Patent GB-1468021-A (GB)
Title: Radiology
IPC: G01N23/08
Filed: 1973-04-07
Abstract: 1468021 Focal viewing EMI Ltd 26 March 1974 [7 April 1973] 16783/73 Heading H4F The invention relates to a method and apparatus for the examination of a body by means of penetrating radiation such as X- or γ-radiation and is concerned with a computation process for the derivation of the absorption coefficients of a matrix of notional elements in the body the process employing a combination of a transformation process (e.g. Fourier transforms or convolution integral processes) and a sequential process of successive approximations, e.g. of the kind disclosed in Specification 1,283,915. As shown, the body to be examined is positioned in an aperture 1 in a disc 5 which carries a frame 2 supporting an X-ray (Collidge) tube 3 and a radiation detector 4 (e.g. a NaI scintillator crystal and associated photomultiplier tube) the disc 5 being rotated in steps by motor 8 and the frame 2 being oscillated by a motor 6 and connecting rod 7, also carried by the disc 5, such that during the dwell time between successive rotational steps of the disc a sweep of parallel rays from tube 3 passes through a planar section of the body to the detector 4 from a series of different angles. Output pulses from detector 4 are counted in a counter 10 and read out in response to pulses from a clock 9 which also controls the motors 6 and 8. The digital numbers derived from counter 10 are indicative of the overall absorption values measured along the respective paths and are termed "edge values" and such values are supplied via a logarithmic amplifier 11 to a store 12. The output of the latter is supplied via gate 13 to transform computer 14 which produces signals indicative of the absorption coefficients of individual ones of a plurality of elements in a matrix of elements notionally delineated in the plane section of the body described by the locus of rays from the source 3, such coefficients being termed "mesh values" and to compensate for variations due to the different angles of different rays, weighting factors (assigned as described in Specification 1,283,915) derived from a store 15 are supplied to the transform computer 14 the output "mesh values" being stored in a store 16. The values stored in the latter store are then subjected to a transformation in transform computer 17 (also supplied with the weighting values from store 15) which is the inverse of that performed by transform computer 14 to derive further "edge-values" (in logarithmic form) indicative of what the original "edge-values" should have been if the "mesh-values" as stored in store 16 were correct. The further "edgevalues" which are stored in store 18 are then compared in a comparator 19 with the original (measured) "edge-values" derived from store 12 and the difference (error signals) is supplied via gate 20 (gate 13 being closed) to the transform computer 14 which operates as before to provide "mesh error signals" for addition to corresponding ones of the "mesh values" stored in store 16, i.e. store 16 is updated. A series of such corrective calculations may be performed so that the "mesh values" converge and become indicative of the actual values, a threshold being included (if desired) to stop the calculations when either the "edge error" signals or their transforms reach a value indicating a required accuracy in the "mesh values". The contents of store 16 may then be displayed on a display device 21 such as a cathoderay tube or a tone printer.