Universal pulses for homogeneous excitation using single channel coils

TY - JOUR

T1 - Universal pulses for homogeneous excitation using single channel coils

AU - Mooiweer, Ronald

AU - Clark, Ian

AU - Maguire, Eleanor

AU - Callaghan, Martina

AU - Hajnal, Jo

AU - Malik, Shaihan

N1 - Funding Information: This work was supported by the EPSRC ( EP/L00531X/1 ), Medical Research Council ( MR/K006355/1 ), a Wellcome Principal Research Fellowship to E.A.M. ( 01759/Z/13/Z ), a Wellcome Strategic Award to the Wellcome Centre for Human Neuroimaging ( 203147/Z/16/Z ), Wellcome/EPSRC Centre for Medical Engineering ( WT 203148/Z/16/Z ), and the National Institute for Health Research (NIHR) Biomedical Research Centre at Guy's and St. Thomas' NHS Foundation Trust and King's College London . We thank Anna Monk, Victoria Hotchin and Gloria Pizzamiglio for assistance with data collection. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health. Funding Information: This research was funded in whole, or in part, by the Wellcome Trust [Grant numbers: 01759/Z/13/Z ; 210567/Z/18/Z ; 203147/Z/16/Z ; 203148/Z/16/Z ]. For the purpose of Open Access, the authors have applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. Publisher Copyright: © 2022

PY - 2022/10

Y1 - 2022/10

N2 - Purpose: Universal Pulses (UPs) are excitation pulses that reduce the flip angle inhomogeneity in high field MRI systems without subject-specific optimization, originally developed for parallel transmit (PTX) systems at 7 T. We investigated the potential benefits of UPs for single channel (SC) transmit systems at 3 T, which are widely used for clinical and research imaging, and for which flip angle inhomogeneity can still be problematic. Methods: SC-UPs were designed using a spiral nonselective k-space trajectory for brain imaging at 3 T using transmit field maps (B 1 +) and off-resonance maps (B 0) acquired on two different scanner types: a ‘standard’ single channel transmit system and a system with a PTX body coil. The effect of training group size was investigated using data (200 subjects) from the standard system. The PTX system was used to compare SC-UPs to PTX-UPs (15 subjects). In two additional subjects, prospective imaging using SC-UP was studied. Results: Average flip angle homogeneity error fell from 9.5 ± 0.5 % for ‘default’ excitation to 3.0 ± 0.6 % using SC-UPs trained over 50 subjects. Performance of the UPs was found to steadily improve as training group size increased, but stabilized after ~15 subjects. On the PTX-enabled system, SC-UPs again outperformed default excitation in simulations (4.8 ± 0.6 % error versus 10.6 ± 0.8 % respectively) though greater homogenization could be achieved with PTX-UPs (3.9 ± 0.6 %) and personalized pulses (SC-PP 3.6 ± 1.0 %, PTX-PP 2.9 ± 0.6 %). MP-RAGE imaging using SC-UP resulted in greater separation between grey and white matter signal intensities than default excitation. Conclusions: SC-UPs can improve excitation homogeneity in standard 3 T systems without further calibration and could be used instead of a default excitation pulse for nonselective neuroimaging at 3 T.

AB - Purpose: Universal Pulses (UPs) are excitation pulses that reduce the flip angle inhomogeneity in high field MRI systems without subject-specific optimization, originally developed for parallel transmit (PTX) systems at 7 T. We investigated the potential benefits of UPs for single channel (SC) transmit systems at 3 T, which are widely used for clinical and research imaging, and for which flip angle inhomogeneity can still be problematic. Methods: SC-UPs were designed using a spiral nonselective k-space trajectory for brain imaging at 3 T using transmit field maps (B 1 +) and off-resonance maps (B 0) acquired on two different scanner types: a ‘standard’ single channel transmit system and a system with a PTX body coil. The effect of training group size was investigated using data (200 subjects) from the standard system. The PTX system was used to compare SC-UPs to PTX-UPs (15 subjects). In two additional subjects, prospective imaging using SC-UP was studied. Results: Average flip angle homogeneity error fell from 9.5 ± 0.5 % for ‘default’ excitation to 3.0 ± 0.6 % using SC-UPs trained over 50 subjects. Performance of the UPs was found to steadily improve as training group size increased, but stabilized after ~15 subjects. On the PTX-enabled system, SC-UPs again outperformed default excitation in simulations (4.8 ± 0.6 % error versus 10.6 ± 0.8 % respectively) though greater homogenization could be achieved with PTX-UPs (3.9 ± 0.6 %) and personalized pulses (SC-PP 3.6 ± 1.0 %, PTX-PP 2.9 ± 0.6 %). MP-RAGE imaging using SC-UP resulted in greater separation between grey and white matter signal intensities than default excitation. Conclusions: SC-UPs can improve excitation homogeneity in standard 3 T systems without further calibration and could be used instead of a default excitation pulse for nonselective neuroimaging at 3 T.

UR - http://www.scopus.com/inward/record.url?scp=85134435313&partnerID=8YFLogxK

U2 - 10.1016/j.mri.2022.07.002

DO - 10.1016/j.mri.2022.07.002

M3 - Article

SN - 0730-725X

VL - 92

SP - 180

EP - 186

JO - Magnetic resonance imaging

JF - Magnetic resonance imaging

ER -