- Email: [email protected]
Biomedical applications of an imaging silicon pixel array (ISPA) tube
Nuclear Instruments and Methods in Physics Research A 387 (1997) 134-136
NUCLEAR INSTRWENTS & METHOOS IN PHVSICS RESEARCH SectIonA
d __-
@/!Z ELSEWIER
Biomedical applications of an imaging silicon pixel array (ISPA) tube D. Puertolasa’b’*,
D. Piedigrossi”, F. de Notaristefanib, C. D’Ambrosio”
H. Leutza, T. Gys”,
“CERN, Geneva, Switzerland ‘INFN Section of Rome, Italy
Abstract Originally developed for HEP applications, the ISPA (Imaging Silicon Pixel Array) tube, a position sensitive photon detector based on the hybrid technology, can also be envisaged for biomedical applications. By coupling it to scintillating crystals or crystal arrays (YAP) we built a y-camera and detected through a collimator the 122 keV y-rays from a “Co source. Depending on the nature of the ISPA tube input window (glass fibre window or quartz window) and of the scintillator (planar YAP or YAP array) spatial resolutions equal or better than 100 p,rn (FWHM) and energy resolutions of 20% have been obtained.
1. Introduction Apart from its applications in particle physics [l-3] and position sensitive photon counting [4], the ISPA tube represents an excellent tool in medical, chemical and micro-biological investigations to image human or animal tissues and chemical compounds labelled with radionucleids. By coupling it to planar scintillators we built a new concept of p-camera. A complete overview of the results can be found in Ref. [5]. Also a -y-camera based on an ISPA tube has been already tested [6,7]. The measured spatial resolution (FWHM) of this ISPA camera amounted in the best case to 310 pm. In the following, we report on improved results obtained with the same y-camera and on preliminary measurements with a camera employing a novel ISPA tube equipped with a quartz input window.
silicon chip anode’. The chip anode is composed of one detector plane divided into 1024 pixel diodes (500 pm X 75 p,rn in size) and one electronics plane also divided into 1024 equally-sized electronic pixels [8]. Each detector pixel is directly bump-bonded to its front end-electronics pixe12. This hybrid configuration of the ISPA tube minimises not only the gain fluctuations and the noise but also the number of leads through the sealed envelope (for details see contribution of T. Gys, same issue). The spatial resolution of electrons emitted from the photocathode is conserved via the electric field linesj. The binary response of individual pixels provides thus a space information and allows 2D-imaging. An important feature of the tube is the possibility to trigger it internally: indeed, the analog signal from the bias contact of the detector chip, arising from the photoelectron pool generated by each event in the scintillator, can be appropriately amplified and shaped. It provides a fast ( - 150 ns peaking time), global information allowing an easy energy calibration and an energy window selection.
2. The ISPA tube The ISPA tube basically consists of a vacuum-sealed cylinder with an optical entrance window on which is evaporated a photocathode. A potential difference (typically 25 kV) is applied between the photocathode and a
* Corresponding author. E-mail: [email protected]; fax: + 41 22 785 0207. 0168-9002/97/$17.00 Copyright PII SO168-9002(96)00976-X
All events
the gating
0 1997 Elsevier
falling
and readout
inside
this energy
window
start
of the tube.
’ Assembled and manufactured by BV Delft Electronische Producten (DEP), NL-9300 AB Roden, The Netherlands. The chips were wire bonded at UC1 MicroBlectronique, F-91946 Les Ulis, France. ‘The chips were developed by the ECP microelectronics group and the RD19 Collaboration at CERN. ’ A magnetic focusing.
