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A method for correctly setting the rf flip angle
Author(s) -
Perman William H.,
Bernstein Matt A.,
Sandstrom John C.
Publication year - 1989
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.1910090104
Subject(s) - flip angle , rf power amplifier , amplitude , echo (communications protocol) , pulse sequence , radio frequency , pulse (music) , signal (programming language) , physics , nuclear magnetic resonance , sequence (biology) , spin echo , power (physics) , optics , acoustics , computer science , telecommunications , magnetic resonance imaging , chemistry , optoelectronics , amplifier , medicine , radiology , computer network , biochemistry , cmos , quantum mechanics , detector , programming language
Currently the accepted method for setting the correct rf power levels to achieve 90° and 180° rf pulses for MR imaging is to peak the echo amplitude of a rf spin‐echo sequence. The echo amplitude of this α‐2α pulse sequence is proportional to sin 3 (α) 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 3α 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° and 180° 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° or 180°, allowing the user to accurately adjust the relative amplitudes of the four rf pulse types within a single pulse sequence. © 1989 Academic Press, Inc.