GB1493594A - Radiography - Google Patents

Abstract

1493594 Radiography EMI Ltd 15 Jan 1975 [31 Jan 1974] 04562/74 Heading H4F Apparatus for examining a slice of a body by X- or γ-radiation which is of the type described in Specification 1,283,915 in which the body is irradiated with a fan shaped spread of radiation which is detected by a plurality of detectors, the radiation source and detectors being orbited around the body so that it is irradiated along a plurality of angular positions, each detector receiving radiation along a narrow but divergent beam, wherein absorption data signals representing the absorption of radiation along paths of uniform width are produced by combining the detector output signals relating to beam paths at angular positions 180 degrees apart. Thus the orbital motion of the source and detectors around the body is not restricted to 180 degrees (which would in theory be sufficient if the divergent beams are implicitly treated as though they are of uniform width) but is continued for 360 degrees so that for every beam disposition of the first 180 degrees of scan there is a second which is identical except for the fact that the direction of passage of radiation is reversed. The average of the two beam absorptions is then taken to produce data corresponding to a beam path of substantially uniform width considering the limited angular spread of each beam. The data signals may either be sorted into parallel sets (i.e. data produced by parallel beams) using a digital computer comprising an address selector and store, or the data signals are directed to a store divided into n sections (there being n detectors) so that each section holds the data of one parallel set i.e. the sorting into parallel sets is effected as the data is derived. The claims in this Specification are directed towards this sorting of the data signals into parallel sets. The combination of the data from parallel sets 180 degrees apart are effected in the reverse order to which they are derived and stored. The detectors may be grouped into categories so that for example the 1st, 6th, 11th &c. detectors are allocated to category one, the 2nd, 7th, 12th &c. to category two &c., i.e. the categories are interlaced, a category of detectors being sampled every 2/15 degrees of orbital movement so that after 2/3 degrees the detectors of the first category are resampled. Thus data for 2/3 degrees spaced parallel sets are derived but with five times the number of beam paths otherwise possible for a given data storage capacity in the computing and storage network.