~ur. J. Nucl. Med. 3, 203-205 (1978) European Nuc lear Journal of

Medicine © by Springer-Verlag 1978

Performance Characteristics of a Gamma Camera Over a Wide Range of Energies

F.R. Hudson, J. Bernard Davis*, and Anthea G. Whittingham** Physics Department, Mount Vernon Hospital, Northwood Middlesex, U.K.

Abstract. The problem of choosing which collimator to use for imaging a new isotope has been approached by collecting resolution and sensitivity data for a selected group of isotopes. These have been chosen to be readily available and to have generally a single ,/-ray only. Resolution and sensitivity plots for a low energy collimator and a high energy collimator are presented and their use with several isotopes of inter- est is discussed. The interpretation of recommenda- tions in the literature on the choice of collimators for newly introduced isotopes would be considerably simpler if data in this format were commonly avail- able.

Introduction

The choice of the optimum gamma camera collimator to use with a given isotope is becoming increasingly important with the growing number of isotopes in common use. An example of current interest is pro- vided by the choice of collimator to use for the 192 KeV 7-rays of s lmKr. Is a low energy collimator or a medium energy collimator more appropriate ?

This question currently tends to be resolved on an ad hoc basis by measuring line source responses for the possible isotope/collimator combinations and then, in conjunction with, possibly subjective, assess- ment of image quality and "pat ient convenience", a choice is made.

Many workers have, in the past, studied the per-

* Present address: Service de M$dicine Nucl6aire, H6pital Can- tonal, Bertigny, CH-1700 Fribourg, Switzerland ** Present address." Physics Department, Royal Berkshire Hospi- tal, Reading, Berkshire, U.K.

Send offprint requests to: F.R. Hudson, Ph .D. , Physics Depart- ment, Mount Vernon Hospital, Northwood, Middlesex, U.K.

formance of gamma camera systems. For example, Payne et al. (1973) measured the field uniformity, re- solution, sensitivity and dead time of three gamma cameras using several isotopes and different collima- tors, and Moretti et al. (1976) measured resolution, field uniformity, distortion, energy resolution, count rate capability and paralysing time of five gamma cameras using three phantoms and the Anger test pattern. There seems, however, to be little systematic data on the change of resolution and sensitivity with energy.

A weakness at present appears to lie in the practice of describing collimators by terms such as " low en- ergy", "medium energy", or "high resolution" with- out giving a full performance specification. A more objective approach to providing recommendations applicable to any camera/collimator combination would be a specification which recognises that a low energy collimator will be used for many isotopes co- vering an energy range from, for example, 80 KeV for '33Xe or 2°tT1 to 192 KeV for 81mKr, and simi- larly for other types of collimator.

The objective of this communication is to help clarify the choice of collimator for use with intermedi- ate energy isotopes. It attempts to demonstrate an objective approach to collimator choice by presenting data on the variation of system sensitivity and resolu- tion with energy.

Materials and Methods

The data has been collected using a Picker Dynacamera IIC. Line source responses were measured using thin polythene tubing ap- proximately 30 cm long with an internal diameter of 1.5 mm as suggested by MacIntyre et al. (1968). This was filled with about 0.5 mCi of each isotope studied; a low energy high resolution collimator (5600 holes) and a medium energy collimator (1900 holes) were studied. The point source sensitivity was measured

0340-6997/78/0003/0203/$01 .00

204

Table 1. Isotopes chosen for the study with their main photopeak energy and intensity

Isotope Main photopeak % per energy (MeV) disintegration

133Xe 0.081 37 99mTc 0.140 90 139Ce 0.165 80 2°3Hg 0.279 77 51Cr 0.320 9 H3mIn 0.393 64

lSF 0.511 194 ~37Cs 0.662 85

The principal p r a y of each isotope is the highest energy p r ay (Except for la3Xe)

by suspending a source of about 1 mCi at 1 m from the face of each collimator, on the axis of the crystal.

