GB1474685A - Radiography - Google Patents

Abstract

1474685 Radiography EMI Ltd 10 July 1974 [12 April 1973] 17732/73 Heading H4F A method of examining a slice of a body by X- or γ-radiation as described in Specification 1,283,915 in which the sum of the weighted estimates of the absorption (or transmission) coefficients of the notionally delineated elements of the slice interserted by each ray of each set of rays is compared with the actual absorption readings, wherein before any resulting error signal is fed back to up-date the stored estimates they are modified in such a way that they will more nearly re-construct the picture. As shown, Fig. 1, the estimates from the store 23 appropriately weighted 25a-25n by a weighting function from store 24 are summed 26 and compared 19 with the actual absorption reading for a ray 8 of radiation passing through a slice of the body 1, but before any resulting error signal is applied via multipliers 27a-27n and dividers 28a-28n back to the store 23 to up-date the estimates stored therein, the error signal from the comparator 19, e.g. 19 (r+1) Fig. 3, (it is assumed in Fig. 3 that there is a comparator for each ray in a set thereof) is applied to a respective combining circuit, e.g. 31 (r+1), together with signals derived (e.g. from 30r and 30r+2) from the errors signals relating to rays of the same set disposed adjacent the particular ray. The factor P by which the error signals are divided in circuits 30 is greater than two. Before the up-dating signals are fed back to store 23 they are divided by a factor which is less than the number of sets of rays to be processed. In addition to dividing each error signal by P and subtracting it from error signals derived from the two adjacent rays, e.g. rays r and (r+2), it may also be divided by q (where q>P) and subtracted from error signals derived from the (r - 1) and (r+3) rays. The weighting functions supplied to multiplers 25a-25n and 27a-27n are related to the degree of overlap between a ray path and each element of the slice to which the coefficient estimates relate. An alternative modification of Fig. 1 is described in which instead of including the arrangement of Fig. 3 in the feedback path back to store 23, the weighting function is chosen to cause the current estimates of the coefficients relating to elements overlapped by the ray path to more than a predetermined amount to be weighted positively and the corresponding estimates of the coefficients relating to elements overlapped by the ray path to less than this predetermined amount to be weighted negatively (Fig. 2(b), not shown).