- Email: [email protected]
A CCD-based tissue imaging system
Nuclear Instruments and Methods in Physics Research A 392 (1997) 220-226 . __+ __ ll!B ELSEVIER
NUCLEAR INSTRUMENTS 8 METIIODS IN PHYSICS RESEARCH Seclm
A
A CCD-based tissue imaging system J.H. MacDonald”, *, K. Wellsb, A.J. Reader”, R.J. Ott” “Joint Department of Physics, Institute of Cancer Research, Royal Marsden NHS Trust, Sutton, Surrey, SM? SPT, UK bDepartment of Medical Physics and Bioengineering. University College. London. WCIA 6JA, UK
Abstract A novel tissue imaging system has been developed utilising a cooled scientific grade charge coupled device (CCD) to detect the low energy X-ray emissions and 0 particle emissions from a variety of isotopes commonly used in tissue autoradiography. Results are presented which illustrate the systems potential for faster and more accurate imaging of tissue samples than is conventionally achieved using film emulsion autoradiography. With position resolution approaching 20 pm and intrinsic sensitivity approaching loo%, the CCD system also surpasses the performance of current digital autoradiography technology. Because of the frame-by-frame composite way in which images are constructed, the system displays excellent linearity over a large dynamic range. Operation of the CCD in the inverted mode at temperatures up to 35°C has been investigated, with promising results.
1. Introduction Conventional film-based autoradiography is routinely used to provide images of radio-labelled biological specimens with spatial resolution of the order of microns. However, it is a time consuming and laborious process, with long exposure times required due to the low sensitivity of photographic emulsion to the radiation. There are also problems with quantitative measurements due to the limited dynamic range and non-linearity of response of the emulsion. Although digital imaging systems are commercially available which display high sensitivity and dynamic range, such as the InstantImager [l] and PhosphorImager [2], the spatial resolution of these systems is limited to -100 pm. Alternative prototype imaging systems have employed a CCD to detect the light emitted by p particle interactions in a scintillator coupled to the CCD via either a lens [3] or an intensifier [4]. The first approach resulted in a system which displayed low resolution (lo&200 pm) and undisclosed detection efficiency over an imaging area of 454 mm’. while the second displayed high resolution (15 pm) and reasonable detection efficiency (56% for S-35) over a restricted imaging area of 60mm’). We have designed and evaluated a system which combines high spatial resolution with almost 100% intrinsic
*Corresponding author. Tel.: + 44 181 642 6011 3721; fax: + 44 181 643 3812; e-mail: [email protected]. 0168-9002/97/$17.00 Copyright PII SO168-9002(97)00297-O
sensitivity to most of the common radio-nuclides used for autoradiography (35S, 33P, 32P, 14C, “‘I). The system utilises the direct irradiation of a cooled CCD, with a pixel size of 22.5 pm. Previous results have shown the system to display spatial resolution of around 30 pm for 35 S, 33P and 125I [S]. In this paper we outline the on-going development of the system, including automation of image acquisition and investigation of system sensitivity.
2. System description A photograph of the CCD system is shown in Fig. 1. The CCD is an 05-20 device supplied by EEV of Chelmsford. UK. It is contained within a liquid nitrogen cooled cryogenic chamber, which allows temperature control of the device, from - 15O’C to f60”C. Integration time and readout of the CCD is governed by an EEV driver board, using correlated double sampling with a user-defined readout between 50 kHz and 1 MHz. Control of the driver board is performed using a National Instruments I/O card which is driven by a custom-designed LabVIEW virtual instrument. The I/O card is installed in a Macintosh Quadra 950 incorporating a PowerPro 601 RISC chip. The analogue video signal from each CCD frame is digitised by a 12 bit ADC card, also supplied by National Instruments. This 1-D digital array is reconstructed into a 2-D image and displayed using Concept VI within the LabVIEW environment.
