- Email: [email protected]
Spectroscopic and imaging capabilities of a pixellated photon counting system
Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
Spectroscopic and imaging capabilities of a pixellated photon counting system S.R. Amendoliaa, M.G. Bisognib, U. Bottiglib, P. Delogub, G. Dipasqualeb, M.E. Fantaccib,*, A. Marchib, V.M. Marzullib, P. Olivab, R. Palmierob, V. Rossob, A. Stefaninib, S. Stumboa, S. Zuccab a
b
Istituto di Matematica e Fisica dell’Universita` di Sassari and Sezione INFN di Pisa, Pisa, Italy Dipartimento di Fisica dell’Universita` and Sezione INFN di Pisa, Via Livornese 1291, 56010 Pisa, Italy
Abstract We are studying the performance of various thickness GaAs pixel detectors bump-bonded to a dedicated photon counting chip (PCC) for medical imaging applications in different energy ranges. In this work we present the experimental results obtained with a 600 mm thick pixel matrix (64 64 square pixels, 170 mm side) in the 60–140 keV energy range to evaluate the possible use of such a system in the nuclear medicine field. In particular, we have measured the spectroscopic properties of the detector (charge collection efficiency, energy resolution and detection efficiency) and evaluated the discrimination capability of the electronics. Then we have measured the imaging properties of the whole system in terms of Point Spread Function and using a home made thyroid phantom. We present also a comparison with a traditional gamma camera and an evaluation, made by both experimental measurements and software simulations, of the imaging characteristics related to the use of a collimation system. # 2001 Elsevier Science B.V. All rights reserved. PACS: 87.66.Pm; 87.58.Mj Keywords: GaAs pixel detector; Photon counting chip; Medical imaging
1. Introduction A 200 mm thick GaAs pixel matrix bumpbonded to a dedicated VLSI chip has already been tested [1] for digital mammography applications (mean energy 20 keV), obtaining very good results both in terms of dose reduction and imaging capabilities. For more energetic photons, as in nuclear medicine field, a thicker detector is required in order to obtain a satisfactory detection *Corresponding author. Tel: +39-050-880-269; fax: +39050-880-317. E-mail address: [email protected] (M.E. Fantacci).
efficiency. So we have chosen a 600 mm thick detector and tested a prototype based on this detector using radioactive sources (241Am, 60 keV photons and 99mTc, 140 keV photons).
2. Experimental setup The detectors are h1 0 0i Semi-Insulating Liquid Encapsulated Czochralski (LEC) GaAs crystals equipped with standard (Ti/Pd/Au multilayer) Schottky contacts and nonalloyed [2] ohmic contacts. Our pixel detector is made by 64 64
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 8 2 7 - 0
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
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square pixels (150 mm side, 20 mm interpixel distance) read out by the Photon Counting Chip (PCC), developed in the framework of the Medipix collaboration which includes CERN, INFN of Napoli and Pisa and the universities of Glasgow and Freiburg. PCC, built in SACMOS 1 mm technology [3], consists of 4096 square cells (170 mm side) connected via bump-bonding to the corresponding detection pixels. The chip is selftriggering. Each cell contains a charge sensitive preamplifier, a latched comparator with an externally tunable threshold, a shaping delay circuit and a 15-bit pseudo random counter. Each cell threshold can be adjusted with a 3-bit resolution in addition to the common threshold. Fig. 1. Charge collection efficiency as a function of the bias voltage.
3. Results and conclusions 3.1. Spectroscopic and electrical properties For the preliminary evaluation of the spectroscopic properties we used single square pads read out by a charge-sensitive preamplifier, a shaper amplifier and a multichannel analyser. The charge collection efficiency as a function of the bias voltage of the pad detector was measured with both 241Am and 99mTc sources and the results are reported in Fig. 1. The behaviour is the same for the different energies and the value at the operating bias voltage of 500 V is 75%. The energy resolution is almost constant and shows a value of about 10% at the same operating bias. In the electrical characterization of the PCC the electrical calibration was made by relating the comparator threshold voltage to the charge in the preamplifier input via the 25 fF test capacitance injected (and so to the equivalent photon energy in GaAs). The effect of the threshold adjustment on the threshold distribution and the linearity of the threshold were also evaluated. Fig. 2 shows the threshold distributions, before and after the 3-bit adjustment, for the mean threshold used in the imaging measurements described in the next section. Fig. 3 shows the calibration curve. This chip can operate from a minimum threshold value corresponding to about 3000 electrons up to a threshold value corresponding to about 13000
Fig. 2. Threshold distributions before and after the threshold adjustment.