Science B.V. All rights reserved
field can also be applied to achieve a very precise
135
D. Puenolas et al. I Nucl. Instr. and Meth. in Phys. Res. A 387 (1997) 134-136
3. Experimental arrangement The two main components of our camera set-up (see Fig. I of Ref. [6]) are the crystal for y-conversion and the ISPA tube for the position-sensitive detection of the crystal scintillations. The 122 keV gammas from a 57Co source 5-mm-thick lead-collimator pass an image-producing (phantom) and are attenuated in three crystal configurations mounted alternately in front of the ISPA tube entrance window. The three configurations of the crystalsJ are the following: - a l-mm-thick planar YAP(Ce) disc that converts 14% of the gammas by photo-effect and 7% by Compton scattering. _ a crystal array composed of YAP(Ce) scintillating elements (or needles) with 0.6 X 0.6 mm2 cross section each and 5 mm thickness. The photo-absorption and Compton scattering fractions are 41% and 22%, respectively. - a crystal array composed of 0.3 X 0.3 mm’ needles with 10 mm thickness. The photo-absorption and Compton scattering fractions are 56% and 30%, respectively. At present, we dispose of two ISPA tubes for our measurements. The first one has a glass fibre input window for collimated light passage with no degradation of the spatial resolution. However, the YAP light emission (peaking at 365 nm) matches badly the spectral sensitivity of the photocathode which results in an overall quantum efficiency of less than 3%. Consequently, the number of photoelectrons per y-event is low and degrades the photopeak energy resolution needed to eliminate Compton scatters. The second tube was built with a quartz input window and an IJV-enhanced photocathode. The window is 2 mm thick and the average quantum efficiency is now 21% over the YAP emission spectrum. In this configuration, the spatial light spread in the quartz window is expected to be compensated by the gain in detected light from the YAP crystals and hence a better energy resolution of the photopeak.
4. Results and discussion Fig. I displays the pulse height distributions of the analog signals arising from the detector chip bias contact of the ISPA tube with a quartz input window and a UV-enhanced photocathode. These distributions represent the energy spectra of “Co measured in the same conditions for the YAP-disc and -arrays (only the measurement with one array is represented since both of them featured within a few percents the same light output). For comparison the distribution obtained in the same conditions
a Produced by Preciosa Crytur Ltd., Tumov, Czech Republic.
0
0
50
loo
150 200 Number of photwlsmms
250
Fig. 1. Pulse height distributions taken from the silicon chip rear side of the ISPA tubes. They represent the energy spectra of the “Co source. The full energy peak corresponding to the 122 keV principal y-emission and the Compton distribution (peak containing backscatter plus X-rays escaping from the lead and Compton edge) are clearly visible with the quartz window ISPA tube.
with the YAP plate and the fibre-optic window ISPA tube is also displayed. Using a quartz window ISPA tube makes clearly visible the Compton part and the full energy peak, both in the case of the plate and the crystal arrays. The number of photoelectrons at the peak is -200 for the plate (in comparison of 12 quoted in Ref. [6]) and - 80 for the arrays (in comparison of 6.4 [6]). The light yield has thus increased by more than a factor of IO, resulting in energy resolutions at the peak of 20% and 40%, respectively. With this configuration, by selecting the appropriate energy window, it becomes easy to trigger the readout of the tube only on y-events corresponding to full energy conversion and suppress Compton effects which could degrade the quality of the image. By imaging a lead-phantom containing 5 holes of - 300 urn diameter with the glass fibre window ISPA tube we obtained spatial resolutions (FWHM) of 700 pm with the YAP plate and 310 p,rn with the YAP array made of 600 pm X 600 pm needles and pointed out that in the case of the array the overall spatial resolution of the system was limited by the crystal element size [6,7]. Repeating the same procedure with the array containing elements of 300 pm X 300 pm improves the spatial resolutions to - 160 km. This result was expected since with this new array we have the same number of photoelectrons per y-event while the cluster size (spread of light) is two times smaller. Actually, here again the spatial resolution is only dictated by the dimensions of the crystal elements. This can be seen on Fig. 2 that displays a 2mm-hole imaged with the YAP array (300 km X 300 pm elements) and the fibre-optic window ISPA tube. One can see clearly each crystal element of the YAP matrix and the imperfections of the
111. HYBRID
PHOTODIODES
136
D. Puertolas
et al. / Nucl. Instr.
and Meth.
in Phys. Res. A 387 (1997)
134-136
conditions, similar values are obtained with the YAP array (600 pm X 600 (*rn elements) suggesting that the overall spatial resolution is no longer governed by the dimensions of the crystal array elements but mainly by the light spread which occurs in the quartz window. Values of 650 to 700 p,rn are obtained with the YAP plate. A complete and comparative study of all the possible configurations will be given in a forthcoming paper.
5. Conclusion
Fig. 2. Image of a lead phantom with one hole of 2 mm diameter taken with the fibre-optic window ISPA tube. The 300 pm X 300 pm individual elements constituting the YAP matrix clearly appear and the small defects in the matrix assembly can also been observed.
