European Journal of

Eur. J. Nucl. Med. 2, 121-128 (1977)

I~ h I f"~ll"lP~l r I ~llt,.,l~i,./l~i..~,lr_,l./

Medicine

© by Springer-Verlag 1977

The Dual Photopeak-Area Method Applied to Scintillation Camera Measurements of Effective Depth and Activity of in vivo 1,2 3i.Distribution s Sven-Erik Strand and Bertil R.R. Persson Department of Radiation Physics, University of Lund, Lasarettet, S-221 85 Lund, Sweden

Abstract. Attenuation curves and photopeak ratios

of 159 keV and 28 keV photons emitted in the decay of 123I have been studied using a scintillation camera equipped with an extra ADC for recording the pulse height distribution of the energy signal. Cavities of various sizes containing 123I-solution were placed at different depths in a water phantom in order to vary the effective depth, i.e., the thickness of attenuating material above the cavity plus the distance to the centre of activity in the cavity. The centre of activity varies with the distribution of activity in the cavity, size of cavity, and the effective attenuation coefficient derived from the attenuation curves. From the photopeak ratio an average effective depth is determined which can be used for calculating the attenuation correction factor. The dual photopeak ratio has been used in clinical evaluation of thyroid 123I uptake measurements where effective depths ranging from 21 to 43 mm were obtained for the activity in the thyroid. Applications of the photopeak ratio method for renography with 123I-hippuran and blood flow studies using 133Xe or 127Xe and studies with many other radionuclides are also discussed.

Introduction

Absolute quantitative determinations of the activity of gamma-emitting radionuclides in various organs by external measurements require corrections for the attenuation of the photons in the intermediate tissue (Johnston, 1971). The depth localization and size of the activity distribution for the organ in the body has to be determined before such corrections can be performed. For offprints contact: Dr. S.-E. Strand

To perform these corrections one has to determine an equivalent depth of the activity independent of its distribution in the organ and overlying tissue. This equivalent depth does not necessarily coincide with the geometrical or anatomical average depth. The method most frequently reported for deriving the depth localization is 'peak-to-compton' measurements (Wellman etal., 1967; Hine etal., 1965; Genna, 1965; Sorenson, 1971; Eversden et al., 1968; Branson et al., 1971). According to Johnston and Brill (1969), however, this method is somewhat dubious in cases where the measuring geometry is not known well. For estimating the depth localization of iodine in the thyroid, distance-dependent detectors have also been used (Myant et al., 1949; Schulz and Rollo, 1970) The double-isotope technique has also been exploited for depth determination by several authors (Van Dilla and Fulwyler, 1963; Dolan and Tauxe, 1968; Ostrowski and Tothill, 1975; Espinasse et al., 1969). This method has, however, a great inherent uncertainty due to different in vivo distributions of the compounds labeled with different radionuclides. An approach to the use of 123I and the two energy method correcting for tissue attenuation in thyroid uptake measurements was reported by L6tter et al., 1972. They used, however, ~23I contaminated with 124I which had a great negative effect on the accuracy of their measurements. By marking the skin surface of the patient with point sources and studying the lateral view with a scintillation camera it is possible to derive an approximate estimate of the localization of the activity distribution, for example in kidneys, and to use this for attenuation corrections (Larsson et al., 1975; Erd et al., 1974). Anterior-posterior measurements for estimating the depth localization of activity and for making corrections for tissue attenuation (Jones et al., 1975; Thomas et al., 1976) have been successfully

122

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo

used in whole body counting by Sorenson (1971), Genna (1965), Evans (1937), Marinelli et al. (1955), in scanning by Heinze et al. (1975), Price et al. (1972), Halsguth et al. (1975), Tothill and Gait (1971), and scintillation camera measurements by Persson et al. (1975, 1977a). Most of these methods are either too complicated and difficult to perform or they are too tedious when applied in routine clinical work. Previously we have used one of these complicated techniques for quantitative measurements of the activity content of the kidney (Larsson et al., 1975 ; Persson et al., 1975). In those studies the great variation of the depth of the kidneys in patients was well demonstrated. Thus when renograms are to be interpreted in quantitative terms they must be corrected for the different degree of attenuation in the tissue between the organ and the body surface. The importance of quantitative estimation of separate kidney function before nefrectomy has shown to be of great clinical importance (Larsson et al., 1975). In the present investigation we have developed a much easier and more accurate method based on a single radionuclide and dual energy attenuation by which one can obtain corrections for tissue attenuation simultaneously with the clinical examination.

