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 Aluminum is handled . The first image shows the big difference of 10mm steel plates where the bad material limits the achievable SNR to about 325 where as a better material is good for a SNR of about 1700 with the same exposure conditions:
A simple test with different IQIs should show the difference, the plate hole IQI of ASTM E1742 for an EPS value of 0.78 for the 1T hole of the .25 IQI and the wire sets where the smallest wire is 50µm thick.
With 64s exposure time at 200kV, 6mA, 1100mm FDD the difference is very well visible.
Reducing the exposure time to 4s will still show a big difference:
and it is quite funny that the 1T hole of the 0.25 IQI is quite good visible – or is it another indication? It is hard to separate between the IQI hole and a “natural imperfection” of the material. Here the wire sets offer an advantage as the wire shall be clear visible along 10mm in minimum reducing the risk to confound with a material imperfection. Next picture shows the setup for the test in the next post:
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