Magnetic Resonance Imaging (MRI) Practice Test

What happens to signal-to-noise ratio (SNR) when the number of phase encodings is increased from 256 to 512?

1. SNR increases

2. SNR decreases

3. SNR remains the same

4. SNR becomes inconsistent

The correct answer is: SNR decreases

When the number of phase encodings is increased, the signal-to-noise ratio (SNR) is affected due to the principles of how MRI data is collected and reconstructed. In MRI, SNR is fundamentally determined by the amount of signal collected relative to the background noise present in the system. Increasing the number of phase encodings results in a higher resolution image because more data points are captured in the phase encoding direction. However, it does not inherently increase the total amount of signal received from the tissues being imaged; instead, the same amount of signal is spread over a greater number of phase steps. This dilution of the signal occurs because the time spent collecting data may remain constant or the repetition time (TR) may not significantly change, leading to a situation where the overall signal contribution does not increase proportionally with the number of phase encodings. As a result of this increased number of phase encodings, while the resolution of the image improves, the SNR tends to decrease because the noise, which is generally constant, becomes more significant relative to the signal now partitioned across the additional phase encodings. This dilution effect leads to a lower SNR when phase encodings are increased from 256 to 512. Therefore, the correct understanding is