Phantom Studies Measurements with a scintillation camera ( P h o / G a m m a III HP, Searle Radiographics Inc.) were performed with different cavities of thicknesses ranging from 10 to 50 m m placed at various depths in a water tank. The heights of these cavities cover the thickness of most organs of interest. The cavities were filled with ~z3I solution and the scintillation camera was equipped with a 1000-hole, parallel-hole collimator which has low septum penetration for the high energy photons from 12aI (Bolmsj6 et al., 1977). The effect of the extension of the cavity in the plane parallel to the collimator is eliminated by using a parallel-hole collimator which gives uniform response over the whole field of view. The energy signals (Z pulses) were analyzed by an extra analog to digital converter (ADC) and a 128-channel pulse height analyzer connected to the routinely used 4096-channel core-memory as shown in Figure 1. This arrangement makes it possible to record simultaneously both scintiphotos, number of counts, and pulse height spectrum from the current activity distribution. The pulse height spectra were evaluated either directly from the oscilloscope (CRT) or in more detail with a computer. Figure 1 also shows the oscilloscope display and corresponding computer plot of the pulse height distribution from a clinical thyroid examination with 123I. This spectrum shows one peak at 159 keV due to g a m m a radiation emitted in the decay of a23I and one peak at 28 keV due to K X-rays from the decay product 123Te. The influence of the choice of collimator and the inhomogencity of the present scintillation camera on the pulse height distribu-

Magnetic tape I recorder

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Analog to digital converter

1E Magnetic core memory 4k

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Material and Methods

tape Pun°h t ..... t

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X,Y CRT

I

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preson,a onJ I presentation Oigta I film

PRINT-OUT

Primary curve presentation

LIST countrate per channel

s-~ (JCi) -~ 0,75

/

0.50

/

0.25

,~...,, ,

0 0

Pinhole collimator

1000 hole collimator

100

. 200

. ,., 300 Energy key

Fig. 1. Block diagram of the scintillation camera system used in this work. The extra A D C for analyzing the Z pulse is connected between the scintillation camera and the core memory. The photopeak ratio can either be evaluated at the cathode ray tube (CRT) or by a computer. The scintiphotos and the pulse height distribution print-out from the computer were obtained from a thyroid I23I uptake measurement of a normal patient

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo Photopeak

- ratio

degrees. This alters the effective attenuation coefficient but the main pattern of the attenuation and photopeak ratio curves still remains (Persson et al., 1977 b).

N ( 1 5 9 ) / N (28)

I

i

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In vivo Measurements

1251/123 I 1.0

123

--

0.2"/,

0.5

0.9~ . ~

O.1 i

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24

36

48

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72

Quantitative measurements of the uptake of ~23I in the thyroid have been performed in a series of patients (Cederquist et al., 1975a and b). The patients were lying in supine position under the scintillation camera and the Z-pulse height spectrum was recorded about 5 h after i.v. injection of the 123I. Patients with low uptake were laid so that recording of high activity concentration areas, i.e., heart and great vessels, was avoided. The fraction of 125I in the ~23I preparation used for the patient varies somewhat from time to time. Therefore a calibration spectrum was recorded of a syringe with the ~z3I preparation in question at a depth of 40 m m in the water tank.

Time after separation hour

Fig. 2. Photopeak ratio of the 159 keV and 28 keV photons of ~23I source as function of time after the separation of the xenon isotopes. The curve illustrates three sources with ~ s I contaminations of 0.2, 0.9, and 5.0 percent of the la31 activity at time of separation

tion have been studied in great detail and are reported elsewhere (Bolmsj6 et al., 1977). That paper also describes the effects of contributions of scattered photons in the 28 keV and 159 keV energy regions. The pulse height distribution was recorded from the entire field of view. In clinical measurements, however, areas of the surrounding tissue of the organ in question with high activity were shielded with lead sheets.

Results

Attenuation Measurements, Centre of Activity Attenuation curves for 159 keV and 28 keV photons from 1231 in the p h a n t o m cavities were measured for several thicknesses. Figure 3 gives the result for cavities of 10 and 50 m m thickness. C o u n t r a t e per unit activity S-1 pCi-1

8t-

2Ix0!

