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Temperature influence on a dual crystal positron camera
578
TEMPERATURE
Nuclear Instruments and Methods in Physics Research A243 (1986) 578-582 North-Holland, Amsterdam
INFLUENCE
ON A DUAL CRYSTAL
POSITRON
CAMERA
Max KESSELBERG Institute of Physics, University of Stockholm, S- 11346 Stockholm, Sweden Received 3 October 1985
Temperature effects on positron camera equipment may cause time dependent sensitivity variations, thus resulting in loss of quantification accuracy. These variations are mostly due to a temperature dependent response of scintillator crystals and photomultiplier tubes (PMT). With two different scintillators mounted on the same PMT, these effects will even more accentuate the problem. This study shows that the energy discriminator settings should not be too close to the full energy peak. That the decay constants decreases differently with increased temperature thereby causing a decreased true to random coincidence ratio for a mixed crystal combination, seems not to be a problem.
1. Introduction
2. Experiment
In nuclear medicine positron emission tomography (PET) is used to study the uptake of positron emitting tracers. The detection device, the positron camera, consists of a n u m b e r of scintillation detectors, mostly organized in a circular array, coupled in coincidence with opposing detectors. A n image of the activity distrib u t i o n can be reconstructed from the recorded data. The reproducibility is defined as the ability of the camera to give the same pixel value of the concentration of administered radionuclides on repeated scans at different times of the same object. Aiming for high reproducibility the gain stability of each scintillation detector is of primary importance. The a m b i e n t temperature is one of several factors that may influence the detector stability. There are three major causes of temperature dependent variations in a scintillation detector [1]: 1. The light yield (conversion efficiency) of the crystal; 2. The efficiency of the p h o t o c a t h o d e of the photomultiplier tube (PMT); 3. The secondary emission ratio of the dynodes. The trend in positron camera design is towards better spatial resolution. This can be achieved with small crystals with high detection efficiency coupled to small PMTs. Unfortunately, small enough PMTs are not commercially available. One of the alternative solutions suggested makes use of two different crystal materials m o u n t e d on each PMT, with identification based on different decay times [2]. This paper reports on to what extent the temperature in the range from + 1 0 to + 4 0 ° C affects the detector responses in such a dual crystal positron camera.
Two crystals (6 × 20 x 30 m m 3) made of b i s m u t h germanate ( B G O ) a n d cerium-activated gadolinium orthosilicate (GSO) respectively were m o u n t e d on a dual a n o d e P M T (fig. 1) from H a m a m a t s u [3]. The reason for not choosing an ordinary P M T was the possibility of collecting the two a n o d e signals separately in order to avoid overlap when displayed on a multichannel analyzer (MCA). The normal case with a single anode P M T can then easily be simulated by adding the anode signals. The crystals and the P M T were placed in a box made of styrofoam, which in turn was placed in a refrigerator, F o u r small light bulbs inside the box were used as heating elements. The heating current was controlled by means of a feedback system based on an i r o n - c o n s t a n tan thermocouple as a probe. This made it possible to adjust the t e m p e r a t u r e to within + 0 . 1 ° C . The a n o d e
0 1 6 8 - 9 0 0 2 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
BGO~
GSO
Fig. 1. The scintillator crystals made of BGO and GSO respectively mounted on the dual PMT R1548.
579
M. Kesselberg / Dual crystal positron camera
the recorded position of the full energy peak one may plot the temperature coefficient, taken as the ratio between the variation of full energy peak position and the associated temperature variation, as a function of temperature (fig. 4). The diagram shows a difference between BGO and GSO where the percentual shifts are on the average - 1 . 5 % / ° C for BGO and - 0 . 7 % / ° C for GSO including a PMT contribution of - 0 . 4 % / ° C according to points 2 and 3 above [4]. The intensity, shows a slight decrease in the BGO case, but for GSO there is no significant change (fig. 5). The fwhm as a function of temperature is an important parameter since it is a measure of the intrinsic spatial resolution. In the GSO case the fwhm is constant within the error limits. In the BGO case, however, a slightly degraded intrinsic resolution is perceived (fig. 6). The error bars in diagrams 4 - 6 correspond to one standard deviation. Because the 511 keV is used in PET, that energy has mostly been chosen in the diagrams. Of course, the other energies measured gave the same resuits. All the measurements were made with both separated and added anode signals. Because of different absorption coefficients and light yield it was, in fact, possible to separate the contributions from the two crystals even in the added case and thereby using it as control [5,6]. No differences were observed. In the case with the coincidence peaks, the variations of the position and fwhm were small and within the limits of error.