C; 1997 Elsevier Science B.V. All rights reserved
221
JH. MacDonald et al. /Nucl. Instr. and Meth. in Phys. Res. A 392 (I 997) 220-226
Fig. 1. The CCD imaging system. The CCD is contained in the aluminium chamber on the right-hand side. CCD drive and readout electronics are shown centre. Images are displayed on the Apple monitor on the left-hand side.
defined as two or more events which result in overlapping or adjacent charge distributions in the CCD. The clustering algorithm used was unable to discriminate between single and multiple events. Multiple events were simply identified as single events. Clearly the likelihood of a multiple event occurring increases with an increase in the number of particles incident, and results in the algorithm identifying less events than would be expected. Longer integration times result in more incident particles, and therefore more multiple events. To determine the optimum integration time for each frame, for a fixed source activity, the CCD was irradiated using a 14C source for integration times varying from 10 s up to several hours, at a temperature of -120°C. The number of events in each frame was counted and plotted against the number of expected events. This is shown in Fig. 2. The resulting curve is described by Eq. (1). m = n, + nexp(
where A tissue sample is placed directly against the surface of the CCD. Image formation consists of the summation of individual frames, each integrated for a period of time dependent on the activity of the sample (see Section 3). Each frame is segmented into a binary image using a threshold set by system noise ( - 50 electrons [S] ). The frames processed in this way are added together as they are acquired to form a composite real-time image of the activity distribution within the sample, with each frame contributing 1 bit of grey level to the image. The number of frames required to form the composite image is userdefined, e.g. 65536 frames could be acquired to produce a 16 bit image. This method of image formation is preferred to one long integration, because /3 emitting isotopes emit particles with a continuous spectrum of energies. These particles deposit a continuous spectrum of charge in the detector. This makes it difficult to quantify the image accurately in terms of number of particles detected. In contrast, our method makes it possible to measure with precision the number of particles detected in the device. The recorded events passing the threshold can be limited to a certain size (i.e. a certain number of pixels), from single pixel events for high resolution, reduced sensitivity imaging to multi-pixel events for high sensitivity, reduced resolution imaging.
3. Optimisation
of frame integration time
The CCD is used to detect the fi and X-ray emissions of the radiolabelled tracer within the tissue sample. These result in many interactions, or events, in the sensitive volume of the device. Clearly, for a fixed activity, the longer the integration time, the greater the probability of multiple events. In this context, a multiple event is
-an)
m = measured
n = expected
(1) number number
of events/mm* of events/mm’,
n,, a = constants. n, represents the number of events detected during the readout sequence, when events are being transferred into the readout register and through the output node. Because the readout time is finite, and there is no shuttering mechanism, events will continue to be recorded during this readout phase. The constant a is equivalent to the dead-urea, which is related to the mean event size. In this case, a = 2786 pm*. This is equivalent to the area of 7.2 CCD pixels. The actual mean event size in this experiment was found to be 2.46 pixels. The discrepancy between this figure and the dead-area, a can be explained by the need for surrounding “empty” pixels which separate identified events. These are included in the dead-area value. It is difficult to predict how many of these should be expected, due to the variability in the shape of each event. For optimum efficiency the frame integration time should be on the initial, i.e. the most linear, portion of the plot. That is, the frame time should be as short as possible. However, a lower limit to the frame time is set by the readout time. As integration time approaches readout time, there is an increased probability of image “smear”. Also, an integration time of the order of the readout time results in a large and undesirable overall system deadtime with respect to imaging, or “live” time. For instance, integrating each frame for 10 s with a readout time of 1 s results in a dead-time/live-time ratio of 10%. Clearly, the optimum integration time is different for each individual tissue sample, and is dependent on the activity of the sample. A high activity sample requires short integration times which result in relatively high dead-time. However, the high activity levels mean that an image can be
VI. CCDs
222
J.H. MacDonald
et al. /Nucl. Instr. arld Meth. in Phys. Rex A 392 (1997) 220- 226
mm~n’m~-m’nm--‘-m*m’
0
0
50
100 expected
Fig. 2. Curve showing of event overlap.
number
of events detected
versus number
obtained relatively quickly anyway. A low activity sample allows long integration times which result in relatively low dead-time. The low activity levels mean that the image is obtained relatively slowly, but the increase in imaging time attributable to dead-time is low.
