[Producing the third dimension of flat radiographic images: analogue tomography - computer tomography]

The inventor of computer tomography, Godfrey N. Hounsfield, mentioned in his Nobel Foundation lecture the following three main problems of conventional radiography: 1. It depicts the sum of shadows from several tissue elements at the same spot of the film. 2. The sensitivity of the employed photographic medium does not allow to differentiate between various soft tissue densities. 3. It gives no exact measure characterizing tissue densities. The first part of this article is dealing with the solution of problem number one. Researchers within medical imaging from many countries developed independently of each other a variety of methods for body-sectional imaging methods, but he did not construct any equipment. Alessandro Vallebona constructed equipment and published the first clinical body-section imaging material ever in 1930, but his method was not ideal. The first clinical material employing an ideal method was published by Bernhard Ziedses des Plantes in 1932. Methods for transverse axial tomography was independently described by William Watson in 1937, Jean Kieffer in 1938, and Shinji Takahashi in 1947. The limitation in sensitivity of the photographic medium was revealed in parallel to this development. In 1963 Allan M. Cormack described the mathematical model for absorption of ionizing rays in inhomogenous tissue. Godfrey N. Hounsfield combined this knowledge of mathematics with the fast developing computer technology and the medical need for a more sensitive registration medium. In 1971 computer tomography was a reality. One of the greatest conquests in medicine of this century had been made. Recent developments within scanner technology predict still better and safer diagnostic possibilities.