Brain MR imaging at ultra-low radiofrequency power

Radiology. 2011 May;259(2):550-7. doi: 10.1148/radiol.11092445. Epub 2011 Feb 25.


Purpose: 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.

Materials and 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.

Conclusion: 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.

MeSH terms

  • Adult
  • Aged
  • Artifacts
  • Brain Mapping / methods*
  • Contrast Media / administration & dosage
  • Female
  • Humans
  • Image Enhancement / methods
  • Imaging, Three-Dimensional
  • Magnetic Resonance Imaging / methods*
  • Male
  • Middle Aged
  • Monte Carlo Method
  • Phantoms, Imaging
  • Prospective Studies
  • Radio Waves
  • Statistics, Nonparametric


  • Contrast Media