signals were amplified and fed into a MCA. The added signals were also collected. For thirteen different temperatures the pulse height spectrum was collected on the MCA. This was done for four different energies; 356 keV (133Ba), 511 keV, 1274 keV (22Na) and 662 keV (13~Cs). After data collection every spectrum was evaluated with a curve fitting program. This procedure gave channel number, intensity (i.e. the area under the full energy peak) and fwhm of each peak, assuming Gaussian peak shapes. Furthermore, in a coincidence setup, the position and fwhm of the timing distribution were measured as a function of temperature in the above mentioned range. All four possible c o m b i n a t i o n s , B G O B G O , B G O - G S O , G S O - G S O and G S O - B G O were recorded.
3.
Results
There is a general tendency to decreased anode pulse amplitude with increasing temperature, here examplified with the result from the 511 keV peak measurement (fig. 2). The MCA calibration plot, for the investigated temperatures, gives the expected linear dependence (fig. 3). These data show that BGO has a stronger temperature dependence then GSO. in order to illustrate the temperature dependence of
~K
-' .. -
40.4
°C
Z ..'. 35.7
0
:..--
4K
,:
°C
• .:'
.
.'.'" 30.4
°C '"'"
.~..,
25.3
°C
..'~.,.
3K
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.
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• . . . . ..... ~ : : , , : , , : : . , . , h , ; , ; , , , : , , . t , : , . : ; i , . i , ; : , ; m , , ~ , , . , ; l : n ,
O
20
40
60
B0
100
120 CHANNEL
140 NUMBER
Fig. 2. The BGO 51l keV peakfrom 22Na displayed onthe MCA forafew differenttemperatures.
M. Kesselberg / Dual crystal positron camera
580 A =
>ntin Z LU
=
40 4 °C ' .0 ."
BGO
,.,'"
.0
10 4 ° C ' •
O. . . . . . . . . .
1000
500"
I
50
I
f
I
100
150
200 CHANNEL
NUMBER
Fig. 3. T h e M C A c a l i b r a t i o n • • • t u s i n g f • u r d • f f e r e n t g a m m a e n e r g • e s f • r t h e t w • e x t r e m e t e m • e r a t u r e s i n t h e p r e s e n t i n v • s t i g a t i • n •
v I-, Z
_= U. LI. UJ O ¢.)
•
BGO
o
GSO
14.1 nl-,< rr-
U.I O. :E U.I I--
•
•
o o
o
7t/
I 10
o
o
o
I 20
I 30
I 40 TEMPERATURE
(°C)
Fig. 4. T h e t e m p e r a t u r e c o e f f i c i e n t as a f u n c t i o n o f t e m p e r a t u r e for B G O a n d G S O , u s i n g the 511 keV p e a k f r o m 22Na.
M. Kesselherg / Dual crystal positron camera
581
8 8 30 Z LU IZ
•
•
Q
•
•
•
BGO
o
GSO
25-
0
20'
o
"•-•/
o
o
o
0
o
0
o
o
0
I
I
I
I
10
20
30
40 TEMPERATURE (°C)
Fig. 5. The intensity as a function of temperature for BGO and GSO. using the 511 keV peak from 22Na. A Z O I'--
40 03 UJ t'r >-
•
•
•
•
•
•
•
•
•
BGO I 356
ILl Z LU
o
o
o
o
o
o
o
o
GSO
keV
J
o
o
o
o
o
•
•
•
o•
• BGO t
30"
,
•
•
•
.
0
0
0
0
0
•
•
0
0
o
o
511 keV
o GSO
20"
~l
I
|
I
I
10
20
30
40 TEMPERATURE
~--
(°C)
Fig. 6. The energy resolution as a function of temperature, for two different energies. The poorer resolution for 356 keV compared to 511 keV is because the former is actually a double peak, not being resolved neither by BGO nor GSO.
