title: Shaped saturation with inherent radiofrequency-power-efficient trajectory design in parallel transmission.
abstract: OBJECTIVE
  A target-pattern-driven (TD) trajectory design is introduced in combination with parallel transmit (pTX) radiofrequency (RF) pulses to provide localized suppression of unwanted signals. The design incorporates target-pattern and B1+ information to adjust denser sampling and coverage in k-space regions where the main pattern information lies. Based on this approach, two-dimensional RF spiral saturation pulses sensitive to RF power limits were applied in vivo for the first time.
  
  
  METHODS
  The TD method was compared with two state-of-the-art spiral design methods. Simulations at different spatial fidelities, acceleration factors and anatomical regions were carried out for an eight-channel pTX 3 Tesla (T) coil. Human in vivo experiments were performed on a two-channel pTX 3T scanner saturating shaped patterns in the brain, heart, and thoracic spine.
  
  
  RESULTS
  Using the TD trajectory, RF pulse power can be substantially reduced by up to 34% compared with other trajectory designs with the same spatial accuracy. Local and global specific absorption rates are decreased in most cases.
  
  
  CONCLUSIONS
  The TD trajectory design uses available a priori information to enhance RF power efficiency and spatial response of the RF pulses. Shaped saturation pulses show improved spatial accuracy and saturation performance. Thus, RF pulses can be designed more efficiently and can be further accelerated.
authors.affiliation: MR Application Development, Siemens Healthcare, Erlangen, Germany; Institute of Biomedical Engineering and Informatics, Ilmenau University of Technology, Ilmenau, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Schneider
authors.forename: Jens
authors.initials: J
authors.lastname: Haueisen
authors.forename: Josef
authors.initials: J
authors.lastname: Pfeuffer
mesh_terms: D000465:Algorithms; D016477:Artifacts; D004735:Energy Transfer; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: RF pulse optimization; k-space trajectory, target-pattern design; parallel transmit; shaped saturation; variable-density sampling