title: Brain MR imaging at ultra-low radiofrequency power.
abstract: OBJECTIVE
  To explore the lower limits for radiofrequency (RF) power-induced specific absorption rate (SAR) achievable at 1.5 T for brain magnetic resonance (MR) imaging without loss of tissue signal or contrast present in high-SAR clinical imaging in order to create a potentially viable MR method at ultra-low RF power to image tissues containing implanted devices.
  
  
  METHODS
  An institutional review board-approved HIPAA-compliant prospective MR study design was used, with written informed consent from all subjects prior to MR sessions. Seven healthy subjects were imaged prospectively at 1.5 T with ultra-low-SAR optimized three-dimensional (3D) fast spin-echo (FSE) and fluid-attenuated inversion-recovery (FLAIR) T2-weighted sequences and an ultra-low-SAR 3D spoiled gradient-recalled acquisition in the steady state T1-weighted sequence. Corresponding high-SAR two-dimensional (2D) clinical sequences were also performed. In addition to qualitative comparisons, absolute signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) for multicoil, parallel imaging acquisitions were generated by using a Monte Carlo method for quantitative comparison between ultra-low-SAR and high-SAR results.
  
  
  RESULTS
  There were minor to moderate differences in the absolute tissue SNR and CNR values and in qualitative appearance of brain images obtained by using ultra-low-SAR and high-SAR techniques. High-SAR 2D T2-weighted imaging produced slightly higher SNR, while ultra-low-SAR 3D technique not only produced higher SNR for T1-weighted and FLAIR images but also higher CNRs for all three sequences for most of the brain tissues.
  
  
  CONCLUSIONS
  The 3D techniques adopted here led to a decrease in the absorbed RF power by two orders of magnitude at 1.5 T, and still the image quality was preserved within clinically acceptable imaging times.
authors.affiliation: Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215, USA. ssarkar@bidmc.harvard.edu
authors.forename: Subhendra N
authors.initials: SN
authors.lastname: Sarkar
authors.forename: David C
authors.initials: DC
authors.lastname: Alsop
authors.forename: Ananth J
authors.initials: AJ
authors.lastname: Madhuranthakam
authors.forename: Reed F
authors.initials: RF
authors.lastname: Busse
authors.forename: Philip M
authors.initials: PM
authors.lastname: Robson
authors.forename: Neil M
authors.initials: NM
authors.lastname: Rofsky
authors.forename: David B
authors.initials: DB
authors.lastname: Hackney
mesh_terms: D000328:Adult; D000368:Aged; D016477:Artifacts; D001931:Brain Mapping / Q000379:methods*; D003287:Contrast Media / Q000008:administration & dosage; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011446:Prospective Studies; D011846:Radio Waves; D018709:Statistics, Nonparametric
chemical_list: D003287:Contrast Media