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Kim DS. Measurements of the noise power spectrum for digital x-ray imaging devices. Phys Med Biol 2024; 69:03TR01. [PMID: 38157548 DOI: 10.1088/1361-6560/ad1999] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2023] [Accepted: 12/29/2023] [Indexed: 01/03/2024]
Abstract
Objective.The noise characteristics of digital x-ray imaging devices are determined by contributions such as photon noise, electronic noise, and fixed pattern noise, and can be evaluated from measuring the noise power spectrum (NPS), which is the power spectral density of the noise. Hence, accurately measuring NPS is important in developing detectors for acquiring low-noise digital x-ray images. To make accurate measurements, it is necessary to understand NPS, identify problems that may arise, and know how to process the obtained x-ray images.Approach.The primitive concept of NPS is first introduced with a periodogram-based estimate and its bias and variance are discussed. In measuring NPS based on the IEC62220 standards, various issues, such as the fixed pattern noise, high-precision estimates, and lag corrections, are summarized with simulation examples.Main results.High-precision estimates can be provided for an appropriate number of samples extracted from x-ray images while compromising spectral resolution. Depending on medical imaging systems, by eliminating the influence of fixed pattern noise, NPS, which represents only photon and electronic noise, can be efficiently measured. For NPS measurements in dynamic detectors, an appropriate lag correction technique can be selected depending on the emitted x-rays and image acquisition process.Significance.Various issues in measuring NPS are reviewed and summarized for accurately evaluating the noise performance of digital x-ray imaging devices.
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Affiliation(s)
- Dong Sik Kim
- Department of Electronics Engineering, Hankuk University of Foreign Studies, Yongin-si, Gyeonggi-do, 449-791, Republic of Korea
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2
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Pineau E, Grynko O, Thibault T, Alexandrov A, Csík A, Kökényesi S, Reznik A. Comparative Analysis of Multilayer Lead Oxide-Based X-ray Detector Prototypes. SENSORS (BASEL, SWITZERLAND) 2022; 22:5998. [PMID: 36015758 PMCID: PMC9412672 DOI: 10.3390/s22165998] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/30/2022] [Revised: 07/31/2022] [Accepted: 08/08/2022] [Indexed: 06/15/2023]
Abstract
Lead oxide (PbO) photoconductors are proposed as X-ray-to-charge transducers for the next generation of direct conversion digital X-ray detectors. Optimized PbO-based detectors have potential for utilization in high-energy and dynamic applications of medical X-ray imaging. Two polymorphs of PbO have been considered so far for imaging applications: polycrystalline lead oxide (poly-PbO) and amorphous lead oxide (a-PbO). Here, we provide the comparative analysis of two PbO-based single-pixel X-ray detector prototypes: one prototype employs only a layer of a-PbO as the photoconductor while the other has a combination of a-PbO and poly-PbO, forming a photoconductive bilayer structure of the same overall thickness as in the first prototype. We characterize the performance of these prototypes in terms of electron-hole creation energy (W±) and signal lag-major properties that define a material's suitability for low-dose real-time imaging. The results demonstrate that both X-ray photoconductive structures have an adequate temporal response suitable for real-time X-ray imaging, combined with high intrinsic sensitivity. These results are discussed in the context of structural and morphological properties of PbO to better understand the preparation-fabrication-property relationships of this material.
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Affiliation(s)
- Emma Pineau
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
| | - Oleksandr Grynko
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
| | - Tristen Thibault
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
| | | | - Attila Csík
- Institute for Nuclear Research, H-4026 Debrecen, Hungary
| | - Sándor Kökényesi
- Department of Electrical and Electronic Engineering, University of Debrecen, H-4026 Debrecen, Hungary
| | - Alla Reznik
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
- Thunder Bay Regional Health Research Institute, Thunder Bay, ON P7B 6V4, Canada
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3
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Grynko O, Thibault T, Pineau E, Reznik A. The X-ray Sensitivity of an Amorphous Lead Oxide Photoconductor. SENSORS 2021; 21:s21217321. [PMID: 34770626 PMCID: PMC8588227 DOI: 10.3390/s21217321] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/14/2021] [Revised: 10/30/2021] [Accepted: 11/01/2021] [Indexed: 11/23/2022]
Abstract
The photoconductor layer is an important component of direct conversion flat panel X-ray imagers (FPXI); thus, it should be carefully selected to meet the requirements for the X-ray imaging detector, and its properties should be clearly understood to develop the most optimal detector design. Currently, amorphous selenium (a-Se) is the only photoconductor utilized in commercial direct conversion FPXIs for low-energy mammographic imaging, but it is not practically feasible for higher-energy diagnostic imaging. Amorphous lead oxide (a-PbO) photoconductor is considered as a replacement to a-Se in radiography, fluoroscopy, and tomosynthesis applications. In this work, we investigated the X-ray sensitivity of a-PbO, one of the most important parameters for X-ray photoconductors, and examined the underlying mechanisms responsible for charge generation and recombination. The X-ray sensitivity in terms of electron–hole pair creation energy, W±, was measured in a range of electric fields, X-ray energies, and exposure levels. W± decreases with the electric field and X-ray energy, saturating at 18–31 eV/ehp, depending on the energy of X-rays, but increases with the exposure rate. The peculiar dependencies of W± on these parameters lead to a conclusion that, at electric fields relevant to detector operation (~10 V/μm), the columnar recombination and the bulk recombination mechanisms interplay in the a-PbO photoconductor.
