Microtomography with ionising radiations

Image reconstruction from projections, which is the basis of x-ray Computerised Tomography (CT), was soon extended to other forms of medical radiographic imaging and later to other imaging modalities. The success of medical CT also led to its application to non medical problems. Despite their enormous advantages medical CT images suffer from relatively poor spatial resolution, due to radiation dose limits to the tissues being scanned. For inanimate materials this dose limitation does not apply and micro CT becomes feasible; many applications of micro CT have been published with resolutions in the range 10-50 micro metre on objects up to about 10 mm in diameter. In order to achieve useful density discrimination at micron-scale spatial resolution, a very intense radiation flux is required if the total imaging time is to be kept within reasonable bounds. In the case of X-ray beams two approaches have generally been followed to achieve adequately intense photon fluxes. synchrotron radiation sources yield parallel beams of photons with fluxes in the range 0.3 - 30 * 10 sup 10 photons mm sup -2 s sup -1 which is many orders of magnitude more intense than from conventional X-ray tubes, however recent work has shown that fluxes of about 1 * 10 sup 10 photons mm sup -2 s sup -1 are available for sub-millimetre samples placed close to a microfocus X-ray tube. An essential requirement of all radiographic imaging is that the mean free path of range of the radiation is of the same order of magnitude as the thickness of the sample being studied to obtain good contrast. For objects in the micron range this suggests using energetic heavy ions which have important advantages over photons beams; they are capable of being focused and steered by electric and magnetic fields, they are easy to collimate, and they can be readily detected and their residual energy accurately measured. Monoenergetic proton beams from electrostatic accelerators, focused down to a few microns, have extended micro CT to samples less than 100 micro metre in diameter. These techniques are compared and some illustrative applications are described. 9 figs., 20 refs. (author)