As mentioned in there is a second parameter which defines the detail visibility: The contrast. While unsharpness shows the resolution perpendicular to the x-ray beam, contrast defines the resolution in direction of the beam. As far as I know there is only one standardized IQI for measurement of the contrast: ASTM E1647 with recesses large enough to exclude unsharpness issues (beside E2597 grooved stepwedge):
All other IQIs measure contrast and spatial resolution at the same time with the IQI, e.g. hole penetrameter, step-hole penetrameter or wires. To show that, an 8mm steel weld with an ISO19232-1 wire set was radiographed with the best film (C1) and with a DDA with 200µm large pixels - both with no magnification. The film was digitized with a high class film scanner and both images highpass filtered to show the differences better on a standard monitor.
The film image can show the wire 16 with 100µm (~1.25% contrast resolution); with the same visibility the DDA with 200µm pixel size can show the 50µm wire (~0.625% contrast resolution) - which is just 1/4 of the pixel size.
There is a second test to prove the sub-pixel visibility. To avoid too high contrast due to the more absorbing wire, grooves were used here with a prototype of the five groove wedge.
The detector has 400µm pixel size (PerkinElmer RID512/400 AF1) and can show the 127µm groove:
You can see the groove - but you could only measure in the range of 400µm. What happened to make the small grooves visible? Let's take an example with a -2% contrast groove which covers just 1/4 of a pixel in one direction:
When the Signal to Noise Ratio (SNR) is high enough, the groove is visible; as the groove is a very long "feature", it would be visible with a Contrast to Noise Ratio (CNR) of 1 - in the example with a SNR of 200 and more.
For evaluation of the visibility of wires (ISO19232-1) a test was done with a 2 focal spot tube (1.0 and 0.4 to EN12543) on a 10mm and 20mm steel plate with a DDA with 200µm pixel size (PerkinElmer XRD1621). The visibility of a wire required as the minimum a 10mm continious visible line; additionally it was measured to have a CNR of 1 for this size. The smallest wire has 50µm diameter. Different SNR levels were archivied with different integration times (detector calibration was done for a SNR of 2000 with a moving steelplate in the beam).
The graph show that the visibility of the wires depends mainly on the SNR. With 10mm steel the focal spot size is nearly independed, with more material thickness the smaller focal spot is better - but of course needed factor 2.5 longer exposure times.
As wires are not so popular in all regions of the world, the test was repeaded with holes. A step hole IQI (ISO19232-2) was used for the next picture with similar conditons as before. The visibility of the holes were evaluated and a hole had to have a CNR >2.5 to be rated as visible. The smallest hole has a diameter of 125µm (5/1000").
With the hole type IQIs the result is very similar - the SNR is the important parameter which determines if a hole is visible (has a stronger signal compared to the noise level).
Conclusion: If the noise level is very low, you can see details much smaller than your pixel. To be honest - you knew that before; if you ever take a picture were a powerline was in, you will see the - compared to the distance - really small wires in your picture (and sometimes have to use the Photoshop to remove them for a perfect image).
In X-ray images you should not rely on the sub-pixel resolution, because sometimes a pixel becomes a bad pixel. Please see the Fig 11 in the DDA guide ASTM E2736 for aqualitative guideline for detectability of a feature with respect to contrast-to-noise (CNR).