I*!x ,0'

1231 Production and Quality Control A mixture of xenon isotopes 122Xe, 123Xe, ~24Xe, and ~2SXe were produced by irradiating NaI with protons of energies 50 65 MeV from the U p p s a l a Cyclotron (Lundquist a n d Malmborg, 1977). During the subsequent transportation to L u n d which takes about 10 h, the 123Xe decays to ~23I, which is then extracted as iodide. The radiochemical and radionuclidic purity of the carrier-free 123I in 0.9% NaC1 solution was studied in detail with G e ( L i ) g a m m a spectrometry and the only long-lived radioactive contaminant detected was 125I (Tl!2=60 days), a m o u n t i n g to 0.2-0.9% of the total 123I activity (Mattsson et al., 1976). The 125I activity will contribute to the count rate in the low energy peak region from photons of average energy 28.4 keV. This contribution will increase with time because the half-life of 12sI (60 days) is m u c h longer compared to that of 123I (13.3 h). In Figure 2 is shown the ratio of recorded counts, N(159) and N(28) in the 159 und 28keV peaks respectively as a function of time for different a m o u n t s of azsI activity present at the time of separation of the Xe isotopes from lz3I. Attenuation Measurements

Attenuation curves were obtained with the cavities at different depths in the water tank. In the recorded pulse height distributions it was possible to select any energy window of interest. In the present investigation we used an energy window of 40% symmetrically over each of the two photopeaks of 159 keV and 28 keV, in order to cover the entire peak area. With energy windows less than 40% the recorded a m o u n t of both primary and scattered photons are decreased to various

centre of activity

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Fig. 3. Attenuation curves for the 159 keV and 28 keV photons of ~z3I. Forty percent energy windows were centered over the photopeak for the two cavities 10 m m (o) and 50 m m (/,) thick. Open dots indicate experimental values where the depths do are measured in the water p h a n t o m from the water surface to the cavity. Filled dots ( e , A) are values where the depths represent effective depths calculated from Equations 3 and 5. The geometry used in the calculations is given in the upper right part of the figure, where c~ and y refer to Equation (4). In the calculations of deff in the present figure is c~= 1

124

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo

The count rates thus recorded under the full energy peaks of the pulse height distributions can be given by the following equation:

N(hv, :, do, a)= Ao.r/- F. D

(1)

where = the thickness of the cavity• do = the thickness of material above the cavity. a = the distance between the collimator surface and the surface of the object. A o = t h e activity in the cavity. r/ = the number of photons of energy hv emitted per disintegration. F = the distance response function of the detector which varies between a constant value (parallelhole collimator) and inverse square law (pinhole collimator). D = the depth attenuation function which approximates closely to an exponential function for the 28 keV photons and at depths greater than 20 mm for the 159 keV photons, as indicated in Figure 3. Thus D can be approximately expressed as: D(#, d)~exp( - #eff-d)

1

(4)

]Jeff

m

in

Centre of activity, X o (ram) Thickness of cavity, : (mm) 10 30 50 70

100

150

200

8 10 12 14 16

5.0 5.0 5.0 4.9 4.9

14.7 14.6 14.6 14.5 14.4

24.2 24.0 23.8 23.6 23.3

33.4 33.0 32.6 32.2 31.8

46.7 45.9 45.1 44.3 43.5

67.6 65.8 64.0 62.3 60.7

86.9 83.9 80.9 78.0 75.3

20 25 30 35

4.9 4.9 4.9 4.9

14.3 14.1 13.9 13.7

22.9 22.4 21.9 21.4

31.0 30.0 29.1 28.2

41.9 40.1 38.3 36.7

57.5 53.8 50.5 47.5

70.2 64.7 59.8 55.6

(m- 1)

distributed activity throughout the whole cavity or organ the above equation is reduced to 1 [ b: : #ef f ] X o = #~ff • in 1 - e x p ( - ~tCff.d')]"

(5)

In Figure 3 this equation has been used for transforming the depth of the cavities do to effective depths do+Xo. The centres of activity, Xo, in cavities of different thicknesses are given in Table 1 for various values of the effective attenuation coefficient.

