Feasibility of a Low-Cost Optical Monitoring System for Physical-Dimension Breath-Hold Threshold Determination In CT Simulation
Abstract
Purpose
To develop a cost-effective, open-source system for patient-specific voluntary breath-hold (VBH) during CT simulation. The goal is to provide accurate gating thresholds for LINAC systems, such as Varian Truebeam Real-time Position Management (RPM) and auto-beam-hold features, without requiring redundant proprietary hardware in the simulation suite.
Methods
The system includes hardware and software. Hardware is comprised of a rigid tracking box with four 16mm ArUco markers and a consumer-grade webcam with an optical zoom lens at 30 FPS. Software components use a vector projection algorithm to overcome the depth uncertainty and perspective scaling errors inherent in single-camera imaging, resolving the box's 3D orientation and projecting motion onto its local Anterior-Posterior (AP) axis. This isolates true physiological motion and converts tracking data into absolute physical units (mm). Breath-hold stability is detected when the signal standard deviation over a 3-second window drops below 0.5 mm, at which point the system logs the stable plateau height and duration. Safety features include visual warnings for marker occlusion and manual threshold overrides.
Results
The system’s accuracy was validated by measuring breathing amplitudes at varying camera distances (100 to 160 cm). Despite the single-lens setup, the AP-projected amplitude measurement remained invariant (<1 mm deviation) across all distances. Stationary testing demonstrated negligible baseline drift (<0.4 mm) over 5 minutes. The 10 Hz data logs confirmed sufficient resolution for retrospective analysis.
Conclusion
This platform provides an accurate and safe method for establishing VBH baselines in real physical dimensions during CT simulation. By utilizing vector projection to eliminate single-camera depth uncertainty, it generates the necessary parameters for LINAC auto-beam-hold features without expensive commercial surrogates. This allows clinical teams to effectively characterize patient breath-hold capability prior to treatment planning and delivery.