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Affiliation(s)
- Oleksandr Grynko
- Chemistry and Materials Science Program, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
- Correspondence:
| | - Tristen Thibault
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada; (T.T.); (E.P.); (A.R.)
| | - Emma Pineau
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada; (T.T.); (E.P.); (A.R.)
| | - Alla Reznik
- Physics Department, Lakehead University, Thunder Bay, ON P7B 5E1, Canada; (T.T.); (E.P.); (A.R.)
- Thunder Bay Regional Health Research Institute, Thunder Bay, ON P7B 6V4, Canada
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Scott CC, Farrier M, Li Y, Laxer S, Ravi P, Kenesei P, Wojcik MJ, Miceli A, Karim KS. High-energy micrometre-scale pixel direct conversion X-ray detector. JOURNAL OF SYNCHROTRON RADIATION 2021; 28:1081-1089. [PMID: 34212871 DOI: 10.1107/s1600577521004835] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/16/2020] [Accepted: 05/07/2021] [Indexed: 06/13/2023]
Abstract
The objective of this work was to fabricate and characterize a new X-ray imaging detector with micrometre-scale pixel dimensions (7.8 µm) and high detection efficiency for hard X-ray energies above 20 keV. A key technology component consists of a monolithic hybrid detector built by direct deposition of an amorphous selenium film on a custom designed CMOS readout integrated circuit. Characterization was carried out at the synchrotron beamline 1-BM-B at the Advanced Photon Source of Argonne National Laboratory. The direct conversion detector demonstrated micrometre-scale spatial resolution with a 63 keV modulation transfer function of 10% at Nyquist frequency. In addition, spatial resolving power down to 8 µm was determined by imaging a transmission bar target at 21 keV. X-ray signal linearity, responsivity and lag were also characterized in the same energy range. Finally, phase contrast edge enhancement was observed in a phase object placed in the beam path. This amorphous selenium/CMOS detector technology can address gaps in commercially available X-ray detectors which limit their usefulness for existing synchrotron applications at energies greater than 50 keV; for example, phase contrast tomography and high-resolution imaging of nanoscale lattice distortions in bulk crystalline materials using Bragg coherent diffraction imaging. The technology will also facilitate the creation of novel synchrotron imaging applications for X-ray energies at or above 20 keV.
