Magnetic Resonance Imaging (MRI) Practice Test

What effect does increasing the flip angle have on signal-to-noise ratio (SNR)?

1. Decreases SNR

2. No effect on SNR

3. Increases SNR up to the Ernst angle

4. Variable effect depending on the sequence

The correct answer is: Increases SNR up to the Ernst angle

Increasing the flip angle in MRI can significantly influence the signal-to-noise ratio (SNR) of the images produced. When the flip angle is increased, the magnetization vector is tilted more toward the transverse plane, leading to a larger signal being produced during the readout phase of the imaging sequence. The relationship between flip angle and SNR continues to increase until it reaches the Ernst angle, which is the optimal flip angle that maximizes the SNR for a given T1 relaxation time of the tissue being imaged. Beyond this angle, further increases in flip angle can actually lead to a decrease in SNR due to various factors, including increased T1 effects and potential saturation of the magnetization. Therefore, the correct choice reflects the principle that increasing the flip angle initially enhances SNR, but the enhancement is maximal up to a certain point, specifically the Ernst angle. After this point, the ability to generate a useful signal may diminish. Understanding this relationship is crucial when optimizing MRI protocols for specific applications, as it helps in balancing image quality and acquisition time.