Rotational artifacts in on-board cone beam computed tomography

Area of Science:

• Medical Physics
• Radiotherapy Imaging
• Image Guidance Systems

Background:

• Rotational artifacts in image guidance systems cause registration errors, impacting non-isocentric treatments and dose delivery to organs-at-risk.
• On-board cone beam computed tomography (CBCT) systems, like XVI (Elekta), are susceptible to these artifacts, necessitating investigation.

Purpose of the Study:

• To identify the cause of rotational artifacts in Elekta XVI CBCT system.
• To characterize artifact dependence on scan speed, pulse width, and focal spot size.
• To investigate potential solutions for mitigating rotational artifacts.

Main Methods:

• Utilized a 30 cm cylindrical phantom on six Elekta linear accelerators across three generations.
• Acquired clockwise and counterclockwise scans at various speeds (120-360 deg/min) and pulse widths.
• Developed a robust analysis to quantify artifacts (0.02° accuracy) and imaging system delays (3 ms accuracy).

Main Results:

• Artifacts stem from mislabeled projections due to imaging system delays, causing a lagging angle.
• At 360 deg/min, artifact is ~0.5°, leading to ~0.25° error per scan direction and 0.5 mm registration error at 11 cm off-center.
• Artifacts increase linearly with scan speed; system delay is independent of speed (34 ± 1 ms at 40 ms pulse width).

Conclusions:

• Rotational artifacts in Elekta XVI CBCT are caused by system delays leading to projection mislabeling.
• A software correction can eliminate rotational errors across all scan speeds and directions.
• Clinical solutions exist to reduce artifacts without compromising image quality pending manufacturer software updates.