Misalignments of a needle from its neighbour as small as l/3 of their dimension can be observed meaning that the spatial resolution is at least as good as 100 Pm. Finally, Fig. 3 shows the image of a 2-holes (- 350 Pm diameter) collimator taken with the YAP array (300 pm X 300 Pm elements) and the quartz window ISPA tube. The two holes with 1.2 mm distance are clearly separated. The light intensity profile along a straight line going through the centre of the collimator holes leads to spatial resolutions (FWHM) of 500 to 550 km. In the same latter.
With a series of scintillating crystals and two different ISPA tubes we have tested different configurations of gamma-cameras. By coupling a fibre-optic window ISPA tube to a 300 Pm X 300 km elements YAP array we achieved spatial resolutions ( I 100 pm) which should satisfy any application where the highest spatial resolution is required. Alternately, by using an ISPA tube with a quartz window we obtained 20% energy resolution combined with spatial resolutions of the order of 700 p,rn (FWHM) with no need of finely segmented crystal matrices. In the future, we plan to integrate an improved silicon chip [9] in a tube and to increase the sensitive detection area; this can be achieved in two complementary ways: tiling of several chips at the anode and electrostatic demagnification of the tube itself.
References 111 T. Gys et al., Nucl. Instr. and Meth. A 3.55 (1995) 386. 121 C. D’Ambrosio et al., Nucl. Instr. and Meth. A 359 (1995) 618.
131 C. D’Ambrosio r41 C. D’Ambrosio
et al., IEEE Trans. Nucl. Sci. 43 (3) (1996). et al., IEEE Trans. Nucl. Sci. 42 (3) (1995)
130.
I51 D. Puertolas
Fig. 3. Image of a lead collimator with two holes (diameter: - 350 pm) taken with the quartz window ISPA tube and the YAP array with 300 urn X 300 Pm needles. The two holes (distance: 1.2 millimeter) are perfectly separated.
et al., IEEE Trans. Nucl. Sci. 43 2477. I61 C. D’Ambrosio et al., Nucl. Instr. and Meth. A 54. 171 D. Puertolas et al., IEEE Trans. Nucl. Sci. 42 2221. 181 E.H.M. Heijne et al., Nucl. Instr. and Meth. A 399. r91 E.H.M. Heijne et al., preprint CERN-ECP/96-03, to Nucl. Instr. and Meth.
(5) (1996) 367 (1995) (6) (1995) 348 (1994) submitted
NUCLEAR INSTRWENTS & METHOOS IN PHVSICS RESEARCH SectIonA
d __-
@/!Z ELSEWIER
Biomedical applications of an imaging silicon pixel array (ISPA) tube D. Puertolasa’b’*,
D. Piedigrossi”, F. de Notaristefanib, C. D’Ambrosio”
H. Leutza, T. Gys”,
“CERN, Geneva, Switzerland ‘INFN Section of Rome, Italy
Abstract Originally developed for HEP applications, the ISPA (Imaging Silicon Pixel Array) tube, a position sensitive photon detector based on the hybrid technology, can also be envisaged for biomedical applications. By coupling it to scintillating crystals or crystal arrays (YAP) we built a y-camera and detected through a collimator the 122 keV y-rays from a “Co source. Depending on the nature of the ISPA tube input window (glass fibre window or quartz window) and of the scintillator (planar YAP or YAP array) spatial resolutions equal or better than 100 p,rn (FWHM) and energy resolutions of 20% have been obtained.
1. Introduction Apart from its applications in particle physics [l-3] and position sensitive photon counting [4], the ISPA tube represents an excellent tool in medical, chemical and micro-biological investigations to image human or animal tissues and chemical compounds labelled with radionucleids. By coupling it to planar scintillators we built a new concept of p-camera. A complete overview of the results can be found in Ref. [5]. Also a -y-camera based on an ISPA tube has been already tested [6,7]. The measured spatial resolution (FWHM) of this ISPA camera amounted in the best case to 310 pm. In the following, we report on improved results obtained with the same y-camera and on preliminary measurements with a camera employing a novel ISPA tube equipped with a quartz input window.
silicon chip anode’. The chip anode is composed of one detector plane divided into 1024 pixel diodes (500 pm X 75 p,rn in size) and one electronics plane also divided into 1024 equally-sized electronic pixels [8]. Each detector pixel is directly bump-bonded to its front end-electronics pixe12. This hybrid configuration of the ISPA tube minimises not only the gain fluctuations and the noise but also the number of leads through the sealed envelope (for details see contribution of T. Gys, same issue). The spatial resolution of electrons emitted from the photocathode is conserved via the electric field linesj. The binary response of individual pixels provides thus a space information and allows 2D-imaging. An important feature of the tube is the possibility to trigger it internally: indeed, the analog signal from the bias contact of the detector chip, arising from the photoelectron pool generated by each event in the scintillator, can be appropriately amplified and shaped. It provides a fast ( - 150 ns peaking time), global information allowing an easy energy calibration and an energy window selection.
