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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
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Water
Fat
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
• artifact
– increases with B
0
– decreases with gradient
strength
Water-fat shift