A Multiwire Proportional Chamber Positron Camera for Studies of the Intracerebral Dopamine Metabolism

Abstract

<p> The recent development of a technique for the synthesis of the molecule 5-(¹⁸F) fluoro-dopa has opened a vast field of research into the study of the intracerebral metabolism. In order to take full advantage of this new tracer compound it will be necessary to use an imaging system which will be capable of providing three dimensional information concerning the rapid changes in activity as the ¹⁸F travels through the brain. This report describes the results os investigations into a possible design for a device which would be capable of this type of dynamic imaging. The design in question employs multiwire proportional chambers (MWPC) as detectors of the .511 MeV gamma rays which result from the annihilation of the positron emitted in the decay of ¹⁸F. </p> <p> For comparison purposes, a breif review of various other types of positron tomographic systems which are presently in use or under development is presented. This review emphasises the resolution, data aquisition speed, and instalation cost for each of these designs. </p> <p> In order to eliminate the need for costly research into the various design aspects of multiwire proportional chambers for use with positron annihilation radiation, the design presented here relies exclusively on methods and designs developed and proven feasible by other groups. These are incorporated into a system to suit the present needs. </p> <p> The design presented uses two pairs of 50x50 cm² MWPC's at 180° to each other and separated by approximately 50 cm. The chambers utilize electromagnetic delay-line readout techniques for the anode and wound bi-filar cathode planes. They are filled with a "magic gas" mixture at a slightly positive pressure. The effeciency of the chambers for .511 MeV photons is increased by employing "sandwich" type converters. Signals from the detectors are transfered to a small computer where they are stored for later tomographic reconstruction off-line. </p> <p> On the basis of the working designs from which the present design has been drawn, one would expect the positron camera to have a sensitivity of better than 1000 counts/second-microcurie. The total costs of development and construction leading up to a working device suitable for clinical use, not including the cost of a dedicated computer, are estimated to be less than seventy five thousand dollars. </p>