In MRI physics, the term 'flip angle' refers to the angle through which the net magnetisation vector is rotated by the RF pulse. A flip angle of 90° is used in conventional spin-echo sequences. In gradient echo (GRE) sequences, a smaller flip angle (e.g., 10–30°) is used. What is the PRIMARY advantage of using a smaller flip angle in GRE sequences?
- A Allows shorter TR (faster imaging) without saturation of longitudinal magnetisation ✓
- B Increases the signal-to-noise ratio (SNR) proportionally
- C Eliminates T2* decay effects and susceptibility artefacts
- D Increases T1 contrast between tissues
Correct answer: A. Allows shorter TR (faster imaging) without saturation of longitudinal magnetisation
Explanation
Using a small flip angle in gradient echo sequences allows the longitudinal magnetisation to recover rapidly between excitations — because only a small fraction is tipped into the transverse plane, the residual longitudinal component remains large. This permits very short repetition times (TR) without saturating the signal, enabling ultra-fast imaging (e.g., FLASH, SPGR). The trade-off is increased susceptibility to T2* effects (no 180° refocusing pulse), not elimination. SNR is reduced with small flip angles.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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