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Affiliation(s)
- Christopher C Scott
- KA Imaging Inc., 560 Parkside Drive, Unit 3, Waterloo, Ontario, Canada N2L 5Z4
| | - Michael Farrier
- Farrier Microengineering LLC, 616 Petoskey Street, Unit 004, Petoskey, MI 49770, USA
| | - Yunzhe Li
- KA Imaging Inc., 560 Parkside Drive, Unit 3, Waterloo, Ontario, Canada N2L 5Z4
| | - Sam Laxer
- KA Imaging Inc., 560 Parkside Drive, Unit 3, Waterloo, Ontario, Canada N2L 5Z4
| | - Parmesh Ravi
- University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
| | - Peter Kenesei
- X-ray Science Division Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
| | - Michael J Wojcik
- X-ray Science Division Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
| | - Antonino Miceli
- X-ray Science Division Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
| | - Karim S Karim
- KA Imaging Inc., 560 Parkside Drive, Unit 3, Waterloo, Ontario, Canada N2L 5Z4
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5
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Scheuermann JR, Howansky A, Hansroul M, Léveillé S, Tanioka K, Zhao W. Toward Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI): Initial fabrication and characterization. Med Phys 2017; 45:794-802. [PMID: 29171067 DOI: 10.1002/mp.12693] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2017] [Revised: 11/10/2017] [Accepted: 11/16/2017] [Indexed: 11/07/2022] Open
Abstract
PURPOSE We present the first prototype Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI). This detector includes a layer of avalanche amorphous Selenium (a-Se) (HARP) as the photoconductor in an indirect detector to amplify the signal and reduce the effects of electronic noise to obtain quantum noise-limited images for low-dose applications. It is the first time avalanche a-Se has been used in a solid-state imaging device and poses as a possible solution to eliminate the effects of electronic noise, which is crucial for low-dose imaging performance of AMFPI. METHODS We successfully deposited a solid-state HARP structure onto a 24 × 30 cm2 array of thin-film transistors (TFT array) with a pixel pitch of 85 μm. The HARP layer consists of 16 μm of a-Se with a hole-blocking and electron-blocking layer to prevent charge injection from the high-voltage bias and pixel electrodes, respectively. An electric field (ESe ) up to 105 V μm-1 was applied across the a-Se layer without breakdown. A 150 μm thick-structured CsI:Tl scintillator was used to form SHARP-AMFPI. The x-ray imaging performance is characterized using a 30 kVp Mo/Mo beam. We evaluate the spatial resolution, noise power, and detective quantum efficiency at zero frequency of the system with and without avalanche gain. The results are analyzed using cascaded linear system model (CLSM). RESULTS An avalanche gain of 76 ± 5 was measured at ESe = 105 V μm-1 . We demonstrate that avalanche gain can amplify the signal to overcome electronic noise. As avalanche gain is increased, image quality improves for a constant (0.76 mR) exposure until electronic noise is overcome. Our system is currently limited by poor optical transparency of our high-voltage electrode and long integrating time which results in dark current noise. These two effects cause high-spatial frequency noise to dominate imaging performance. CONCLUSIONS We demonstrate the feasibility of a solid-state HARP x-ray imager and have fabricated the largest active area HARP sensor to date. Procedures to reduce secondary quantum and dark noise are outlined. Future work will improve optical coupling and charge transport which will allow for frequency DQE and temporal metrics to be obtained.
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Affiliation(s)
- James R Scheuermann
- Department of Radiology, Stony Brook University, Stony Brook, NY, 11794, USA
| | - Adrian Howansky
- Department of Radiology, Stony Brook University, Stony Brook, NY, 11794, USA
| | | | | | - Kenkichi Tanioka
- Department of Radiology, Stony Brook University, Stony Brook, NY, 11794, USA
| | - Wei Zhao
- Department of Radiology, Stony Brook University, Stony Brook, NY, 11794, USA
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6
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Amorphous lead oxide (a-PbO): suppression of signal lag via engineering of the layer structure. Sci Rep 2017; 7:13272. [PMID: 29038544 PMCID: PMC5643314 DOI: 10.1038/s41598-017-13697-2] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2017] [Accepted: 09/25/2017] [Indexed: 11/19/2022] Open
Abstract
Presence of a signal lag is a bottle neck of performance for many non-crystalline materials, considered for dynamic radiation sensing. Due to inadequate lag-related temporal performance, polycrystalline layers of CdZnTe, PbI2, HgI2 and PbO are not practically utilized, despite their superior X-ray sensitivity and low production cost (even for large area detectors). In the current manuscript, we show that a technological step to replace nonhomogeneous disorder in polycrystalline PbO with homogeneous amorphous PbO structure suppresses signal lag and improves time response to X-ray irradiation. In addition, the newly developed amorphous lead oxide (a-PbO) possesses superior X-ray sensitivity in terms of electron-hole pair creation energy \documentclass[12pt]{minimal}
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\begin{document}$${W}_{\pm }$$\end{document}W± in comparison with amorphous selenium – currently the only photoconductor used as an X-ray-to-charge transducer in the state-of-the-art direct conversion X-ray medical imaging systems. The proposed advances of the deposition process are low cost, easy to implement and with certain customization might potentially be applied to other materials, thus paving the way to their wide-range commercial use.
