Signal-to-noise ratio and absorbed power as functions of main magnetic field strength, and definition of “90°” RF pulse for the head in the birdcage coil
✍ Scribed by Christopher M. Collins; Michael B. Smith
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 409 KB
- Volume
- 45
- Category
- Article
- ISSN
- 0740-3194
- DOI
- 10.1002/mrm.1091
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✦ Synopsis
Abstract
Calculations of the RF magnetic (B~1~) field as a function of frequency between 64 and 345 MHz were performed for a head model in an idealized birdcage coil. Absorbed power (P~abs~) and SNR were calculated at each frequency with three different methods of defining excitation pulse amplitude: maintaining 90° flip angle at the coil center (center α = π/2), maximizing FID amplitude (Max. A~FID~), and maximizing total signal amplitude in a reconstructed image (Max. A~image~). For center α = π/2 and Max. A~image~, SNR increases linearly with increasing field strength until 260 MHz, where it begins to increase at a greater rate. For these two methods, P~abs~ increases continually, but at a lower rate at higher field strengths. Above 215 MHz in MRI of the human head, the use of FID amplitude to set B~1~ excitation pulses may result in apparent decreases in SNR and power requirements with increasing static field strength. Magn Reson Med 45:684–691, 2001. © 2001 Wiley‐Liss, Inc.