What happens to SNR when TR is increased?

When the repetition time (TR) is increased, the signal-to-noise ratio (SNR) generally increases. This occurs because a longer TR allows more time for the spins of the protons in the tissue to return to their equilibrium state before the next radiofrequency pulse is applied. As a result, the signal received during the next acquisition is stronger.

A longer TR gives more protons the opportunity to relax fully, which adds to the number of available spins contributing to the signal. Consequently, when we conduct our imaging with a higher number of coherent signals, we enhance the SNR, leading to clearer and more defined images.

Other factors, such as the inherent noise of the MRI system, remain relatively constant regardless of TR, further emphasizing how the increase in signal outweighs any minor increases in noise, leading to improved SNR.