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

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.