The regulations specify a minimum optical density of 1.2 at the center of the phantom image when exposed under typical clinical conditions. Is there an upper limit on this density and why does it have to be measured in the center of the phantom?
No, there is no specified upper limit. A trend has developed in recent years towards increasing the optical density of mammographic images, albeit at a somewhat higher dose, to take advantage of the resulting higher contrast. The amount of density increase will always be limited by the fact that the film contrast will peak (and so will the visibility of image details) before the density gets too high. Measurement of the optical density is specified to be in the center of the phantom because this location is most representative of the mean density and is easy to find consistently.