Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 45, No. 12, p. 1347–1352 (2008)
ARTICLE
Optimal Optical Conditions and Positioning Scheme for an Ultrahigh-Resolution Silicon Drift Detector-Based Gamma Camera Jinhun JOUNG1 , Kisung LEE2; , Debora HENSELER1 , Wilhelm METZGER3 , Yong CHOI4 , Young Bok AHN5 and Yongkwon KIM2 2
1 Siemens Medical Solutions, Hoffman Estates, IL, 60195 U.S.A. Department of Radiologic Science, Korea University, JeongReung3-dong, Seongbuk-gu, Seoul 136-703, Korea 3 Siemens AG, Munich, Germany 4 School of Medicine, Sungkyunkwan University, 50 Ilwon-dong, Kangnam-gu, Seoul 135-710, Korea 5 Department of Electronics, Konkuk University, 1 HwaYang-dong, KwangJin-gu, Seoul 143-701, Korea
(Received May 1, 2008 and accepted in revised form September 1, 2008)
In this study, we optimized the optical conditions and associated positioning scheme for an ultrahighspatial-resolution, solid-state gamma detector. The detector module consisted of an array of seven hexagonal silicon drift detectors (SDDs) packed hexagonally and coupled to a single slab of crystal via a light guide glass. Because the optical behavior and requirements of the detector module and noise characteristics of the SDD sensor are different from those of conventional photomultiplier tube (PMT)-based detectors, the following parameters were studied to determine the optimum condition: scintillator selection, the effect of cooling on signal-to-noise ratio (SNR), the depth dependence of the scintillation light distribution, and optimum shaping time. To that end, a modified, Anger-style positioning algorithm with a denoise scheme was also developed to address the estimation bias (pincushion distortion) caused by the excessively confined light distribution and the leakage current induced by the SDD sensor. The results of this study proved that the positioning algorithm, together with the optimized optical configuration of the detector module, improves the positioning accuracy of the prototype detector. Our results confirmed the ability of the prototype to achieve a spatial resolution of about 0.7 mm in full width at half maximum (FWHM) for 122 keV gamma rays under the equivalent noise count (ENC) of 100 (e- rms) per SDD channel. The results also confirmed NaI(Tl) to be a more desirable scintillator for our prototype with an energy resolution performance of about 8%. KEYWORDS: gamma camera, solid-state detector, silicon drift detector, positioning scheme, high resolution
I. Introduction The standing of solid-state detector technology in the field of nuclear medicine has been raised due to the following benefits over the conventional, photomultiplier tube (PMT)based technology: better energy and spatial resolution, compactness, lightweight, system design freedom, insensitivity to magnetic field and more. However, economical viability is one of the key challenging factors that must be overcome before such technologies can be successfully applied in the field of nuclear medicine as a commercial product. Silicon drift detector (SDD) technology is more attractive than other popular, solid-state technologies used in the nuclear medicine field, such as CZT-, PIN-, and APD-based detectors.1) The two major advantages are an extremely low noise despite its relatively large diameter (5–10 mm), due to the electron drift mechanism with a small collecting anode,2–4) and the ability to design a cost-effective gamma
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detector by using reduced electronics and nonpixelated crystals. In addition, its sub-millimeter spatial resolution allows a new and innovative, pinhole-type, collimator design that is capable of maintaining system resolution with a relatively small detector and a small focal length.5) However, the relatively long drift time of 4–6 ms that is necessary to collect charges and the extreme cooling requirement of 10 to 20 C are the major drawbacks of the technology. A prototype miniature gamma camera using an array of 5 mm2 SDD sensors has been presented by Fiorini et al.6,7) In addition, we previously investigated the feasibility of SDD for nuclear medical applications.8) In this study, as an extension of our previous research on SDDs, we developed a prototype detector module consisting of an array of seven hexagonal SDDs (about 1 cm in diameter) packed hexagonally (Fig. 1) and coupled to a single slab of crystal via a light guide glass. Unlike other solid-state detectors where a photosensor is coupled directly to a pixelated crystal, the SDD detector module employed conventional gamma camera positioning principles in which the positioning of a gamma event was determined by the relationship of
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tance of 0.3 pF were used. The depth of the interaction effect of the impinging gamma rays was also taken into account based on the thickness and stopping power of a given scintillator.