4. System sensitivity Several grade 5, scientific grade CCDs have been characterised in the system; a standard CCD05-20, a standard CCD05-20 with a 2 urn thick layer of polyamide (to protect the front surface of the device from mechanical damage), an inverted mode CCD05-20 with a 10 urn coating of CsI on the front surface, and an inverted mode CCD05-20 with a 100 urn coating of CsI on the front surface. System sensitivity was measured using a calibrated 3.74 kBq 14C source, consisting of a 16mm diameter anodised activated aluminium foil, supplied by Amersham International. This was suspended 8 cm above the CCD surface. The CCD was irradiated for a fixed time, and the number of detected events counted. The number of events expected was calculated using simple geometry. The results are shown in Table 1. These results are uncorrected for backscatter, which explains the > 100%
150
200
events/mm*
of events expected. The curve drops off at high event densities because
detection efficiency values obtained for the standard device and the polyamide coated device. At 14C @energies (end-point 156 keV), around 14% of emitted particles are backscattered from the aluminium foil in the direction of the CCD [6]. There is also a large uncertainty associated with the estimate of the number of particles incident on the detector. However, one can conclude from these results that the standard and polyamide coated CCDs display very high (-100%) detection efficiency to “C l3 particles. This would be expected given the large signalto-noise ratio associated with a b particle event which generates several thousand electronhole pairs [S], compared to 50 electron noise levels. Clearly, the uncoated device is far more sensitive to p particles than the scintillator coated device, and slightly more sensitive than the polyamide coated device. The former result is explained by the fact that a p particle
Table
1
CCD
Standard
Polyamide coated
10 pm CsI coated
100 pm CsI coated
Efficiency
120 * 30
114+28
6.9 k 1.7
4.7 f 1.2
(%)
J.H. MacDonald et al. /Nuci. hstr. and Meth. in Piws. Res. A 392 (1997) 220-226
incident on the CsI layer converts only 11.9% of its energy to photons of energy 560-580nm, at least half of which do not reach the CCD. With the quantum efficiency of the CCD at these wavelengths being only -35%, the eventual number of electron-hole pairs produced by the fl particle is -1% of that produced if it entered the silicon directly. Thus, many b particles do not generate a signal above system noise, and sensitivity consequently suffers, The polyamide coated device is slightly less efficient than the uncoated device because of absorption in the polyamide layer.
5. System performance at high temperature Operation of a CCD in the inverted mode reduces the generation of dark current in the device by suppressing the most significant contribution, i.e. the contribution from surface states. The measured dark current generation in a standard device and an inverted mode device is compared in Fig. 3. With an inverted mode device, it is feasible that the CCD system could be operated without the need for cooling. This was investigated by irradiating both the standard CCD and the 10 urn CsI coated inverted mode CCD for a fixed time (10s) with a fixed
223
particle flux at a range of temperatures, using the calibrated “C source. As illustrated in Fig. 4 a constant number of events was identified at all temperatures up until a certain threshold. At this point the number of events identified increased rapidly. This was due to “hot” pixel and line defects; CCD defects which display increased dark current generation at relatively low temperatures. These were mis-registered as genuine events when the temperature was such that the dark current associated with them rose above system noise. Integrating for a shorter time than 10 s would have alleviated this problem, but was undesirable due to the finite readout time of the detector (1 s minimum), and the need to avoid image smear. As expected, spurious events became apparent in the standard device at a lower temperature ( - 250 K) than in the inverted mode device ( - 280 K). However, according to Fig. 3, an inverted mode device at 280 K should exhibit the same rate of dark current generation as a standard device at -235 K and not 250 K. This higher than expected rate of spurious event generation is attributed to the greater number of line and pixel defects found in the scintillator coated CCDs, compared with the uncoated and polyamide coated CCDs. It is thought that the performance of the scintillator coated CCDs may have
1 o6
1 o5 s 5 $ z!
