Multiple echo chemical shift imaging

Abstract

In the disclosed pulse and gradient switching sequences, refocusing pulses are timed to provide spectral encoding so that a resulting series of spin echoes each include both spatial and spectral information for chemical shift imaging. Atoms within an object are excited and may then be spatially encoded, as by a phase encoding gradient. A series of refocusing pulses is then applied, each followed by an observation gradient field. Each of the refocusing pulses after the first one may be displaced in time relative to the observation gradients in order to provide spectral encoding. In an alternative embodiment for limited spectral resolution of fat and water, the observation gradients may also be timed so that echoes may be added to compensate for the decreasing amplitudes of the successive spin echoes. In order to increase the spatial resolution, a multiple shot excitation sequence may be applied, each shot beginning with an excitation pulse, with the first refocusing pulse after each excitation pulse occurring after a corresponding initial time interval. In order to obtain the desired resolution, the initial time interval may be incremented by TE/2M+dt/M for each successive excitation pulse, where TE is the time between successive spin echoes, dt is the delay time of subsequent refocusing pulses and M is the total number of excitation pulses. As a result, any desired level of resolution may be obtained.