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Detector physics in photoconductor and phosphor materials

Winston Chi Ji

1. Photoconductor and phosphor detectors

The materials used to detect X-rays can be classified into two categories:

  • photoconductor detectors such as amorphous selenium (a-Se); and
  • phosphor detectors such as Gd2O2S.

Photoconductor detectors convert the input X-ray into charges. Phosphor detectors convert the input X-ray into light photons. Different recording devices such as film, CMOS and photo-diodes can read out both the charges and the photons to obtain the medical image.
Our goal is to understand the physical processes in these detectors to optimize the detector design and to get a compromise between the image quality and X-ray dose. At this time, we are focusing on the detector physics of the phosphor detector.

2. Physical quantities to be studied

  • Modulation transfer function

The phosphor detector is characteristic of modulation transfer function (MTF), which describes the resolution of the obtained medical image.

  • Noise power spectrum and detective quantum efficiency

Detective quantum efficiency (DQE) is another important indicator of the performance of a detector, which presents the signal-to-noise ratio of the obtained image. It determines the image quality and visibility of a microcalcification.
We need to study the noise power spectrum (NPS) in order to study DQE.

  • Pulse height spectrum

Investigating the pulse height spectrum (PHS) is an alternative approach to exploring the detector physics. Same physical processes and the related physical parameters for MTF and NPS also determine the measured PHS. Pulse height spectrum is another subject in our study of the detector physics.

  • Reflectance

Similar to MTF, NPS and PHS, the optical reflectance of a detector screen is also determined by similar physical processes and material parameters. Reflectance (R) of the screen is another physical quantity in our study.

3. Physical processes in our study

We have studied the following physical processes:

  • X-ray’s motion in a detector including its absorption and the K-fluorescence effect when the input X-ray energy is above the K-edge;
  • the creation of electrons by the absorbed X-ray and its trapping by the trapping centres in the screen;
  • the creation of photons from a fluorescent centre by the electron-hole recombination and the diffusion motion of photons;
  • the noise correlation between different channels when the K-fluorescence effect exists; and
  • the noise propagation in the cascade process of the detector.

4. Approaches

  • Experiments

We have accomplished systematical measurement in the BaFBr phosphor screens having different grain sizes, thicknesses and backing conditions. We have studied two groups of screens developed by Agfa and Fuji. The measured physical quantities include reflectance R, MTF, NPS and PHS.

  • Analytical formulation

We have developed a set of analytical formulas based on the Swank model for reflectance R, MTF, NPS and PHS. We have implemented the related physical processes listed in 2.

  • Monte Carlo simulations

We use the Monte Carlo (MC) approach to validate the analytical formulation and examine the limitation of the analytical formula. We have developed a complete set of C-codes of MC simulation for R, MTF, NPS and PHS. As in the analytical formulation, we have implemented the related physical processes listed in 2.

5. Main progresses of our study

  • We presented a systematical approach to study a phosphor screen and show the feasibility in its accomplishment.
  • We determined the optical parameters of phosphor screens including the scattering cross-section and absorption cross-section of photons and the boundary conditions.
  • We verified the analytical Swank model to be an accurate and practical model in application. We have also determined its limitation.
  • We developed a complete set of analytical formula in calculation of R, MTF and NPS.
  • We developed an analytical expression of PHS and examined the corresponding MC simulation.
  • We developed a complete set of MC codes in calculation of R, MTF, NPS and PHS.
  • We validated the developed analytical and MC results by cross-examination between them and by comparison with our measurements.
  • We have proposed a new mechanism of noise source in a phosphor screen, which we named the inter-grain trapping variation. We presented the importance of this noise term in NPS, and developed and validated its expressions and its propagation in a cascade chain process of a detector. We also developed and examined the implementation of its effect to the analytical formula of PHS.

6. Other related topics

  • We studied similar physical quantities for a CsI-like detector with cylindrical structure and developed related analytical formula and MC codes.
  • We studied MTF and NPS for a stimulated phosphor computed radiography detector such as BaFBr and CsBr (cylindrical structure).
  • We studied the effect of the pixel size, sampling and aliasing in a pixilated flat-panel detector.
  • We studied the role of the observer in the visibility of micro-calcifications and the decision theory.

Publications

  1. W. Zhao, W.G. Ji, A. Debrie and J.A. Rowlands. “Imaging Performance of Amorphous selenium Based Flat-Panel Detectors for Digital Mammography: Characterization of A Small Area Prototype Detector”, Med. Phys. 30, 254-263 (2003).
  2. J.A. Rowlands, W. G. Ji and W. Zhao, “Effect of Depth Dependent Modulation Transfer Function and K-Fluoresecence Reabsorption on the Detective Quantum Efficiency of Indirect Conversion Flat-Panel X-ray Imaging System Using CsI”, Proc SPIE 4320 (2001).
  3. W. Zhao, W. G. Ji, J. A. Rowlands and A. Debrie, “Investigation of Imaging Performance of Amorphous selenium Flat-Panel Detectors for Digital Mammography”, Proc SPIE 4320 (2001).
  4. W. Zhao, WG Ji and JA Rowlands, “Effects of Characteristic X Rays on the Noise Power Spectra And Detective Quantum Efficiency of Photoconductive X-ray Detectors”, Med.Phys, 28, 2039-2049(2001).
  5. J. A. Rowlands, W. G. Ji, W. Zhao and D. L. Y. Lee, “Direct-conversion flat-panel X-ray imaging: Reduction of noise by presampling filtration”, Proc. SPIE 3977, 446-455 (2000).
  6. W. Zhao, W.G. Ji, J.A. Rowlands and A. Debrie “Relationships between detector design parameters and imaging performance of amorphous selenium based flat-panel detectors for digital mammography”, Canadian Association of Radiologists Annual Meeting (June 10-14, 2000, Toronto).
  7. J. A. Rowlands and W. G. Ji, Optimization of the presampling modulation transfer function of flat-panel detectors for digital radiology, Proc. SPIE Medical Imaging, 3659, 66-75 (1999).
  8. W.G. Ji, W. Zhao and J.A. Rowlands, X-ray Imaging with Amorphous selenium: Reduction of Aliasing Med.Phys. 25, 2148 -2162 (1998).
  9. J.A. Rowlands, W. Zhao, I. Blevis, G. Pang, W.G. Ji, S. Germann, D. Waechter and Z. Huang, Flat panel detector for digital radiology using active matrix readout of amorphous selenium Proc. SPIE 3032, 97-108 (1997).
  10. J.A. Rowla