A spin echo chemical shift MR imaging technique

Abstract

A new method is described that produces images of either the fat or water component in tissues in magnetic resonance imaging. Only a single scan is required, with scan times of a few minutes. Chemical shift selectivity is achieved in the spin echo process by controlling the spectral content of the 180 degree pulse that induces the spin echoes. A theoretical analysis of the selective spin echo process for the case of a radio frequency pulse of constant amplitude shows that spin echoes will be suppressed for certain values of offset frequency that are similar to, but different from, the frequencies at which the Fourier spectrum of the pulse vanishes. The theory was confirmed by experiment on a water phantom. The imaging technique was tested on both a phantom of oil and water and on a human forearm. Excellent suppression of the water signal was found in the fat images, and the small fat component seen in the water images is attributable to components of the triglyceride molecule for which spectral lines overlap those of water. The forearm images also showed blood flow effects in the water image that were not visible in the fat image. The relationship of this method to other proposed methods of chemical shift imaging is discussed.