electrons. In order to compare the charge collection efficiency of the GaAs pixel with the values obtained with the single pad detector, and to evaluate the detection efficiency of this system, we have acquired the integral spectra of the 241Am source by a threshold scan at different detector bias voltages. In Fig. 4 are shown the corresponding curves, in which the number of counts registered as a function of the threshold are reported for each bias voltage. By differentiating these curves we obtained the differential spectra (Fig. 5) and evaluated the charge collection efficiency and the detection efficiency. The values SECTION II.
76
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
Fig. 3. Threshold calibration of the PCC.
Fig. 5. Differential spectrum of the voltages.
241
Am source at 500 V bias
Fig. 4. Integral spectra of the 241Am source at different detector bias voltages. Fig. 6. Detection efficiency as a function of the bias voltage.
of the charge collection efficiency are in agreement with the previous measurements. The detection efficiency as a function of bias voltage is reported in Fig. 6. The behaviour of the active thickness as a function of the bias voltage is linear, in agreement with theoretical models [4]. Taking into account the source activity, the irradiation geometry and the attenuation properties of the GaAs, already evaluated by Monte Carlo simulations [5], we have calculated that, at the maximum bias voltage used of 500 V, the detector is not fully depleted and the active thickness is about 290 mm. 3.2. Imaging capabilities The imaging properties of the GaAs pixel detector were evaluated in terms of spatial resolution, by measuring the Point Spread Function
(PSF), and also using a home made thyroid phantom. We also made a comparison between these results and those obtained with a traditional gamma camera. In photon emission imaging, as in nuclear medicine, a collimation system is used in order to select the direction of the incoming photons and detection parameters such as spatial resolution and overall detection efficiency are critically related to the physical characteristics and geometry of this collimation system. So we have evaluated, by both experimental measurements and software simulations, the influence of the collimation geometry and the capabilities of our prototype in standard clinical conditions. The PSF measurements used a phantom consisting of a
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
77
Table 1 PSF obtained with our prototype Distance (mm)
FWHM (mm)
LPSF (mm)
25 40 50 60
1.39 0.02 2.28 0.05 3.00 0.06 3.9 0.1
1.1 0.1 2.1 0.2 2.9 0.3 3.8 0.4
Table 2 PSF obtained with a traditional gamma camera
Fig. 7. PSF images obtained with the pixel GaAs detector (left) and with the gamma camera (right).
Distance (mm)
FWHM (mm)
LPSF (mm)
25 40 50 60
2.95 0.04 3.31 0.08 3.75 0.08 4.05 0.10
2.8 0.3 3.2 0.3 3.6 0.35 4 0.4
9 mm thick lead block with a 800 mm diameter hole. The 241Am source (and then the 99mTc source) was placed on the block and the FWHM of the image of the hole measured for various distances between the block and the GaAs pixel detector. The same measurements were then made with a clinical gamma camera (Transcam POLARIS). By considering the geometry of the hole, we evaluated in both the situations the contribution to the FWHM, LPSF, due to the detection system. The results are given in Tables 1 and 2. Fig. 7 [6] shows the images obtained at a distance of 25 mm. We can conclude that our prototype shows very good imaging capabilities in comparison with the traditional gamma camera, allowing for a better spatial resolution. To evaluate the effect of the collimator we have used a low energy high resolution (LEHR) one, consisting of a 4 cm thick lead block in which are present hexagonal holes with a diameter of 1.8 mm with 200 mm thick septa. We report here the experi-mental images (acquired with the pixel GaAs dete-ctor) and the simulated images of this collimator irradiated with the 241Am source placed at a distance of 0 cm (Fig. 8) and 5 cm (Fig. 9) from the collimator. Our simulation reproduces correctly the geometrical and attenuation properties of the collimator. Fig. 10 shows the images of a clinical phantom in which a 3 mm diameter hole in a plexiglass
Fig. 8. Experimental (left) and simulated (right) images images of the collimator (distance sourcecollimator=0 cm).
