- The T1 values for fat are much shorter than those of water
- The hydrogen protons of water resonate slightly faster than those of fat. This difference in resonance frequency is known as the water-fat chemical shift.
A large number of methods have been developed to suppress, augment, or separate water and fat signals for MR imaging and spectroscopy. These techniques accomplish their goal by exploiting differences in T1, resonant frequency, or both.
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Although for decades just considered a storage site, most modern physiologists now consider adipose tissue to be a major endocrine "organ". Adipose tissues produce hormones (leptin, resistin, and TNF-α) and help regulate levels of blood sugar, cholesterol, and triglycerides.
The structure and function of adipose tissue varies by location, including subcutaneous white and brown fat, visceral fat, marrow fat, and fat within organs. All adipose tissue is not the same. Although mostly composed of triglycerides, adipose tissue also contains variable amounts of water, cells, blood vessels, and connective tissues. Even if a perfect "fat sat" pulse were devised, we would not expect it to suppress these other tissues.
Ren J, Dimitrov I, Sherry AD, Malloy CR. Composition of adipose tissue and marrow fat in humans by ¹H NMR at 7 Tesla. J Lipid Res 2008; 49:2055-2062.
What is meant by the fat/water chemical shift? How is it calculated?