Parallel Transmit Imaging with Real-time Multislice-to-volume Motion Correction for Task-based Functional MRI at 7T

Winata, S. , Hoinkiss, D.C., Keith, G.A. , Williams, S.N. , Ding, B.Y., Al-Wasity, S., Gunamony, S. and Porter, D.A. (2023) Parallel Transmit Imaging with Real-time Multislice-to-volume Motion Correction for Task-based Functional MRI at 7T. 39th Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2023), Basel, Switzerland, 4-7 October 2023. pp. 214-15.

Abstract

Introduction: The radiofrequency (RF) wavelength decreases with increasing magnetic field strength. This poses an issue for routine human imaging at ultra-high field strengths, such as 7 T, due to greater RF field (B1 ?) inhomogeneity. Parallel transmission (pTx) uses a dedicated RF coil with independent transmit channels that generate individual RF fields. These fields can be tailored for a homogenous, combined excitation that is simultaneously subject to SAR constraints. While the improved SNR in 7 T increases the capacity for higher resolution imaging, it is also more prone to motion-induced artefacts. Prospective, real-time motion correction techniques, can help to reduce these effects. MS-PACE [1] is a technique adapted from Prospective Acquisition CorrEction (PACE). In-plane and throughplane motion are estimated by registering a subset of equidistant EPI slices to a Reference volume, differing from the volumetric registration in PACE [2]. This allows for sub-TR motion detection and higher temporal resolution of imaging system updates. This abstract presents an integration of pTx with real-time MS-PACE motion correction for a functional MRI (fMRI) protocol, harnessing both techniques to deliver more homogenous and motion-insensitive imaging. This study integrated a subject-specific pTx workflow [3] with slice-by-slice B1 ? field shimming [4]. Methods: The study was performed on a MAGNETOM Terra 7 T scanner (Siemens Healthineers, Erlangen, Germany) with an 8Tx64Rx head coil (MR CoilTech Ltd, Glasgow, Scotland, UK) [5, 6] using an in-house-developed GRE-EPIsequence on 5 healthy subjects (age 49 ± 15). Following localisation and B0 shimming, B1 ? and B0 field mapping was performed at the slice positions used by the EPI protocol, and the results were used in MATLAB (MathWorks, Natick, MA, USA) offline to generate pTx slice-specific B1 ? shim weights. T1-weighted data were also acquired using a 3D FLASH sequence. After the structural scan was completed, 4 EPI scans were acquired: 2 in circularly-polarised (CP) mode (equivalent to conventional single transmission) and 2 in pTx shim mode. In each case, one scan was acquired with motion correction and one without. The scan parameters were otherwise identical: voxel size 2 9 2 9 2 mm3, resolution 96 9 96, GRAPPA factor 3, 51 slices, 110 volumes, echo spacing 580 ms, TR 4 s, TE 18 ms and total acquisition time 7 m 32 s. A simple paradigm with resting and finger-tapping segments alternating every 10 volumes was instructed to the subject with the aid of PsychoPy [7]. A 3-slice registration subset was used for real-time motion correction. Estimated motion parameters were fed back to the scanner and the imaging gradients were updated accordingly. The correction robustness was evaluated by calculating residual rigid-body motion parameters with the multislice-to-volume method. Online and offline processing was done within the Image Calculation Environment (Siemens Healthineers, Erlangen, Germany) using ITK open-source libraries. Results: Figure 1 compares the mean voxel displacement in the pTx scans in all subjects. Figure 2 compares temporal SNR (tSNR) maps and percentage differences in tSNR (dtSNR) from the motion corrected scans in both CP and pTx modes across all subjects Fig. 3 shows the mean dtSNR of pTx compared to CP in all subjects, normalised into Montreal Neurological Institute (MNI) space. Discussion: Figure 1 shows that the motion correction capability was preserved when pTx was used. For the acquisitions with real-time motion correction, Fig. 2 demonstrates tSNR improvements when pTx is used, although this observation was not universal in all slices acquired from all subjects. In particular, there was a marked tSNR pTx improvement for slices in which the tSNR of the corresponding CP acquisition was low, as in Subject 3. The averaged difference across all subjects, when normalised into MNI space (Fig. 3), also shows tSNR improvements in pTx versus CP across the central slices, although there was a decrease in tSNR in the temporal lobe on some slices. This preliminary analysis of the data has focused on assessing the performance of pTx when they are integrated into a real-time motion correction procedure. The results confirm that the overall benefit of pTx and motion correction is preserved. Conclusion: This study has evaluated an integrated method of EPI acquisition that combines real-time motion correction with subject specific pTx using slice-by-slice B1 + shimming. An overall improvement in tSNR was demonstrated with the motion-corrected data acquired with pTx compared to acquisitions using standard CP pulses.

• Item Type: Conference or Workshop Item
• Additional Information: Abstract - available at Magnetic Resonance Materials in Physics, Biology and Medicine 36(Suppl 1): S214-S215
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Gunamony, Dr Shajan and Williams, Dr Sydney and Keith, Dr Graeme and Porter, Professor David and Winata, Mr Steven and Al-Wasity, Mr Salim
• Authors: Winata, S., Hoinkiss, D.C., Keith, G.A., Williams, S.N., Ding, B.Y., Al-Wasity, S., Gunamony, S., and Porter, D.A.
• College/School: College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
• Research Group: Imaging Centre of Excellence

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