Optimal slice thickness for cone-beam CT with on-board imager

Area of Science:

• Medical Physics
• Radiological Imaging
• Radiation Oncology

Background:

• Accurate patient positioning is critical for effective radiation therapy.
• Kilovoltage cone-beam CT (kV-CBCT) is widely used for image-guided radiation therapy (IGRT).
• Slice thickness impacts kV-CBCT image quality and registration accuracy.

Purpose of the Study:

• To determine the optimal slice thickness (Δτ) for kV-CBCT patient registration on a Varian OBI system.
• To investigate the relationship between slice thickness, automatic registration accuracy, and contrast-to-noise ratio (CNR).

Main Methods:

• Automatic registration accuracy was assessed using head and pelvis phantom studies with known offsets.
• Images were reconstructed with slice thicknesses ranging from 1.0 to 5.0 mm.
• Contrast-to-noise ratio was measured using inserts with known CT numbers at various slice thicknesses.

Main Results:

• Registration residual error increased for slice thicknesses greater than 3.0 mm.
• Contrast-to-noise ratio was proportional to slice thickness up to 2.5 mm.
• No significant difference in residual error was observed below the planning CT slice thickness threshold.

Conclusions:

• Automatic registration accuracy is maximized with slice thicknesses between 1.0 mm and 3.0 mm.
• Optimal contrast-to-noise ratio is achieved with slice thicknesses between 2.5 mm and 5.0 mm.
• A slice thickness of 2.5-3.0 mm is recommended for kV-CBCT patient registration when using 3.0 mm planning CT.