As Albert Rose already showed in 1946 Spatial Resolution and Contrast to Noise could be exchanged. He did several tests on visibility based on size and contrast of holes of a flat plate:

(From left to right the size is reduced and from top to buttom the depth of the hole)
He found, that the Contrast multiplied with the Object Size devided by the Noise in the image is constant:

Hu published to following graph (please have in mind that at that time FILM was used with it's dose limit):


The formula can be used and with some effort it can be shown that the visibility of holes (D) depends on:


This is the base for most of the IQIs who measure contrast to noise and spatial resolution: Wires, plate hole penetrameter, hole type step wedges. It also paves the way for the .

Here is the link in the knowledge base to .