Iterative RF pulse refinement for magnetic resonance imaging.

Selective RF pulses are needed for many applications in magnetic resonance imaging (MRI). The waveform required to produce a desired excitation profile is, to first-order, its Fourier transform. This approximation is most valid for small tip angles and the quality and accuracy of such excitations decreases with increasing tip angle. Since large-tip-angle excitations are required in most types of imaging, a better synthesis technique is necessary. While a variety of analytical and numerical synthesis techniques based on solution of the Bloch equations are available, these techniques fail to consider the effect of the physical scanner hardware and are often accompanied by computational complexity. We present a technique for selective RF pulse refinement which uses real-time feedback techniques in lieu of a solution to the Bloch equations. Physical experiments are conducted to demonstrate the effectiveness of this algorithm and an extension to pulses of 90 degrees is investigated.