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1.2. Comparison between Analog and Digital Radiography……………………………………...4

1.3. Avalanche x-ray detectors………………………………………………………………………………...4

1.4. Thesis targets…………………………………………………………………………………………………....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

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’, presented by 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 an AMA (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.

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].

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.

1. 
   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]
   
2. 
   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]
   
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]
   
4. 
   Measurement of the performance of the proposed multilayer direct conversion avalanche x-ray detector (e.g., dark current, sensitivity, MTF and DQE).[3]
   
5. 
   Optimizing the design of the proposed detector will be done by analyzing the DQE performance. [3]
   

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 amorphous silicon (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.

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.

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]

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]

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]

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.

[^] 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).