2. The ISPA tube The ISPA tube basically consists of a vacuum-sealed cylinder with an optical entrance window on which is evaporated a photocathode. A potential difference (typically 25 kV) is applied between the photocathode and a
* Corresponding author. E-mail: [email protected]; fax: + 41 22 785 0207. 0168-9002/97/$17.00 Copyright PII SO168-9002(96)00976-X
All events
the gating
0 1997 Elsevier
falling
and readout
inside
this energy
window
start
of the tube.
’ Assembled and manufactured by BV Delft Electronische Producten (DEP), NL-9300 AB Roden, The Netherlands. The chips were wire bonded at UC1 MicroBlectronique, F-91946 Les Ulis, France. ‘The chips were developed by the ECP microelectronics group and the RD19 Collaboration at CERN. ’ A magnetic focusing.
Science B.V. All rights reserved
field can also be applied to achieve a very precise
135
D. Puenolas et al. I Nucl. Instr. and Meth. in Phys. Res. A 387 (1997) 134-136
3. Experimental arrangement The two main components of our camera set-up (see Fig. I of Ref. [6]) are the crystal for y-conversion and the ISPA tube for the position-sensitive detection of the crystal scintillations. The 122 keV gammas from a 57Co source 5-mm-thick lead-collimator pass an image-producing (phantom) and are attenuated in three crystal configurations mounted alternately in front of the ISPA tube entrance window. The three configurations of the crystalsJ are the following: - a l-mm-thick planar YAP(Ce) disc that converts 14% of the gammas by photo-effect and 7% by Compton scattering. _ a crystal array composed of YAP(Ce) scintillating elements (or needles) with 0.6 X 0.6 mm2 cross section each and 5 mm thickness. The photo-absorption and Compton scattering fractions are 41% and 22%, respectively. - a crystal array composed of 0.3 X 0.3 mm’ needles with 10 mm thickness. The photo-absorption and Compton scattering fractions are 56% and 30%, respectively. At present, we dispose of two ISPA tubes for our measurements. The first one has a glass fibre input window for collimated light passage with no degradation of the spatial resolution. However, the YAP light emission (peaking at 365 nm) matches badly the spectral sensitivity of the photocathode which results in an overall quantum efficiency of less than 3%. Consequently, the number of photoelectrons per y-event is low and degrades the photopeak energy resolution needed to eliminate Compton scatters. The second tube was built with a quartz input window and an IJV-enhanced photocathode. The window is 2 mm thick and the average quantum efficiency is now 21% over the YAP emission spectrum. In this configuration, the spatial light spread in the quartz window is expected to be compensated by the gain in detected light from the YAP crystals and hence a better energy resolution of the photopeak.
4. Results and discussion Fig. I displays the pulse height distributions of the analog signals arising from the detector chip bias contact of the ISPA tube with a quartz input window and a UV-enhanced photocathode. These distributions represent the energy spectra of “Co measured in the same conditions for the YAP-disc and -arrays (only the measurement with one array is represented since both of them featured within a few percents the same light output). For comparison the distribution obtained in the same conditions
a Produced by Preciosa Crytur Ltd., Tumov, Czech Republic.
0
0
50
loo
150 200 Number of photwlsmms
250
Fig. 1. Pulse height distributions taken from the silicon chip rear side of the ISPA tubes. They represent the energy spectra of the “Co source. The full energy peak corresponding to the 122 keV principal y-emission and the Compton distribution (peak containing backscatter plus X-rays escaping from the lead and Compton edge) are clearly visible with the quartz window ISPA tube.
