengths (B0 ≥ 7 Tesla) using a parallel transmit system. Methods Multiple interleaved mode saturation employs interleaving of 2 complementary phase sets during the saturation pulse train. Phase differences of 45° (first mode) and 90° (second mode) between 2 adjacent transmitter coil channels are used. The influence of the new saturation scheme on the CEST contrast was analyzed using Bloch‐McConnell simulations. The presented method was verified in phantom and in vivo measurements of the healthy human brain. The relayed nuclear Overhauser effect was evaluated, and the inverse magnetic transfer ratio metric was calculated. Results were compared to a published B1+ correction method. All measurements were conducted on a whole‐body 7 Tesla MRI system using an 8 transmitter and 32 receiver channel head coil. Results Simulations showed that the inverse magnetic transfer ratio metric contrast of relayed nuclear Overhauser effect shows a smaller dependency on the relative amplitudes of the 2 different modes than the contrasts of Cr and amide proton transfer. Measurements of an egg white phantom showed markedly improved homogeneity compared to the uncorrected inverse magnetic transfer ratio metric (relayed nuclear Overhauser effect) images and slightly improved results compared to B1+ corrected images. In vivo multiple interleaved mode saturation images showed similar contrast compared to B1+ corrected images. Conclusion Multiple interleaved mode saturation can be used as a simple method to mitigate B1+ inhomogeneity effects in CEST MRI at ultra‐high magnetic field strengths. Compared to previous B1+ correction methods, acquisition time can be reduced because an additional scan, usually required for B1+ correction, can be omitted.

Whole‐brain quantitative CEST MRI at 7T using parallel transmission methods and correction

“…In addition, the MIMOSA FOCUS sequence shows fewer artifacts in particular in regions of extremely low rB 1 and FA values. The phantom measurements show an overestimation of MTR Rex around areas with both extreme high (rB 1 > 1.3) and extreme low rB 1 (rB 1 < 0.5) that is in agreement with our previous results 21 . The overestimations in MTR Rex (APT) in vivo for acquisitions with N corr = 2 around areas of extremely low FA and rB 1 in Full CP acquisitions are in agreement with results presented by Akbey et al 11 …”

“…As expected for all sequences the application of the B + 1 correction decreases the mean wCoV for both contrasts that is in agreement with our previous results. 21 The overestimations in MTR Rex (APT) in vivo for acquisitions with N corr = 2 around areas of extremely low FA and rB 1 in Full CP acquisitions are in agreement with results presented by Akbey et al 11 The sequences applying the CP readout show similar SNR values, as the SNR is mostly dependent on the readout FA. The higher mean FA of the MIMOSA FOCUS sequence results in a broadening of the PSF.…”

“…WB‐CEST MRI was performed using a pulsed CEST GRE sequence 9,28 . For the Gaussian pre‐saturation pulse train, MIMOSA was applied as presented in previous work 21 …”

Whole brain snapshot CEST at 3T using 3D‐EPI: Aiming for speed, volume, and homogeneity

“…Still, in this case the fitted Lorentzian amplitudes were visibly altered. If parallel transmit systems are available, B 1 inhomogeneity might be further mitigated using B 1 shimming 29 or the MIMOSA approach, 30 which could directly be combined with our protocol.…”

Whole‐brain snapshot CEST imaging at 7 T using 3D‐EPI

“…Tse et al recently introduced a CEST approach applying pTx that significantly reduces the contrast variation related to inhomogeneity. Alternatively, it may be possible to combine the snapshot‐EPI approach with the multiple interleaved mode saturation (MIMOSA) method of Liebert et al to mitigate the dependency.…”