(3)

where Xo is the 'centre of activity' in the activity distribution, as seen by the detector in the present setup. It can easily be shown that the centre of activity, determined with a scintillation camera equipped with a parallel-hole collimator is given by the expression:

Xo =

Photon attenuation coefficient in water, g

(2)

where #~ef=the effective attenuation coefficient for the experimental condition in question. The depth of the activity distribution can be given as an effective depth defined by d~"(f= d o + Xo

Table 1. The centre of activity X0 for different cavity thicknesses calculated from Equation (5). The listed photon attenuation coefficients in water or soft tissue cover most radionuclides used with a scintillation camera in nuclear medicine

(: + ( ~ - 1). y). #elf . ~).]] [el + (c~- 1)-exp(-/~eff' ( : - Y ) i - ~ " exp(-#eff

where it is assumed that the activity is distributed in a cavity of rectangular shape of thickness : and where the activity concentration in a fraction of the cavity with length y is c~ times the activity concentration in the other part (see Fig. 3). The dimensions of the cavity are such that #eff is approximately constant in the depth interval in question. The situation described above simulates the state, for example, of kidneys with different activity concentration in the pelvis and Darenchyma or of thyroids with cold and hot nodules. For uniformly

Double Energy Ratios, Centre of Activity, and Effective Depth Determination With radionuclides emitting at least two photons of different energies hVl and by2 it is possible to use the ratio of counts under the corresponding photopeaks of the pulse height spectrum for determination of the effective depth. The ratio of the number of counts N(1) and N(2) under the two photopeaks can be derived from equations 1 and 2 as follows: N(1) _ ~/1.F1 • exp(-/2,f (1)e •A(1) t~eff) N(2) r]2.F2 • exp(-- #elf" (2) ~eff]'4(2)]"

(6)

Provided there is negligible septum penetration in the collimator used with the scintillation camera the ratios of the distance response functions F1 and F2 are reduced to a ratio of detector efficiences which is independent of the distance between the detector and the activity distribution. The photopeak ratio thus reduced to:

N(1)/N(2) ~-constant, exp ((~t(~2~-/~eff!"(1)].deff )

(7)

and deff ~- average effective depth at the two energies.

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo The N(1)/N(2) ratio was experimentally determined for 123I. A 1000-hole collimator was used in order to get negligible septum penetration valid also for the high energy p h o t o n s o f 529 keV. Figure 4 gives the N(159)/N(28) ratios for different cavity sizes as a function of the arithmetic m e a n deff according to E q u a t i o n (8) d 1 5 9 A_A28 d e f f - - t*eff " ~ e f f

125

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It can be seen f r o m Figure 4 that when 6~ef f is equal to or greater t h a n the centre o f activity, Xo, the N(159)/N(28) ratio is independent of cavity size. This is one of the great advantages of the double energy m e t h o d used in the present investigation, because in the clinical case the thickness and the distribution o f activity are not usually known.

Attenuation Correction The effective attenuation coefficient includes a contribution f r o m scattered radiation and therefore varies slightly with depth and with the energy w i n d o w selected in the measurements. By recording the N(159)/N(28) ratio and using the diagram in Figure 4, one obtains an average value of the effective depth, d~ff. But in order to correct for tissue attenuation in the 159 keV channel which is used for generating the image, the effective depth for the 159 keV p h o t o n s is needed. F o r example, according to Figure 3 the effective attenuation coefficients for 159 keV and 28 keV radiation can be determined asp~ff= 8.1 m ~ and 21.3 m respectively. If the given cavity has a thickness o f Photopeak- ratio N(159) / N (28)

1.05

m X ~ O ~ ~=50m =30 m m ~

1.00

;

0

,

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L

,

,

100

. . . . . .

150

•

200

Average effective depth,(teff

Fig. 5. The ratio of effective depth for 159 keV, dle~g and the average effective depth doff for 4 cavities of thicknesses 30, 50, 100, and 200 mm respectively, calculated from Equations (3), (5), and (8). These ratios indicate the uncertainty using the average effective depth deff correcting the count rate in the 159 keV window. For example, with the activity distributed in a 100 mm thick layer with an average effective depth of 130 mm, the effective depth d~fs9 will be 1.02 x130= 133 mm

