Florent Eggenschwiler1, Kieran Robert O'Brien2,3, Daniel Gallichan4, Rolf Gruetter4,5, José Pedro Marques6. 1. Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Station 6, 1015, Lausanne, Switzerland. florent.eggenschwiler@epfl.ch. 2. Centre for Advanced Imaging, University of Queensland, Brisbane, Australia. 3. Siemens Healthcare Pty Ltd, Brisbane, QLD, Australia. 4. Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Station 6, 1015, Lausanne, Switzerland. 5. Department of Radiology, Universities of Lausanne and Geneva, Vaud, Switzerland. 6. Donders Center for Cognitive Neuroimaging, Radboud University, Nijmegen, The Netherlands.
Abstract
OBJECTIVES: For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B 1 (+) inhomogeneity. Previously, it was shown that a single kT-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static kT-points). This work demonstrates that the B 1 (+) dependence of T 2-weighted imaging can be further mitigated by designing a specific kT-point pulse for each pulse of a 3D TSE sequence (dynamic kT-points) even on single-channel transmit systems MATERIALS AND METHODS: By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic kT-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic kT-points were inserted into the TSE sequence to acquire in vivo images at 7T. RESULTS: The improvement provided by the dynamic kT-points over the static kT-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic kT-points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. CONCLUSION: Designing dynamic kT-points for a 3D TSE sequence allows the acquisition of T 2-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 (+) inhomogeneity.
OBJECTIVES: For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B 1 (+) inhomogeneity. Previously, it was shown that a single kT-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static kT-points). This work demonstrates that the B 1 (+) dependence of T 2-weighted imaging can be further mitigated by designing a specific kT-point pulse for each pulse of a 3D TSE sequence (dynamic kT-points) even on single-channel transmit systems MATERIALS AND METHODS: By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic kT-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic kT-points were inserted into the TSE sequence to acquire in vivo images at 7T. RESULTS: The improvement provided by the dynamic kT-points over the static kT-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic kT-points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. CONCLUSION: Designing dynamic kT-points for a 3D TSE sequence allows the acquisition of T 2-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 (+) inhomogeneity.
Entities:
Keywords:
B 1 + inhomogeneity correction; Dynamic kT-points; Spatially resolved extended phase graph; T 2-weighted imaging; TSE sequence
Authors: Florent Eggenschwiler; Tobias Kober; Arthur W Magill; Rolf Gruetter; José P Marques Journal: Magn Reson Med Date: 2011-08-29 Impact factor: 4.668
Authors: Arian Beqiri; Hans Hoogduin; Alessandro Sbrizzi; Joseph V Hajnal; Shaihan J Malik Journal: Magn Reson Med Date: 2018-02-24 Impact factor: 4.668