Fig. 9. Experimental (left) and simulated (right) images images of the collimator (distance sourcecollimator=5 cm).
block was filled with 99mTc to reproduce the emission of a marked hot nodule. The images were realized in the presence of the collimator with our prototype (left) and with the gamma camera (right). In this configuration the spatial resolution of the whole system is dominated by the geometry of the standard high resolution collimator, so we can conclude that the very good imaging SECTION II.
78
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
to S. Bozzi (www.bozzi.net) for making the PSF phantom.
References
Fig. 10. Images of the phantom obtained with the pixel GaAs detector (left) and with the gamma camera (right).
capabilities of our system could be useful in actual imaging in nuclear medicine only with the use of a dedicated collimator.
Acknowledgements We would like to thank the U.O. of Nuclear Medicine of Livorno and Pisa hospitals and the U.O. of Medical Physics of Livorno hospital for the collaboration in this work. Thanks are also due
[1] S.R. Amendolia et al., Low contrast imaging with a GaAs pixel digital detector, presented at IEEE Nuclear Science Symposium and Medical Imaging Conference, November 1999, Seattle, WA, IEEE Trans. Nucl. Sci. NS-47 (2000) 1478. [2] M. Alietti et al., Performance of a new ohmic contact for GaAs particle detectors, Nucl. Instr. and Meth. A 362 (1995) 344. [3] M. Campbell et al., A readout chip for a 64 64 pixel matrix with 15-bit single photon counting, IEEE Trans. Nucl. Sci. NS-45 (1998) 751. [4] A. Cola et al., Microscopic modelling of semi-insulating GaAs detectors, Nucl. Instr. and Meth. A 395 (1997) 98. [5] S.R. Amendolia et al., GaAs detector optimization for different medical imaging applications, Nucl. Instr. and Meth. A 434 (1999) 14. [6] S.R. Amendolia et al., Evaluation of the imaging properties of a direct detection single photon counting based system, presented at the Eighth Pisa Meeting on Advanced Detectors, May 2000, La Biodola, Italy, Nucl. Instr. and Meth. A 461 (2001) 422.
Spectroscopic and imaging capabilities of a pixellated photon counting system S.R. Amendoliaa, M.G. Bisognib, U. Bottiglib, P. Delogub, G. Dipasqualeb, M.E. Fantaccib,*, A. Marchib, V.M. Marzullib, P. Olivab, R. Palmierob, V. Rossob, A. Stefaninib, S. Stumboa, S. Zuccab a
b
Istituto di Matematica e Fisica dell’Universita` di Sassari and Sezione INFN di Pisa, Pisa, Italy Dipartimento di Fisica dell’Universita` and Sezione INFN di Pisa, Via Livornese 1291, 56010 Pisa, Italy
Abstract We are studying the performance of various thickness GaAs pixel detectors bump-bonded to a dedicated photon counting chip (PCC) for medical imaging applications in different energy ranges. In this work we present the experimental results obtained with a 600 mm thick pixel matrix (64 64 square pixels, 170 mm side) in the 60–140 keV energy range to evaluate the possible use of such a system in the nuclear medicine field. In particular, we have measured the spectroscopic properties of the detector (charge collection efficiency, energy resolution and detection efficiency) and evaluated the discrimination capability of the electronics. Then we have measured the imaging properties of the whole system in terms of Point Spread Function and using a home made thyroid phantom. We present also a comparison with a traditional gamma camera and an evaluation, made by both experimental measurements and software simulations, of the imaging characteristics related to the use of a collimation system. # 2001 Elsevier Science B.V. All rights reserved. PACS: 87.66.Pm; 87.58.Mj Keywords: GaAs pixel detector; Photon counting chip; Medical imaging
1. Introduction A 200 mm thick GaAs pixel matrix bumpbonded to a dedicated VLSI chip has already been tested [1] for digital mammography applications (mean energy 20 keV), obtaining very good results both in terms of dose reduction and imaging capabilities. For more energetic photons, as in nuclear medicine field, a thicker detector is required in order to obtain a satisfactory detection *Corresponding author. Tel: +39-050-880-269; fax: +39050-880-317. E-mail address: [email protected] (M.E. Fantacci).
efficiency. So we have chosen a 600 mm thick detector and tested a prototype based on this detector using radioactive sources (241Am, 60 keV photons and 99mTc, 140 keV photons).