10’
2 ‘E.1000 2 E 5 100 2? Q) E E 2 -z G
10
1
0.1
0.01 200
220
240
260
260
300
320
temperature (K) Fig. 3. Comparison of dark current levels in standard and inverted mode devices. The inverted operation than the standard device because of decreased dark current levels.
mode device allows higher temperature
VI. CCDs
224
J.H. MacDonald
et al. / Nucl. Instr. and Meth. in Ph.vs. Rex A 392 (1997) 220-226
200
250
300
350
temperature (K) Fig. 4. Number of events detected at various temperatures, for a fixed activity. The dramatic increase is due to CCD defects causing spurious events. This happens at a higher temperature in the inverted mode device than the standard device.
been degraded by exposure to moisture. is known to be hygroscopic.
The scintillator
6. System linearity The linearity of the CCD system was investigated by imaging a calibrated 14C microscale supplied by Amersham International. The microscale consisted of eight separate bands of ascending activity, from 31 to 883 Ci/g. Each band measured 3.5 mm x 1.9mm x 120nm. This form of calibrated source is usually placed on photographic film and left for several weeks along with the tissue sections to be imaged. This allows quantitation of the final image. A CCD image of the microscale, composed of 800 fifteen second frames for a total imaging time of only 3 h 20 min, is shown in Fig. 5. This image was obtained with the polyamide coated device. The mean counts/mm’ detected by the CCD in each band of the image is plotted against the quoted activity for each band in Fig. 6. This figure illustrates the excellent linearity of the CCD system over a wide range of activities. Also, the short image acquisition time illustrates the extremely high sensitivity of the system in comparison to film.
7. Conclusions and further work The ICR digital tissue imaging system is nearing the end of the evaluation stage. The acquisition process has been automated to enable composite image acquisition. This allows very accurate quantification of the images, since each event (i.e. each incident particle) is individually identified. This is illustrated in Section 6 by the accurate quantification of a i4C microscale. This image also indicates the speed with which the CCD system can obtain accurate quantitative images. It is clear that a standard (i.e. with no scintillator coating) CCD displays much greater detection efficiency than a scintillator coated CCD, at least to 14C particles. However, a scintillator coated device may yet prove to be a more efficient detector of tritium than the standard device. Tritium p particles do not penetrate the front surface of the CCD, but convert in the scintillator and generate a detectable photon flux. Other options such as back-illumination and thinned electrode or open electrode architectures would improve the performance of a direct-detection silicon system and will be investigated in future work. The possibility of room temperature operation has been investigated with an inverted mode device. The highest temperature achieved whilst preserving reliable
6000
400
600
activity (nCi/g) Fig. 6. Plot of events detected/mm* versus band activity
operation of the system was 280K. Above this temperature, pixel and line defects were wrongly identified as legitimate events and degraded accurate quantification of the image. However, the uncommonly high level of spurious event generation in the CsI devices may be a (undesirable) feature of such coated devices, and there is no reason to believe that an uncoated device would behave in the same way. The results shown in Fig. 3 for the standard device indicate that an uncoated inverted mode device could operate at 300K without such image degradation.
Further improvement still will be achieved with faster readout of the pixel array, which will allow shorter integration periods. and a concurrent reduction in dark current.
Acknowledgements This work has been undertaken with financial support from the Institute of Cancer Research. We would like to thank Mr. Craig Cummings from the ICR workshop,
VI. CCDs
226
J.H. MacDonald
et al. /Nucl. In&.
and Meth. in Phw. Rex A 392 (1997) 220-226
who constructed the cryogenic chamber for the CCD systern. EEV Ltd, for their advice and help, and the F.C. Hunter Trust, which provided funding for one of us (JMD).
References [l] D. Englert, N. Roes&r, A. Yeavons and S. Fairless. Cellular and Molecular Biology 41 (1995) 57. [2] Johnston et al., Electrophoresis 11 (1990) 355.
[3] A. Karellas, H. Liu. C. Reinhardt, L.J. Harris and A.B. Brill.
IEEE Trans. Nucl. Sci. 40 (1993) 979. [4] Charon et al. Nucl. Instr. and Meth. A 310 (1991) 319. [S] J.H. MacDonald, K. Wells and R.J. Ott. Proc. IEEE Symp. on Nuclear Science and Medical Imaging, San Francisco (1995) p. 1741. [6] T. Tabata. R. Ito and S. Okabe, Nucl. Instr. and Meth. 94 (1971) 509.
