TABLE OF CONTENTS
Chapters Page no
ABSTRACT………………………………………………………………………………………………………………. 2
1. Introduction ………………………………………………………………....…………………………………….3
1.1. Literature review……………………………………………………………………………………………...4
1.2. Comparison between Analog and Digital Radiography……………………………………...4
1.3. Avalanche x-ray detectors………………………………………………………………………………...4
1.4. Thesis targets…………………………………………………………………………………………………....5
2. HARP-AMFP………………………………………………………………………………………………………....5
2.1 CsI and a-Se…………………………………………………………………………………………………….….6
2.2 Performance of Amorphous Selenium Imaging in Indirect
Conversion X-ray Image Sensors……………………………………………………………………………...7
2.3 Lubberts Effect……………………………………………………………………………………………….…..7
2.4 Effect of MTF (Modulation Transfer Function)…………………………………………….………8
3. Relationship of charge collection efficiency and DQE in
Direct avalanche detector…………………………………………………………………………………………9
4. Pending/future works……………………………………………………………………………………………9
5. Conclusion…………………………………………………………………………………………………………….10
6. Personal understanding and comments……………………………………………………………….11
7. References……………………………………………………………………………………………………………12
ABSTRACT
The seminar report that I am going to present here is the succinct explanation of the PhD research proposal entitled as ‘Modeling and Characterization of a-Se Avalanche Detectors for Low Dose Medical X-ray Imaging’, presentedby Salman Moazzem Arnab and supervised by Dr. M. Z. Kabir. The research mainly deals with an existing technology but in a different approach. Flat-panel x-ray image detectors consisting anAMA (active matrix array) is used extensively in modern day x-ray technology. To extract meaningful images, a high SNR (sound to noise ratio) is required which is difficult in low x-ray exposure. Direct avalanche multiplication can increase the SNR but causes avalanche gain fluctuations whereas the indirect avalanche multiplication deteriorates the resolution. So this proposal encompasses of new device architecture for amorphous selenium (a-Se) based direct avalanche x-ray detector. This proposed scheme contains a hole trapping layer which separates the x-ray absorption layer from the avalanche gain region. This will remove the avalanche gain fluctuations and at the same time the proposed structure will improve the spatial resolution of the detector compared to that of indirect avalanche detectors.The goal of the thesis is to design the most efficient a-Se detectors for low dose medical x-ray imaging.
1. Introduction
Modern day x-ray technology extensively deals with flat-panel x-ray image detectors. Low cost and low radiation dose has a huge demand for in various medical radiation imaging modalities especially in general x-ray radiography and real-time imaging such as fluoroscopy. An ideal imager should support both radiographic and fluoroscopic modes of operation [5]. At the same time, taking human sensitivity due to exposure to x-ray radiation under consideration, intense research is underway to minimize the patient’s exposure to the x-ray ionizing radiation. Selenium based x-ray detectors are now available in the market for general radiography and mammography, and are being contemplated for use in fluoroscopy.
The exponentially distributed absorption profile of the x-ray beam in the absorption layer results a significant deterioration of resolution (in terms of modulation transfer function, MTF), which is known as Lubberts effect. This dissertation proposal involves a theoretical modeling based on a parallel cascaded linear-system which incorporates optical photon scattering with Lubberts effect, K-fluorescence reabsorption and geometric scattering. [3]
The indirect conversion avalanche x-ray detectors separate the absorption and the avalanche gain regions to eliminate the avalanche gain variation. However, in the indirect conversion x-ray detectors, the resolution deteriorates due to an extra optical stage. Therefore, the direct conversion detector is a better choice for higher resolution. This dissertation proposal comprises of new device architecture for amorphous selenium (a-Se) based direct avalanche x-ray detector. The proposed architecture contains a hole trapping layer which separates the x-ray absorption layer from the avalanche gain region. This will remove the avalanche gain variation and at the same time the proposed structure will improve the spatial resolution of the detector compared to that of indirect avalanche detectors. The viability of the proposed detector architecture will be ensured by measuring the transient dark current and the electric field across the total length. The developed parallel cascaded linear-system model will be modified to calculate the frequency dependent detective quantum efficiency, DQE (f) and MTF as functions of electric field and spatial frequency for the proposed multilayer direct conversion avalanche x-ray detector architecture [2].
1.1 Literature review
X-ray imaging mostly relies on radiography. Radiography is the examination of any part of the body for diagnostic purposes by means of x-rays with the record of the findings usually exposed onto photographic film[4]. It basically consists of two parts. i) Analog radiography ii) Digital radiography.
Analog radiography is usually less expensive and stores images using x-ray films whereas the digital counterpart stores image digitally, has less radiation, less expensive and environment friendly; Digital radiography can be directed or computed [3].
1.2 Comparison between Analog and Digital Radiography:
Dynamic range (signal response of a detector): It is different for analog and digital radiography. Analog system has a narrow range exposure whereas the digital system has a wider and linear dynamic range [3].
