On MRI, fat appears bright on T1-weighted images. When a fat-suppression technique (chemical shift fat sat) is applied, the signal from fat drops significantly. Which MRI property of fat is responsible for its T1 hyperintensity?

• A Fat has a very short T1 relaxation time due to efficient dipole-dipole interactions between methylene groups ✓
• B Fat protons have a very short T2 relaxation time
• C Fat has a high proton density compared to water
• D Fat absorbs more radiofrequency energy at the Larmor frequency

Explanation

On T1-weighted MRI, substances with short T1 relaxation times appear bright (hyperintense). Fat has a very short T1 due to efficient dipole-dipole interactions among closely packed methylene (-CH2-) protons of lipid chains, which match well the tumbling frequency for T1 relaxation. This makes fat consistently T1-bright. Fat also has intermediate T2 but the T1 shortening dominates T1-weighted contrast. Chemical shift fat suppression exploits the small resonance frequency difference between fat and water protons (~3.5 ppm) to selectively null fat signal. High proton density increases signal but does not specifically cause T1 brightness.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.