Potential problems with selective pulses in NMR imaging systems

Most nuclear magnetic resonance (NMR) imaging systems require pulses whose frequency spectrum is shaped so as to selectively excite a given plane in the presence of a magnetic field gradient. We demonstrate by both computer simulation and experiment that linear Fourier transform theory is not a reliable guide to the uniformity of flip angle in the slice. We show by simulation that the nonuniformity can have serious implications for the measurement of relaxation time T1 if selective 180 degrees pulses are used; the exact results depend also on the details of data analysis and criteria for adjusting the rephasing gradients. We describe an experiment and a phantom in which the axial nonuniformity can be demonstrated on clinical NMR imaging machines.