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7
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Semeniuk O, Grynko O, Decrescenzo G, Juska G, Wang K, Reznik A. Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors. Sci Rep 2017; 7:8659. [PMID: 28819287 PMCID: PMC5561065 DOI: 10.1038/s41598-017-09168-3] [Citation(s) in RCA: 31] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2017] [Accepted: 07/21/2017] [Indexed: 12/04/2022] Open
Abstract
While polycrystalline lead oxide (poly-PbO) is known to be one of the most promising photoconductors for utilization in X-ray detectors, its major performance parameters such as charge yield and mobility-lifetime product (μτ) are still not well established and require further investigation. Combining the conventional X-ray induced photocurrent and pulse height spectroscopy techniques we examine the X-ray photogeneration and recombination processes in poly-PbO. The measurements indicate that the amount of energy required to release a single electron hole pair W± (inverse of charge yield) strongly depends on applied electric field and at 10 V/μm reaches ~20 eV/ehp. Fitting the measured pulse height spectra with the Hecht formula provided μτ for holes and electrons to be 4.1 × 10−8 cm2/V and 10−9 cm2/V, respectively. Obtained μτ values combined with recently reported mobility values of charge carriers in PbO suggest a new direction towards improvement of PbO technology by incorporation of Frisch grid or X-ray transistor architectures.
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Affiliation(s)
- O Semeniuk
- Chemistry and materials science program, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada. .,Advanced detection devices department, Thunder Bay Regional Health Research Institute, 290 Munro Street, Thunder Bay, ON, P7A 7T1, Canada.
| | - O Grynko
- Chemistry and materials science program, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada.,Advanced detection devices department, Thunder Bay Regional Health Research Institute, 290 Munro Street, Thunder Bay, ON, P7A 7T1, Canada
| | - G Decrescenzo
- Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada
| | - G Juska
- Department of Solid State Electronics, Vilnius University, Saulėtekio 9 III k., 10222, Vilnius, Lithuania
| | - K Wang
- SYSU-CMU Joint Institute of Engineering, School of Electronics and Information Technology, Sun Yat-sen University, 132 Waihua Rd., Guangzhou, 510006, P. R. China.
| | - A Reznik
- Advanced detection devices department, Thunder Bay Regional Health Research Institute, 290 Munro Street, Thunder Bay, ON, P7A 7T1, Canada.,Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada
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8
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Scaduto DA, Tousignant O, Zhao W. Experimental characterization of a direct conversion amorphous selenium detector with thicker conversion layer for dual‐energy contrast‐enhanced breast imaging. Med Phys 2017; 44:3965-3977. [PMID: 28543761 DOI: 10.1002/mp.12358] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2016] [Revised: 03/29/2017] [Accepted: 05/09/2017] [Indexed: 12/17/2022] Open
Affiliation(s)
- David A. Scaduto
- Department of Radiology Stony Brook Medicine Stony Brook NY 11794‐8460 USA
| | | | - Wei Zhao
- Department of Radiology Stony Brook Medicine Stony Brook NY 11794‐8460 USA
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9
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Abbaszadeh S, Scott CC, Bubon O, Reznik A, Karim KS. Enhanced detection efficiency of direct conversion X-ray detector using polyimide as hole-blocking layer. Sci Rep 2013; 3:3360. [PMID: 24285255 PMCID: PMC3842535 DOI: 10.1038/srep03360] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2013] [Accepted: 11/11/2013] [Indexed: 01/07/2023] Open
Abstract
In this article we demonstrate the performance of a direct conversion amorphous selenium (a-Se) X-ray detector using biphenyldisnhydride/1,4 phenylenediamine (BPDA/PPD) polyimide (PI) as a hole-blocking layer. The use of a PI layer with a-Se allows detector operation at high electric fields (≥10 V/μm) while maintaining low dark current, without deterioration of transient performance. The hole mobility of the PI/a-Se device is measured by the time-of-flight method at different electric fields to investigate the effect of the PI layer on detector performance. It was found that hole mobility as high as 0.75 cm2/Vs is achievable by increasing the electric field in the PI/a-Se device structure. Avalanche multiplication is also shown to be achievable when using PI as a blocking layer. Increasing the electric field within a-Se reduces the X-ray ionization energy, increases hole mobility, and improves the dynamic range and sensitivity of the detector.
