Applications of a Capacitor-Based Respiratory Position Sensing Device: Implications for Radiation Therapy

Research Article

Austin J Med Oncol. 2014;1(2): 4.

Applications of a Capacitor-Based Respiratory Position Sensing Device: Implications for Radiation Therapy

Weng Y1, Westover MB2, Speier C3, Sharp G3, Bianchi MT2 and Westover KD4*

1UT Southwestern Medical School, Dallas, TX, USA

2Department of Neurology, Massachusetts General Hospital, Boston, MA, USA

3Division of Radiation Physics, Department of Radiation Oncology, Massachustets General Hospital, Boston, MA, USA

4Department of Radiation Oncology, UT Southwestern Medical School, Dallas, TX, USA

*Corresponding author: Westover, KD, UT Southwestern Medical Center, 5323 Harry Hines Blvd, L4.268, Dallas, TX, USA, Tel: 75390-9038

Received: August 26, 2014; Accepted: October 22, 2014; Published: October 24, 2014


Respiratory motion may significantly affect the outcome in a number of medical imaging techniques and some radiation therapy applications. 4-dimensional computed tomography (4DCT) and respiratory gating technology, which account for the dynamics of respiration, are expensive and often unavailable in smaller radiation treatment centers. Here we evaluate the ability of an inexpensive, technology comprised of two capacitors placed next to the skin to provide real-time respiratory phase information. Three subjects were simultaneously monitored by the new capacitor-based device (CBD) and a commercially available Real time Position Management (RPM) system by Varian. All respiratory phases detected by the RPM system were also detected by the CBD. Automatically detected peaks were not significantly different in timing when comparing RPM and CBD-derived respiratory amplitudes. The anatomic locations of the CBD were varied to evaluate the change in signal quality across the abdomen and thorax. CBD signals were reliable on the abdomen and lower thorax but degraded when recorded from the upper thorax. We also used computed tomography (CT) to assess the imaging characteristics of CBD and found that there were minimal artifacts. We therefore conclude that CBD respiratory amplitude measurements may be useful for tracking respiratory movements as part of a number of advanced radiation therapy technologies including 4DCT image resorting, adaptive radiation therapy and gated radiation therapy.

Keywords: Respiratory gating; 4DCT; RPM


Measuring respiratory phase is important in several types of medical imaging, especially in circumstances where respiratory motion can degrade image quality, or when treatment optimization requires real-time knowledge of respiratory phase. Examples include 4-dimensional computed tomography (4DCT), gated positron emission tomography (Gated PET) and several MRI-based methods [1–4]. Respiratory phase information is also commonly used when delivering gated therapeutic radiation to minimize radiation treatment volumes and enable dose escalation [5,6].

Several systems have been developed to acquire respiratory information including the real time position management (RPM) system and Vision RT system [7,8]. The widely used RPM system utilizes an infrared marker affixed to the supine patient’s abdomen which rises and falls with respiration. The marker is detected by a camera mounted near the patient’s feet and respiratory amplitudes are extracted in real time by automated image processing algorithms. The Vision RT system consists of a camera mounted above the patient couch and relies on extraction of visual cues from rapid sequence imaging of the patient’s thorax surface to construct respiratory amplitudes. In both of these systems, tumor motion observed in rapid sequence imaging has been shown to correlate with respiratory output from these systems [7,9]. Thus, radiation oncologists routinely use real-time respiratory effort information when designing radiation plans that are expected to be impacted by respiration.

There is also now interest in using multiple respiratory signal inputs to improve tumor motion or deformation models. The RPM system is not designed to provide multiple inputs. The Vision RT system may be used to provide multiple respiratory signals but is complex, relying on real time image processing. Capacitor-based technology has long been used to sense changes in position and to obtain motion information because physical stresses on a capacitor alter its capacitance due to geometric changes. In this study, we employed a capacitor-based medical device (CBD) to monitor the patient’s chest wall position during respiration. Two versions of the CBD were used: 1) a lycra shirt with two small, adjacent capacitors supported by a printed vinyl layer (Figure 1a); 2) the capacitors were adapted to fit on an adhesive bandage allowing for placement of the sensor on multiple locations of the body (Figure 1b). The change of capacitance over time has been shown to correlate with the respiratory cycle. The advantage that CBD has over the two existing methods discussed previously is that it is inexpensive and utilizes an electronic readout, which is computationally inexpensive to analyze. This enables multiple respiratory signals monitoring via the strategic and simultaneous placing of CBD sensors in different anatomic locations.