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) imaging for equivalent effective echo times, fast spin-echo (FSE) imaging 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 signal 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 single-echo images and spectra at identical echo times. One- and two-dimensional spectroscopy experiments confirm that it is the generation of 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 signal is due to the maintenance of in-phase magnetization in the coupled spin systems by this Hamiltonian. In CSE, the weak coupling Hamiltonian allows development of antiphase coherences which, in the presence of the line broadening due to the imaging gradients, result in signal loss.