with the YAP plate and the fibre-optic window ISPA tube is also displayed. Using a quartz window ISPA tube makes clearly visible the Compton part and the full energy peak, both in the case of the plate and the crystal arrays. The number of photoelectrons at the peak is -200 for the plate (in comparison of 12 quoted in Ref. [6]) and - 80 for the arrays (in comparison of 6.4 [6]). The light yield has thus increased by more than a factor of IO, resulting in energy resolutions at the peak of 20% and 40%, respectively. With this configuration, by selecting the appropriate energy window, it becomes easy to trigger the readout of the tube only on y-events corresponding to full energy conversion and suppress Compton effects which could degrade the quality of the image. By imaging a lead-phantom containing 5 holes of - 300 urn diameter with the glass fibre window ISPA tube we obtained spatial resolutions (FWHM) of 700 pm with the YAP plate and 310 p,rn with the YAP array made of 600 pm X 600 pm needles and pointed out that in the case of the array the overall spatial resolution of the system was limited by the crystal element size [6,7]. Repeating the same procedure with the array containing elements of 300 pm X 300 pm improves the spatial resolutions to - 160 km. This result was expected since with this new array we have the same number of photoelectrons per y-event while the cluster size (spread of light) is two times smaller. Actually, here again the spatial resolution is only dictated by the dimensions of the crystal elements. This can be seen on Fig. 2 that displays a 2mm-hole imaged with the YAP array (300 km X 300 pm elements) and the fibre-optic window ISPA tube. One can see clearly each crystal element of the YAP matrix and the imperfections of the
111. HYBRID
PHOTODIODES
136
D. Puertolas
et al. / Nucl. Instr.
and Meth.
in Phys. Res. A 387 (1997)
134-136
conditions, similar values are obtained with the YAP array (600 pm X 600 (*rn elements) suggesting that the overall spatial resolution is no longer governed by the dimensions of the crystal array elements but mainly by the light spread which occurs in the quartz window. Values of 650 to 700 p,rn are obtained with the YAP plate. A complete and comparative study of all the possible configurations will be given in a forthcoming paper.
5. Conclusion
Fig. 2. Image of a lead phantom with one hole of 2 mm diameter taken with the fibre-optic window ISPA tube. The 300 pm X 300 pm individual elements constituting the YAP matrix clearly appear and the small defects in the matrix assembly can also been observed.
Misalignments of a needle from its neighbour as small as l/3 of their dimension can be observed meaning that the spatial resolution is at least as good as 100 Pm. Finally, Fig. 3 shows the image of a 2-holes (- 350 Pm diameter) collimator taken with the YAP array (300 pm X 300 Pm elements) and the quartz window ISPA tube. The two holes with 1.2 mm distance are clearly separated. The light intensity profile along a straight line going through the centre of the collimator holes leads to spatial resolutions (FWHM) of 500 to 550 km. In the same latter.
With a series of scintillating crystals and two different ISPA tubes we have tested different configurations of gamma-cameras. By coupling a fibre-optic window ISPA tube to a 300 Pm X 300 km elements YAP array we achieved spatial resolutions ( I 100 pm) which should satisfy any application where the highest spatial resolution is required. Alternately, by using an ISPA tube with a quartz window we obtained 20% energy resolution combined with spatial resolutions of the order of 700 p,rn (FWHM) with no need of finely segmented crystal matrices. In the future, we plan to integrate an improved silicon chip [9] in a tube and to increase the sensitive detection area; this can be achieved in two complementary ways: tiling of several chips at the anode and electrostatic demagnification of the tube itself.
References 111 T. Gys et al., Nucl. Instr. and Meth. A 3.55 (1995) 386. 121 C. D’Ambrosio et al., Nucl. Instr. and Meth. A 359 (1995) 618.
131 C. D’Ambrosio r41 C. D’Ambrosio
et al., IEEE Trans. Nucl. Sci. 43 (3) (1996). et al., IEEE Trans. Nucl. Sci. 42 (3) (1995)
130.
I51 D. Puertolas
Fig. 3. Image of a lead collimator with two holes (diameter: - 350 pm) taken with the quartz window ISPA tube and the YAP array with 300 urn X 300 Pm needles. The two holes (distance: 1.2 millimeter) are perfectly separated.
et al., IEEE Trans. Nucl. Sci. 43 2477. I61 C. D’Ambrosio et al., Nucl. Instr. and Meth. A 54. 171 D. Puertolas et al., IEEE Trans. Nucl. Sci. 42 2221. 181 E.H.M. Heijne et al., Nucl. Instr. and Meth. A 399. r91 E.H.M. Heijne et al., preprint CERN-ECP/96-03, to Nucl. Instr. and Meth.
(5) (1996) 367 (1995) (6) (1995) 348 (1994) submitted