There are a few different materials for Image Quality Indicators (IQIs). The reason is, that the IQIs should fit to the absorption of the material in test. E.g. in ASTM E1025 nine different material groups are defined increases in attenuation, from Material Group 001 for polymer matrix composite materials, over 03 for magnesium, 02 aluminum, 01 titanium to the more heavy material groups without the Zero in the beginning. Examples are the Material Group 1 for stainless steel, Material Group 3 for Inconel, and Material Group 5 for phosphor bronze like gun metal.
As long as the IQI is only a percentage of the total material in the beam, the quality of the material is not of highest interest. But some standards as ASTM E2597 or E2737 use IQIs which are totally exposed and all material imperfections will be visible and may influence the results. As detectors - especially DDAs - improved a lot in quality, these imperfections of the "base material" are on a level that they dominate the results. An example could be seen where the material imperfections of the five groove wedge dominates the visibility of the grooves at longer exposure times.

Therefore a task group at ASTM was found to evaluate materials for light metal (e.g. Aluminum) and heavy metal (e.g. Steel) which have high homogeneity or uniformity and low internal structures. The topic for Steel is handled .
With Aluminum the problems of the internal material structures are dominant, as AL6061 is mandatory for ASTM E2597 stepwedge - which limits the performance of the detectors in test. In the past 15 years (since first E2597) the Contrast Sensitivity of the DDAs has improved a lot. This leads to the strange situation that the quality of the test equipment becomes the limiting factor for testing of CS and SMTR with the Aluminum stepwedge.
To give a feeling what I am talking about, have a look at the first picture, where the material limit of AL6061 is compared to material limit of AL7022 of a 40mm flat plate. Please notice, that the window width is identical for both images. The material limit of AL6061 is about 350 and of AL7022 more than 1600 (factor 4.6).


As with the example of steel I start with images with IQIs on a base plate of 40mm each, left with AL6061 and right with AL7022. The 1T hole of the .75 plate hole IQI correspondents to an EPS value of 0.65 and the 2T hole to a value of 1.0. The thinnest wire of the wire set has 50µm diameter.

With 30s exposure time the 1T hole is visible on AL7022 and borderline on AL6061. From the wire set wire #16 is clearly visible for 10mm length on AL6061, on AL7022 wire #18 is clearly visible and on a good monitor also the smallest wire with 50µm can be seen.
Reducing the exposure time to 0.5s shows a lot more noise in both images, but still the visibility of the 1T hole with 0.65 EPS is visible and the wire #16 clearly on AL7022. On AL6061 the 1T hole has gone and the 2T hole may be confounded with an internal structure; wire #15 is clearly visible.

The material structures influence the visibility of details already at shorter exposure times.
To get quantitative values a similar test as for the steel plates was performed with the radiation quality of E2597 for Aluminum:

Within the test, four different alloys of Aluminum are tested:
AL6061 (todays reference in E2597 and E2737)
AL2024
AL7075 and
AL7022.
In the next images the reference AL6061 is compared to AL7022. The Live-Image (1/15s) does not show big differences

but already at 1s exposure time the difference in notable:

With 30s the difference is factor 5 (!). This material will limit the CS test in E2597 to values below 11 in the spidernet diagram.

The maximum achievable SNR with the four different alloys of Aluminum is shown in the next diagram. The calculation was done as describes in the parallel .

We can see that with AL6061 a SNR value above 500 is difficult to reach. This is a better value than film of CR can achieve, but for today’s DDAs this is not more a real limit. With AL7022 the limit increases to values above 2500 – which is still today a challenge.
From extrapolation of the 1s value we could calculate the best possible SNR when structures of the material does not influence the result. We can see that up to 8s exposure time with this radiation quality the AL7022 does not have any influence to the results and we get already a SNR value of 1350.


Just for fun I defined a Material Efficiency – the 100% score would be the perfect material. To do this I took the maximum SNR value of each of the four alloys from the difference image at identical positions (blue column - X-ray SNR). The red columns are the SNR values of the difference when the plates are moved – this is the X-ray SNR and the Material SNR. The right diagram shows the Material SNR divided by the X-ray SNR in percent. AL6061 is here below 20%, AL7022 is above 80%.

If an Aluminum alloy should be taken for solid IQIs as in E2597 or E2737 we should take AL7022 for today’s detectors.