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Modeling of X-ray Photoconductors for X-ray Image Detectors

by Kabir, Mohammad Zahangir

Abstract (Summary)

Direct conversion flat panel x-ray image sensors based on using a photoconductor with an active matrix array provide excellent images. These image sensors are suitable for replacing the present day x-ray film/screen cassette to capture an x-ray image electronically, and hence enable a clinical transition to digital radiography. The performance of these sensors depends critically on the selection and design of the photoconductor. This work quantitatively studies the combined effects of the detector geometry (pixel size and detector thickness), operating conditions (x-ray energy and applied electric field) and charge transport properties (e.g., carrier trapping and recombination) of the photoconductor on the detector performance by developing appropriate detector models. In this thesis, the models for calculating the x-ray sensitivity, resolution in terms of the modulation transfer function (MTF), detective quantum efficiency (DQE), and ghosting of x-ray image detectors have been developed. The modeling works are based on the physics of the individual phenomena and the systematic solution of the fundamental physical equations in the photoconductor layer: (1) semiconductor continuity equation (2) Poissons equation (3) trapping rate equations. The general approach of this work is to develop models in normalized coordinates to describe the results of different photoconductive x-ray image detectors. These models are applied to a-Se, polycrystalline HgI_2 and polycrystalline CdZnTe photoconductive detectors for diagnostic medical x-ray imaging applications (e,g., mammography, chest radiography and fluoroscopy). The models show a very good agreement with the experimental results.

The research presented in this thesis shows that the imaging performances (e.g., sensitivity, MTF, DQE and ghosting) can be improved by insuring that the carrier with higher mobility-lifetime product is drifted towards the pixel electrodes. The carrier schubwegs have to be several times greater, and the absorption depth has to be at least two times smaller than the photoconductor thickness for achieving sufficient sensitivity. Having smaller pixels is advantageous in terms of higher sensitivity by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes.

A model for calculating zero spatial frequency detective quantum efficiency, DQE (0), has been developed by including incomplete charge collection and x-ray interaction depth dependent conversion gain. The DQE(0) analyses of a-Se detectors for fluoroscopic applications show that there is an optimum photoconductor thickness, which maximizes the DQE(0) under a constant voltage operation. The application of DQE(0) model to different potential photoconductive detectors for fluoroscopic applications show that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an x-ray image detector.

An analytical expression of MTF due to distributed carrier trapping in the bulk of the photoconductor has been derived using the trapped charge distribution across the photoconductor. Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the other type of carriers improves the sharpness of the x-ray image.

The large signal model calculations in this thesis show an upper limit of small signal models of x-ray image detectors. The bimolecular recombination between drifting carriers plays practically no role on charge collection in a-Se detectors up to the total carrier generation rate q0 of 10^18 EHPs/m^2-s. The bimolecular recombination has practically no effect on charge collection in a-Se detectors for diagnostic medical x-ray imaging applications.

A model for examining the sensitivity fluctuation mechanisms in a-Se detectors has been developed. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers, electric field dependent charge carrier generation and x-ray induced new deep trap centers are mainly responsible for the sensitivity fluctuation in biased a-Se x-ray detectors.

The modeling works in this thesis identify the important factors that limit the detector performance, which can ultimately lead to the reduction of patient exposure/dose consistent with better diagnosis for different diagnostic medical x-ray imaging modalities. The quantitative analyses presented in this thesis show that the detector structure is just as important to the overall performance of the detector as the material properties of the photoconductor itself.

Bibliographical Information:

Advisor:Xiao, Chijin; Salt, J. Eric; Singh, Jai; Kasap, Safa O.; Chowdhury, Nurul A.

School:University of Saskatchewan

School Location:Canada - Saskatchewan

Source Type:Master's Thesis

Keywords:trapping and recombination ghosting mechanisms detective quantum efficienciency resolution sensitivity physics modeling x ray photoconductors image detectors

ISBN:

Date of Publication:08/15/2005

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