What effect does doubling the NEX have on SNR?

Doubling the number of excitations (NEX) in an MRI scan leads to an increase in the signal-to-noise ratio (SNR) by a factor of the square root of 2 (√2). This relationship arises because SNR is dependent on both the amount of signal generated and the noise present in the system.

When you increase NEX, you effectively average the signal across multiple acquisitions. This averaging increases the signal strength but does not fully increase the noise in the same proportion because the noise sources are normally distributed. Therefore, while the signal increases linearly with NEX, the noise increases less significantly, following a statistical principle that leads to the square root relationship. Consequently, the actual improvement in SNR is √NEX, which specifically means that doubling NEX results in an increase in SNR by about 41%, or √2.

Understanding this relationship is essential for optimizing image quality in MRI, as it allows practitioners to balance scan time and image clarity effectively.