IMPROVEMENT IN POSITRON CAMERAS

Abstract: Positron cameras in which an area (17, 18) of detectors (15, 16) are arranged to detect gamma photon emissions from a positron emitting isotope injected in an object to be examined. The detected emission is electronically treated and converted into readings such as images. By increasing the number of detectors (15, 16) the resolution is enhanced. The detectors (15, 16) are constituted by scintillation detectors or crystals optically connected to photomultiplier tubes (14). In accordance with the present invention the number of the scintillation detectors (15, 16) are increased without increase of the number of photomultiplier tubes (14), the increase of the number of scintillation detectors (15, 16) for each photomultiplier tube (14) being possible by using scintillation detectors (15, 16) which all for each photomultiplier tube (14) show its own characteristics.

Description: IMPROVEMENT IN POSITRON CAMERAS The present invention relates to positron cameras and more closely to an improvement increasing the resolution of such a camera. Positron cameras are primarily used for tomography, i.e. for examining the human body. A short-lived positron emitting radiopharmaceutical is introduced into the patient's blood system for circulating with the blood around the whole body. Each emitted positron interacts with an electron causing annihilation of the two particles. At the interaction two gamma photons are generated and emitted in almost exactly opposite directions. These gamma photons are detected by ring detectors arranged around the patient. Each such ring detector consists of a large number of scintillation detectors such as scintillation crystals arranged in a circle and optically connected to the face plate of photomultiplier tubes (PM tubes). The patient lies in an aperture in the gantry and a certain percentage of the gamma photons will therefore strike the scintillation detectors in the detector rings. When this occurs, the scintillation detector emits a tiny flash of light which is detected and amplified by the PM tube. The outputs or signals from the PM tubes are coincidence checked which means that if one gamma photon is received by one scin illation detector then the other simultaneously emitted gamma photon must have been received by an oppositely located scintillation detector. If so, the coincident signals are fed to the main computer via an intermediate storage memory and sampling circuit. OMPI fa WIPO From the large volume of positional and event data, the main computer is able to reconstruct an image and/or provide quantitative information corresponding to the local concentration of the isotope. The number of detector channels determines the resolution and accordingly, the more detector channels in the positron camera, the better resolution of the images. Up to now, a scintillation detector or scintillation crystal has been connected to its own PM tube to form a detector channel in positron cameras in general use. However, researches are going on in order to find other solutions. As the PM tubes are rather expensive and as a positron camera of today contains about 400 PM tubes it is a desideratum at least not to increase the number thereof and it is, of course, a desideratum to increase the resolution especially if this can be done for a reasonable price. The PM tubes which are in general use within this field are rather bulky and accordingly, primarily due to lack of space, it is not practically possible to increase the number of PM tubes in the positron camera rings. However, in order to enhance the resolution it is, as is clear from what is stated above, necessary to increase the number of detector channels then including an increase of the number of scintillation detectors or crystals. The problems mentioned above are well-known to the man skilled in the art and researches are being made in order to find solutions to the problems. One attempt has been to reduce the size of the PM tube. The result thereof has been that the smaller PM tube has lower sensitivity and higher price than the conventional PM tube. Further, in such a solution the basic concept, namely one scintillation detector - one PM tube is still valid. OMPI Accordingly, such a solution may to some extent solve the space problem but gives rise to other problems such as lower PM tube amplification and higher price. Another attempt has been to make multiple PM tubes, which practically means that more than one PM function has been included in one and the same tube. Also said PM tubes seem to be less satisfactory and they are neither yet generally available. A further attempt has been to mount several scintillation detectors on one and the same PM tube in which case solid state light detectors have been arranged one in connection with each scintillation detector in order to indicate which of the detectors on the tube being struck by a gamma photon. However, it has not yet been possible to find such a light detector sufficiently effective definitely to indicate a gamma photon strike on the scintillation detector. Also