NUCLEAR INSTRUMENTS 8 METIIODS IN PHYSICS RESEARCH Seclm
A
A CCD-based tissue imaging system J.H. MacDonald”, *, K. Wellsb, A.J. Reader”, R.J. Ott” “Joint Department of Physics, Institute of Cancer Research, Royal Marsden NHS Trust, Sutton, Surrey, SM? SPT, UK bDepartment of Medical Physics and Bioengineering. University College. London. WCIA 6JA, UK
Abstract A novel tissue imaging system has been developed utilising a cooled scientific grade charge coupled device (CCD) to detect the low energy X-ray emissions and 0 particle emissions from a variety of isotopes commonly used in tissue autoradiography. Results are presented which illustrate the systems potential for faster and more accurate imaging of tissue samples than is conventionally achieved using film emulsion autoradiography. With position resolution approaching 20 pm and intrinsic sensitivity approaching loo%, the CCD system also surpasses the performance of current digital autoradiography technology. Because of the frame-by-frame composite way in which images are constructed, the system displays excellent linearity over a large dynamic range. Operation of the CCD in the inverted mode at temperatures up to 35°C has been investigated, with promising results.
1. Introduction Conventional film-based autoradiography is routinely used to provide images of radio-labelled biological specimens with spatial resolution of the order of microns. However, it is a time consuming and laborious process, with long exposure times required due to the low sensitivity of photographic emulsion to the radiation. There are also problems with quantitative measurements due to the limited dynamic range and non-linearity of response of the emulsion. Although digital imaging systems are commercially available which display high sensitivity and dynamic range, such as the InstantImager [l] and PhosphorImager [2], the spatial resolution of these systems is limited to -100 pm. Alternative prototype imaging systems have employed a CCD to detect the light emitted by p particle interactions in a scintillator coupled to the CCD via either a lens [3] or an intensifier [4]. The first approach resulted in a system which displayed low resolution (lo&200 pm) and undisclosed detection efficiency over an imaging area of 454 mm’. while the second displayed high resolution (15 pm) and reasonable detection efficiency (56% for S-35) over a restricted imaging area of 60mm’). We have designed and evaluated a system which combines high spatial resolution with almost 100% intrinsic
*Corresponding author. Tel.: + 44 181 642 6011 3721; fax: + 44 181 643 3812; e-mail: [email protected]. 0168-9002/97/$17.00 Copyright PII SO168-9002(97)00297-O
sensitivity to most of the common radio-nuclides used for autoradiography (35S, 33P, 32P, 14C, “‘I). The system utilises the direct irradiation of a cooled CCD, with a pixel size of 22.5 pm. Previous results have shown the system to display spatial resolution of around 30 pm for 35 S, 33P and 125I [S]. In this paper we outline the on-going development of the system, including automation of image acquisition and investigation of system sensitivity.
2. System description A photograph of the CCD system is shown in Fig. 1. The CCD is an 05-20 device supplied by EEV of Chelmsford. UK. It is contained within a liquid nitrogen cooled cryogenic chamber, which allows temperature control of the device, from - 15O’C to f60”C. Integration time and readout of the CCD is governed by an EEV driver board, using correlated double sampling with a user-defined readout between 50 kHz and 1 MHz. Control of the driver board is performed using a National Instruments I/O card which is driven by a custom-designed LabVIEW virtual instrument. The I/O card is installed in a Macintosh Quadra 950 incorporating a PowerPro 601 RISC chip. The analogue video signal from each CCD frame is digitised by a 12 bit ADC card, also supplied by National Instruments. This 1-D digital array is reconstructed into a 2-D image and displayed using Concept VI within the LabVIEW environment.
