Abstract
The use of heavy charged particles, such as protons, a-particles and heavy ions, in computed tomography CT is explored. The technique is based on certain physical properties of tissue, the stopping power, which has never been used by any available diagnostic method. Advantages and limitations of this modality are compared to those of X-ray CT. In a detailed study of their interaction with matter, the stopping power and range of charged particles are calculated for different materials of biological interest. A correction to the standard Rossi formula, defining the root mean square angle arising from small-angle multiple scattering, has been derived. The correction is more important for proton and a-particles, particularly when they traverse thick targets. The dose advantage and mass resolution improvement for particles over X-rays are presented quantitatively. The object surface dose, and to a lesser extent the dose at the centre, are remarkably reduced when using charged particles. The effect of small angle scattering on spatial resolution is examined and shown to be improved by employing an exit position detection system. The potential of several particles for computed tomography have been compared. The particles performance together with design considerations indicate the feasibility of a clinically useful particle accelerator which could be implemented for tomography. Simulated data for reconstructed tomography using particles were generated for a head phantom using the general purpose SNARK reconstruction programme, basically designed for X-rays. The programme was implemented to accommodate the way particles convey information about the sample traversed and to account for the influence of the beam divergence. Reconstructions obtained for different particles demonstrate the possibility of producing images of comparable spatial resolution to X-rays, especially with the heavier ions. Charged particle tomography would add a new dimension to the practice of diagnostic radiology for differentiating and imaging the body structure sensitively and relatively safely.