title: Application of the limited-memory quasi-Newton algorithm for multi-dimensional, large flip-angle RF pulses at 7T.
abstract: OBJECTIVE
  Ultrahigh field MRI provides great opportunities for medical diagnostics and research. However, ultrahigh field MRI also brings challenges, such as larger magnetic susceptibility induced field changes. Parallel-transmit radio-frequency pulses can ameliorate these complications while performing advanced tasks in routine applications. To address one class of such pulses, we propose an optimal-control algorithm as a tool for designing advanced multi-dimensional, large flip-angle, radio-frequency pulses. We contrast initial conditions, constraints, and field correction abilities against increasing pulse trajectory acceleration factors.
  
  
  METHODS
  On an 8-channel 7T system, we demonstrate the quasi-Newton algorithm with pulse designs for reduced field-of-view imaging with an oil phantom and in vivo with scans of the human brain stem. We used echo-planar imaging with 2D spatial-selective pulses. Pulses are computed sufficiently rapid for routine applications.
  
  
  RESULTS
  Our dataset was quantitatively analyzed with the conventional mean-square-error metric and the structural-similarity index from image processing. Analysis of both full and reduced field-of-view scans benefit from utilizing both complementary measures.
  
  
  CONCLUSIONS
  We obtained excellent outer-volume suppression with our proposed method, thus enabling reduced field-of-view imaging using pulse trajectory acceleration factors up to 4.
authors.affiliation: Department of Chemistry, Center for Ultrahigh-Field NMR Spectroscopy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000, Aarhus, Denmark. msv@inano.au.dk.
authors.forename: Mads S
authors.identifier: http://orcid.org/0000-0002-0149-583X
authors.initials: MS
authors.lastname: Vinding
authors.affiliation: German Center for Neurodegenerative Diseases DZNE, Bonn, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Brenner
authors.affiliation: Department of Neuropsychology and Psychopharmacology, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
authors.forename: Desmond H Y
authors.initials: DH
authors.lastname: Tse
authors.affiliation: Experimental Physics III, TU Dortmund University, 44221, Dortmund, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Vellmer
authors.affiliation: Department of Chemistry, Center for Ultrahigh-Field NMR Spectroscopy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000, Aarhus, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Vosegaard
authors.affiliation: Experimental Physics III, TU Dortmund University, 44221, Dortmund, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Suter
authors.affiliation: German Center for Neurodegenerative Diseases DZNE, Bonn, Germany.|Department of Physics and Astronomy, University of Bonn, Bonn, Germany.
authors.forename: Tony
authors.initials: T
authors.lastname: Stöcker
authors.affiliation: Experimental Physics III, TU Dortmund University, 44221, Dortmund, Germany. ivan@e3.physik.tu-dortmund.de.
authors.forename: Ivan I
authors.initials: II
authors.lastname: Maximov
mesh_terms: D000465:Algorithms; D001931:Brain Mapping; D001933:Brain Stem / Q000000981:diagnostic imaging* / Q000473:pathology; D017352:Echo-Planar Imaging; D006801:Humans; D007089:Image Enhancement; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D015233:Models, Statistical; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results
keywords: MRI; Parallel transmit; RF pulse design; Ultrahigh field MRI