C; 1997 Elsevier Science B.V. All rights reserved
221
JH. MacDonald et al. /Nucl. Instr. and Meth. in Phys. Res. A 392 (I 997) 220-226
Fig. 1. The CCD imaging system. The CCD is contained in the aluminium chamber on the right-hand side. CCD drive and readout electronics are shown centre. Images are displayed on the Apple monitor on the left-hand side.
defined as two or more events which result in overlapping or adjacent charge distributions in the CCD. The clustering algorithm used was unable to discriminate between single and multiple events. Multiple events were simply identified as single events. Clearly the likelihood of a multiple event occurring increases with an increase in the number of particles incident, and results in the algorithm identifying less events than would be expected. Longer integration times result in more incident particles, and therefore more multiple events. To determine the optimum integration time for each frame, for a fixed source activity, the CCD was irradiated using a 14C source for integration times varying from 10 s up to several hours, at a temperature of -120°C. The number of events in each frame was counted and plotted against the number of expected events. This is shown in Fig. 2. The resulting curve is described by Eq. (1). m = n, + nexp(
where A tissue sample is placed directly against the surface of the CCD. Image formation consists of the summation of individual frames, each integrated for a period of time dependent on the activity of the sample (see Section 3). Each frame is segmented into a binary image using a threshold set by system noise ( - 50 electrons [S] ). The frames processed in this way are added together as they are acquired to form a composite real-time image of the activity distribution within the sample, with each frame contributing 1 bit of grey level to the image. The number of frames required to form the composite image is userdefined, e.g. 65536 frames could be acquired to produce a 16 bit image. This method of image formation is preferred to one long integration, because /3 emitting isotopes emit particles with a continuous spectrum of energies. These particles deposit a continuous spectrum of charge in the detector. This makes it difficult to quantify the image accurately in terms of number of particles detected. In contrast, our method makes it possible to measure with precision the number of particles detected in the device. The recorded events passing the threshold can be limited to a certain size (i.e. a certain number of pixels), from single pixel events for high resolution, reduced sensitivity imaging to multi-pixel events for high sensitivity, reduced resolution imaging.
3. Optimisation
of frame integration time
The CCD is used to detect the fi and X-ray emissions of the radiolabelled tracer within the tissue sample. These result in many interactions, or events, in the sensitive volume of the device. Clearly, for a fixed activity, the longer the integration time, the greater the probability of multiple events. In this context, a multiple event is
-an)
m = measured
n = expected
(1) number number
of events/mm* of events/mm’,
n,, a = constants. n, represents the number of events detected during the readout sequence, when events are being transferred into the readout register and through the output node. Because the readout time is finite, and there is no shuttering mechanism, events will continue to be recorded during this readout phase. The constant a is equivalent to the dead-urea, which is related to the mean event size. In this case, a = 2786 pm*. This is equivalent to the area of 7.2 CCD pixels. The actual mean event size in this experiment was found to be 2.46 pixels. The discrepancy between this figure and the dead-area, a can be explained by the need for surrounding “empty” pixels which separate identified events. These are included in the dead-area value. It is difficult to predict how many of these should be expected, due to the variability in the shape of each event. For optimum efficiency the frame integration time should be on the initial, i.e. the most linear, portion of the plot. That is, the frame time should be as short as possible. However, a lower limit to the frame time is set by the readout time. As integration time approaches readout time, there is an increased probability of image “smear”. Also, an integration time of the order of the readout time results in a large and undesirable overall system deadtime with respect to imaging, or “live” time. For instance, integrating each frame for 10 s with a readout time of 1 s results in a dead-time/live-time ratio of 10%. Clearly, the optimum integration time is different for each individual tissue sample, and is dependent on the activity of the sample. A high activity sample requires short integration times which result in relatively high dead-time. However, the high activity levels mean that an image can be
VI. CCDs
222
J.H. MacDonald
et al. /Nucl. Instr. arld Meth. in Phys. Rex A 392 (1997) 220- 226
mm~n’m~-m’nm--‘-m*m’
0
0
50
100 expected
Fig. 2. Curve showing of event overlap.
number
of events detected
versus number
obtained relatively quickly anyway. A low activity sample allows long integration times which result in relatively low dead-time. The low activity levels mean that the image is obtained relatively slowly, but the increase in imaging time attributable to dead-time is low.
