Publication Date

2010

Document Type

Thesis

Committee Members

Thomas Dufresne (Committee Member), Thomas Hangartner (Advisor), Julie Skipper (Committee Member)

Degree Name

Master of Science in Engineering (MSEgr)

Abstract

Computed tomography (CT) has various applications in different fields. In our case, the cone-beam CT scanner is used in the industrial field for qualitative and quantitative assessment of Procter and Gamble products. The Flash CT scanner, on which we do our research, has a microfocus x-ray tube, a detector size of 30 cm x 40 cm and a rotating stage for the object to be imaged. The x-ray tube can be operated up to 225 kV. Non-linearities in the response of the detector, scatter and beam hardening may cause false interpretations of the image values. To allow appropriate assessment of the images, the scanner needs to be characterized. We measured the linearity of the detector and the geometric resolution of the scanner. To reduce the effect of scatter, we designed a collimator. For the beam-hardening effect we propose a software solution to improve the accuracy of the CT values in the reconstructed images. The linearity of the detector elements was tested by acquiring images with no objects in the beam at voltages between 50-150 kVp in intervals of 25 kVp and at a number of anode current settings. An R-squared test of detector reading versus anode current allowed the identification of bad pixels, and we found 5,660 pixels that were below the decided threshold (R-squared value of 0.98). No correction was done for this, as the manufacturer's software already provides a correction, and this was confirmed as non-linear pixels did not appear in the calibrated file format. A forearm phantom made from Plexiglas and aluminum was reconstructed to perform modulation transfer function (MTF) measurements. Because a step phantom creates streaks in the reconstructed images, a cylindrical phantom (forearm phantom) is preferable for this purpose. The images contained five circular profiles, and the MTFs of these five profiles were measured by using an error spread function (ESF)-based fitting procedure, in which parameters are optimized using a non-linear least-squares method. The 10% MTF value was higher at a stage distance of 300 mm from the source (7.2 cycles/mm to 8.5 cycles/mm) than at a stage distance of 550 mm from the source (2.9 cycles/mm to 3.5 cycles/mm) and at a stage distance of 700 mm from the source (2.2 cycles/mm to 2.76 cycles/mm). To obtain a higher cut-off value for the 10% MTF, the stage position should be as close as possible to the source A step phantom made from Delrin and the forearm phantom made from Plexiglas and aluminum were used for scatter measurements in the projections and reconstruction images, respectively. Three stage positions (300 mm, 550 mm and 700 mm from the source) and three regions of the detector (region 1, region 2 and region 3) were used to conduct experiments for this part of our research. A precise collimator was designed to limit the cone beam to the detector size by calculating the source position and determining the necessary size of the cone beam. Data for both the step phantom in projection images and the forearm phantom in reconstructed images were compared with and without collimator Scatter causes an increase in the photon counts at the detector, resulting in decreased projection values and decreased reconstructed image values. The collimator prevents scatter from structures outside the x-ray cone resulting in increased projection and image values. At a distance of 300 mm from the source, the rotating stage was completely outside of the collimated beam; therefore, scatter emanating from the rotating stage was not present for data collected at this stage position. Our results indicate that the effect of scatter is more prominent for stage positions closer to the detector. The software beam-hardening correction was based on a fourth-order polynomial, representing measured projection value versus step-wedge thickness. As the beam-hardening correction needs to only correct for beam-hardening and not scatter, the scatter collima...

Page Count

79

Department or Program

Department of Biomedical, Industrial & Human Factors Engineering

Year Degree Awarded

2010


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