Scott Brian Reeder, Presenter: Nothing to Disclose

Angel R Pineda, Abstract Co-Author: Nothing to Disclose

Huanzhou Yu MS, Abstract Co-Author: Nothing to Disclose

Zhifei Wen MS, Abstract Co-Author: Nothing to Disclose

Ann Shimakawa MS, Abstract Co-Author: Nothing to Disclose

Norbert Joseph Pelc ScD, Abstract Co-Author: Nothing to Disclose

To optimize the SNR performance of “Dixon” fast spin-echo water-fat separation by choice of echo time shifts.

Three point “Dixon” water-fat separation techniques are increasingly being with fast spin-echo (FSE) imaging for applications that require uniform water-fat separation despite the presence of field inhomogeneities. Theoretical calculations, however, predict that noise performance of water-fat decomposition depends strongly on the water:fat ratio (WFR) within a voxel and the choice of echo shifts. Noise performance can be described using the effective number of signal averages (NSA). For a three image acquisition, the highest possible NSA is three; a method achieving this is considered efficient.Validation of theory was made using a phantom containing peanut oil layered on saline containing 5mM NiCl2. An oblique slice oriented 30o to the water-fat interface created a continuum of WFRs. Imaging was performed on a 1.5T GE-TwinSpeed scanner with an extremity coil. Parameters: 256x256, ETL=16, FOV=20cm, slice=9mm, BW=32kHz, TR/TE=700/13. Each scan was repeated 200 times to measure the NSA for every pixel in the image.Water-fat decomposition was performed with an iterative least-squares “Dixon” method that permits the use of arbitrary echo times. For each echo combination, NSA was calculated for each pixel from the variance of the signal from source images divided by the variance of the calculated water images, determined from all 200 images.

Theoretical predictions show that NSA is maximized (equal to 3) for all WFRs when echoes are acquired with fat-water phase shifts of –π/6, π/2, and 7π/6. At 1.5T these shifts correspond to time shifts of –0.40ms, 1.19ms, and 2.78ms, with respect to the spin echo. For symmetrically acquired echoes, NSA approached zero when water and fat are equal. Experimental plots of NSA vs WFR showed very close agreement to theoretical predictions for all combinations of echo shifts.

Reliable water-fat separation is highly dependent on the WFR within a voxel and the choice of echo shifts. Asymmetric echoes greatly improve the noise performance of “Dixon” water-fat separation methods used with FSE imaging.

Reeder, S, Pineda, A, Yu, H, Wen, Z, Shimakawa, A, Pelc, N, Asymmetric Echoes for Optimal SNR Performance of “Dixon” Water-Fat Separation with Fast Spin-Echo Imaging.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4412759.html