Abstract: A reduction in motion-induced image artifact, occurring when imaging moving subjects using Fourier transform (FT) NMR techniques, is achieved by recognizing that the greatest contribution to the artifacts is simple geometric motion (such as respiration). The method relies on avoiding the synchrony between the sequence repetition time of an FT pulse sequence, such as spin warp, and the period of the motion. In one preferred embodiment, the different amplitude values of the phase-encoding gradient are selected randomly, rather than sequentially. In another preferred embodiment, gradient amplitude values are selected by alternating high and low values in the temporal implementation of the pulse sequence.

Abstract: A method for providing NMR images free of chemical shift artifacts, obtained from selected nuclei of a sample containing nuclei having chemically-shifted NMR frequencies is described. Non-selected nuclei of the sample are selectively saturated prior to applying an NMR imaging sequence by applying to the sample an RF pulse containing frequencies equal to the chemically-shifted frequencies of the non-selected nuclei and having a narrow frequency spectrum about the chemically-shifted frequencies. Alternatively, selected nuclei are directly excited during application of an NMR imaging sequence by applying to the sample an RF pulse containing frequencies equal to the chemically-shifted frequencies of the selected nuclei, and having a narrow frequency spectrum about the chemically-shifted frequencies so as to exclude signal contributions from the non-selected nuclei. These selective pulses are applied in the absence of the imaging magnetic field gradients.

Abstract: Improved nuclear magnetic resonance (NMR) imaging methods based on a Carr-Purcell-Meiboom-Gill (CPMG) RF pulse sequence and pulsed magnetic field gradients applied to both multiple angle projection (MAP) imaging techniques, and to the two-dimensional Fourier transform (2DFT) or spin warp imaging techniques, are disclosed. The improved methods involve the generation of multiple spin echoes induced by a repetitive sequence of phase alternated 180.degree. nonselective pulses, wherein the resulting spin echoes are used to provide substantive improvements in the pixel signal to noise ratio, and/or to generate images which reflect substantially the transverse relaxation time T.sub.2. The improved imaging methods are particularly useful when directed to the detection and localization of various disease states of biological tissue, which exhibit altered T.sub.2 relaxation times indicative of changes at the molecular level.

Abstract: Selective excitation is used to excite to resonance nuclear spins located in a predetermined region of an NMR sample. Variable amplitude gradients are then used to phase encode spatial information into the NMR signal produced by the excited nuclear spins. Fourier transformation of the NMR signal, observed in the absence of magnetic field gradients, yields information of the spatial distribution of chemical shifts in the predetermined region of the NMR sample. In NMR chemical shift imaging methods using multiple angle projection reconstruction techniques, projections are built up along a gradient point-by-point by use of selective RF pulses having variable frequency content. Non-selective 180.degree. RF pulses are used with the imaging methods to produce NMR spin echo signals.

Abstract: A method for eliminating the effects of a spurious free induction decay (FID) NMR signal due to imperfect 180.degree. RF pulses comprises applying a large magnitude, short duration magnetic field gradient pulse, termed the "crusher" pulse immediately following the 180.degree. pulse. When the method is employed with NMR pulse sequences in which the 180.degree. pulse is part of a spin echo type refocusing RF pulse sequence, the 180.degree. pulse is preceded by a magnetic field gradient pulse termed the "primer", having an equal integral with respect to time as the crusher pulse. The method is effective in removing NMR image artifacts produced by spurious FID in both planar and three-dimensional NMR imaging methods.

Abstract: Selective irradiation is used to isolate a planar slice of nuclear spins in a nuclear magnetic resonance (NMR) sample. Selective radio frequecy pulses, applied in the presence of magnetic field gradients, are then employed to define a smaller sensitive volume in a predetermined region of the isolated planar slice. An NMR spin echo signal produced by excited nuclear spins situated in the thusly localized volume is observed in the absence of magnetic field gradients and, upon Fourier transformation, yields the desired localized chemical shift spectra.

Abstract: The T.sub.2 * effects due to inherent inhomogeneities in the static magnetic field are overcome by the use of a 180.degree. RF pulse to produce a nuclear spin echo suitable for obtaining spatial information and by the adjustment of the pulsed gradients such that the occurrence of the spin echo due to the applied gradients coincides with the occurrence of the nuclear spin echo derived from the rephasing of spins in the presence of the inhomogeneities inherent in the main static magnetic field. The 180.degree. RF pulse is applied with the imaging gradients turned off so that long RF pulse lengths can be used, thus reducing RF power requirements. Exemplary applications of the pulse method to multiple angle projection reconstruction and two-dimensional Fourier transform (spin warp imaging) imaging techniques are also described.

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.