The Nyquist theorem in MRI states that to avoid aliasing (wrap-around artifact), the sampling frequency must be:

• A Equal to the highest frequency component in the signal
• B Half the bandwidth of the received signal
• C At least twice the highest frequency component (2 × maximum frequency) ✓
• D Equal to the Larmor frequency of the nucleus being imaged

Explanation

The Nyquist-Shannon sampling theorem states that to accurately reconstruct a signal without aliasing, the sampling frequency must be at least twice the highest frequency component present in the signal (fs ≥ 2 × fmax). In MRI, if an anatomical structure extends beyond the field of view (FOV), its signal frequencies exceed the Nyquist limit and appear 'wrapped' to the opposite side of the image — this is the aliasing or fold-over artifact (also called ghosting in phase direction). It can be eliminated by increasing FOV, using anti-aliasing oversampling, or swapping phase/frequency directions. The Larmor frequency is the fundamental precession frequency determined by B0 field strength.

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

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Written and medically reviewed by the StethoPrep medical team.