A method for correctly setting the rf flip angle

✍ Scribed by William H. Perman; Matt A. Bernstein; John C. Sandstrom

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 524 KB
• Volume: 9
• Category: Article
• ISSN: 0740-3194
• DOI: 10.1002/mrm.1910090104

✦ Synopsis

setting the correct rf power levels to achieve YO" and 180" rf pulses for MR imaging is to peak the echo amplitude of a rf spin-echo sequence. The echo amplitude of this 01-201 pulse sequence is proportional to sin3( a) 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 3a 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 YO" and 180" rf pulses. In this method, based upon the stimulated echo pulse sequence, we are able to accurately set the rfpower to within kO.1 dB by minimizing the signal amplitude of the third spin echo. This null method works for both selective and nonselective rfpulses of flip angle 90" or 180", allowing the user to accurately adjust the relative amplitudes of the four rfpulse types within a single pulse sequence.