Application of a coincidence counting technique in a fixed geometry whole-body counter

International Journal of Nuclear Medicine and Biology, 1974, Vol. I, pp. 175-180.

Pergamon Press. Printed in Northern Ireland

Application of a Coincidence Counting Technique in a Fixed Geometry Whole-Body Counter* N. S. CHENT,

K. J. ELLIS,

H. R. PATE and S. H. COHN

Medical Research Center, Brookhaven National Laboratory, Upton, L.I., New York 11973, U.S.A. (Accepted11 October 1973) The coincidence counting technique has been applied to the Brookhaven 54-detector wholebody counter for the localization of positron-emitting radionuclides. With the spatial resolution limited by detector size, administered isotopes can be located to within 85 (17 x 5) compartments or grid-points. The dose required for a study can be as low as 1 @i. Because of the fast data handling facilities available with the “on-line” computer, the technique is most suitable for studies involving rapidly distributed radionuclides. APPLICATION

DE LA TECHNIQUE

UN COMPTEUR

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DU CALCUL SEULE

PIECE

DES COINCIDENCES A GEOMETRIE

DANS

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La technique du calcul des coincidences a 6tt appliqute sur un Compteur dttecteur-54 de Brookhaven, pour la localisation de radionuclei emetteurs de positron. Avec la resolution spatiale limitte par la taille du detecteur, les isotopes en question peuvent ttre localis dam 85 (17 x 5) compartiments ou points d’intersection. La dose ntcessaire pour une etude peut &tre aussi basse que 1 ,uCi. A cause de la vitesse avec laquelle les don&es peuvent Ctre utilistes dans cet ordinateur, la technique convient plus particulierement aux etudes comportant des radionuclei distribues rapidement. 1. INTRODUCTION

labelled with gamma emitting radionuclides and used as tracers in biomedical and clinical investigations can be traced and localised by using collimated gamma radiation detectors. The spatial resolution, i.e. the ability to distinguish between locations of different activities, depends primarily on the degree of collimation of the gamma rays. Therefore, in making scan images and scintillograms, gamma energies in the region of 150 keV are usually preferred for their ability to escape the body and because they can be fairly readily collimated. If positron emitters are used as tracers, the 510-keV annihilation gammas that they emit would require much COMPOUNDS

* Research supported by the U.S. Atomic Energy Commission. t Fogarty International Fellow, Department of Physics, University of Birmingham, Birmingham 15, England.

thicker collimators for achieving good spatial resolution. However, because of the property that annihilation radiations are always emitted in pairs and in exactly opposite directions, the coincidence gamma counting technique can be employed to make more definitive localization of positron emitters. W ( Tl12 = 20 min), 13N ( TIpa = 10 min), I50 ( Tllz = 2 min) and ‘sF ( Tlla = 110 min) are positron emitters produced in accelerators, and there are potentially a large number of compounds that can be labelled with these. Positron gamma cameras, scanners, and multiple-detector assemblies have been constructed and applied in studies of cerebral blood flow(*) and brain tumors,(z) lung air flow,t3) and bone and liver scanning,(*-” using conventional and coincidence counting techniques. This paper describes an attempt made to adapt the Brookhaven whole-body counter for coincidence counting and discusses the capabilities and limitations of the technique.

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N. S. Chen, K. J. Ellis, H. R. Pate and S. H. Cohn 2. METHODS