50 m m at 50 m m depth, according to Table 1, the centers o f activity, X0, in this case are 24.2 m m and 22.8 m m for 159 keV and 28 keV respectively. The corresponding effective depths d e f f = d o - } - X o are 74.2 m m and 72.8 mm, and the average effective depth d~ff=73.5 mm. The differences between these values are within the limits of uncertainty. It is therefore possible to use the arithmetic m e a n derived from E q u a t i o n (8) when calculating the attenuation correction factor according to E q u a t i o n (9). d159~ N 0 = N(159)-exp(plef59 "~eff ]

~_ N(159). exp(#~f~ 9. dell) Cavity height 10

'

water

air

5

o 10 m m x 50ram + 2 0 mm 3 9 mm o / o "

o

Avera~le effective depth,deff mm

Fig. 4~ P h o t o p e a k r a t i o o f t h e e n e r g y i n t e r v a l s 1 5 9 ± 3 2 k e V a n d 28.1 _ + 5 . 6 k e V as a f u n c t i o n o f t h e a v e r a g e effective d e p t h deff

for four cavities with thickness 10, 50, 20, and 39 mm. The two curves represent two ~z3I sources with different ~25Icontamination

(9)

where No is the count rate which would be recorded with the ~23I activity free in air. F o r u n i f o r m in vivo distribution of activity, however, it is possible to derive a relation between te ,#59 f f and ~ f f by using Equa159/A tions 3, 5, and 8. Figure 5 gives the ratio A-eff /"eff obtained for various sizes o f cavity. F r o m this figure it is seen that for cavity thicknesses smaller than 100 m m the difference is less than 5%. In Table 2 some examples are given on the effect of using deer instead of d eff 159 in the calculations o f the attenuation correction factor. F r o m Table 2 it can be seen that the differences between using d-eff o r ,~159 "eff in the correction for the tissue attenuation are about 2% for 100 m m thick cavities and a b o u t 8% for 200 m m thick cavities,

126

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo

Table 2. The average effective depth d-ere determined from Figure 4 for different photopeak ratios. The corresponding effective depth for 159 keV photons, de~59 has been calculated from Figure 5. Attenuation correction factors were derived from both 2/,rf and d~e~/59, and an effective attenuation coefficient of 8.1 m - * for 159 keV according to Figure 2. The differences in correction factors obtained by using different d~ff are given in percent parentheses below corresponding values for 159 keV

N(159) N(28)

d~ff (mm)

Effective depth for 159 keV dlef~9 (mm)

Cavity size (ram) 30 50

Correction factor for 159 keV photons using ~ f r 200

-

-

-

1.050 1.157

.

.

1.00 1.25

6.0 18.0

. 18.2

1.50

28.5

28.7

29.2

-

-

1.260

2.00

47.5

47.7

48.2

50.1

--

1.469

2.50

69.0

69.2

69.7

71.6

--

1.749

3.00

96.0

96.2

96.7

98.6

105.0

2.176

which is well within the limits of uncertainty in the clinical case where the thickness of organs are usually under 100 ram. Thus, the average depth a~eef determlned by the N(159)/N(28) photopeak ratio from Figure 4 can be used in calculating the correction factor, according to Equation (9). This is a further advantage because in practice the distribution of activity is usually very inhomogeneous and it is only possible to determine an average value of d28.

Influence of

125_/Contamination

The difference between the two curves in Figure 4 is due to varying degrees of ~25I contamination in the 123I preparation used in the present experiments. The importance of 125I contamination is shown in Figure2 where the ratio N(159)/N(28) is given at various times and with various degree of 125I contamination. In practice a calibration measurement with the given 123I solution is performed on a syringe at a known depth in water. The value of the photopeak ratio thus obtained gives a reference point to which the curve in Figure 4 is normalized. The shift merely reflects the changed count rate in the 28 keV window due to various 125I contaminations. By adopting this procedure the influence of varying 125i contamination can be eliminated.

In vivo Measurements The method described in this work for determination of the effective depth has been used in clinicat studies

,

Cavity size (mm) 30 50

100

.

