Computer assisted tomography in orbital disease.

Computer assisted tomography (CAT) in the transverse axial plane, a fundamentally new method of x-ray diagnosis, was developed from I967 to 1972 by G. N. Hounsfield (I973) at the Central Research Laboratories of E.M.I. Ltd. Clinical evaluation of this technique by J. Ambrose (I973) at Atkinson Morley's Hospital has indicated the enormous value of the investigation in patients with intracranial disease. Minor modifications of the examination technique have enabled us to study the orbit, and this report presents our initial findings. A conventional radiograph is obtained by passing an x-ray beam through a structure and measuring the emergent x rays by their ability to expose photographic film. All the component tissues of the structure will absorb energy from the beam, but the absorption increases proportionally with increasing tissue density. A radiograph thus presents a record in two dimensions of the varying tissue densities encountered by an x-ray beam traversing a three-dimensional structure. Only when component tissues differ quite markedly in density can they be distinguished, and all tissues lying in the long axis of the beam appear superimposed on the radiograph. This difficulty in resolving tissues of similar density, particularly 'soft tissues', and the problem of envisaging the original three-dimensional anatomy have been serious limitations in radiology. In order to define tissues of similar density by conventional means, it has been necessary to alter the density of one tissue by administering a contrast medium. Radiographs in several planes and conventional tomography give only an incomplete picture in three dimensions. These difficulties have limited the investigation of soft tissue masses within the orbit. Computer assisted tomography replaces relatively insensitive photographic film by a sodium iodide crystal which serves as the x-ray detector. The crystal gives off visible light (scintillation) in a proportional fashion when exposed to x-rays and an electrical signal is generated via a photomultiplier system. In this way a very large number of individual readings are taken ofa collimated x-ray beam scanning transverse axial "slices" ofthe head. The readings are processed by a computer and the anatomy of the "slice" is reconstructed