Patent GB-1147384-A (GB)
Title: Radiation camera
Inventor: HARVEY PETER JAMES
IPC: G21G4/08
Filed: 1967-01-06
Abstract: 1,147,384. Television. EKCO ELEGTRONICS Ltd. 5 Jan., 1968 [6 Jan., 1967], No. 929/67. Heading H4F. In a radiation camera, gamma photon emissions from an object O, Fig. 1, pass via a multihole collimator 1 to a crystal detector matrix 2 which converts the emissions into optical signal pulses, i.e. converts the gamma image into a light image. These optical signal pulses are fed with demagnification by optical fibres 3 to a photo-cathode 4 of a 2-stage image intensifier 5 of the phosphor/photo-cathode sandwich type. The amplified optical signal pulses from phosphor 6 are fed by lens or fibre optics to an optical gate 12 which is responsive to a sensing signal derived from the output phosphor 6 of the image intensifier across resistive load 7. Electrical sensing signal pulses are developed across this load proportional to the optical signal pulses released from the detector which correspond to the energy of the incident gamma photons. Each sensing signal pulse is processed in a pulse amplifier 8 and a pulse height analyser 9. Pulses in a predetermined amplitude range corresponding to the wanted gamma energy range are accepted by the analyser to give rise to standard output pulses which are amplified 10 and used to switch the optical gate. This gating operation also rejects scattered and background radiation and limits the noise from the image intensifier. The light output from the optical gate 12 is coupled by lens 17 or fibre optics to the input photo-cathode 19 of storage tube 18, the sequential pulses of electrons from which are accelerated by electrode 20 on to target 22 where proportional positive electrical charges are set up by secondary emission, the excess electrons being collected by the collector grid 21. The storage tube integrates the sequential optical signal pulses to form an electrical charge image on the target of the original radioisotope distribution in the body O. Read-out of the charge image is by means of raster scanning by beam 24 and the output is fed to a conventional CRT, the presentation being in monochrome or colour, various colours being used to display various levels of signal intensity. The storage tube 18 operates on the principle of the image orthicon but with relatively non-destructive read-out. The storage target 22 is a mesh with an insulating coating on the photo-cathode side which releases secondary electrons in excess of the electrons incident from the photocathode thus setting up a positive charge on the insulator. The secondary electrons are collected by the collector grid or mesh 21 which is held positive. Beam 24 of electrons scan the target at a slow rate (1 frame/sec.) and the electrons are de-accelerated to near zero velocity by electrode 23 and then modulated by the positive charge pattern on the target, some of the reading beam passing through the target to the collector 21, the remainder being reflected and amplified by multiplier 25 to form the output signal. For operation with positron-emitting isotopes, gating is on the basis of coincident detection of the annihilation gamma photons. The multi-hole collimator is removed and the gamma image is formed by detecting one of the annihilation gamma photons in the crystal matrix 2 as before, and the second in a scintillation counter 43. The pulses of amplitude proportional to the incident gamma energy are produced by this counter, amplified 44 and analysed 45 and the outputs of the analysers 45 and 9 are fed to a coincidence circuit 15 (switch 16 being in the other position). Gate 12 is thus opened by the coincident detection of the two gamma photons. In a second embodiment, Fig. 4, a singlestage image intensifier 5 is optically coupled directly to a crystal detector 2 or a detector matrix. When the detector 2 is a single crystal detector the light image is formed at the output of the detector by a fibre optic face-plate 3a which is integral to the construction of both detector and intensifier. The intensified light pulses from output phosphor 6 are coupled by lens 11 to an intensifier gate 12a which is a 2-stage device comprising a first photo-cathode/ phosphor gap intensifying stage and a second photo/phosphor gap gating stage switched by the gating pulse amplifier 10 as in Fig. 1.