Four topics are considered - each associated with a different aspect of imaging.

Using X-ray diffraction it is possible to say much about the structure of molecules. Two models for the D.N.A. molecule are analysed with respect to the diffraction data. The models are Watson and Crick's "Double Helix" and Rodley's, recently developed, "Side-by-Side". It is demonstrated that the side-by-side is a viable alternative model for D.N.A. However the low quality of the data precludes a definitive decision as to the actual structure of the molecule.

Conventional X-ray computed tomography body scanners, while producing impressive results when imaging stationary objects, can not image rapidly moving organs such as the beating heart. As the heart motion is periodic, it has been suggested that stroboscopic techniques be employed. However the resulting imaging quality is poor when standard image reconstruction methods are used. By taking account of the fact that the region surrounding the heart is stationary, though, a significant improvement in image quality can be obtained. A simple procedure for achieving this improvement is presented here.

Ultrasonic transmission tomography is more complicated than the X-ray case because ultrasonic rays, unlike X-rays, are diffracted as they pass through a body. Therefore they are generally curved. It is shown how ray curvature makes it impossible to image certain types of objects exactly. Nevertheless it seems that useful results can be obtained by treating the rays as being straight, and using X-ray computed tomography image reconstruction algorithms.

Imaging using electric currents is examined. The types of independent measurements that can be made are discussed. The imaging problem is far from. trivial and, in the general case, largely unsolved. Here a method for uniquely imaging circularly symmetric conductivity distributions is outlined.