(a) The Brookhaven whole-body counter Figure 1 shows the general view of the 54-detector assembly. It consists of a bed placed between two banks of detectors, one above and the other below. The 15 x 5 cm NaI(T1) detectors in each bank are arranged in a 3 (column) x 9 (row) grid, and the corresponding detectors in the two banks are situated exactly opposing each other. The separation between the banks is usually 71 cm, but the position of either bank is adjustable independently. Gamma pulses recorded in each of the detectors are amplified, digitized and stored in an “on-line” Sigma IT computer (24K memory). When a pulse is transmitted as a 16-bit word to the computer, it carries the following information : (i) gamma energy: a channel number (l-127), (ii) detector of origin: upper or lower bank and a detector number (l-27), and (iii) whether it is in coincidence with another pulse recorded in any one detector in the opposite bank. The coincidence resolution time is about 5.7 ,us. When one pulse is recorded in any one detector, no information from the rest of the detectors in the same bank are handled until after that pulse is In the case of two coincident processed. pulses recorded in different banks, one word waits its turn in a buffer while the other one is being analysed and stored in the computer In a normal whole-body count, memory. only the information on gamma energy and detector of origin is stored. So, of the 24K memory locations, only 6858 (127 x 54) locations are required for storage, while the rest are used for basic counter operations and for servicing other facilities sharing the same computer. Roughly the same number of memory locations are available to accommodate coincidence counting information. (b) Adaption to coincidence counting Since each detector in one bank can be in coincidence with any of the 27 opposing detectors, there are 729 (27 x 27) possible combinations of detector pairs, leaving little room for energy information. The computer is therefore instructed to scan the energy

information on the incoming pulses and to accept only those with energy falling in between one of the three pre-selected energy ranges. (One of these ranges covers the 510 keV peak of the annihilation radiations, while the other two are used for coincidences between gamma radiations of other energies.) While this manipulation reduces the number of memory locations required to an acceptable 6561 (3 x 3 x 27 x 27), it has caused an increase in the counter dead time during which no further counts can be recorded. With reference to Figs. 2 and 3, the up.per detectors are designated U(R, C) and the lower ones L(R, C), where R and C are the row number and column number respectively. For simplicity in data interpretation, only the coincidences between the two opposing detectors and between the immediate neighboring detectors in the opposite bank are considered. If the subject under investigation occupies a volume between the two banks of detectors, annihilation radiations originating in volumes

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Applicationof a coim%encecountingtechniquein a fixed geometrywhole-bodycounter

TABLE 1. Normalizing factors and distances between centers of the detectors

ROW NUMBER

FIG. 3. Relative Positions of the detectors and the different contributing volumes. D would produce

coincidences between opposing detectors. Those in volumes B would produce coincidences between U(R, C) and ~3, C & I), and for volumes C, between U(R, C) and L(R f 1, C). Volumes A correspond to the intersections between diagonally opposing detectors. By considering only these fractions of the total number of possible coincidence counts, the plane of the bed is divided into 85 (17 x 5) compartments or grid points, and positron emitting radionuclides can be located to within these compartments. Because of the different numbers of coincidence pairs involved in the various volumes (for example, only one coincidence pair for volume D and 4 pairs for volume A) and the different geometry factors involved in the gamma radiation detection, to obtain a true I

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distribution profile of the radionuclides, normalizing factors have to be applied to the various coincidence counts. These factors were determined experimentally using a standard saNa point source. The source was carefully placed on the bed at

the various A, B, C and D positions. The coincidence counts between the pairs of appropriate detectors were printed out, and the normalizing factors were so determined as to give equal count rates for all the source positions. The factors and the separations between the corresponding detectors are listed in Table 1. These factors were incorporated into the program for analysing the data stored in the computer memory and give print-outs representing two-dimensional radionuclide distribution profiles (see Figs. 4 and 5). (c) System operation

For a typical run, the three channel limits, within which coincidence counts are recorded, are first set. Data are collected for a pre-set I

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N. S. Chen, K. J. Ellis, H. R. Pate and S. H. Cohn

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5. Spatial localization of isF in human subject 10 min after ingestion of a 0.5 &i

time, and then written on magnetic tape or drum for storage. Counter background due to cosmic radiations or random coincidences is practically zero for all positions in a 15-min count. The cycle can be repeated as required At the end of data by the experiment. collection, the choice for displaying coincidence counts between any one of the nine possible combination of gamma radiations of the three different energies can be made. (It is set for the coincidence between 510-keV gamma radiations unless specified otherwise.) Then the magnetic tape or drum records are read back and analysed by the “on-line” computer, and radionuclide distribution profiles are printed out by a fast line-printer. 3.