Correction factor for 159 keV photons using ,4159 --eff and within parentheses the difference in % from using defe

1.159 (+0.2) 1.262 (+0.2) 1.472 (+0.2) 1.752 (+0.2) 2.180 (+0.2)

100

200

--

-

-

1.267 (+0.6) 1.478 (+0.6) 1.759 (+0.6) 2.189 (+0.6)

-

--

1.501 (+2.1) 1.786 (+2.1) 2.223 (+2.1)

2.341 (+7.6)

Table 3. The average effective depths of the thyroid in 12 patients as calculated from the observed 159 keV photopeak ratio from ~23I. The geometric Iimits of the activity were derived from lateral views with the scintillation camera

Patient

Average depth (mm)

Side view (mm)

sex

age (years)

M F F M

59 54 40 66

31 ± 3 34+3 33 ± 3 70 ± 7

8-34 27-72

F F F F

35 44 27 79

30_+ 3 28 ± 3 21 ± 2 43 + 4

10 38 3-49 3-42 4-58

F F F F

35 61 62 53

30 _+3 29 _+3 39± la 25± 1 ~

"

-

5 51 8-59 9 75 5-53

Corrected for 125I contamination

of the quantitative '23I uptake of the thyroid (Cederquist et al., 1975a and b). The results of the effective depths for 123I in the thyroid which were obtained in that study are given in Table 3. The effective depths of the thyroid ranges from 21 to 43 mm which is in good agreement with corresponding values found by other authors (Wellman et al., 1967; Schultz and Rollo, 1970; Rollo, 1971). In one patient, however, with a much enhanced thyroid, an effective depth of 70 mm was recorded. A geometric estimation of the extension of the activity distribution can be obtained from lateral

S.-E. Strand and B.R.R. Persson: Dual Photopeak Method for Activity Measurements in vivo views with the scintillation camera. The limits of the activity distribution estimated in this way are also given in Table 3 and as seen f r o m this table the effective depths are all within these limits. This confirms the validity of this m e t h o d ' s ass u m p t i o n that the calculated effective depths lie within the a n a t o m i c limits. The difference between the geometric average depth and the effective depth o f the activity distribution was also confined in some of the patients.

127

the pelvis. By using 123I-labelled hippuric acid and list m o d e storage for the X, Y, and Z pulses it is possible to m o n i t o r the shift of the effective depth with time and thus also to m a k e accurate corrections for the tissue attenuation in different parts of the kidneys.

Acknowledgements. Thanks are due to H. Lundquist and P. Maimborg at the Gustaf Werner Institute, Uppsala, Sweden, for supplying us with 123I. This study has been supported by grants from the John and Augusta Perssons Foundation for Medical and Scientific Research, Lund, Sweden.

Discussion and Conclusions I n quantitative measurements o f the uptake of radionuclides in various organs o f the body, one often has to deal with an i n h o m o g e n e o u s activity distribution o f u n k n o w n thickness. The thicknesses of organs in the b o d y range f r o m a few millimeters (e.g., lymphnodes, b l o o d vessels) up to about 100 m m or more (e.g., liver, lungs). The m e t h o d described in the present investigation for the determination o f an effective depth o f the activity distribution has, however, the great advantage of being independent of the volume inhomogeneity o f the activity distributionl The dual p h o t o p e a k m e t h o d developed here can be used for all radionuclides emitting two or m o r e g a m m a rays with sufficient difference in attenuation coefficients in tissue. Radionuclides suitable for this purpose are, for example, lt3Inm, 133Xe, 127Xe, 758e, 99Tcm, 12~I, 131I, i9aAu, 197Hg, and so on. F o r example the characteristic X-rays f r o m 99Tc have an energy of 18 keV and could be applicable for the above p u r p o s e if the thickness of the covers for crystal and collimators are thin enough. With a thickness o f 1 m m A1 the transmission o f 18 keV photons is still a b o u t 30 per cent (Oldendorf, 1970). It is also possible to apply the dual p h o t o p e a k m e t h o d to distinguish between deep and superficial circulation by using 133Xe(Crawley and Veall, 1971; Duyl et al., 1976) and 127Xe (Bolmsj6 et al., 1975). Using a scintillation camera with two energy windows makes it possible to obtain the p h o t o p e a k ratio f r o m the scalers o f the c a m e r a simultaneously with the recording of the clinical image. By discriminating the X- and Y-pulses f r o m the camera it is also possible electronically to select certain regions o f interest and determine the p h o t o p e a k ratio in those regions only (Strand and Larsson, 1977). The p h o t o p e a k m e t h o d can be used particularly well in renography, where the distribution o f activity in the kidneys changes with time (Ostrowski and Tothill, 1975). In the uptake phase, m o s t of the activity is normally f o u n d in the p a r e n c h y m a and later, in the secretory phase, most of the activity resides in

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Received July 30, 1976