The isotopes were obtained from the Radiochemical Centre, Amersham, England, and the M.R.C. Cyclotron Unit, Hammer- smith Hospital, London. They have been chosen to have the princi- pal 7-ray as the highest energy y-ray, Lederer et al. (1968), thus only the quoted energy will be reflected in the measured line source responses. (The higher energy y-rays of 13axe are of intensity N0.1% in total.) For each measurement the analyser window was set for 20% of the photopeak energy. This was achieved in the Dynacamera IIC by calibrating the variable window for several isotopes in each energy range, and a + 10% window for each isotope was then selected from the calibration curves. The isotopes selected are summarised in Table 1.

F.R. Hudson et al. : Energy Characteristics of a G a m m a Camera

Results and Discussion

Figures 1 to 3 depict the variation in full width at half maximum (FWHM) and full width at tenth maxi- mum (FWTM) with energy for each camera-collima- tor combination. The data has been collected in air since the system characteristics are the prime interest. High energy data has been examined for the low en- ergy collimator because it is often important even when it only arises from trace impurities. This is due to the filtering effect of the collimator which, being effectively transparent to the higher energy photons, will greatly enhance their contribution relative to those low energy photons accepted by the collimator.

The point source sensitivities of the system are shown in Figure 4. Their shapes reflect the balance between the increase in septal penetration with energy and the corresponding decrease in detector sensitivity. The sensitivities are expressed relative to that for 99mTc measured with the low energy collimator. This was found to be 7.0 cts/s gCi for a point source a t l m .

Reference to these plots should enable the system performance for a new isotope to be predicted and if similar plots are available for other camera-collima- tor combinations a fair assessment should be possible of recommendations derived from measurements made on other systems.

e m

30

20

10

Low energy col[imcltor FWTM I surface / /~. t" ~[-

i/

/ . .....+'-'-+ _ / ~ . x FW.M

-l-=I~q I r 200 400 600 KeY

c m

30

20

10

Medium energy cotlimotor ~C (

FWT M i t / /~ i /

/ / ! ~/ , /

200 4.00 600 KeV

3

c r n

6

4

2

1.5

1.0

0.5

0

4

Medium energy .1 col[imator {'l

FW HM C / / [

. / / ~lJ

--%. /

. " i - { ' i - - ~ - ~ - t - ~ { ~ t

I I 200 400 600 KeV

Relative / ~ - ' ~ sensitiv- ,4 i X

ity , / / w energy x ~

. ~ ± collimator

./}"~'..~. Medium energy / t "-~} collimator

I I I 200 400 600 KeY

F i g . 1. Low energy collimator. Variation of response as measured by Full Width Half Max imum (FWHM) and Full Width Tenth Max imum (FWTM) with energy. Both measurements were carried out on the collimator surface

Fig. 2. Medium energy collimator. Variation of response as measured by Full Width Half Max imum (FWHM) with energy

Fig. 3. Medium energy collimator. Variation of response as measured by Full Width Tenth Max imum (FWTM) with energy. In both cases measurements were carried out (A) on the surface of the collimator, (B) at 10 cm from the collimator in air, and (C) at 25 cm from the collimator in air

Fig. 4. Comparison of the variations of relative sensitivity, as measured by a point source at 1 m, with energy for the Low Energy Collimator and the Medium Energy Collimator. The sensitivity for 99mTc was measured to be 7.0 counts/s gCi at l m for the low energy collimator

F.R. Hudson et al. : Energy Characteristics of a Gamma Camera 205

The present study was undertaken to clarify our choice of collimator for use with the 190 KeV y-rays of s am Kr. Impurities are not a problem and the typical imaging time with our medium energy collimator is ~2 min for 200,000 counts. The resolution curves show that at this energy the low energy collimator is just beginning to break down; using the medium energy collimator reduces the system sensitivity by a factor of 2.2 but the F W H M is reduced to 0.7 cm and the F W TM even more radically. As the require= ment of the study is an image with optimum resolu- tion the medium energy collimator is favoured; the increased count rate from the lower energy collimator arising partly from counts which are degrading the image. Other camera systems will, of course, show a different balance of sensitivity and resolution partic- ularly in the energy range where a low energy collima- tor is just failing.

