COHERENCE TRANSFER BY ISOTROPIC MIXING IN CARR-PURCELL-MEIBOOM-GILL IMAGING - IMPLICATIONS FOR THE BRIGHT FAT PHENOMENON IN FAST SPIN-ECHO IMAGING

It is well known that when compared to conventional spin-echo (CSE) im aging for equivalent effective echo times, fast spin-echo (FSE) imagin g experiments yield higher signal intensities for coupled spin systems , such as that for lipid. One hypothesis put forth for this phenomenon is the removal of scalar coupling-based echo amplitude modulation by the FSE pi pulse train. This would result in the maintenance of signal intensity in the late echoes, with an overall increase in image signa l when the multiecho train data is combined to form the image data, it will be shown that in images and spectra obtained from the final echo of a Carr-Purcell-Meiboom-Gill (CPMG) pi pulse train, an increase in signal in coupled spin systems occurs, when compared to conventional s ingle-echo images and spectra at identical echo times. One- and two-di mensional spectroscopy experiments confirm that it is the generation o f an isotropic mixing Hamiltonian by the pi pulse train in FSE that is responsible for the increased signal in images of a simple AX system and of corn oil, a model for human fat. This relative increase in sign al is due to the maintenance of in-phase magnetization in the coupled spin systems by this Hamiltonian. In CSE, the weak coupling Hamiltonia n allows development of antiphase coherences which, in the presence of the line broadening due to the imaging gradients, result in signal lo ss.