In MRI, the phenomenon of 'chemical shift artefact' (Type 1) occurs at fat-water interfaces because:
- A Fat and water protons precess at slightly different Larmor frequencies, causing spatial misregistration along the frequency-encoding direction ✓
- B Fat has a shorter T1 than water, causing phase cancellation
- C Fat suppression pulses fail to null all fat signal
- D The slice thickness produces volume averaging at fat-water boundaries
Explanation
Chemical shift Type 1 artefact occurs because fat protons resonate at a frequency approximately 3.5 ppm (220 Hz at 1.5T, 440 Hz at 3T) lower than water protons. During frequency encoding, the MRI scanner spatially encodes position by frequency, so fat and water protons in the same location are mapped to slightly different pixel positions along the frequency-encoding direction. This creates a bright band on one side of fat-containing structures and a dark band (signal void) on the other side — classically seen around the kidneys or orbit (fat-water interface). Higher field strength (3T vs 1.5T) worsens this artefact; fat-suppression sequences eliminate it.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.