2. Experimental setup The detectors are h1 0 0i Semi-Insulating Liquid Encapsulated Czochralski (LEC) GaAs crystals equipped with standard (Ti/Pd/Au multilayer) Schottky contacts and nonalloyed [2] ohmic contacts. Our pixel detector is made by 64 64
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 8 2 7 - 0
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
75
square pixels (150 mm side, 20 mm interpixel distance) read out by the Photon Counting Chip (PCC), developed in the framework of the Medipix collaboration which includes CERN, INFN of Napoli and Pisa and the universities of Glasgow and Freiburg. PCC, built in SACMOS 1 mm technology [3], consists of 4096 square cells (170 mm side) connected via bump-bonding to the corresponding detection pixels. The chip is selftriggering. Each cell contains a charge sensitive preamplifier, a latched comparator with an externally tunable threshold, a shaping delay circuit and a 15-bit pseudo random counter. Each cell threshold can be adjusted with a 3-bit resolution in addition to the common threshold. Fig. 1. Charge collection efficiency as a function of the bias voltage.
3. Results and conclusions 3.1. Spectroscopic and electrical properties For the preliminary evaluation of the spectroscopic properties we used single square pads read out by a charge-sensitive preamplifier, a shaper amplifier and a multichannel analyser. The charge collection efficiency as a function of the bias voltage of the pad detector was measured with both 241Am and 99mTc sources and the results are reported in Fig. 1. The behaviour is the same for the different energies and the value at the operating bias voltage of 500 V is 75%. The energy resolution is almost constant and shows a value of about 10% at the same operating bias. In the electrical characterization of the PCC the electrical calibration was made by relating the comparator threshold voltage to the charge in the preamplifier input via the 25 fF test capacitance injected (and so to the equivalent photon energy in GaAs). The effect of the threshold adjustment on the threshold distribution and the linearity of the threshold were also evaluated. Fig. 2 shows the threshold distributions, before and after the 3-bit adjustment, for the mean threshold used in the imaging measurements described in the next section. Fig. 3 shows the calibration curve. This chip can operate from a minimum threshold value corresponding to about 3000 electrons up to a threshold value corresponding to about 13000
Fig. 2. Threshold distributions before and after the threshold adjustment.
electrons. In order to compare the charge collection efficiency of the GaAs pixel with the values obtained with the single pad detector, and to evaluate the detection efficiency of this system, we have acquired the integral spectra of the 241Am source by a threshold scan at different detector bias voltages. In Fig. 4 are shown the corresponding curves, in which the number of counts registered as a function of the threshold are reported for each bias voltage. By differentiating these curves we obtained the differential spectra (Fig. 5) and evaluated the charge collection efficiency and the detection efficiency. The values SECTION II.
76
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
Fig. 3. Threshold calibration of the PCC.
Fig. 5. Differential spectrum of the voltages.
241
Am source at 500 V bias
Fig. 4. Integral spectra of the 241Am source at different detector bias voltages. Fig. 6. Detection efficiency as a function of the bias voltage.
of the charge collection efficiency are in agreement with the previous measurements. The detection efficiency as a function of bias voltage is reported in Fig. 6. The behaviour of the active thickness as a function of the bias voltage is linear, in agreement with theoretical models [4]. Taking into account the source activity, the irradiation geometry and the attenuation properties of the GaAs, already evaluated by Monte Carlo simulations [5], we have calculated that, at the maximum bias voltage used of 500 V, the detector is not fully depleted and the active thickness is about 290 mm. 3.2. Imaging capabilities The imaging properties of the GaAs pixel detector were evaluated in terms of spatial resolution, by measuring the Point Spread Function
(PSF), and also using a home made thyroid phantom. We also made a comparison between these results and those obtained with a traditional gamma camera. In photon emission imaging, as in nuclear medicine, a collimation system is used in order to select the direction of the incoming photons and detection parameters such as spatial resolution and overall detection efficiency are critically related to the physical characteristics and geometry of this collimation system. So we have evaluated, by both experimental measurements and software simulations, the influence of the collimation geometry and the capabilities of our prototype in standard clinical conditions. The PSF measurements used a phantom consisting of a
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
77
Table 1 PSF obtained with our prototype Distance (mm)
FWHM (mm)
LPSF (mm)
25 40 50 60
1.39 0.02 2.28 0.05 3.00 0.06 3.9 0.1
1.1 0.1 2.1 0.2 2.9 0.3 3.8 0.4
Table 2 PSF obtained with a traditional gamma camera
Fig. 7. PSF images obtained with the pixel GaAs detector (left) and with the gamma camera (right).
