Patent GB-1283915-A (GB)
Title: A method of and apparatus for examination of a body by radiation such as x or gamma radiation
Inventor: HOUNSFIELD GODFREY NEWBOLD
IPC: G01N23/08
Filed: 1968-08-23
Abstract: 1283915 Radiation absorption testing E M I Ltd 7 Aug 1969 [23 Aug 1968] 40317/68 Heading G1A [Also in Divisions H4 and H5] The absorption co-efficient for radiation, such as X or gamma rays, as a function of position throughout a given body is determined by exposing the body to the radiation from a plurality of different directions and measuring with a detector the transmission of the radiation by each of a plurality of paths of limited cross-section through the body, the directions and numbers of the paths being such that the body may be notionally divided into a matrix of elements, Fig.3, of dimensions comparable to the cross-sections of the paths, each element is intersected by a group of non- parallel paths, and the different paths of any given group intersect different sets of elements whereby, by a successive approximation method of solving a set of simultaneous equations derived from the detector readings, the values of absorption to be assigned to each element of the matrix may be computed to a desired degree of accuracy. The detector output signal, in digital form, is applied to a logarithmic converter - whereby the absorption contributions of all the elements in a given path may be summed to give that of the complete path - and subsequently to a digital computer for processing: the method of computing the absorption values to be assigned to each volume element is outlined. The digital output of the computer may be converted to analogue form to produce a pictorial representation on a cathode ray tube of the absorption throughout the body Fig.8a, (not shown). For a plane slice of the body, the radiation paths are arranged to lie in that plane, as in Fig. 3; for the complete body, either successive parallel slices are examined, either in sequence, or simultaneously by using a plurality of X-ray sources and detectors Figs.2a to f, (not shown), or oblique radiation paths are employed. To reduce the radiation dose when examining living matter, either a circular scan, the axis of which itself moves slowly in a circle Fig. 5 (not shown), or a spiral scan traversed rapidly at its outer edge and slowly near the centre, may be employed, to give high resolution only in the neighbourhood of interest. Apparatus. In the apparatus of Fig. 6a the body 32 to be investigated is mounted in a hole 27 in a plastics block 26 which lies in the path of one of two beams of X-rays which are derived from a source 20, are alternately interrupted by a chopper 33, and one defined by two pairs of collimators 21, 23, and 22, 24 respectively, the other beam path incorporating a dummy attenuator 25 to provide a reference signal. Each beam path terminates in a scintillator 28 and 29, respectively, the signals from which are fed via a light pipe 30 to a photomultiplier 31. The apparatus, apart from the source 20, is oscillated through the angle subtended by the block 26, while the whole apparatus is slowly rotated about the body 32, the space between the body and the block 26 being filled with water to reduce the range of readings handled by the photomultiplier. A plurality of radiation paths arranged in a fan-shaped pattern Fig. 7 (not shown), each having a scintillator and separate photomultiplier, may be employed, to reduce the angle through which the collimator and detector system must be oscillated.