title: Intersubject local SAR variation for 7T prostate MR imaging with an eight-channel single-side adapted dipole antenna array.
abstract: OBJECTIVE
  Surface transmit arrays used in ultra-high field body MRI require local specific absorption rate (SAR) assessment. As local SAR cannot be measured directly, local SAR is determined by simulations using dielectric patient models. In this study, the inter-patient local SAR variation is investigated for 7T prostate imaging with the single-side adapted dipole antenna array.
  
  
  METHODS
  Four-dedicated dielectric models were created by segmenting Dixon water-fat separated images that were obtained from four subjects with a 1.5T scanner and the surface array in place. Electromagnetic simulations were performed to calculate the SAR distribution for each model. Radio frequency (RF) exposure variations were determined by analyzing the SAR(10g) distributions (1) with one element active, (2) using a Q-matrix eigenvalue/eigenvector approach, (3) with the maximum potential SAR in each voxel, and (4) for a phase shimmed prostate measurement.
  
  
  RESULTS
  Maximum potential local SAR levels for 1 W time-averaged accepted power per transmit channel range from 4.1 to 7.1 W/kg.
  
  
  CONCLUSIONS
  These variations show that one model is not sufficient to determine safe scan settings. For the operation of the surface array conservative power settings were derived based on a worst-case SAR evaluation and the most SAR-sensitive body model.
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, the Netherlands.
authors.forename: Ozlem
authors.initials: O
authors.lastname: Ipek
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Raaijmakers
authors.forename: Jan J
authors.initials: JJ
authors.lastname: Lagendijk
authors.forename: Peter R
authors.initials: PR
authors.lastname: Luijten
authors.forename: Cornelis A T
authors.initials: CA
authors.lastname: van den Berg
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D008297:Male; D008954:Models, Biological; D019047:Phantoms, Imaging; D011467:Prostate / Q000033:anatomy & histology*; D011829:Radiation Dosage; D011835:Radiation Protection / Q000295:instrumentation* / Q000379:methods; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: parallel transmit; segmented body models; specific absorption rate; surface body coil array; ultra-high field MRI