582
M. Kesselberg / Dual cO'stal positron camera
4. Discussion The decreasing amplitude of the anode signals with increasing temperature for B G O and G S O is caused by the light yield temperature dependence. A n o t h e r contribution may be due to the temperature d e p e n d e n c e of the spectral matching between the emission spectrum from the crystal and the absorption spectrum of the photocathode. The results found are in agreement with other investigations [7,8]. The results indicate the necessity of a wide energy window setting in a dual crystal positron camera. The use of a too narrow window will cause a temperature d e p e n d e n t variation in sensitivity which is different for the two crystals. The slight degradation of the fwhm (fig. 6) for B G O is due to the decrease in ligtit yield, resulting in reduced p h o t o n statistics [8]. No degradation was found for GSO. F u r t h e r m o r e the lifetime of the scintillation decay is not constant, but decreases with increasing temperature [8]. For a fixed time window setting, usually chosen as the fwtm of the timing distribution, a shift in decay
c o n s t a n t might result in a dislocated coincidence peak relative the window for the G S O - B G O c o m b i n a t i o n and thus a reduced true to r a n d o m ratio. This study shows, however, that this is not a serious problem in the actual temperature interval.
References [1] A. Meessen, J. de Phys. Radium 4 (1958) 437. [2] L. Eriksson, Chr. Bohm, M. Kesselberg, J.-E. Litton, M. Bergstr6m and G. Blomqvist, in: Metabolism of the Human Brain Studied with PET (Raven press, New York, 1985) p. 33. [3] H. Uchida, Y. Yamashita, T. Hamashita and T. Hayashi, IEEE Trans. Nucl. Sci. NS-30 (1983) 451. [4] Hamamatsu catalog on PM-tubes (1985). [5] O.H. Nestor and C.Y. Huang, IEEE Trans. Nucl. Sci. NS-22 (1975) 68. [6] K. Tagaki and T. Fukazava, Appl. Phys. Lett. 42 (1983) 43. [7] Hitachi Ltd., private communication. [8] M.J. Weber and R.R. Monchamp, J. Appl. Phys. 44 (1973) 5495.
TEMPERATURE
Nuclear Instruments and Methods in Physics Research A243 (1986) 578-582 North-Holland, Amsterdam
INFLUENCE
ON A DUAL CRYSTAL
POSITRON
CAMERA
Max KESSELBERG Institute of Physics, University of Stockholm, S- 11346 Stockholm, Sweden Received 3 October 1985
Temperature effects on positron camera equipment may cause time dependent sensitivity variations, thus resulting in loss of quantification accuracy. These variations are mostly due to a temperature dependent response of scintillator crystals and photomultiplier tubes (PMT). With two different scintillators mounted on the same PMT, these effects will even more accentuate the problem. This study shows that the energy discriminator settings should not be too close to the full energy peak. That the decay constants decreases differently with increased temperature thereby causing a decreased true to random coincidence ratio for a mixed crystal combination, seems not to be a problem.
1. Introduction
2. Experiment
In nuclear medicine positron emission tomography (PET) is used to study the uptake of positron emitting tracers. The detection device, the positron camera, consists of a n u m b e r of scintillation detectors, mostly organized in a circular array, coupled in coincidence with opposing detectors. A n image of the activity distrib u t i o n can be reconstructed from the recorded data. The reproducibility is defined as the ability of the camera to give the same pixel value of the concentration of administered radionuclides on repeated scans at different times of the same object. Aiming for high reproducibility the gain stability of each scintillation detector is of primary importance. The a m b i e n t temperature is one of several factors that may influence the detector stability. There are three major causes of temperature dependent variations in a scintillation detector [1]: 1. The light yield (conversion efficiency) of the crystal; 2. The efficiency of the p h o t o c a t h o d e of the photomultiplier tube (PMT); 3. The secondary emission ratio of the dynodes. The trend in positron camera design is towards better spatial resolution. This can be achieved with small crystals with high detection efficiency coupled to small PMTs. Unfortunately, small enough PMTs are not commercially available. One of the alternative solutions suggested makes use of two different crystal materials m o u n t e d on each PMT, with identification based on different decay times [2]. This paper reports on to what extent the temperature in the range from + 1 0 to + 4 0 ° C affects the detector responses in such a dual crystal positron camera.