7

EXPERIMENTAL

RESULTS

Using the 160(3H, n) l*F reaction, l*F in water solution was prepared by bombarding water with tritons from a Van de Graaff accelerator. An Alderson (plastic) phantom, with the full complement of major internal organs, was used for the experiment. The radioactive solution was diluted and then injected into the organs of the phantom, one at a time at first, and two or more together later for determining the spatial resolution possible between different organs. Figure 4 shows the distribution profile for a counting time of 2 min, with approx. 2 ,uCi of l*F in the liver and gonads of the phantom. This, with other similar profiles for other organ(s), indicated that by carefully noting the position of the phantom on the bed, it was possible to determine the location of the activities in the specific organs. Random coincidence

close.

count rates caused by the high count rates in the individual detectors were quite low. Distinguishing organs, for example, between the thyroids and the spleen or between the liver and the gonads, was possible, but it seemed unlikely that organs as close together like the liver and the stomach could be separated by just viewing the distribution profile. In an experiment designed for studying the dynamic aspects of the technique, about O-5 ,uCi of i*F in water solution was ingested by a volunteer with a full stomach. Except for a brief period for emptying the bladder, the subject remained perfectly still in the wholebody counter and successive 5-n& counts were taken for 2 hr. l*F-activity distribution profiles were printed out, one of which (taken at 10 min after the dose administration) is shown in Fig. 5. From the sequence of profiles, it could be seen that the activity was first confined to the stomach for some 10 min, and hereafter began to appear down the intestinal tract. The activity then started to build up slightly in the heart position (column 2, row 7) and in the general area of the kidneys and bladder. The changes in the activities (corrected for radioactive decay) at the stomach and bladder positions were plotted against time as shown in Fig. 6. The emptying of the bladder was demonstrated very clearly, while the stomach was shown retaining nearly 10 per cent of the original activity at the end of the 2 hr. The latter could probably be partially accounted for by the l*F now present in the circulating blood. In comparison, the uptake of iBF by bones in the legs was very low (lo-12 counts) with some possibly in the blood.

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4.

DISCUSSION

Without any modification to the instrument, a valuable operational feature was incorporated in the highly versatile Brookhaven whole-body counter, namely the capability to produce distribution profiles of positron emitters by a coincidence counting technique. The coincidence resolving time of 5.7 ys of the counter is probably too high, but could easily be reduced by the addition of a Coincidence Unit (electronics) before the main amplifier. In the determination of normalization factors, the different gamma attenuation through the slightly different path length in the body is not considered. This variation is, however, generally accepted in most gamma scanning techniques, the main application of which is to obtain a qualitative distribution profile of certain radionuclides. The spatial localization information offered by the present technique is limited by the size of the NaI(T1) detectors and so could not be applied in cases where high resolution is required for several closely situated organs. One advantage of this technique lies in the very small dose required; about 1 ,&i of out an activity is sufficient for carrying investigation. The dead time of the counter actually limits the usable dose to about 10 PCi. Moreover, this technique offers coverage of the whole body in the same count, and is particularly suitable for dynamic studies in-

volving fast changing radionuclide distributions. The data accumulation time can be made as short as desired, and only a few seconds are required to write the record on magnetic tape before the start of the next cycle. 5. CONCLUSION The feasibility for using the coincidence circuitry of the Brookhaven whole-body counter for spatial localization of internally deposited positron emitters has been demonstrated. With the convenient half-lives of radionuclides like llC and l*F and the availability of increasing number of llC-labelled organic compounds, the coincidence counting feature of the wholebody counter will no doubt find many useful applications in the fields of biological, nutritional and medical research. authors wish to thank Mr. M. J. STRAVINOfor his assistance in the operation of the whole-body counter. One of us, N.S.C., also wishes to thank the National Institute of Health for the award of a Fogarty International Fellowship. The research was supported by the U.S. Atomic Energy Commission. Acknowledgements-The

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