other and more sophisticated attempts have been carried out but up to our invention no one has reached a solution which has been practically and economically acceptable. One example of positron cameras in which our invention is possible to bring into practical use is the positron camera model PC384-7B manufactured by Instrument AB Scanditronix in Uppsala, Sweden. Said camera is an emission tomographic scanner designated to provide as highly resolved images as previously possible of the distribution of injected positron emitting pharmaceuticals, primarily in the brain of humans. The apparatus consists of a gantry containing detectors mounted in a mechanical wobble system, a patient support couch and electronic equipment. A total of 384 scintillation crystals are arranged in four rings of 96 scintillation crystals each with a PM tube mounted to each crystal. The outside diameter of the ring is 48 centimeters and the inside diameter, i.e. the aperture diameter, is 27 centimeters. The scintillation crystals are of bismuth germanate (BGO) type. This type of apparatus is well-known within the hospital field and accordingly, a detailed description thereof does not seem to be motivated in this connection. For an understanding of the invention it is sufficient to mention that the scintillation crystals are arranged in a cylindrical area to be surrounding the object to be examined. In order to obtain a high collection efficiency of the emitted gamma photons the scintillation crystals are packed together as close as possible. The wobbling action of the detector rings increases to some extent the spatial resolution but in order to further increase the resolution the size of the individual scintillation crystals must be reduced and, in order to maintain a high collection efficiency, the number of scintillation crystals in a ring must be increased. The object of the present invention is to .reduce the above problems as much as possible and to make possible an increase of the number of crystals without increasing the number of PM tubes. Thereby the resolution of the positron camera is enhanced without any great increase of costs of the positron camera. This object is reached by a structure of the type referred to in the claims and in accordance with which each PM tube is optically connected to at least two scintillation detectors or crystals, the novel feature being the fact that the scintillation detectors are of to * OMPI V such compositions that they convert the gamma photon strikes into tiny flashes of light the light from each scintillation detector in each detector channel showing its own characteristics. The invention is closer described in the following, reference being made to the accompanying drawings, in which Fig. 1 is a schematical front view, partially in section, of a gantry comprising four ring detectors, Fig. 2 is a schematical side view of the gantry shown in Fig. 1, Fig. 3 is a side perspective view of a multiple PM tube having one scintillation crystal for each photomultiplier included in the tube, Fig. 4 is a side perspective view of a single PM tube provided with several scintillation crystals, each crystal including a solid state light detector, Fig. 5 is a perspective view of a PM tube to which two scintillation detectors or crystals are optically connected, the structure being in accordance with one embodiment of the invention, and Fig. 6 is a diagram showing the transient light output obtained from two scintillation detectors of different characteristics. The gantry 1 shown in Figs. 1 and 2 comprises four detector rings 8 each consisting of twelve casettes 2 in turn containing eight photomultiplier tubes (PM tubes). A scintillation crystal 4 is mounted to each PM tube. A ^ head 5 of a human is shown within the gantry aperture 6 and arrows 7 show schematically gamma photon emissions from the annihilation of the injected emitting isotopes. Such a gantry is conventional. In Fig. 3 there is shown a multiple PM tube 9 provided with four scintillation crystals 10, all of the same characteristics. Such an arrangement operates in the same way as four single PM tubes having one scintillation crystal each. The difference being that several PM structures are enclosed in a common vacuum tube. In Fig. 4 there is shown a single PM tube 11 provided with eight scintillation crystals 12, all of the same characteristics. In order to find out which of the eight crystals 12 that is scintillating each crystal 12 is combined with a solid state light detector 13. The structures shown in Figs. 3 and 4 have not yet, if ever, been sufficiently developed to be practically useful. A useful structure is shown in Fig. 5 and the embodiment of the invention shown therein comprises a conventional PM tube 14 provided with two scintillation crystals 15, 16 each having its own characteristics. In this embodiment one of the crystals 15, 16 is of BGO type and the other of GSO type. The gamma photon detecting areas 17, 18 are each of about half the size of the corresponding areas of the crystals 4 included in the gantry rings shown in Figs. 1 and 2. As the