Development of a real-time multi-scenario monitoring method for respiratory motion based on capacitive sensors

This study aimed to improve the accuracy of the tumor radiotherapy dose and reduce the irradiation dose to the surrounding organs, by developing a versatile respiratory motion monitoring system with multi-scenario applications to enhance the clinical efficacy of radiotherapy for thoracic and abdominal tumors.

The respiratory motion monitoring system comprised an airbag structure integrated with a parallel-plate capacitive sensor. The airbag comprised a polymer balloon encapsulated in a nonelastic flexible membrane, and had a projected area of 120 mm × 120 mm. The capacitive sensor adopts a concentric configuration of beryllium copper (thickness: 0.1 mm, diameter: 10 mm) and oxygen-free copper (thickness: 1 mm, diameter: 10 mm) foils. Real-time monitoring of the respiratory motion was achieved by detecting the capacitance variations corresponding to changes in the internal air pressure within the airbag. The system performance was rigorously evaluated using a dynamic thorax phantom capable of simulating various patterns, including sinusoidal and fourth-power cosine waveforms.

The capacitive sensor-based respiratory motion monitoring system demonstrated a displacement measurement range of up to 10 mm, capable of detecting movements as small as 0.1 mm (signal-to-noise ratio: 2.18). Real-time displacement conversion was performed using the fitted model y = ax+bx ³ (a = 26.105 ± 0.398, b = 285.868 ± 22.147). The system exhibited a high stability, with a standard deviation of only 0.0011 in capacitance measurements over 10 repeated tests with a 5 mm amplitude sinusoidal waveform, and further maintained the amplitude consistency within 2.25%-96.05% of the maximum value throughout 10 min of continuous operation. A reliable performance was confirmed across various respiratory waveforms, including sinusoidal and fourth-power cosine profiles. Furthermore, the non-metallic airbag structure enhances the adaptability to multiple clinical scenarios. However, two types of signal distortions were observed, originating from the airbag deformation and the limitations of the capacitive sensor electrode, both of which are thoroughly explained in the manuscript.

Overall, this study developed a capacitive sensor and airbag-based respiratory monitoring system that combines the advantages of abdominal pressure belts with infrared-based monitoring technologies. This integrated approach offers a cost-effective, structurally simple, and versatile solution for monitoring respiratory motion across multiple clinical scenarios.