Fig. 1 Single- and seven-channel prototype devices: (a) single SDD channel with preamp board by Ketek (Munich, Germany) and (b) seven-channel prototype with front-end electronics
the light distributions of the SDD sensors coupled to a single scintillator. This enables the construction of a detector with fewer channels and a nonpixelated single slab of scintillation crystal. In addition, its smaller diameter (about 1 cm) than those of PMTs and the noise behaviors of the SDD sensor force the optical configuration of the module to be tailored such that its performance is optimally balanced. The objective of this study is to investigate the optimal parameters that affect the performance of the SDD gamma camera module. Simulation studies and experiments were conducted to determine the optimal performance parameters, such as scintillator selection, the effect of cooling on SNR, the depth dependence of the scintillation light distribution, and optimum shaping time. To that end, a modified, Anger-style positioning algorithm with a denoise scheme was also developed to address the estimation bias (pincushion distortion) caused by the excessively confined light distribution and the leakage current induced by the SDD sensor.
II. Materials and Methods 1. Simulation Studies Among the scintillation materials, thallium-activated sodium iodide, i.e., NaI(Tl), is the most widely used material for a gamma camera because of its reasonable wavelength, matching with alkaline PMTs (25% QE), and relatively fast decay time. However, the CsI(Tl) scintillator is more frequently used with solid-state detectors because of its higher QE with silicon sensors that typically have QE-max in the 500 to 600 nm range. Our ultimate goal of SDD research was to develop a detector module consisting of an array of about 80 SDDs coupled to a single slab of scintillator plate with a detector size of about 13 9 cm2 . The simulation condition was determined in consideration of the dimension and requirements of the final prototype. To investigate the aforementioned optimum conditions, a Monte Carlo simulator, DETECT,9) was used. To incorporate the noise condition into the simulation, simulated photons were converted into a corresponding electron based on light photon energy (3.0 and 2.2 eV for NaI(Tl) and CsI(Tl), respectively), the sensitivity of the SDD sensor (0.32 A/W at 0 C), and the quantum efficiency (QE) of the SDD sensor for a photon wavelength (60 and 80% for NaI(Tl) and CsI(Tl), respectively). The total light collection efficiency was set to be 60% for both scintillator cases. For the calculation of the noise induced by the sensor, leakage currents of 800 pA at 20 C and 1.4 pA at 0 C, and a capaci-
2. Experimental Studies Prototype detector modules consisting of single and seven SDD sensors were developed for the experimental study. The single- and seven-channel devices are shown in Figs. 1(a) and 1(b), respectively. The prototype device was designed to couple scintillators, such as NaI(Tl) and CsI(Tl). A scintillator cube (10 10 10 mm3 , hermetically sealed by encapsulation in aluminum with 2-mm-thick Pyrex glass) was coupled to one of the SDD sensors of the 7-channel device. The SDD and scintillator were optically coupled using an optical gel (Bicron, BC630). The modules were operated inside a climate chamber in which the temperature could be controlled down to 50 C. Nitrogen gas was also supplied to the unit to prevent condensation.
III. Results 1. Crystal Consideration In most previous solid-state detector technologies in nuclear medicine application, a solid-state photon sensor is directly coupled to the same or similar size pixelated scintillator. In such configurations, scintillator decay time and afterglow effect are less problematic for the performance of the detector. However, when a single slab of scintillator is coupled to an array of sensors, such as an Anger camera-type system, the scintillator should be carefully selected based on its decay characteristics in order to achieve the desired count rate performance. Particularly for the CsI(Tl) scintillator, its slow 2nd decay component limits the count rate of a detector. Furthermore, if the crystal is cooled down, the crystal decay time increases significantly. Figure 2(a) shows decay curves of CsI(Tl) at 0 C obtained using Eq. (1), which is the simplified BollingerThomas method.10) DðtÞ ¼ A1 e1 t þ A2 e2 t
ð1Þ
At a temparature of 0 C, the 1st decay components A1 and 1 are 0.610 and 784 ns, while the 2nd decay components A2 and 2 are 0.413 and 4.31 ms, respectively.10) Figure 2(b) shows the simulated photon collection curve of NaI and CsI as a function of time. For CsI, a shaping time of about 20 ms was required to fully collect photons compared with the shaping time of about 2 ms for NaI(Tl). Although a shorter shaping time can be used by giving up full photon collection, energy resolution degradation and the crystal afterglow effect caused by residual photons should be considered. It was notable that the collection time increased when the CsI was operated at 0 C. 2. Signal and Noise: Optimum Shaping Time Previous studies have suggested that NaI(Tl) is more desirable for optimizing the count rate performance. However, since NaI(Tl) (max ¼ 420 nm) produces fewer photons than JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
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Fig. 2 (a) CsI(Tl) decay curve and (b) photon collection as a function of shaping time. When CsI(Tl) was cooled down, the decay time increased (solid line with an tick mark)
Fig. 3 (a) Signal vs noise for NaI(Tl) and CsI(Tl) at 0 C, and (b) SNR of CsI vs NaI at 0 and 20 C. NaI(Tl) SNR is comparable to that of CsI(Tl)
