A comparison of selective saturation and selective echo chemical shift imaging techniques

We report on a comparative study of two methods of chemical shift imaging which can be used to selectively image fat and water in vivo. Both methods require a B 0 field sufficiently homogenous to resolve the methylene and water spectral lines. One method, called CHESS, uses a chemically selective pulse to saturate the unwanted spectral line. The other method, called SECSI, achieves chemical selectivity by using a soft 180 degree RF pulse in forming a spin echo image. Both methods require that the strength of the B 1 RF field be accurately calibrated and homogenous. We show by theoretical analysis that the suppression of unwanted spectral lines is sensitive to the first power of B 1 errors in the CHESS method but to the second power of B 1 errors in the SECSI method. Experiments with phantoms confirmed the expected non linearity of the SECSI method, and showed superior water suppression factors in phantoms with it. Experiments with a large phantom and a living rabbit showed superior results using the SECSI method, and the best results were obtained using a combination of the two techniques.