title: Stray light in cone beam optical computed tomography: II. Reduction using a convergent light source.
abstract: Optical cone beam computed tomography (CBCT) using a broad beam and CCD camera is a fast method for densitometry of 3D optical gel dosimeters. However, diffuse light sources introduce considerable stray light into the imaging system, leading to underestimation of attenuation coefficients and non-uniformities in CT images unless corrections are applied to each projection image. In this study, the light source of a commercial optical CT scanner is replaced with a convergent cone beam source consisting of almost exclusively image forming primary rays. The convergent source is achieved using a small isotropic source and a Fresnel lens. To characterize stray light effects, full-field cone beam CT imaging is compared to fan beam CT (FBCT) using a 1 cm high fan beam aperture centered on the optic axis of the system. Attenuating liquids are scanned within a large 96 mm diameter uniform phantom and in a small 13.5 mm diameter finger phantom. For the uniform phantom, cone and fan beam CT attenuation coefficients agree within a maximum deviation of (1  ±  2)% between mean values over a wide range from 0.036 to 0.43 cm(-1). For the finger phantom, agreement is found with a maximum deviation of (4  ±  2)% between mean values over a range of 0.1-0.47 cm(-1). With the convergent source, artifacts associated with refractive index mismatch and vessel optical features are more pronounced. Further optimization of the source size to achieve a balance between quantitative accuracy and artifact reduction should enable practical, accurate 3D dosimetry, avoiding time consuming 3D scatter measurements.
authors.affiliation: Department of Medical Biophysics, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario N6A 5C1, Canada.
authors.forename: Kurtis H
authors.initials: KH
authors.lastname: Dekker
authors.forename: Jerry J
authors.initials: JJ
authors.lastname: Battista
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Jordan
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D008027:Light; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods; D012031:Refractometry