Reducing the duration of broadband excitation pulses using optimal control with limited RF amplitude

Combining optimal control theory with a new RF limiting step produces pulses with significantly reduced duration and improved performance for a given maximum RF amplitude compared to previous broadband excitation by optimized pulses (BEBOP). The resulting pulses tolerate variations in RF homogeneity relevant for standard high-resolution NMR probes. Design criteria were transformation of I-z --> I-x over resonance offsets of +/-20 kHz and RF variability of +/-5%, with a pulse length of 500 mus and peak RF amplitude equal to 17.5 kHz. Simulations transform I-z to greater than 0.995 I-x, with phase deviations of the final magnetization less than 2degrees, over ranges of resonance offset and RF variability that exceed the design targets. Experimental performance of the pulse is in excellent agreement with the simulations. Performance tradeoffs for yet shorter pulses or pulses with decreased digitization are also investigated. (C) 2003 Elsevier Inc. All rights reserved.