title: 3D T 2-weighted imaging at 7T using dynamic kT-points on single-transmit MRI systems.
abstract: OBJECTIVE
  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
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Station 6, 1015, Lausanne, Switzerland. florent.eggenschwiler@epfl.ch.
authors.forename: Florent
authors.initials: F
authors.lastname: Eggenschwiler
authors.affiliation: Centre for Advanced Imaging, University of Queensland, Brisbane, Australia.|Siemens Healthcare Pty Ltd, Brisbane, QLD, Australia.
authors.forename: Kieran Robert
authors.initials: KR
authors.lastname: O'Brien
authors.affiliation: Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Station 6, 1015, Lausanne, Switzerland.
authors.forename: Daniel
authors.initials: D
authors.lastname: Gallichan
authors.affiliation: Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Station 6, 1015, Lausanne, Switzerland.|Department of Radiology, Universities of Lausanne and Geneva, Vaud, Switzerland.
authors.forename: Rolf
authors.initials: R
authors.lastname: Gruetter
authors.affiliation: Donders Center for Cognitive Neuroimaging, Radboud University, Nijmegen, The Netherlands.
authors.forename: José Pedro
authors.initials: JP
authors.lastname: Marques
mesh_terms: D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging; D001931:Brain Mapping / Q000379:methods*; D003198:Computer Simulation; D006257:Head / Q000000981:diagnostic imaging*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D015233:Models, Statistical
keywords: B 1 + inhomogeneity correction; Dynamic kT-points; Spatially resolved extended phase graph; T 2-weighted imaging; TSE sequence