IEEE Transactions on Nuclear Science, Vol. NS-30, No. 1, February 1983 DIGITAL IMAGING OF REGIONAL GLUCOSE METABOLISM OF THE HEART WITH A MWPC R. Bellazzini, A. Del Guerra, M.M. Massai, M. Ragadini, G. Spandre, G. Tonelli Istituto di Fisica dell'Universita di Pisa, Piazza Torricelli 2 - 56100 Pisa (Italy) I.N.F.N. - Istituto Nazionale di Fisica Nucleare - 56010 San Piero a Grado (Italy) and P. Camici, G. Coppini, G. Valli Istituto di Fisiologia Clinica del C.N.R. - Via Savi 8 - 56100 Pisa (Italy) ABSTRACT The digital imaging of carbohydrate consumption in myocardial tissue has been performed by using a depo sit tracer of glucose metabolism (3H-deoxyglucose) and a specially designed Multiwire Proportional Chamber (MWPC) that acts as a position sensitive radioactivity detec- tor. The resolving power (=1.5 mm FWHM), sensitivity (10OlBq/cm2), efficiency (10%) and uniformity (±4%) of the system are shown and MWPC digital autoradiogra- phs of canine heart in different pathophysiological con ditions are presented. INTRODUCTION The renewed interest in myocardial metabolism stimulated by positron emission tomography has increa- sed the need for further assessment of the behaviour of metabolic tracers in physiological and pathological con ditions. The availability of labelled deoxyglucose (DG), a deposit tracer of glucose metabolism, h3s recently ma- de possible to study myocardial carbohydrate distribu- tion (1). A heterogeneous glucose uptake might be, in- deed, the consequence of differences in work and stress in different regions of the heart. In order to obtain tissue distribution of a ra- dioactive tracer, conventional autoradiography has the advantage of the highest spatial resolution (200-300 pm) as compared to other nuclear techniques. Nevertheless it presents serious limitations when quantitative infor mations on the regional radioactivity content are reque sted in very short time (2). Furthermore, the sensitivi ty of the radiographic film limits the choice of physio logical tracers to high energy 6- emitters, often very expensive. To overcome these limits we have constructed a specially designed Multiwire Proportional Chamber with a cathode coupled delay-line read-out (3,4). With this detection system, very sensitive to low energy r emit- ters such as the quite cheaper 3H, statistically signi ficant data are obtained in a few minutes compared to the several weeks needed for autoradiography. The indi- vidual S- rays emitted by the samples and detected by the chamber are directly counted and their position is digitally recorded on a magnetic memory for a subse- quent image processing and statistical analysis of the data. THE MWPC DESIGN AND PERFORMANCE A crucial problem in S- activity imaging with ga seous detectors is related to the range of the S3 rays inside the detector volume. The area of the point-source image, defined as theareaon which 50% of the detected events are collec ted, is shown if fig. 1 as function of the linear absor ption coefficient of the S- rays for a typical set of geometrical parameters of the MWPC (see ref. (4) for a detailed discussion of the 6- imaging model). As one can see the lower the absorption coeffi- cient, the higher is the resolving power. To improve the sensitivity to the very low ener- gy S rays such those emitted by a 3H source (EM 18 KeV, pi = 1 mm in Argon S.T.P.) we have designed a MWPC that can be operated as a windowless detector (i.e. the sample can be easily placed inside and removed from the detector) and with a minimized (<200 iam) gas layer between the source and the active gas volume. E 30- 20 10- 6S 7. 0 ff 2 6 10 14 18 22 i (mm) Fig. 1 - The area of the point source image as a func- tion of the linear absorption coefficient. The main MWPC design characteristics and intrinsic performance (4) are shown in Table I. Table I - Main Parameters and Intrinsic Performance of the MWPC Active area 25 x 25 cm2 Anode-cathode gap 4 mm Anode wires diameter 20 pm Cathode wires diameter 100 pm Wire spacing 2 mm Gas filling Argon/Isobutane or C02 Energy resolution at 5.9 KeV 14% FWHM (Single wire) Spatial resolution at 5.9 KeV (400±50) pm (FWHM) Linearity Better than 0.5% Noise level 10-4 c.p.s./mm2 The pick-up and processing of the signals are obtai ned by means of low noise, charge sensitive pre-ampli- fiers which also constitute the "cold" termination of the delay-line. An accurate timing system based on Con- stant Fraction Discriminators (CFD) is used to measure the time of arrivals of the signals at both ends of the delay-line with an accuracy better than 2 ns. Two flexi ble, parallel, data acquisition systems, one analog and one digital, have been implemented. The analog branch is conveniently used for trouble shooting as well as for control and immediate feedback on the experiment, while the digital branch is used for off-line data reduction and analysis. The logical signals from the two ends of each delay-line are the start and stop of two Time to Amplitude Converters (TAC) that drive the x and y deflec tion plates of an oscilloscope. The z axis of the oscil- loscope is intensified by the AND of the two TAC's and a Polaroid camera is used as a permanent storage of the ob tained analog information. At the same time the output 0018-9499/83/0200-0686$01.00 © 1983 IEEE 686