Detective Quantum Efficiency (DQE): DQE is one of the fundamental physical variables related to image quality in radiography and refers to the efficiency of a detector in converting incident x-ray energy into an image signal [4]. DQE of the digital system (both CsI and a-Se based) is better than the analog system.
Modulation Transfer Function (MTF): MTF is thespatialfrequency response of an imaging system or a component. It is the contrast at a given spatial frequency relative to low frequencies[4]. Indirect conversion detector (based on CsI) exhibits an improved MTF at higher spatial frequencies but direct conversion detector (based on a-Se) provides the best resolution [3].
1.3 Avalanche x-ray detectors
It can be of two types- i) indirect conversion ii) direct conversion
i)Indirect conversion: Here, the x-ray photons strike scintillator. The scintillator then converts the x-rays into light photons. Photoconductor converts light photons into electrical charge. Indirect conversion has a reduced resolution [3].
ii) Direct conversion: In this process the x-ray photons strike photoconductor and then the photoconductor converts light photons into electrical charge [3].
1.4Thesis targets
- Determination of physical mechanism causing frequency dependent DQE(f) of phosphor (i.e., CsI) based x-ray detectors to deteriorate at higher frequency in indirect conversion detectors.[3]
- Determination of the effects of charge collection efficiency on DQE in a direct conversion x-ray imager with a built-in high-gain avalanche rushing photoconductor (HARP) layer. [3]
- Proposing a multilayer detector structure for direct conversion avalanche detectors and developing mathematical models for calculating dark current, sensitivity, DQE and MTF in the proposed x-ray detectors.[3]
- Measurement of the performance of the proposed multilayer direct conversion avalanche x-ray detector (e.g., dark current, sensitivity, MTF and DQE).[3]
- Optimizing the design of the proposed detector will be done by analyzing the DQE performance. [3]
2. HARP-AMFP
The low dose X-ray detectors face performance deterioration due to noise of thin film transistor (TFT) which can be rectified by avalanche gain. AMFPI, direct-conversion active matrix flat-panel imager, deals with two active regions which are drift and gain regions. HARP, high-gain avalanche rushing photoconductor, is the gain region [5].
In this thesis proposal a direct conversion x-ray imager is used with a built-in high-gain avalanche rushing photoconductor layer to determine the effects of charge collection efficiency in DQE. The imaging performance of the system is measured by a modulation transfer function (MTF).
Fig-1: The structure of a HARP-AMFPI [4]
2.1 CsI and a-Se
CsI, ionic compound of cesium and iodine, is often used in fluoroscopy where CsI works as the input phosphor of an x-ray image intensifier tube. CsI is highly efficient at extreme ultraviolet wavelengths. [1]
A-Si, non-crystalline form of silicon, is used extensively in thin-film transistors. Though amorphoussilicon (a-Si) has a comparatively lower efficiency yet it is adored worldwide because of its eco-friendly nature,
2.2 Performance of Amorphous Selenium Imaging in Indirect Conversion X-ray Image Sensors
Fig-2: Cross sectional diagram showing an indirect FPI [3]
Here, we can see that there is a hole blocking layer in between the CsI and a-Se and an electron blocking layer. The CsI has a columnar structure with high atomic number of cesium and iodide. The a-Se has a low dark current and supports large area depositions. The reason why we are using CsI and a-Se is indirect conversion detector (based on CsI) exhibits an improved MTF at higher spatial frequencies and direct conversion detector (based on a-Se) provides the best resolution.
2.3 Lubberts Effect
In granular x-ray imaging phosphor screens the noise transfer is not proportional to the square of the magnitude of the signal transfer when the transfer properties are considered for the entire screen thickness, unless appropriately weighted at each depth of interaction. This property, known as the Lubberts effect, has not yet been studied in columnar structured screens because of a lack of a generalized description of the depth-dependent light transport [2]
Due to the Lubberts effect X-ray photons are consumed in different layers. The light photons also tend to follow diverse distances. Due to these phenomenon there occurs numerous spread in the optical photons.Depending on the thickness of the CsI layer, spatial frequency and other parameters the DQE increases.
