Gamma Cameras
Deviations from the NEMA standard
Source position
Some manufacturers including SMV (now GE) and Siemens use a point source at considerably less than 5 x FOV but include a curvature correction. This corrects for the difference in the photon flux between the centre and the edge of the field of view.
The image on the right was acquired using a Siemens ecam. It will need a geometric correction to be applied before true uniformity results can be calculated.
Total counts
When performed as a daily check the total number of counts is often reduced to 3, 5 or 10 million counts per detector allowing much faster acquisition. Reducing the total counts will mask some any subtler changes in the uniformity but will still show if any major defects have arisen.
Standard Deviation
Often the standard deviation of the pixel values will be quoted as part of the results. This is because the NEMA values show the results for the worst pixels only and do not give an indication of the extent of the non-uniformity. A similar indicator that is sometimes used is the number of pixels lying outside 5% of the mean value.
