Computerized transverse axial scanning (tomography): Part 2. Clinical application*

A new and fundamentally different X-ray method is described. The cranium is scanned in successive layers by a narrow beam of X rays, in such a way that the transmission of the X-ray photons across a particular layer can be measured, and by means of a computer, used to construct a picture of the internal structure. Employing a suitably designed scanning gantry, a continuously operating X-ray tube, and a narrow collimated X-ray beam, the transmissions of X-ray photons across a slice of tissue may be measured by a system of crystal detectors in such a way that 28,800 readings are obtained. These form the basis of 28,000 simultaneous equations which are solved by a computer. The solutions are transformed into absorption coefficients and by means of a suitable algorithm related to their correct cells in a matrix of chosen size. The results are stored, computed, and then made available from a magnetic disc to construct a picture on a CRT. The numerical results are available from a print-out. The examination is, therefore, qualitative and quantitative. Pictures thus obtained are looked at in much the same way as radiographs. Structures are identified and shape, size, and position defined. Changes in tissue density are then looked for. Lesions are seen as alterations of normal density and are interpreted in the light of pathological changes which are known to occur. Increased density may be due to blood clot, calcium deposition in tumours, and other lesions. In haemorrhage, once clotting has occurred, the concentrated blood constituents show up as an area of high density. Average tissue density is lowered in tissue necrosis, oedema, cyst formation, and haemorrhage where clotting has not occurred. Tissue density may be artificially enhanced by the intravenous injection of substances containing large atoms;sodium iothalamate has been found to be ideal and the tissue density of a variety of tumours may be enhanced by this means.