A method for correctly setting the rf flip angle

Abstract

Currently the accepted method for setting the correct rf power levels to achieve 90 degrees and 180 degrees rf pulses for MR imaging is to peak the echo amplitude of a rf spin-echo sequence. The echo amplitude of this alpha-2 alpha pulse sequence is proportional to sin3 (alpha) and has a relatively broad maximum. Recently another method for setting the rf flip angle by maximizing the ratio of the stimulated echo to the primary echo amplitudes (in a 3 alpha sequence) demonstrated accuracy similar to that of the spin-echo method using a shorter repetition time. We present a new, more sensitive, and more accurate method for setting the correct rf power levels for 90 degrees and 180 degrees rf pulses. In this method, based upon the stimulated echo pulse sequence, we are able to accurately set the rf power to within +/- 0.1 dB by minimizing the signal amplitude of the third spin echo. This null method works for both selective and nonselective rf pulses of flip angle 90 degrees or 180 degrees, allowing the user to accurately adjust the relative amplitudes of the four rf pulse types within a single pulse sequence.