Medical CT scanners like the one below employ an x-ray source and the detector that rotates around the patient. The rotation is usually only 180 degrees in order to limit exposure to radiation.
Industrial CT systems employ a stationary x-ray source and detector. The sample to be examined is rotated usually through 360 degrees.
Unlike the traditional transmission x-ray images that can be visualized in real time, the images produced by a CT scan must be reconstructed after the scan is complete. A computer algorithm called back projection is used to reconstruct the CT image using the data collected during the CT scan. This is often a very time intensive process and parallel computer clusters are used to speed the process.
The image below shows a transmission image and reconstructed image of the same part. The black regions in the CT image are pores or voids in the aluminum part left behind from the casting process.
A computer simulation of a CT scan is useful in several ways. First, as a training tool for users who find it difficult to obtain time on actual systems that are very expensive and are heavily used. Second, for users who wish to optimize their CT set-ups before they try to use an actual CT system. Third, a computer simulator can be programmed to simulate common errors allowing the user to see the effects on the images. Fourth, simulated data can be used to fill in gaps in real data that would otherwise be unattainable. And, lastly, a wide variety of real or manufactured defects can be inserted in any orientation into any CAD model and tested to ascertain its detectability in a real CT scan.
The next image is from the main CT Scan set up page in SimCT. It shows the green CAD part above the detector (light blue). A simulated radiograph appears in the upper right and had the CT slices selected by the user superimposed over the radiograph. The color image on the bottom right is the thickness map of the part with the flaw shown in red.
In a computer simulation, the first calculated result is the thickness map. This shows the result of the ray trace through the CAD model. The user can refer to the thickness table and determine what the appropriate x-ray machine settings should be to penetrate the part and give a good radiograph.
The image below shows SimCT’s main CT setup page after the CT scan has been simulated. The part with the flaw inserted is shown in a different orientation. The radiograph and thickness maps have been replaced with the sinograms for the first slice. A sinogram is created when the data from each slice and each rotation is combined into one data set. Each vertical line in the images below represents one slice from each of the 360 transmission images.
At this point in the simulation, the CT scan is complete and it is time to reconstruct the data to get the cross sectional CT images.
The next image shows seven of the ten slice images that were generated by the reconstruction algorithm.