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1471531 Radiography EMI Ltd 8 April 1974 [25 April 1973 11 Aug 1973 17 Aug 1973 21 Aug 1973 11 Oct 1973] 47507/73 39420/73 39070/73 38817/73 19528/73 Heading H4F A method of investigating a planar slice of body by measuring the logarithm of the absorption of each beam forming part of a parallel set of at least 50 such beams and repeating these measurements with sets of parallel beams having a different orientation with respect to the body comprises the steps of not only forming the sum of all the absorptions suffered by all the beams passing through a particular elementary area, but also correcting this to take account of the fact that each beam passing through that area suffers absorption at other points along its path. This correction is achieved by calculating the absorption properties of such an elementary area using the measurements derived from beams passing through that area and from beams passing through other areas of the slice of the body. The measurement of absorption suffered by an individual beam at a particular orientation is modified by adding a sum of a series consisting of the measurements of absorption suffered by adjacent beams in the same parallel set and each multiplied by an "L-factor". These L- factors are derived as solutions of equations resulting from a theoretical analysis of the problem set out at length in the Specification. Briefly, these L-factors are defined by convolutional functions or a function declining monotonically as the distance of the adjacent beam from the individual beam increases, although there may be minor perturbations in the lower order factors. This calculation is repeated for each individual beam in the parallel set then all the modified measurements relating to a particular elementary area are added together. The whole procedure is then carried out for every elementary area enabling ultimately a plot or map of the absorption properties of the whole slice to be drawn or displayed, e.g. on a cathode-ray tube. A digital data processing system is described which uses standard computer techniques to handle and store data between the radiation detectors and the final display and there is also disclosed a specialized computer circuit for performing the repeated multiplications and additions required, Fig. 9 (not shown) (see Division G4). Apparatus.-A suitable apparatus for measuring the absorption of radiation in a body for use in the method described above is shown in Fig. 1. A source of radiation 11 and a detector 12 are scanned horizontally as shown in synchronism by a motor 18 driving a belt 15 connected to the source 11 which is mechanically connected by a yoke 19 to the detector 12. After each scan the source and detector assembly is rotated, e.g. by 1 degree, by a motor 10 driving a rotatable mount 1 and another scan takes place and so on until a complete revolution of the mount 1 has been made. The object under examination is held in a pouch 4 in a tank 6 containing water. A photo-cell 22 controlled by a graticule on yoke 19 determines when a reading from the detector 12 should be taken, thus defining the beam paths. The reading from detector 12 is compared with that from another detector 24 which receives direct radiation from the source, variations in which are therefore compensated. Fig. 2 (not shown) shows how signals from the detectors are integrated, converted to digital form and stored and compared and at the appropriate times are gated to the digital data processing system which performs the calculations as set out above.
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