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1 Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa, USA; Department of Radiology, University of Iowa, Iowa City, Iowa, USA
2 Department of Radiology, University of Iowa, Iowa City, Iowa, USA; Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa, USA
* To whom correspondence should be addressed. E-mail: joseph-reinhardt{at}uiowa.edu.
In order to objectively quantify airway geometry from 3D Computed Tomographic (CT) images, an idealized (circular cross-section) airway model is parameterized by airway lumenal caliber, wall thickness, and tilt angle. Using a 2D CT slice, an initial guess for the airway center and the full-width-halfmax principle we form an estimate of the inner and outer airway wall locations. We then fit ellipses to the inner and outer airway walls using a direct least squares fit and use the major and minor axes of the ellipses to estimate the tilt and in-plane rotation angles. Convolving the airway model, initialized with these estimates, with the 3D scanner point spread function, forms the predicted image. The difference between predicted and actual images is minimized by refining the model parameter estimates using a multidimensional, unconstrained, non-linear minimization routine. When optimization converges, airway model parameters estimate the airway inner and outer radii and tilt angle. Results using a plexiglass phantom show that tilt angle is estimated to within ±4°, and both inner and outer radii to within 1/2 pixel when a "standard" CT reconstruction kernel is used. By opening up the ability to measure airways which are not oriented perpendicular to the scanning plane, this method provides for the ability to now evaluate a greater sampling of airways in a 2D CT slice than has been previously possible. In addition, by combining the tilt angle compensation with the deconvolution method, we provide significant improvement over the previous fullwidth-half-maximum method for assessing location of the lumenal edge but not the outer edge of the airway wall.
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