Distance (mm)
FWHM (mm)
LPSF (mm)
25 40 50 60
2.95 0.04 3.31 0.08 3.75 0.08 4.05 0.10
2.8 0.3 3.2 0.3 3.6 0.35 4 0.4
9 mm thick lead block with a 800 mm diameter hole. The 241Am source (and then the 99mTc source) was placed on the block and the FWHM of the image of the hole measured for various distances between the block and the GaAs pixel detector. The same measurements were then made with a clinical gamma camera (Transcam POLARIS). By considering the geometry of the hole, we evaluated in both the situations the contribution to the FWHM, LPSF, due to the detection system. The results are given in Tables 1 and 2. Fig. 7 [6] shows the images obtained at a distance of 25 mm. We can conclude that our prototype shows very good imaging capabilities in comparison with the traditional gamma camera, allowing for a better spatial resolution. To evaluate the effect of the collimator we have used a low energy high resolution (LEHR) one, consisting of a 4 cm thick lead block in which are present hexagonal holes with a diameter of 1.8 mm with 200 mm thick septa. We report here the experi-mental images (acquired with the pixel GaAs dete-ctor) and the simulated images of this collimator irradiated with the 241Am source placed at a distance of 0 cm (Fig. 8) and 5 cm (Fig. 9) from the collimator. Our simulation reproduces correctly the geometrical and attenuation properties of the collimator. Fig. 10 shows the images of a clinical phantom in which a 3 mm diameter hole in a plexiglass
Fig. 8. Experimental (left) and simulated (right) images images of the collimator (distance sourcecollimator=0 cm).
Fig. 9. Experimental (left) and simulated (right) images images of the collimator (distance sourcecollimator=5 cm).
block was filled with 99mTc to reproduce the emission of a marked hot nodule. The images were realized in the presence of the collimator with our prototype (left) and with the gamma camera (right). In this configuration the spatial resolution of the whole system is dominated by the geometry of the standard high resolution collimator, so we can conclude that the very good imaging SECTION II.
78
S.R. Amendolia et al. / Nuclear Instruments and Methods in Physics Research A 466 (2001) 74–78
to S. Bozzi (www.bozzi.net) for making the PSF phantom.
References
Fig. 10. Images of the phantom obtained with the pixel GaAs detector (left) and with the gamma camera (right).
capabilities of our system could be useful in actual imaging in nuclear medicine only with the use of a dedicated collimator.
Acknowledgements We would like to thank the U.O. of Nuclear Medicine of Livorno and Pisa hospitals and the U.O. of Medical Physics of Livorno hospital for the collaboration in this work. Thanks are also due
[1] S.R. Amendolia et al., Low contrast imaging with a GaAs pixel digital detector, presented at IEEE Nuclear Science Symposium and Medical Imaging Conference, November 1999, Seattle, WA, IEEE Trans. Nucl. Sci. NS-47 (2000) 1478. [2] M. Alietti et al., Performance of a new ohmic contact for GaAs particle detectors, Nucl. Instr. and Meth. A 362 (1995) 344. [3] M. Campbell et al., A readout chip for a 64 64 pixel matrix with 15-bit single photon counting, IEEE Trans. Nucl. Sci. NS-45 (1998) 751. [4] A. Cola et al., Microscopic modelling of semi-insulating GaAs detectors, Nucl. Instr. and Meth. A 395 (1997) 98. [5] S.R. Amendolia et al., GaAs detector optimization for different medical imaging applications, Nucl. Instr. and Meth. A 434 (1999) 14. [6] S.R. Amendolia et al., Evaluation of the imaging properties of a direct detection single photon counting based system, presented at the Eighth Pisa Meeting on Advanced Detectors, May 2000, La Biodola, Italy, Nucl. Instr. and Meth. A 461 (2001) 422.