New approaches to selective pulse design

Abstract:: Selective pulse design for noninteracting spins is equivalent to inversion of the Bloch equations. Until recently, few analytical solutions to this problem were known. However, approaches based on inverse-scattering theory have now led to general solutions that offer ever higher precision in meeting target responses. The concept of soliton pulses (pulses that leave the spin system unaffected) turns out to be a particularly valuable one because half-solitons (both π/2 and π pulses) are inherently phase compensated. Such pulses are important for observation of shortT2 species, where substantial signal loss could occur in any refocusing period. Multiply-selective pulses, suitable for simultaneous suppression of several “solvent” lines have been generated by inverse-scattering theory and have considerable potential in bothin vivo magnetic resonance spectroscopy and in routine high-resolution NMR. Although analytical solutions show great promise, it is likely that optimization methods will continue to be of value for the foreseeable future. The use of the SPINCALC scheme that operates in a switched stationary reference frame is illustrated through its use to design adiabatic refocusing pulses that do not lead to cumulative errors when used in multiple-echo trains.

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