Title page for ETD etd-01262011-153300

Type of Document Dissertation
Author Fritz, Shannon
Author's Email Address shannon.fritz@gmail.com
URN etd-01262011-153300
Title Characterization of Photon Counting CZT Detectors for Medical X-ray Imaging and Spectroscopy
Degree Doctor of Philosophy (Ph.D.)
Department Physics & Astronomy
Advisory Committee
Advisor Name Title
Shikhaliev, Polad Committee Chair
Blackmon, Jeffrey Committee Member
Hogstrom, Kenneth Committee Member
Matthews, Kenneth "Kip" Committee Member
Murray, Kermit Dean's Representative
  • energy weighting
  • breast CT
  • dual energy subtraction
  • characteristic x-ray escape
  • hole trapping
  • capacitive grid detector
  • tilted angle irradiation
Date of Defense 2010-12-03
Availability unrestricted
Purpose: The purpose of this research was to characterize a photon counting cadmium zinc telluride (CZT) detector for medical x-ray imaging and spectroscopy purposes. The overall aim was to characterize CZT detector properties and to develop modifications and correction methods to address the limiting factors, making the detector clinically viable.

Methods: Hole trapping issues were investigated through simulation and experiments with a large area single pixel CZT detector under three different irradiation geometries: edge-on, surface-on, and tilted angle irradiation. Characteristic x-ray escape was simulated using Monte Carlo methods and compared to measurements with a small pixel CZT imaging detector. Monoenergetic sources were measured with the small pixel CZT imaging detector to investigate energy blurring. Spectroscopy measurements were made both with and without hole trapping and characteristic escape corrections, and compared.

Results: Tilted angle geometry improved the energy resolution compared to surface-on irradiation; peak-to-total ratios increased from 38% to 83% at 10 tilt for 122 keV. An increase was also seen for 59 keV, from 73% to 97% at 10; simulation confirmed that tailing is a minor concern below 60 keV, even for surface-on irradiation. Characteristic x-ray escape simulations showed that side escape was less for strip pixels compared to square pixels (47% vs. 66% at 110 keV for 0.1 mm pixel width). Back escape increased from 27% for surface-on irradiation to 51% for 10 irradiation at 30 keV. For spectroscopy, modifications in detector geometry and electric field, as well as escape corrections, reduced the integral deviation between measured and true 120 kVp spectrum to 19% compared to 47% for operation without modification or correction.

Conclusions: We showed that a CZT detector for diagnostic x-ray spectroscopy is clinically feasible, requiring simple modifications to current technology. However, the energy resolution of small pixel CZT imaging detectors requires substantial improvement to achieve clinical utility; several avenues of development are available to drive a CZT imaging detector towards its desired performance level. With these developments, advanced imaging systems using tilted-angle strip geometry, e.g., for photon-counting breast imaging, are likely to become feasible for routine clinical use.

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