Spatially selective RF pulses and the effects of digitization on their performance

Spectrometers make use of D/A converters to generate RF and gradient shapes. This paper examines by exact simulations the time and amplitude digitization effects, inherent to the use of D/A converters, on the performance of amplitude modulated (AM) frequency selective RF pulses. By making use of Fourier theory and the small tip angle approximation, an approximate model of these effects on the magnetization slice profiles is derived and verified for several pulse types by computer simulations. This approximate model will be used to derive requirements for D/A converters with respect to spatial localization. The dynamics of the spin system allows pulse width modulation (PWM) as an alternative to AM for pulse envelope encoding. The effects of PWM on the slice profile are examined and compared with conventional AM pulses. It is shown by simulation and measurement that adiabatic PWM pulses can be found. In contrast to AM modulated adiabatic pulses, adiabatic PWM pulses have side bands with the same slice quality as the main slice and might therefore be useful as multislice selective pulses.