CsI(Tl) (max ¼ 550 nm), and its QE is lower than that of CsI(Tl) when they are coupled to silicon sensors (500 max 600 nm), the energy resolution performance was an interesting factor to be examined. Therefore, to investigate the feasibility of using NaI(Tl) with SDD sensors, the SNR of both NaI(Tl) and CsI(Tl) crystals, i.e., the index correlated to energy resolution performance, was calculated. Figure 3(a) shows the noise (0 and 20 C) and signal components for NaI(Tl) and CsI(Tl), and Fig. 3(b) shows SNR as a function of shaping time. At 0 C, the SNR of NaI(Tl) was comparable to that of CsI(Tl) crystal with a shorter shaping time. The SNR of CsI(Tl) at a shaping time of around 12 ms was similar to that of NaI(Tl) at 6–7 ms, because the photons from NaI(Tl) had higher energy (shorter wavelength) and the photons from CsI(Tl) were not fully collected at a shaping time of 12 ms. We confirmed that the analytical optimum shaping time results well agreed with the experimental results (Fig. 4) for both NaI(Tl) and CsI(Tl). 3. Energy Resolution Performance: CsI(Tl) vs NaI(Tl) Figures 4(a) and 4(b) plots the experimental results of spectral analysis for CsI(Tl) and NaI(Tl) crystals, respectively. A scintillator cube was coupled to one of the SDD sensors of the 7-channel device and the energy spectrum was VOL. 45, NO. 12, DECEMBER 2008
measured. Both were measured at 20 C and shaping times of 12 and 6 ms was the best for CsI(Tl) and NaI(Tl), respectively. The results demonstrated the energy resolutions of 7.6 and 7.9% for CsI(Tl) and NaI(Tl), respectively. 4. Cramer-Rao (CR) Lower Bound CR-lower bound evaluation11) was performed to study the best attainable spatial resolution as a function of the crystal thickness and equivalent noise count (ENC) of SDDs, as shown in Fig. 5. The result indicated that the scintillator thickness should not exceed 7 mm in order to achieve submillimeter spatial resolution, considering the noise condition of 100 e- or less. 5. Depth-Dependent Light Response Function and Distortion In the previous studies on SDD modules,2,3) a relatively thin crystal (1.4–3 mm) plate was coupled directly to the array of sensors without light guide components. In the present study, however, the use of a relatively thin crystal plate without a light guide component between the crystal and the sensor array severely distorted the pincushion toward the center of the sensor. Figure 6 demonstrates an example of positioning distor-
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Fig. 4 Energy spectra of CsI vs NaI with 57 Co, measured at 20 C: (a) spectrum of CsI(Tl) at an energy resolution of 7.6% with a shaping time of 12 ms and (b) spectrum of NaI(Tl) at an energy resolution of 7.9% with a shaping time of 6 ms
Fig. 5 CR-lower bound of spatial resolution as a function of thickness and ENC: (a) CsI(Tl) and (b) NaI(Tl)
Fig. 6 Example of nonlinear bias of positioning depending on depth of gamma event inside a crystal. Positioning error can be seen in the composted image, particularly at the center
tion for a 5-mm-thick crystal coupled to an array of 1 cm2 SDD sensors without a light guide component. Noise was not added in this simulation in order to demonstrate the problem better. The distortion clearly progressed as the depth of the gamma ray interaction occurred toward the deep part of the crystal, because the photon distribution of the deep event was too narrow to be shared by the sensors. In Fig. 7, the first row of images also shows the distortion effect as a function of crystal thickness. As expected, the severity of the distortion increased as the crystal became thinner. 6. Denoise Positioning Scheme Unlike conventional PMT-based detectors, the solid-state detector has to deal with the relatively high electrical noise
induced by the photon sensors. This noise degrades both the energy resolution and the spatial resolution performance. Figure 7 shows point spread functions (PSFs) for various optical and noise conditions. The images represent the PSF of 6 test points measured diagonally in 1 cm2 areas. The center of the images corresponds to the center of an SDD array packed hexagonally. Each column from the left shows the PSFs of the 7-, 5-, and 3-mm-thick CsI(Tl) crystals. As mentioned in the previous section, if the electric noise is not properly addressed by the positioning method, the spatial resolution is severely degraded, as demonstrated in the 4th row of Fig. 7. We developed a modified, Anger-type positioning estimator that reflects noise from the sensor. The method allows nonlinear weighting to the observed SDD signals as a funcJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
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In Eq. (2), xi and Mi are the coordinate and observed photon counts from the ith sensor, respectively. The summing upper bound ‘K’ is set such that the number of sensors contributing to the position calculation becomes 5 to 7 in order to balance the energy and spatial resolution tradeoff. E½ENC is the expected ENC value that can be practically measured by a blank scan. ‘Tth ’ is an energy threshold constant determining the upper bound ‘K’. The value subtracted from Mi (i.e., inside the parenthesized term in Eq. (3)) should be a function of gamma energy. The results show that the PSF in the full width at half maximum (FWHM) improved from 1.5 to 0.74 mm with the denoise positioning algorithm for the 7-mm-thick crystal with 100 e- ENC noise. Some of the profiles taken from the results of denoise positioning (i.e., images in the third row of Fig. 7) are displayed in Fig. 8.