2.4 Effect of MTF (Modulation Transfer Function)
If we do pre-sampling MTF of CsI we can see that when the scintillator gets thicker the MTF increases.The thick CsI layer contributes to the vigorous drop in pre-sampling MTF at higher frequencies.. The optical scattering in scintillator is the main reason for MTF degradation in an indirect conversion x-ray imager. MTF of CsI is more vulnerable to the optical scattering with Lubberts effect. Furthermore the x-ray absorption can be improved by increasing the thickness of the CsI. The following graph consisting MTF and spatial frequency shows the MTF without optical blurring with Lubberts effect (dash-dotted line) and MTF without K-fluorescence reabsorption scattering. [3]
Fig-3: Pre-sampling MTF (solid line), MTF without optical blurring with Lubberts effect (dash-dotted line) and MTF without K-fluorescence reabsorption scattering (dashed) (thickness of CsI is 600 μm and x-ray energy is 54 keV[3]
3. Relationship of charge collection efficiency and DQE in direct avalanche detector
Insufficient charge collection leads to the decrease of DQE. If we compare DQE with spatial frequency with the influence of different avalanche gain we can see the different DQE measurements. If we have a higher avalanche gain the DQE reduces accordingly. We can say that Avalanche multiplication overcomes the effect of charge collection efficiency [4]
Fig-4:Effect of charge collection on DQE (f) at different avalanche gain (at an exposure of 0.1 ΜR) [3]
4. Pending/future works
The works which are to be completed can be divided into three parts:
i) Structure optimization: After considering a few parameters, eg: applied voltage, permittivity, trapped hole concentration and elementary charge, the calculations showed that a high electric field is required in a-Se to initiate the avalanche multiplication and the architecture is feasible. [3]
ii) Minimizing dark current density: A multilayer direct conversion a-Se avalanche detector is proposed. The bias-dependent transient dark current will be scrutinized under some considerations, which include actual electric field profile, metal-semiconductors contact properties, carrier injection at high electric field, thermal generation. The proposed architecture is as follows.
Fig-5:Multilayer direct conversion a-Se avalanche detector (not to scale) with blocking layers [3]
The blocking layers work as an obstacle against the upcoming carriers from the electrodes and thus minimizes the occurrence of the dark currents. The selected material for the photoconductor is expected have some characteristics, such as, it should have minimum dark current, it needs to support avalanche multiplication and corresponding optimum trap density, it also needs to be stable so that it can have a uniform attributes. [1]
iii) Required Characteristics: The required trap density and length of the trap layer for the avalanche multiplication to operate in the gain region needs to be calculated. The effects of trap density and length of the trap layer on the overall MTF of the detector also needs to be calculated, Effects of trap charges in successive exposures and theeffects of traps in sensitivity need to be carefully calculated. To fill up the traps in the trapping layer bias voltage needs to be applied along with flooding the traps with x-ray photons.
5. Conclusion
The thesis proposal emphasizes on the new device architecture for amorphous selenium (a-Se) based direct avalanche x-ray detector. High quantum efficiency at low energy and high spatial resolution of a-Se makes it an apt scheme for photoconductor materials in x-ray detectors. Further, the proposal depicts some architecture with multilayer technology which is supposed to minimize dark current density as well as support avalanche multiplication.The measuring of the transient dark current and the electric field across the total length can assure the feasibility of the proposed scheme. The concern is that the avalanche x-ray detector is somewhat restricted and is not yet practicable for medical imaging devices, however, widespread studies and researches are ongoing to make avalanche x-ray detectors feasible and known.
6. Personal understanding and comments
By going through and studying the research proposal I got a clear understanding about x-ray imaging, especially about low dose medical x-ray imaging. Mr. Salman Moazzem Arnab tried to emphasize about the different technologies that are being used for medical x-ray imaging purposes. He discussed about the a-Se Avalanche Detectors for Low Dose Medical X-ray Imaging both in direct and indirect format and emphasized on the direct method. Mr. Salman researched and found out a way to improve the architecture by adding layers in between the components which is really feasible. He also mentioned about calculating the performances of dark currents, MTF, DQE if time permits, but I think it is really essential for the research to calculate the performances of these prior mentioned parameters and it should be given a higher priority.
7.References
[] S. Kasap, J. B. Frey, G. Belev, O. Tousignant, H. Mani, J. Greenspan, L. Laperriere, O. Bubon, A. Reznik, G. DeCrescenzo, K. S. Karim and J. A. Rowlands, “Amorphous and polycrystalline photoconductors for direct conversion flat panel x-ray image sensors,” Sensors 11, 5112-5157 (2011).
[2] M. Wronski, W. Zhao, K. Tanioka, G. DeCrescenzo and J. A. Rowlands, “Scintillator high-gain avalanche rushing photoconductor active-matrix flat panel imager: Zero-spatial frequency x-ray imaging properties of the solid-state SHARP sensor structure,” Med. Phys. 39, 7102-7109 (2012).
[3] PhD research proposal by Salman Moazzem Arnab, ‘’Modeling and Characterization of a-Se Avalanche Detectors for Low Dose Medical X-ray Imaging’’
[4] Wronski, M. M., and J. A. Rowlands. "Direct-conversion flat-panel imager with avalanche gain: Feasibility investigation for HARP-AMFPI." Medical physics 35.12 (2008): 5207-5218.
[5] D. C. Hunt, K. Tanioka, and J. A. Rowlands, “X-ray imaging using avalanche multiplication in amorphous selenium: Investigation of intrinsic avalanche noise,” Med. Phys. 34, 4654–4663 (2007).
1 | Page