4. System sensitivity Several grade 5, scientific grade CCDs have been characterised in the system; a standard CCD05-20, a standard CCD05-20 with a 2 urn thick layer of polyamide (to protect the front surface of the device from mechanical damage), an inverted mode CCD05-20 with a 10 urn coating of CsI on the front surface, and an inverted mode CCD05-20 with a 100 urn coating of CsI on the front surface. System sensitivity was measured using a calibrated 3.74 kBq 14C source, consisting of a 16mm diameter anodised activated aluminium foil, supplied by Amersham International. This was suspended 8 cm above the CCD surface. The CCD was irradiated for a fixed time, and the number of detected events counted. The number of events expected was calculated using simple geometry. The results are shown in Table 1. These results are uncorrected for backscatter, which explains the > 100%
150
200
events/mm*
of events expected. The curve drops off at high event densities because
detection efficiency values obtained for the standard device and the polyamide coated device. At 14C @energies (end-point 156 keV), around 14% of emitted particles are backscattered from the aluminium foil in the direction of the CCD [6]. There is also a large uncertainty associated with the estimate of the number of particles incident on the detector. However, one can conclude from these results that the standard and polyamide coated CCDs display very high (-100%) detection efficiency to “C l3 particles. This would be expected given the large signalto-noise ratio associated with a b particle event which generates several thousand electronhole pairs [S], compared to 50 electron noise levels. Clearly, the uncoated device is far more sensitive to p particles than the scintillator coated device, and slightly more sensitive than the polyamide coated device. The former result is explained by the fact that a p particle
Table
1
CCD
Standard
Polyamide coated
10 pm CsI coated
100 pm CsI coated
Efficiency
120 * 30
114+28
6.9 k 1.7
4.7 f 1.2
(%)
J.H. MacDonald et al. /Nuci. hstr. and Meth. in Piws. Res. A 392 (1997) 220-226
incident on the CsI layer converts only 11.9% of its energy to photons of energy 560-580nm, at least half of which do not reach the CCD. With the quantum efficiency of the CCD at these wavelengths being only -35%, the eventual number of electron-hole pairs produced by the fl particle is -1% of that produced if it entered the silicon directly. Thus, many b particles do not generate a signal above system noise, and sensitivity consequently suffers, The polyamide coated device is slightly less efficient than the uncoated device because of absorption in the polyamide layer.
5. System performance at high temperature Operation of a CCD in the inverted mode reduces the generation of dark current in the device by suppressing the most significant contribution, i.e. the contribution from surface states. The measured dark current generation in a standard device and an inverted mode device is compared in Fig. 3. With an inverted mode device, it is feasible that the CCD system could be operated without the need for cooling. This was investigated by irradiating both the standard CCD and the 10 urn CsI coated inverted mode CCD for a fixed time (10s) with a fixed
223
particle flux at a range of temperatures, using the calibrated “C source. As illustrated in Fig. 4 a constant number of events was identified at all temperatures up until a certain threshold. At this point the number of events identified increased rapidly. This was due to “hot” pixel and line defects; CCD defects which display increased dark current generation at relatively low temperatures. These were mis-registered as genuine events when the temperature was such that the dark current associated with them rose above system noise. Integrating for a shorter time than 10 s would have alleviated this problem, but was undesirable due to the finite readout time of the detector (1 s minimum), and the need to avoid image smear. As expected, spurious events became apparent in the standard device at a lower temperature ( - 250 K) than in the inverted mode device ( - 280 K). However, according to Fig. 3, an inverted mode device at 280 K should exhibit the same rate of dark current generation as a standard device at -235 K and not 250 K. This higher than expected rate of spurious event generation is attributed to the greater number of line and pixel defects found in the scintillator coated CCDs, compared with the uncoated and polyamide coated CCDs. It is thought that the performance of the scintillator coated CCDs may have
1 o6
1 o5 s 5 $ z!
