Micro CT is a computed tomography system that utilizes a micro-focus X-ray tube as the source and typically uses a flat panel detector array. The combination can render an image with resolution as low as 2 microns. Of course to do this one needs a manipulator that is capable of even more accuracy. This is why diondo uses a solid granite frame construction.
Metrology and Granite
It has long been recognized by master machinists that granite provides the best surface for metrology. Granite is an igneous rock formed by volcanic eruptions. By comparison, marble is metamorphosed limestone. For metrology use, the granite selected meets specific requirements outlined in Federal Specification GGG-P-463c, Part 3.1 3.1 . Granite should be a fine to medium-grained texture and gray in color. Young’s Modulus of Elasticity is used to express the flexibility or indication of the hardness of the stone. Gray granite averages 5-7 points. Making it soft enough not to chip easily for every day use.
One might ask why we spend so much time and money to encapsulate the micro CT system in granite. It is evident that multiple axis movements on a less rigid structure will cause deviations in the measured accuracy of the CT machine. To assure the quality of our calibrations over time, it is necessary to use the best materials and positioners available. The diondo micro CT system uses granite to maintain an accuracy of movement to +/- 1 micron.
The focal spot to detector distance (FDD) is an important factor in the quality of your CT image because it provides you with enough space to get the highest magnification factor possible. With micro CT the focal spots can be as small as 2 microns. There is not allot of radiation because Power [W] = Resolution [µm]. Having FDD continuously variable from 1200mm to 400mm will allow one to increase the dose to the detector and thus shorten scan time. Let's say engineering is testing a process for butt welds and has 40 samples that they need scanned at a resolution of 50 microns. If scanning at a mag factor of 2.68X, you are able to set your integration time to 2000ms per projection at a distance of 1200mm FDD. Each scan is about 60 minutes. However, because of the inverse square law, set the FDD closer to 600mm the same 50 micron resolution can be attained in about 500 milliseconds per projection and the total scan time is now 15 minutes. The total project scan time goes from 40 hours down to 10 hours. This makes a great deal of difference in the material sciences because so many samples require CT. We should come away with the points as follows:
Micro CT Systems Need Maximum Flexibility
Highest dose rates for fast CT scans at small FDD
Higher resolution due to large FDD
Larger 3D scan field due to large FDD
The micro CT system is used to analyze multiple materials such as composites and new alloys developed in conjunction with additive manufacturing processes. The micro CT has become invaluable to the quality lab as a tool to look for flaws in materials. When used in conjunction with metrology testing such as tensile testing and environmental chambers, a volumetric record can be kept of crack migration and characteristics of material weakness.