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Affiliation(s)
- Shiva Abbaszadeh
- Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada
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10
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Li D, Zhao W, Nanba M, Egami N. Scintillator avalanche photoconductor with high resolution emitter readout for low dose x-ray imaging: Lag. Med Phys 2009; 36:4047-58. [DOI: 10.1118/1.3187227] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
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11
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Zhao B, Zhao W. Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system. Med Phys 2008; 35:1978-87. [PMID: 18561674 DOI: 10.1118/1.2903425] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
In breast tomosynthesis a rapid sequence of N images is acquired when the x-ray tube sweeps through different angular views with respect to the breast. Since the total dose to the breast is kept the same as that in regular mammography, the exposure used for each image of tomosynthesis is 1/N. The low dose and high frame rate pose a tremendous challenge to the imaging performance of digital mammography detectors. The purpose of the present work is to investigate the detector performance in different operational modes designed for tomosynthesis acquisition, e.g., binning or full resolution readout, the range of view angles, and the number of views N. A prototype breast tomosynthesis system with a nominal angular range of +/-25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85 microm. The detector can be read out in full resolution or 2 x 1 binning (binning in the tube travel direction). The focal spot blur due to continuous tube travel was measured for different acquisition geometries, and it was found that pixel binning, instead of focal spot blur, dominates the detector modulation transfer function (MTF). The noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector were measured with the exposure range of 0.4-6 mR, which is relevant to the low dose used in tomosynthesis. It was found that DQE at 0.4 mR is only 20% less than that at highest exposure for both detector readout modes. The detector temporal performance was categorized as lag and ghosting, both of which were measured as a function of x-ray exposure. The first frame lags were 8% and 4%, respectively, for binning and full resolution mode. Ghosting is negligible and independent of the frame rate. The results showed that the detector performance is x-ray quantum noise limited at the low exposures used in each view of tomosynthesis, and the temporal performance at high frame rate (up to 2 frames per second) is adequate for tomosynthesis.
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Affiliation(s)
- Bo Zhao
- Department of Radiology, State University of New York at Stony Brook, L-4 Health Science Center, Stony Brook, New York 11794-8460, USA.
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12
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Sakellaris T, Spyrou G, Tzanakos G, Panayiotakis G. Energy, angular and spatial distributions of primary electrons inside photoconducting materials for digital mammography: Monte Carlo simulation studies. Phys Med Biol 2007; 52:6439-60. [PMID: 17951854 DOI: 10.1088/0031-9155/52/21/007] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Materials such as a-Se, a-As(2)Se(3), GaSe, GaAs, Ge, CdTe, CdZnTe, Cd(0.8)Zn(0.2)Te, ZnTe, PbO, TlBr, PbI(2) and HgI(2) are potential candidates as photoconductors in direct detectors for digital mammography. The x-ray induced primary electrons inside a photoconductor's bulk comprise the initial signal that propagates and forms the final signal (image) on the detector's electrodes. An already developed model for a-Se has been properly extended to simulate the primary electron production in the materials mentioned. Primary electron characteristics, such as their energy, angular and spatial distributions that strongly influence the characteristics of the final image, were studied for both monoenergetic and polyenergetic x-ray spectra in the mammographic energy range. The characteristic feature in the electron energy distributions for PbI(2) and HgI(2) is the atomic deexcitation peaks, whereas for the rest of the materials their shape can also be influenced by the electrons produced from primary photons. The electrons have a small tendency to be forward ejected whereas they prefer to be ejected perpendicular (theta = pi/2) to the incident beam's axis and at two lobes around phi = 0 and phi = pi. At practical mammographic energies (15-40 keV) a-Se, a-As(2)Se(3) and Ge have the minimum azimuthal uniformity whereas CdZnTe, Cd(0.8)Zn(0.2)Te and CdTe the maximum one. The spatial distributions for a-Se, a-As(2)Se(3), GaSe, GaAs, Ge, PbO and TlBr are almost independent of the polyenergetic spectrum, while those for CdTe, CdZnTe, Cd(0.8)Zn(0.2)Te, ZnTe, PbI(2) and HgI(2) have a spectrum dependence. In the practical mammographic energy range and at this primitive stage of primary electron production, a-Se has the best inherent spatial resolution as compared to the rest of the photoconductors. PbO has the minimum bulk space in which electrons can be produced whereas CdTe has the maximum one.
