Water-fat separation by diffusion weighted imaging
Fat suppression or water-fat separation is usually based on the difference in chemical shift between water and lipid protons. This approach is, however,difficult to implement, if the spectral separation between the water and lipid signals is compromised by low magnetic field strength and/or excessive field inhomogeneity. Another approach, based on differences in T1 relaxation (STIR), is not affected by the spectral separation between water and lipid protons, but is not practical for obtaining images with standard T1-weighted contrast.
It is demonstrated here how differences in the apparent diffusion coefficient (ADC) can be exploited for robust, straightforward, water- fat separation, even in low and not perfectly homogeneous magnetic fields, preserving T1-weighted contrast in the fat-suppressed image.
The method is based on the following steps:
1. Image at low b-value
Water and fat at full intensity
2. Image at high b-value
Water significantly suppressed. Fat at close to full intensity.
Fat image
3. Subtract 2 from 1
Water image
Demonstration of principle
Consisting of four 1.5ml vials in following arrangement: OIL, 0.5 mM NiCl2, 4 mM NiCl2, WATER
Phantom:
b ~ 0
b = 1233 s/mm2
subtraction
Images were acquired with a PGSE sequence, TR=400, TE=17,Slice = 6 mm, FOV = 6.4 cm, 256x256, NEX = 3, = 4 ms, = 7.2 ms, interleaving low-and high b-value acquisition by alternating the values of the diffusion-sensitizing gradient.
Live mouse:
SE reference image
Axial plane perpendicular to reference:
The axial images were acquired using a first-order-motion-compensated SE sequence (with bipolar diffusion gradients), TR=400, TE=21, slice= 6 mm, FOV=4.5 cm, 128x128, NEX = 3.
Implemented on Aspect M2 scanner housed in the Department of Veterinary Resources at the Weizmann Institute of Science. (1T permanent magnet).
