Abstract: The effects of a spurious free induction decay (FID) NMR signal due to imperfect 180.degree. RF pulses are eliminated by phase alternating the selective 90.degree. RF pulses and subtracting alternate ones of the NMR signals. The desired signals due to the 90.degree. RF pulses reinforce, while the signals produced by the imperfect 180.degree. pulses cancel. This method also has the beneficial effect of eliminating DC offset in the desired signal which, when present, causes artifacts in NMR images. Another method to eliminate the effects of spurious FID signals due to imperfect 180.degree. pulses is to phase alternate these 180.degree. pulses on successive NMR imaging pulse sequences and add successive NMR signals. In this case, the phase shifted spurious FID signals cancel, while the desired NMR signals reinforce. The latter method does not eliminate the effects of DC offset, however.

Abstract: Selective excitation is used to define a thick planar slab of excited nuclear spins in a nuclear magnetic resonance (NMR) imaging sample. The thick slab is selected such that the excited spins are contained well within the optimum sensitive region defined by the radio frequency (RF) transmitter and receiver coils. Three-dimensional spatial information of an NMR imaging parameter, such as nuclear spin density or nuclear spin relaxation time, is collected simultaneously from the excited slab and can be used to construct a series of several tomographic section images of the slab. The spatial information is encoded in the NMR signal by application of pulsed gradient magnetic fields subsequent to excitation. Image picture information is obtained from the NMR signals via three-dimensional Fourier transformation.