Flow imaging by high speed transmission tomography.
Radiation transmission tomography has found widespread use in cross sectional imaging of industrial processes, particularly for research purposes. Many laboratories have built their own 3rd generation mechanical scanners based on one fan beam collimated source and several detectors in an arc shaped array. Such systems are sufficient for imaging of slow processes or where temporal averaging of the process dynamics is acceptable. Applications where the process dynamics is to be investigated is more demanding.
Fourth generation medical x-ray (CT) scanners utilize rotating X-ray source and a fixed circular detector array as sensor head. The minimum time required for one rotation of the source is in the range of half a second for the fastest scanners. However, even this is not sufficient for imaging of fast processes such as pipe flows. When the rotation is too slow compared to the process dynamics, the reconstructed image will become motion blurred because process features enter and leave the image plane before the data acquisition is completed. In order to avoid this type of inconsistent measurement, all measurements should be carried in a short time compared to the time constants of the process dynamics. Several approaches have been reported on high speed imaging: One is to use several fixed radiation sources each of which faces an array of detectors on the opposite side of the process, and do all measurements simultaneously. A system based on five 241Am gamma-ray sources and five corresponding arrays of detectors is capable of gas/liquid pipe flow imaging at rates of about 500 frames per second. The next approaches utilize electronic source scanning. Two flow imaging systems are based on a ring of miniature, switchable X-ray sources are developed and are capable of image rates of 2000 and 50 frames per second. Another approach is the application of a scanned electron beam source which focuses the beam on a linear tungsten target and at the same time periodically deflected with a high frequency. This system provides image rates up to 104 frames per second. These systems will be presented and their performance will be discussed in the light that imaging of industrial processes and their dynamics implies a trade-off between three conflicting requirements; the measurement resolution of time, space and matter.