Selective properties of trains of real and complex RF micropulses in slice-selective spin-echo MRI

The spin nutation properties of frequency selective (space selective in combination with a magnetic field gradient) trains of radiofrequency micropulses were studied in a numeric model. Two cases were considered, one simulating the 90 degrees excitation pulse in spin-echo MRI, the other the 180 degrees spin inversion pulse. Image reconstruction according to the 2-D Fourier transform technique requests that the effect of the 180 degree pulse is independent of the initial phase of the spin vector relative to the radiofrequency field. It was found that with a train of phase-stable radiofrequency micropulses with an envelope of the traditional 'sinc' type the result of the spin inversion pulse was depending on the initial phase whereas this was not the case for a train of phase-shifted (complex) micropulses. The complex train of radiofrequency micropulses also had better selective properties both for excitation and spin inversion than the real one.