Fat/Water Chemical Shift

What is meant by the fat/water chemical shift? How is it calculated?  

As described in the prior Q&A, the strongly electronegative oxygen atom of a water molecule pulls away the protective electron clouds covering the hydrogen nuclei. This deshielding effect exposes the hydrogen protons to a relatively stronger local magnetic field than they would otherwise experience. Hence they resonate slightly faster than the more shielded protons in triglyceride (fat) molecules. This difference in resonance frequencies between water and fat increases with increasing magnetic field strength through a scaling factor called the chemical shift.

Deshielding of hydrogen nuclei in water by electronegative oxygen

The chemical shift between fat and water has been measured to be approximately 3.5 parts per million (ppm). The suffix ppm is interchangeable with x10⁻⁶, just as the symbol % is interchangeable with 1/100, x0.01 or x10⁻². The chemical shift is given as a ratio because the actual frequency difference (in Hz) will depend upon the applied magnetic field.

Calculating the actual fat-water frequency difference is easy — you simply multiply the Larmor frequency at whatever field strength you are using by 3.5 ppm or 3.5 x 10⁻⁶.

Spectral peaks for fat and water over a large anatomic area broadened due to inhomogeneity effects. The average chemical shift difference is about 3.5 ppm.

In a 1.5T scanner (operating at 64 MHz), the fat-water frequency difference (Δf) will therefore be

Δf = (64 MHz)(3.5 ppm) = (64 x 10⁶ Hz)(3.5 x10⁻⁶) ≈ 220 Hz

At 3.0T where the Larmor frequency is 128 MHz, the fat-water frequency shift would be twice as large, or about 440 Hz.

Advanced Discussion (show/hide)»

Water molecules have two equivalent ¹H nuclei with the same chemical shift (δ=4.65). Triglycerides, however, contain at least, a dozen non-equivalent ¹H nuclei with different values of δ. The most plentiful ¹H nuclei in fat are those in methylene (−CH2−, δ=1.3) and methyl (−CH3, δ=0.9) groups. Fat thus has a broad spectrum and a single fat-water chemical shift value does not exist. Instead, a weighted average value in the range of 3.3 to 3.5 ppm is generally quoted. 

Multiple chemically non-equivalent ¹H nuclei in a triglyceride molecule give it a broad spectrum. The two dominant peaks are methylene (−CH₂−) and methyl (−CH₃) with chemical shifts (δ) = 1.3 and 0.9 respectively.

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References
     "Chemical Shift". Wikipedia, the Free Encyclopedia.

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