IV. Discussion and Conclusion
Fig. 7 Point spread functions (PSF) in FWHM for 7-mm-, 5-mm-, and 3-mm-thick CsI(Tl) crystals from the 1st , 2nd , and 3rd columns, respectively. 1st row: without light guide and zero noise. 2nd row: with 2-mm-thick light guide and zero noise. 3rd row: with 2-mm-thick light guide and 100 e- ENC added with the Anger denoise scheme. 4th row: with 2-mm-thick light guide and 100 e- ENC with the conventional Anger scheme. The numbers in the image represent the mean FWHM values in the x and y directions for the six point sources in the image
tion of incoming gamma energy and background noise. The benefit of the method can be clearly demonstrated by comparing the images in the 3rd and 4th rows. The former used the positioning algorithm we developed, while the latter used the simple Anger position method. The 1st row represents PSF in the case of no light guide and zero noise, the 2nd row represents PSF in the case of the 2-mm-thick light guide placed between the sensors and the crystal without noise, the 3rd row represents the same condition as the 2nd row but with 100 e- ENC noise, and the 4th row represents the same condition as the 3rd but with conventional Anger-type positioning. The positioning estimator is expressed as K X
x^ ¼
ðxi M~ i Þ
i¼1 K X
;
ð2Þ
M~ i
i¼1
where
M~ i ¼ Mi E½ENC þ
n X i¼1
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Acknowledgements
! Mi Tth :
The important performance parameters required for an ultrahigh resolution, SDD-based detector were investigated in this study. Our experiments demonstrated the energy resolutions of 7.6 and 7.9% for CsI(Tl) and NaI(Tl), respectively, and indicated that the thickness of the crystal should not exceed 7 mm in order to maintain a sub-millimeter spatial energy resolution. Thus, the thickness should be carefully considered based on the sensitivity and resolution tradeoff. The effects of the light guide and denoise positioning algorithm on the spatial resolution performance were also addressed. The results demonstrated that the module achieves a PSF of 0.74 mm in FWHM with electronics noise up to 100 e- per channel condition by using the denoise positioning scheme while the conventional Anger logic yielded a significant amount of artifact. The NaI(Tl) scintillator coupled with the prototype was also explored to prove its feasibility for SDD detectors. Compared with CsI(Tl), the NaI(Tl) crystal demonstrated the benefit of a high count rate capability without sacrificing energy resolution performance. The factor limiting count rate performance was the drift time of the sensor, i.e., the inherent characteristic of the SDD sensor that requires the time needed for collecting electrons at its small anode located at the center of the device. The drift time of the current device was about 6 ms. Further improvement in count rate performance is expected with the next prototype. In conclusion, the present results have proven the feasibility of making a high-performance, cost-effective, solid-state detector using SDD technology for nuclear medicine applications. In future research, we will improve the prototype by expanding the number of channels up to 67 to cover a wider field of view and conduct experiments with phantom studies to prove the feasibility of the SDD technology for gamma imaging.
ð3Þ
We thank Dr. Carlo Fiorini and his team for providing us electronics for the 7-channel device and assistance needed for operating the system.
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Fig. 8 Profiles of PSFs by denoise positioning in Fig. 7. CsI(Tl) with thicknesses of (a) 7 and (b) 5 mm
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