Deriving motion from megavoltage localization cone beam computed tomography scans

Area of Science:

• Medical Imaging
• Radiotherapy Physics
• Computational Anatomy

Background:

• Accurate tracking of internal organ motion is crucial for effective radiotherapy.
• Cone beam computed tomography (CBCT) offers volumetric imaging capabilities during treatment delivery.
• Existing methods for motion tracking may have limitations in precision and applicability.

Purpose of the Study:

• To develop and validate a novel method for reconstructing 4D trajectories of moving points using CBCT data.
• To assess the accuracy of the proposed method in tracking cranio-caudal and other motion components.
• To explore the potential applications of this technique in radiotherapy and motion surrogate calibration.

Main Methods:

• Utilizing CBCT projection data to identify extreme motion points (inhale/exhale) via ray tracing.
• Constructing a motion-bounding box and interpolating point positions between extreme coordinates.
• Calculating room coordinates as a function of time using view timestamps and interpolated positions.
• Validating CBCT-derived trajectories against known movements of a tungsten pin.

Main Results:

• CBCT-derived trajectories for a tungsten pin showed deviations of at most 1.06 mm from expected paths.
• Mean errors were reduced to less than 0.2 mm when accounting for imaging geometry deviations.
• Cranio-caudal position accuracy was insensitive to the choice of identification views.
• Optimal bounding box determination requires specific view separations (15-163 degrees), with inhale views of largest amplitude preferred.

Conclusions:

• The developed CBCT-based method accurately reconstructs 4D trajectories of moving points.
• This technique holds significant potential for calibrating motion surrogates and improving radiotherapy gating.
• The findings support the use of CBCT for precise motion management in radiation oncology, including 4D CBCT applications.