Use of frequency-modulated radiofrequency pulses in MR imaging experiments.

With increasing field strength for whole-body MR scanners there is a need for rf pulses of higher bandwidth to perform selective excitation or inversion. AM pulses will have to become shorter thus leading to an enormous requirement for the peak power of the power amplifier. To reduce these requirements the use of frequency modulation is investigated. The effect of pulses based on a linear frequency sweep on the spin system is analyzed by calculation and computer simulation. It turns out that the nonlinear phases induced by those pulses may compensate each other, that therefore spin-echo experiments as usual in MR imaging may be performed, and that the required peak power may be reduced by up to a factor of 50.