Image Noise Image noise is the degree of variation of pixel values caused by the statistical nature of radioactive decay and detection processes. The counting noise in nuclear medicine is Poisson noise, so that the pixel noise variance is equal to the mean number of counts expected in a given region of the image.
Standard deviation is the square-root of the variance.
A lesion SNR of about 2 would be marginally detectable. A SNR of about 3 would be fairly easily detectable. Therefore, ideal imaging systems are as efficient as possible.