What happens to SNR when only half of the phase encodings are performed?

When half of the phase encodings are performed in MRI scanning, the signal-to-noise ratio (SNR) is impacted due to the way MRI images are reconstructed. Each phase encoding contributes to the sampling of the k-space, the data matrix that forms the basis of the image. When only half of the required phase encodings are used, the amount of signal collected is reduced, as there is less information contributing to the final image.

SNR is fundamentally influenced by the amount of signal collected relative to the amount of noise present. When phase encodings are halved, the signal intensity is effectively reduced by a factor as well, which leads to a decrease in SNR. Specifically, the reduction in the number of phase encodings means only √2 of the original signal is collected, resulting in the SNR decreasing by a factor of √2.

This principle illustrates the relationship between signal acquisition and image quality in MRI. Fewer phase encodings result in a lower SNR, which can negatively affect image clarity and increase the likelihood of noise dominating the image representation.