2016_Imaging for Physicists

Water-fat shift

Magnetic field at the nucleus depends on magnetic shielding of surrounding electron clouds, depends on molecular environment example: resonant frequencies of protons in fat and water differ by 220 Hz 220 Hz = 3.4 ppm at 1.5 T 3.4 10 -6 ·1.5T = 5.1 µ T at 1.5 T =5.1 µ T / 3 mT/m = 1.7 mm at 1.5 T and read out gradient 3 mT/m

Water

• artifact

– increases with B 0 – decreases with gradient strength

Fat

Signal intensity

-9

-6

-3

0

3

6

9

Chemical Shift (ppm)