The first 2D images were films. The definition of image quality parameter for film exposures are described in the ASTM Guide E94 for film:
The parameters for digital images are quite similar.
The unsharpness values from the film structure has as equivalent the detector unsharpness - or (pixel size)*2. The unsharpness due to the limited size of the focal spot und geometric magnification is identical.
On the other side the subject contrast is identical also (same object) and the film contrast has it's equivalent in the detector contrast.
The biggest difference is the middle part. When buying the film, the film granularity is fixed with the film class (slow film with small grains).
If you never did an X-ray film, you may remember with your film photo camera, that the best image quality came with the slow films of ISO50 or ISO100 (or 50 / 100 ASA). The films with ISO 400 or even worse ISO 800 were very noisy that you could see the grains already on 10x15cm (0.4x0.6") print. And there was the problem, that it was difficult to change the film if you would need another sensitivity or different ISOs. I used to have two bodies, one with the 50 ASA and the landscape lens and one with 200 ASA with the tele lens. Today with the digital camera I can change the ISOs as easy as the shutter speed 
.This is similar with digital detectors (DDAs); you can set the ISO speed (which is normally called "sensitivity" or "gain") dynamically at the detector where you may have the factor of 50 in sensitivity. Of course, the high sensitivity images are noisy, too - but you can take several hunderts of them and add them together and the noisy will be by the square root less compared to a single image.
Photographers who do the amazing do the same: taking a lot of pictues and add them in the PC - with a motorized tripod to compensate the rotation of the earth - or GPS-based AstroTracer in some cameras.
