4714884 Method of eliminating effects of spurious NMR signals caused by imperfect 180 degree RF pulses

Abstract

In an NMR imaging method employing multiple spin-echo pulse sequences, a method is disclosed for eliminating ghosting artifacts in an NMR image. The method comprising the steps of (1) maintaining a static magnetic field along a first axis of an NMR imaging sample, (2) exciting a plurality of nuclear spins in a region of said NMR sample by irradiating said sample with an RF pulse, (3) irradiating said NMR sample with a series of 180° RF pulses such that each RF pulse inverts the orientation of a first group of the excited nuclear spins by 180°, whereupon inherent imperfections in at least one of the 180° pulses cause a second group of excited nuclear spins to change orientation by an angle other than 180°, (4) applying first and second magnetic field gradient pulses to the NMR sample before and after, respectively, the application of at least one of the 180° RF pulses thereby causing no net accumulation of phase in the first group of excited nuclear spins, (5) applying an imaging gradient after the 180° RF pulse in order to focus the plurality of nuclear spins and thereby create an NMR spin-echo signal, (6) sampling the NMR signal during a predetermined time interval between 180° pulses in order to detect the spin-echo signal; and (7) modulating each pair of first and second magnetic field gradients with respect to other pairs associated with the series of 180° RF pulses in order to prevent refocusing of the second group of nuclear spins for the duration of all of the time intervals during which the NMR signal from the sample is detected for the purpose of constructing an image.