10’
2 ‘E.1000 2 E 5 100 2? Q) E E 2 -z G
10
1
0.1
0.01 200
220
240
260
260
300
320
temperature (K) Fig. 3. Comparison of dark current levels in standard and inverted mode devices. The inverted operation than the standard device because of decreased dark current levels.
mode device allows higher temperature
VI. CCDs
224
J.H. MacDonald
et al. / Nucl. Instr. and Meth. in Ph.vs. Rex A 392 (1997) 220-226
200
250
300
350
temperature (K) Fig. 4. Number of events detected at various temperatures, for a fixed activity. The dramatic increase is due to CCD defects causing spurious events. This happens at a higher temperature in the inverted mode device than the standard device.
been degraded by exposure to moisture. is known to be hygroscopic.
The scintillator
6. System linearity The linearity of the CCD system was investigated by imaging a calibrated 14C microscale supplied by Amersham International. The microscale consisted of eight separate bands of ascending activity, from 31 to 883 Ci/g. Each band measured 3.5 mm x 1.9mm x 120nm. This form of calibrated source is usually placed on photographic film and left for several weeks along with the tissue sections to be imaged. This allows quantitation of the final image. A CCD image of the microscale, composed of 800 fifteen second frames for a total imaging time of only 3 h 20 min, is shown in Fig. 5. This image was obtained with the polyamide coated device. The mean counts/mm’ detected by the CCD in each band of the image is plotted against the quoted activity for each band in Fig. 6. This figure illustrates the excellent linearity of the CCD system over a wide range of activities. Also, the short image acquisition time illustrates the extremely high sensitivity of the system in comparison to film.
7. Conclusions and further work The ICR digital tissue imaging system is nearing the end of the evaluation stage. The acquisition process has been automated to enable composite image acquisition. This allows very accurate quantification of the images, since each event (i.e. each incident particle) is individually identified. This is illustrated in Section 6 by the accurate quantification of a i4C microscale. This image also indicates the speed with which the CCD system can obtain accurate quantitative images. It is clear that a standard (i.e. with no scintillator coating) CCD displays much greater detection efficiency than a scintillator coated CCD, at least to 14C particles. However, a scintillator coated device may yet prove to be a more efficient detector of tritium than the standard device. Tritium p particles do not penetrate the front surface of the CCD, but convert in the scintillator and generate a detectable photon flux. Other options such as back-illumination and thinned electrode or open electrode architectures would improve the performance of a direct-detection silicon system and will be investigated in future work. The possibility of room temperature operation has been investigated with an inverted mode device. The highest temperature achieved whilst preserving reliable
6000
400
600
activity (nCi/g) Fig. 6. Plot of events detected/mm* versus band activity
operation of the system was 280K. Above this temperature, pixel and line defects were wrongly identified as legitimate events and degraded accurate quantification of the image. However, the uncommonly high level of spurious event generation in the CsI devices may be a (undesirable) feature of such coated devices, and there is no reason to believe that an uncoated device would behave in the same way. The results shown in Fig. 3 for the standard device indicate that an uncoated inverted mode device could operate at 300K without such image degradation.
Further improvement still will be achieved with faster readout of the pixel array, which will allow shorter integration periods. and a concurrent reduction in dark current.
Acknowledgements This work has been undertaken with financial support from the Institute of Cancer Research. We would like to thank Mr. Craig Cummings from the ICR workshop,
VI. CCDs
226
J.H. MacDonald
et al. /Nucl. In&.
and Meth. in Phw. Rex A 392 (1997) 220-226
who constructed the cryogenic chamber for the CCD systern. EEV Ltd, for their advice and help, and the F.C. Hunter Trust, which provided funding for one of us (JMD).
References [l] D. Englert, N. Roes&r, A. Yeavons and S. Fairless. Cellular and Molecular Biology 41 (1995) 57. [2] Johnston et al., Electrophoresis 11 (1990) 355.
[3] A. Karellas, H. Liu. C. Reinhardt, L.J. Harris and A.B. Brill.
IEEE Trans. Nucl. Sci. 40 (1993) 979. [4] Charon et al. Nucl. Instr. and Meth. A 310 (1991) 319. [S] J.H. MacDonald, K. Wells and R.J. Ott. Proc. IEEE Symp. on Nuclear Science and Medical Imaging, San Francisco (1995) p. 1741. [6] T. Tabata. R. Ito and S. Okabe, Nucl. Instr. and Meth. 94 (1971) 509.