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

When the number of phase encodings in MRI increases, the overall signal-to-noise ratio (SNR) is affected primarily due to the inherent properties of how images are reconstructed. Increasing the number of phase encodings typically results in finer spatial resolution, which means each individual phase encoding contributes disproportionately to the SNR at higher counts.

However, while more phase encodings can yield higher resolution images, it doesn't directly increase SNR in a linear manner. In fact, doubling the phase encodings does not lead to a doubling of SNR; rather, the relationship is more complex. Specifically, SNR improvement from additional phase encodings exists, but it is tempered by the overall increase in noise as the imaging time or the required signal averaging increases, often leading to a decrease in effective SNR per voxel.

Thus, the choice that states the SNR decreases reflects a more accurate understanding of the interactions in MRI signal acquisition and processing when increasing phase encodings.