Patent GB-1489652-A (GB)
Title: Hybrid scintillation scanning apparatus
IPC: G01T1/164
Filed: 1975-01-07
Abstract: 1489652 Scintillation scanning apparatus COMPAGNIE GENERALE DE RADIOLOGIE 7 Jan 1975 [10 Jan 1974] 671/75 Heading G1A [Also in Division H4] An apparatus for locating a radio-active source lying on a linear field of view C, Fig.3, has an elongate scintillator bar 1 which receives radiation via a collimator 18; a set of photo-multipliers 101-112 spaced along the bar are used to determine the position of the maximum scintillation and hence of the source. The apparatus may be moved to produce a two dimensional location of radioisotopes in a human body.  The bar 1, e.g. of Tl doped NaI, may be of rectangular, trapezoidal or semicircular crosssection, and is connected to the photo-multipliers by a light guide 16 in contact with a flat face of the bar; the remaining faces of the bar are frosted, except the end faces which are in contact with non scintillating members 9, e.g. of undoped NaI. A metallic case 17 permeable to the radiation from the source but not to visible radiation seals out damp. The collimator 18 has a set of elongate openings 19 which conduct γ or X rays to the bar. The photo-multipliers are connected to the guide 16, preferably by an optical joint, and their glass cases not contacting the guide 16 are made opaque to light, as is the rest of the guide. Circuitry: the photo-multiplier outputs are combined at 30 in one of two ways described below to allow a circuit 40 to compute the position of the source in the X direction, i.e. along the line C. A threshold circuit 50 activates circuit 40 via unit 60 when the total photo-multiplier outputs indicate a scintillation, and unit 60 also operates a record or display 80 via unit 70 at this time. A transducer 90 supplies a signal giving the Y coordinate of the apparatus, so that two co-ordinate information can be recorded or displayed. One form of combination of photo-multiplier outputs is shown in Fig.6, where after preamplification at 31 the signals are summed at 35 to operate the threshold device 50, and are also fed to adjustable threshold circuits comprising amplifiers 32 and  diodes 33. These latter circuits discriminate against noise signals and their outputs are summed at 37 and 38. The summer 37 receives its inputs via resistors 300, all of different value and selected according to the positions of the photo-multipliers along the bar; hence the outputs of the threshold circuits 32, 33 are weighted before summation at 37. The input resistors 380 of summer 38 are all equal however, so the inputs to it are not weighted. The ratio of the outputs of 37 and 38 is practically linearly related to the required X co-ordinate, and is found at 41 (corresponding to 40 of Fig.3). An alternative form of combination is shown in Fig.7 where all the photo-multiplier outputs are preamplified, hiffered and supplied to different points of a delay line 340. The photo-multiplier nearest the scintillation produces the largest signal, and this arrives at the ends of the delay line at different times; by measuring the time which elapses between the arrival of these two largest signals the X coordinate is found. Units 367,368 convert the signals to bi-polar ones whose zero-crossings are detected at 369, 370. The time between the signals is found at 42 by observing the difference between the voltage of a ramp generator at the moment when the first signal is detected and at the moment when the second one is detected, the generator being initially triggered by the summed photo-multiplier signals via units 35, 50 and 60. If desired, only the signal at one end of the delay line need be used, with a single bi-polar converter and zero crossing  detector, the X co-ordinate being found by the time taken for the zero crossing signal to occur after the initiation by unit 60. The display 80 may be an oscilloscope gated by unit 50; the display may include a computer memory for storing the coordinates.