volume of each scintillation crystal 15, 16 is about half the volume of the crystals 4 just mentioned also the price thereof is about the half. In Fig. 6 there are shown the different responses obtained from the two scintillation crystals 15, 16 when struck by gamma photons of the same energy (511 keV). Graph A belongs to the BGO crystal and graph B to the GSO crystal. The graphs A and B represent light waves emitted from the scin illation crystals and detected and amplified in the PM tube for transferring to electronic equipment for further treatment and reading. From the above it is clear that each PM tube can be provided with as many different scintillation crystals as there is space for, of course of a size necessary for practical use. The only requirement is the fact that the scintillation detectors or crystals show different characteristics such as emitted light decay constants which in turn leads to different shapes in the corresponding electrical signals from the PM tube to which they are optically connected, and the electrical signals are separated by the electronic circuitry into different channels. The following table shows five different scintillation detector compositions which in different combinations are useful for realizing the present invention. Said compositions are only examples and in no way delimiting the invention. In the described embodiment of the invention we have suggested BGO and GSO as there is a difference of five times of the decay constant which is sufficient to obtain separable outputs and such scintillation crystals are generally available. -- KEAtr OMPI , Λr- IPO x TABLE Properties of potential scintillation detectors for posi- ron computed tomography Bi 4 Ge 3 0 12 NaΙ(Tl) GsF BaF 2 Gd SiO (Cβ) Effective atomic number 74 50 53 54 59 Density (g/cm 3 ) 7.13 3.67 4.64 4.89 6.71 Decay constant (ns) 300 230 2.5 0.8/620 60 Light yield (photons/Mev) 4800 40000 2500 2000/6500 6400 Emission wave¬ length (nm) 480 410 390 225/310 430 Index of refraction 2.15 1.85 1.48 1.57/1.55 1.9 Energy resolution at 511 keV (FWHM %) 11 9 25 13 14 Hygroscopic no yes yes little no te _. ( , - OMH As mentioned, the signals from the PM tubes are treated in electronic circuitry equipment, e.g. in an ORTEC 460 amplifier and an ORTEC 458 pulse shape analyzer, and after computer treatment of all the signals from all the detector channels the gamma photons emitted from the distribution of injected positron emitting isotopes have been- converted into images of higher resolution than before and carrying more information about the state of the object being examined. An important advantage of the present invention is the fact that the number of scintillation detectors or crystals can be increased several times without increasing the number of PM tubes. On the contrary, it is possible to decrease the number of PM tubes. This means that further to the advantage of the enhanced resolution, the costs for the apparatus can be lowered or at least maintained at the same level as before. Depending of the costs of the scintillation detectors the decreased costs for the PM tubes compensate more than well for the higher costs of the scintillation detectors, if higher. As is clear from the above, the present invention involves a great progress within the positron camera field and it fulfills the object mentioned in the preamble of the description. However, the above description is only intended to explain the invention and is in no way delimiting the same. A man skilled in the art may realize many modifications of the invention. Accordingly, it is not necessary to make use only of scintillation crystals but also other detectors are useful provided they fulfill the characteristics mentioned above, and they can be connected directly to as well as by e.g. optical conductors to the PM tubes. All such modifications are intended to be within the scope of the accompanying claims. OMPI - wipo

Claims: CLAIMS 1. Improvement in positron cameras having gamma photons sensing detectors, said detectors" including scintillation detectors (4, 15, 16) optically connected to photomultiplier tubes (3, 14), which scintillation detectors convert gamma photon emissions into light, the light being detected and amplified in the photomultiplier tubes (3, 14) and the signal characteristics of the light being electronically converted into readings, c h a r a c t e r i z e d by the fact that there are at least two scintillation detectors (15, 16) optically connected to each photomultiplier tube (14), and that each scintillation detector (15, 16) gives its output its own characteristics . 2. Improvement in accordance with claim 1, c h a r a c t e r i z e d by the fact that scintillation detectors are scintillation crystals. 3. Improvement in accordance with claim 2 , c h a r a c t e r i z e d by the fact that the different scintillation detectors show different decay constants.

Inventors: BOHM CHRISTIAN, ERIKSON LARS, BOHM, CHRISTIAN, ERIKSON, LARS
Applicants: SCANDITRONIX INSTR, INSTRUMENT AB SCANDITRONIX
Application year: 1983
Main classification: G01T1
IPC codes: G01T1