Parallel transmit kT-points pulses for improving high resolution quantitative multi-parameter mapping (MPM)
We use quantitative multi-parameter mapping (MPM) to obtain detailed information about brain microstructure. This information includes maps of MR parameters, like longitudinal relaxation rate (R1), proton density (PD), and effective transverse relaxation rate R2*. Higher magnetic field strengths (such as 7 Tesla MRI) help us measure these parameters at ultra-high resolutions, but at 7T bias and shading artifacts can be observed. To address this issue, we use the parallel transmit infrastructure of our 7T Terra scanner and integrated the pTx kt-points approach into the MPM acquisition.
In the pilot phase, we compared the performance of the kt-points MPM approach with the conventional MPM approach in a group of 8 volunteers at 7T. Two different non-selective RF excitation pulses were used: pTx 4 kt-points and sinc pulses, referred to as pTx and True Form acquisitions, respectively. The data were processed using the hMRI toolbox, resulting in maps of local flip angle distribution, R1, PD, and R2*.
As the images show, the pTx approach significantly improved excitation flip angle homogeneity, resulting in lower bias and reduced coefficient of variation (CoV) in R1 maps. Similar improvements were observed in PD maps, but not in R2* maps. The improvements were most noticeable in areas with signal dropouts, such as the inferior temporal lobes and cerebellum. The pTx approach did not significantly impact R2* maps due to the small dependence of R2* fits on initial signal intensity.
In conclusion, the pTx approach improves the effective coverage and sensitivity for anatomical studies and can be easily integrated into scanning and data processing workflows. It adds approximately 2.5 minutes of scan time and is compatible with established quantitative MRI processing pipelines.