Stray light in cone beam optical computed tomography: I. Measurement and reduction strategies with planar diffuse...

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Imaging Science

Background:

• Optical cone-beam computed tomographic (CBCT) scanning offers potential for 3D dose verification in radiation therapy using 3D radiochromic dosimeters.
• Stray light in cone-beam geometry contaminates projection images, reducing the accuracy of reconstructed linear attenuation coefficients.

Purpose of the Study:

• To investigate and mitigate the impact of stray light on the accuracy of CBCT reconstructions for 3D radiochromic dosimeters.
• To evaluate the effectiveness of scanner modifications and stray light subtraction techniques in improving dose verification accuracy.

Main Methods:

• Stray light was quantified using beam pass aperture array (BPA) and structured illumination methods.
• CBCT scanner modifications included relocating the spectral filter and adjusting source-object distance to reduce stray light.
• Reconstructed linear attenuation coefficients were compared to spectrometer measurements and fan-beam CT scans using phantom studies.

Main Results:

• Scanner modifications reduced the stray-to-primary ray ratio (SPR) from 0.24 to 0.06.
• Reduced stray light improved the accuracy of linear attenuation coefficient measurements and agreement between CBCT and fan-beam CT reconstructions.
• Structured illumination was more effective than BPA for measuring stray light in small phantom geometries.

Conclusions:

• Modifying CBCT scanner geometry and implementing stray light reduction techniques significantly enhance the accuracy of 3D radiochromic dosimeter dose verification.
• CBCT, with these improvements, shows promising agreement with fan-beam CT for radiation therapy dosimetry applications.
• Further research into stray light measurement and correction is crucial for advancing CBCT in clinical dosimetry.