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For non-rectangular pulses, the flip angle is proportional to the area under the RF-pulse rather than simply B1tp. And for adiabatic pulses, this relationship breaks down completely.
A variety of clever methods using multiple simultaneously acquired echoes have been developed to speed up the process of finding the optimal transmitter gain. See the patent literature in "References" below where several of these methods are explained.
King KF, Licato PE, Slayman BE. Nutation angle measurement during MRI prescan. US Patent # 5,416,412A, dated 16 May 1995.
Feimeier T, Heubes P, Speckner T. Method and magnetic resonance system for adjustment of the field strength of RF pulses. US Patent # 7,573,266 B2, dated 11 Aug 2009.
Sattin W. High signal-to-noise, rapid calibration of RF pulses. US Patent # 4,866,386, dated 12 Sept 1989.
What is the scanner doing during the automatic pre-scan period?
What is meant by flip angle?