The characteristic shape of Figures 1-3, showing a rapid deterioration in resolution above the nominal 'working range', reflects the different components of the observed signal. This is the sum of the collimated primary radiation, scattered radiation and radiation that has penetrated the collimator septa. In the work- ing range the primary and scattered radiation completely dominate the signal; however, the attenua- tion coefficient for y-rays in Pb is a steeply falling monotonic function of energy in the range 100 KeV to 1 MeV. Once an energy is reached at which the penetration fraction has become comparable with the primary and scattered radiation any further increase in energy produces a characteristic which reflects the logarithmic rate of change of the attenuation coeffi- cient with energy.

Data in the form presented in Figures 1, 2, 3 and 4 is also useful in interpreting the effects of low intensity high energy y-rays. It is clear, for example, that a 99~Tc brain scan will be particularly prone to interference by traces of ~ 311 as reported by Makler et al. (1976) as the peak of the system sensitivity lies in the 300-400 KeV region when the low energy colli- mator is used.

Groch and Lewis (1976) have shown how the im- portance of a low intensity high energy y-ray is en- hanced by the filtering effect of a collimator on the lower energy y-rays. Thus the low energy collimator system will be ~ 1.6 more sensitive to the 439 KeV 2°2T1 y-ray than to the 80 KeV X-rays from 2°1T1. While the ratio of the septal penetration fractions may be ~104 for the 439 KeV:80 KeV photons it is perhaps more important to refer to a system sensi- tivity curve of the type shown in Figure 4.

lZ3I is another isotope for which these curves can provide useful information. Problems arise from the presence both of other iodine isotopes and from the

presence of a 529 KeV y-ray in the 123I spectrum with intensity ~ 1% of that of the 159 KeV y-ray, McKeighen et al. (1974); Bolmsjo et al. (1977). The response characteristics of the low and medium en- ergy collimators show that at 160 KeV the medium energy collimator has ~0.5 the sensitivity of the low energy collimator but at 530 KeV the ratio falls to

0.15. While the sensitivity of the low energy colli- mator system is so much higher for the 530 KeV y- rays they will not, however, be effectively imaged and will appear as a general increase in background.

Conclusions

If the choice of collimator for use with a given gamma camera system is to be made systematically it appears to be necessary for data to be available in a format of the type shown in Figures 1, 2, 3 and 4. This does not appear to be currently commonly available which can make the assessment of recommendations from the literature confusing when different cameras and collimators are to be compared.

The data has been used to assess the system perfor- mance for several isotopes of current interest and a set of commonly available standardisation isotopes has been chosen which avoids problems arising from weak high energy y-rays.

Acknowledgements. The Authors wish to acknowledge the help and encouragement which they have received from Dr. D.M. Thomson and their other colleagues, and in particular the careful preparation of the manuscript by Mrs. Angela Forster.

References

Bolmsjo, M.S., Persson, B.R.R., Strand, S.-E.: Imaging of 123I with a scintillation camera. A study of detection performance and quality factor concepts. Phys. Med. Biol. 22, 266-277 (1977)

Groch, M.W., Lewis, G.K.: Thallium 201: Scintillation camera imaging considerations. J. Nuc. Med. 17, 142-145 (1976)

Lederer, C., Hollander, J., Perlman, I., (eds.): Table of isotopes New York: Wiley 1968

MacIntyre, W.J., Fedoruk, S.O., Harris, C.C., Kuhl, D.E., Mal- lard, J.R.: Sensitivity and resolution in radio-isotope scan- n i n g - A report to the International Commission on Radiation Units and Measurements. Medical Radio-isotope Scintigraphy (Salzburg) I.A.E.A. Vol. I: 391M35 (1968)

McKeighen, R.E., Muehllehner, G., Moyer, R.A. : Gamma camera collimator considerations for imaging with 123I. J. Nucl. Med. 15, 328-331 (1974)

Makler, Jr., P.T., Eymontt, M.J., Block, P., Koch, K.: Effect of small amounts of 131I on 99mTc detection by the scintillation camera. J. Nucl. Med. 17, 1100 1101 (1976)

Moretti, J.L., Mensch, B., Guey, A., Desgrez, A.: Comparative assessment of scintillation camera performance. Radiology 119, 157 165 (1976)

Payne, J.T., Williams, L.E., Ponto, R.A., Goldberg, M.E., Loken, M.K. : Comparison and performance of anger cameras. Radiol- ogy 109, 381-386 (1973)

Received April 7, 1978