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.

Optical CT imaging of solid radiochromic dosimeters in mismatched refractive index solutions using a scanning laser and large area detector

“…This is different from a camera-based system, where mismatched refractive indices would cause a complete loss of information in large regions of projection images due to ray rejection by the imaging lens and aperture. 27 The measured intensity is dependent upon the position of the beam on the diffuser, but this is taken into account by calculating transmission as the ratio of post-irradiation to pre-irradiation measurements. This system has a single ray-single detector property; recorded light intensities can be assigned along unique ray paths.…”

Technical Note: Evaluation of an iterative reconstruction algorithm for optical CT radiation dosimetry

“…This is a key result for optical CT dosimetry, as it has been a long‐standing challenge to manufacture a dosimeter vessel that performs well both optically (refractive index, imperfections) and dosimetrically (thin walls, water‐like x‐ray attenuation, relatively impermeable to oxygen). Furthermore, the impact of vessel imperfections, in terms of streak artifacts, increases as stray light is removed from the system …”

“…Additionally, optical CT is sensitive to the effects of refraction. As scatter production and detection is reduced in optical CT, systems become more sensitive to the effects of optical imperfections in dosimeters and dosimeter vessels, which deflect primary image‐forming rays away from their intended path. These imperfections create considerable streaking artifacts when reconstructing datasets using filtered backprojection, which is currently the most common reconstruction algorithm used in gel dosimetry.…”

Optical and X-ray computed tomography scanning of 3D dosimeters

“…Figure 1, shows reconstructed transverse slice from an 11 cm diameter vessel with 0.2 mm thick PET wall containing a carbon black micelle solution. The scanner was a modified Vista 10 scanner with a Fresnel lens convergent cone beam source [16]. Because the vessel wall has a different refractive index from the hydrogel, no single refractive index liquid will provide minimum deflection of all transmitted rays at increasing radius [6,17].…”

“…Stray light in certain scanner geometries essentially masked the effects of scratches, see [16]. By increasing the diameter of the source many optical artifacts such as missing primary rays near the vessel wall can be minimized.…”