Limits to signal-to-noise improvement by FID averaging in NMR imaging.

We studied the improvement of signal-to-noise ratio (SNR) in NMR images by free induction decay (FID) averaging. We found that as the number of FIDS averaged increases, a region is reached where the image SNR levels off asymptotically. A theoretical explanation for this behaviour is offered. Agreement between experimental results and theoretical prediction is excellent. It is also pointed out that the exact location of the point at which further averaging does not pay-off is dependent on the individual NMR imaging system used.