Two crystals (6 × 20 x 30 m m 3) made of b i s m u t h germanate ( B G O ) a n d cerium-activated gadolinium orthosilicate (GSO) respectively were m o u n t e d on a dual a n o d e P M T (fig. 1) from H a m a m a t s u [3]. The reason for not choosing an ordinary P M T was the possibility of collecting the two a n o d e signals separately in order to avoid overlap when displayed on a multichannel analyzer (MCA). The normal case with a single anode P M T can then easily be simulated by adding the anode signals. The crystals and the P M T were placed in a box made of styrofoam, which in turn was placed in a refrigerator, F o u r small light bulbs inside the box were used as heating elements. The heating current was controlled by means of a feedback system based on an i r o n - c o n s t a n tan thermocouple as a probe. This made it possible to adjust the t e m p e r a t u r e to within + 0 . 1 ° C . The a n o d e
0 1 6 8 - 9 0 0 2 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
BGO~
GSO
Fig. 1. The scintillator crystals made of BGO and GSO respectively mounted on the dual PMT R1548.
579
M. Kesselberg / Dual crystal positron camera
the recorded position of the full energy peak one may plot the temperature coefficient, taken as the ratio between the variation of full energy peak position and the associated temperature variation, as a function of temperature (fig. 4). The diagram shows a difference between BGO and GSO where the percentual shifts are on the average - 1 . 5 % / ° C for BGO and - 0 . 7 % / ° C for GSO including a PMT contribution of - 0 . 4 % / ° C according to points 2 and 3 above [4]. The intensity, shows a slight decrease in the BGO case, but for GSO there is no significant change (fig. 5). The fwhm as a function of temperature is an important parameter since it is a measure of the intrinsic spatial resolution. In the GSO case the fwhm is constant within the error limits. In the BGO case, however, a slightly degraded intrinsic resolution is perceived (fig. 6). The error bars in diagrams 4 - 6 correspond to one standard deviation. Because the 511 keV is used in PET, that energy has mostly been chosen in the diagrams. Of course, the other energies measured gave the same resuits. All the measurements were made with both separated and added anode signals. Because of different absorption coefficients and light yield it was, in fact, possible to separate the contributions from the two crystals even in the added case and thereby using it as control [5,6]. No differences were observed. In the case with the coincidence peaks, the variations of the position and fwhm were small and within the limits of error.
signals were amplified and fed into a MCA. The added signals were also collected. For thirteen different temperatures the pulse height spectrum was collected on the MCA. This was done for four different energies; 356 keV (133Ba), 511 keV, 1274 keV (22Na) and 662 keV (13~Cs). After data collection every spectrum was evaluated with a curve fitting program. This procedure gave channel number, intensity (i.e. the area under the full energy peak) and fwhm of each peak, assuming Gaussian peak shapes. Furthermore, in a coincidence setup, the position and fwhm of the timing distribution were measured as a function of temperature in the above mentioned range. All four possible c o m b i n a t i o n s , B G O B G O , B G O - G S O , G S O - G S O and G S O - B G O were recorded.
3.
Results
There is a general tendency to decreased anode pulse amplitude with increasing temperature, here examplified with the result from the 511 keV peak measurement (fig. 2). The MCA calibration plot, for the investigated temperatures, gives the expected linear dependence (fig. 3). These data show that BGO has a stronger temperature dependence then GSO. in order to illustrate the temperature dependence of
~K
-' .. -
40.4
°C
Z ..'. 35.7
0
:..--
4K
,:
°C
• .:'
.
.'.'" 30.4
°C '"'"
.~..,
25.3
°C
..'~.,.
3K
.'" . •
2K
, .:.',:" .., .. .'.,
. • • ..,:',...
"'..:::::.,.:...
.
.
'
"::::::i)..."
~K
•
.
. "'..
,,
"'.....
"..
• . . . . ..... ~ : : , , : , , : : . , . , h , ; , ; , , , : , , . t , : , . : ; i , . i , ; : , ; m , , ~ , , . , ; l : n ,
O
20
40
60
B0
100
120 CHANNEL
140 NUMBER
Fig. 2. The BGO 51l keV peakfrom 22Na displayed onthe MCA forafew differenttemperatures.
