B1+$$ {B}_1^{+} $$‐correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

Abstract

Purpose: Magnetization transfer saturation (MTsat) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased B + 1 -inhomogeneity at ≥ 7 T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous MTsat B + 1 correction methods are applicable. Here, we develop a calibration-based correction model and procedure, and validate and evaluate it in postmortem 7T data of whole chimpanzee brains. Theory: The B + 1 dependence of MTsat was investigated by varying the off-resonance saturation pulse flip angle. For the range of saturation pulse flip angles applied in typical experiments on postmortem tissue, the dependence was close to linear. A linear model with a single calibration constant C is proposed to correct bias in MTsat by mapping it to the reference value of the saturation pulse flip angle. Methods: C was estimated voxel-wise in five postmortem chimpanzee brains. "Individual-based global parameters" were obtained by calculating the mean C within individual specimen brains and "group-based global parameters" by calculating the means of the individual-based global parameters across the five brains. Results: The linear calibration model described the data well, though C was not entirely independent of the underlying tissue and B + 1 . Individual-based correction parameters and a group-based global correction parameter (C = 1.2) led to visible, quantifiable reductions of B + 1 -biases in high-resolution MTsat maps. Conclusion: The presented model and calibration approach effectively corrects for B + 1 inhomogeneities in postmortem 7T data. K E Y W O R D S calibration, chimpanzee, magnetization transfer, MRI, postmortem, transmit field, ultra high-field This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Brain structure and function: a multidisciplinary pipeline to study hominoid brain evolution

“…These images were then used to calculate MT sat as described in Helms et al (2008) and R1 as described in Dathe and Helms (2010) and proton density PD was calculated as the signal amplitude described by Dathe and Helms (2010) . The MT sat images were additionally B1 + bias corrected ( Lipp et al, 2023 ).…”

Sourcing high tissue quality brains from deceased wild primates with known socio‐ecology

“…Quantitative maps of MRI parameters were reconstructed with the help of an optimised pipeline in the hMRI toolbox optimised for 7 T post mortem imaging (http://www.hMRI.info, Tabelow et al, 2019). To correct the bias in the MTsat maps induced by inhomogeneity of the transmit radio‐frequency field we developed the calibration approach described in detail in (Lipp et al, 2022).…”

“…( To correct the bias in the MTsat maps induced by inhomogeneity of the transmit radio-frequency field we developed the calibration approach described in detail in (Lipp et al, 2022).…”

Neuroinfectiology of an atypical anthrax-causing pathogen in wild chimpanzees

“…QMRI was performed using a multi-parametric mapping method 45,47,48 . Scanning parameters are provided in 45 and in Supplementary Materials. The quantitative maps of R1, MTsat and R2* with isotropic resolution of 300 µm were obtained using the open source hMRI toolbox 49 , adapted for the analysis of ultra-high resolution postmortem images 45 .…”

“…Macroscopic neuropathological changes caused by Bcbva we accessed used ultra-high resolution quantitative MRI (qMRI), providing microstructural information about tissue pathology [45][46][47] . We obtained maps of three qMR parameters: magnetisation transfer saturation (MTsat) reflecting tissue macromolecular content, longitudinal relaxation rate (R1) reflecting myelin and iron content, effective transverse relaxation rate R2* strongly sensitive to tissue iron 46 .…”