The fat-water chemical shift difference is approximately which value in ppm?

• A. 1.5 ppm
• B. 3.5 ppm
• C. 5.0 ppm
• D. 7.0 ppm

Answer: (B)

Fat and water protons resonate at slightly different frequencies because their electronic environments shield the nuclei differently. This difference is about 3.5 parts per million, a standard reference value used across MR imaging. At typical field strengths this corresponds to a measurable frequency difference (about 220 Hz at 1.5 Tesla), which explains why fat and water signals can be in-phase or out-of-phase depending on the echo time. This 3.5 ppm separation is the number you’ll see cited most often when discussing chemical shift artifacts and fat–water separation techniques.