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The term microtomography generally refers to x-ray computed tomography with a resolution of better than 100 microns. Because of the range of different specimens that can be studied, it is important that resolution, x-ray energy and exposure are matched to the composition and size of the specimen. Where control over specimen size is possible, it should be made as small as possible since there is a third order relationship between size and required exposure for a given resolution and contrast. In first generation microtomography systems the specimen is stepped though a single collimated x-ray beam and the attenuation recorded by a single detector. The ability of such systems to record both number and energy of transmitted photons facilitates quantitative evaluation of noise and errors in the detection system and their propagation through to the reconstructed image. Most microtomography scanners are similar in principle to third generation medical scanners, except that the specimen, rather than the source/detector system, rotates. Images from such scanners are subject to ring artifacts because of slight differences in response of the detector elements. Fourth generation medical scanners overcome this problem by using fixed ring of detectors with only the source rotating, but it is impractical to build a microtomography scanner using the same principle. We have designed a fourth generation microtomography scanner which uses a CCD camera operated in time delay integration mode. This overcome the problem of ring artifacts and allows specimens larger than the CCD imaging area to be scanned.
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