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Affiliation(s)
- T Sakellaris
- University of Patras, School of Medicine, Department of Medical Physics, 265 00 Patras, Greece
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Bloomquist AK, Yaffe MJ, Mawdsley GE, Hunter DM, Beideck DJ. Lag and ghosting in a clinical flat-panel selenium digital mammography system. Med Phys 2006; 33:2998-3005. [PMID: 16964878 DOI: 10.1118/1.2218315] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
We present measurements of lag and ghosting in a FDA-approved digital mammography system that uses a dielectric/selenium based detector structure. Lag is the carryover of signal from a previous image, whereas ghosting is the reduction of sensitivity caused by previous exposure history of the detector. Data from six selenium units were acquired. For the type of selenium detector tested, and under typical clinical usage conditions, the lag was as high as 0.15% of source signal and the ghosting could be as high as 15%. The amount of lag and ghosting varied from unit to unit. Results were compared with data acquired on a phosphor-based full-field digital mammography system. Modifications in the technology of the selenium detectors appear to have resulted in a marked decrease in both lag and ghosting effects in more recent systems.
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Affiliation(s)
- Aili K Bloomquist
- Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario M4N 3M5, Canada
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14
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Abstract
Medical images are created by detecting radiation probes transmitted through or emitted or scattered by the body. The radiation, modulated through interactions with tissues, yields patterns that provide anatomic and/or physiologic information. X-rays, gamma rays, radiofrequency signals, and ultrasound waves are the standard probes, but others like visible and infrared light, microwaves, terahertz rays, and intrinsic and applied electric and magnetic fields are being explored. Some of the younger technologies, such as molecular imaging, may enhance existing imaging modalities; however, they also, in combination with nanotechnology, biotechnology, bioinformatics, and new forms of computational hardware and software, may well lead to novel approaches to clinical imaging. This review provides a brief overview of the current state of image-based diagnostic medicine and offers comments on the directions in which some of its subfields may be heading.
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Affiliation(s)
- Anthony B Wolbarst
- Department of Radiation Medicine, Georgetown University Medical School, Washington, DC, USA
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15
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Using a Homogeneity Test as Weekly Quality Control on Digital Mammography Units. DIGITAL MAMMOGRAPHY 2006. [DOI: 10.1007/11783237_36] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/07/2022]
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16
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Rau AW, Bakueva L, Rowlands JA. The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers. Med Phys 2005; 32:3160-77. [PMID: 16279070 DOI: 10.1118/1.2042248] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously, (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghosting in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/ microm, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S0) of the a-Se layers was 63 +/- 2 nC cm(-2) cGy(-1). It was found that S decreases to 30% of S0 after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25 +/- 0.1 x 10(22) ehp m(-3) s(-1) and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to approximately 80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.
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Affiliation(s)
- A W Rau
- Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario, M4N 3M5, Canada.
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Zhao W, DeCrescenzo G, Kasap SO, Rowlands JA. Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium. Med Phys 2005; 32:488-500. [PMID: 15789596 DOI: 10.1118/1.1843353] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
Direct flat-panel detectors using amorphous selenium (a-Se) x-ray photoconductors are gaining wide-spread clinical use. The goal of our investigation is to understand the physical mechanisms responsible for ghosting, i.e., x-ray induced change in sensitivity that results in image persistence, so that the knowledge can be used to consistently minimize ghosting artifacts in a-Se flat-panel detectors. In this paper we will discuss the effect on x-ray sensitivity of charge trapping in a-Se, which is the dominant source for ghosting in a-Se flat-panel detectors. Our approach is to correlate ghosting in electroded a-Se detectors with the trapped charge concentration measured by the "time-of-flight" (TOF) method. All measurements were performed as a function of radiation exposure X of up to approximately 20 R at electric field strength's of E(Se)=5 and 10 V/microm. The results showed that the x-ray sensitivity decreased as a function of X and the amount of ghosting decreased with increasing E(Se). The shape of the TOF curves changed as a result of irradiation in a manner indicating trapped electrons in the bulk of a-Se. The density of trapped electrons n(t) increases as a function of X. A method was developed to determine the values of n(t) in the bulk of a-Se from the TOF measurements, and to predict the corresponding change in x-ray sensitivity. Our results showed that a recombination coefficient consistent with that predicted by Langevin produced good agreement between calculated and measured x-ray sensitivity changes. Thus it can be concluded that the trapping of electrons in the bulk of a-Se and their subsequent recombination with x-ray generated free holes is the dominant mechanism for ghosting in a-Se.
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Affiliation(s)
- Wei Zhao
- Department of Radiology, State University of New York at Stony Brook, L-4 Health Sciences Center, Stony Brook, New York 11794-8460, USA.
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