Modelling optical scattering artefacts for varying pathlength in a gel dosimeter phantom

Area of Science:

• Medical physics
• Optical imaging
• Radiotherapy physics

Background:

• Cone-beam optical computed tomography (CT) is utilized in gel dosimetry.
• Optical properties of phantoms can be affected by scattering.
• Accurate dose assessment in radiotherapy requires understanding image artifacts.

Purpose of the Study:

• To investigate light-scattering-induced artifacts in cone-beam optical CT.
• To analyze the impact of scattering on optical density (OD) measurements in gel phantoms.
• To model the observed 'dishing' and 'doming' artifacts.

Main Methods:

• A gelatin phantom with a funnel-shaped region of elevated optical density was created.
• Colloidal scatterers were added to the gelatin to simulate polymer gel dosimeters.
• 3D reconstructions of the phantom were analyzed using line profiles of OD.

Main Results:

• Scattering leads to an underestimation of optical density and, consequently, radiation dose.
• Line profiles showed 'dishing' (cupping) artifacts for long pathlengths through high OD regions.
• Line profiles showed 'doming' artifacts for short pathlengths through high OD regions.

Conclusions:

• Light scattering is a significant source of artifacts in optical CT gel dosimetry.
• A model incorporating stray scattered light can account for observed 'dishing' and 'doming' phenomena.
• Understanding these artifacts is crucial for accurate dose verification in radiotherapy.