M. Kesselberg / Dual crystal positron camera
580 A =
>ntin Z LU
=
40 4 °C ' .0 ."
BGO
,.,'"
.0
10 4 ° C ' •
O. . . . . . . . . .
1000
500"
I
50
I
f
I
100
150
200 CHANNEL
NUMBER
Fig. 3. T h e M C A c a l i b r a t i o n • • • t u s i n g f • u r d • f f e r e n t g a m m a e n e r g • e s f • r t h e t w • e x t r e m e t e m • e r a t u r e s i n t h e p r e s e n t i n v • s t i g a t i • n •
v I-, Z
_= U. LI. UJ O ¢.)
•
BGO
o
GSO
14.1 nl-,< rr-
U.I O. :E U.I I--
•
•
o o
o
7t/
I 10
o
o
o
I 20
I 30
I 40 TEMPERATURE
(°C)
Fig. 4. T h e t e m p e r a t u r e c o e f f i c i e n t as a f u n c t i o n o f t e m p e r a t u r e for B G O a n d G S O , u s i n g the 511 keV p e a k f r o m 22Na.
M. Kesselherg / Dual crystal positron camera
581
8 8 30 Z LU IZ
•
•
Q
•
•
•
BGO
o
GSO
25-
0
20'
o
"•-•/
o
o
o
0
o
0
o
o
0
I
I
I
I
10
20
30
40 TEMPERATURE (°C)
Fig. 5. The intensity as a function of temperature for BGO and GSO. using the 511 keV peak from 22Na. A Z O I'--
40 03 UJ t'r >-
•
•
•
•
•
•
•
•
•
BGO I 356
ILl Z LU
o
o
o
o
o
o
o
o
GSO
keV
J
o
o
o
o
o
•
•
•
o•
• BGO t
30"
,
•
•
•
.
0
0
0
0
0
•
•
0
0
o
o
511 keV
o GSO
20"
~l
I
|
I
I
10
20
30
40 TEMPERATURE
~--
(°C)
Fig. 6. The energy resolution as a function of temperature, for two different energies. The poorer resolution for 356 keV compared to 511 keV is because the former is actually a double peak, not being resolved neither by BGO nor GSO.
582
M. Kesselberg / Dual cO'stal positron camera
4. Discussion The decreasing amplitude of the anode signals with increasing temperature for B G O and G S O is caused by the light yield temperature dependence. A n o t h e r contribution may be due to the temperature d e p e n d e n c e of the spectral matching between the emission spectrum from the crystal and the absorption spectrum of the photocathode. The results found are in agreement with other investigations [7,8]. The results indicate the necessity of a wide energy window setting in a dual crystal positron camera. The use of a too narrow window will cause a temperature d e p e n d e n t variation in sensitivity which is different for the two crystals. The slight degradation of the fwhm (fig. 6) for B G O is due to the decrease in ligtit yield, resulting in reduced p h o t o n statistics [8]. No degradation was found for GSO. F u r t h e r m o r e the lifetime of the scintillation decay is not constant, but decreases with increasing temperature [8]. For a fixed time window setting, usually chosen as the fwtm of the timing distribution, a shift in decay
c o n s t a n t might result in a dislocated coincidence peak relative the window for the G S O - B G O c o m b i n a t i o n and thus a reduced true to r a n d o m ratio. This study shows, however, that this is not a serious problem in the actual temperature interval.
References [1] A. Meessen, J. de Phys. Radium 4 (1958) 437. [2] L. Eriksson, Chr. Bohm, M. Kesselberg, J.-E. Litton, M. Bergstr6m and G. Blomqvist, in: Metabolism of the Human Brain Studied with PET (Raven press, New York, 1985) p. 33. [3] H. Uchida, Y. Yamashita, T. Hamashita and T. Hayashi, IEEE Trans. Nucl. Sci. NS-30 (1983) 451. [4] Hamamatsu catalog on PM-tubes (1985). [5] O.H. Nestor and C.Y. Huang, IEEE Trans. Nucl. Sci. NS-22 (1975) 68. [6] K. Tagaki and T. Fukazava, Appl. Phys. Lett. 42 (1983) 43. [7] Hitachi Ltd., private communication. [8] M.J. Weber and R.R. Monchamp, J. Appl. Phys. 44 (1973) 5495.