The “Air-bag System”: A Novel Respiratory Monitoring Device Collaborated with RPM System

Abstract

Respiratory motion is an issue that is becoming increasingly important in the era of image-guided radiotherapy. The Real-Time Position Management System (RPM, Varian Medical System, Palo Alto, CA) is commercially available gating equipment which is popular around the world. This system uses a plastic box with a reflective marker which is placed on the patient's abdomen to detect the vertical motion by an infrared camera. However, this method with such a single point detector has several limitations to obtain the accurate respiratory motion. We developed the novel substitution device for this plastic box of the RPM system to monitor the patient's body volume change as an external respiration signal. The “Air-bag System”, a novel respiratory monitoring device, consists of a non-elastic air bag connected with an elastic air bag. A non-elastic air bag is placed between the patient's body and the stereotactic fixation frame. A current meter located between two bags measures the air flows back and forth between two bags. The data are processed by personal computer and translated the respiratory motion into swinging decoy markers displayed on the computer screen so that an infrared camera of RPM system can capture the swinging marker instead of an infrared reflective marker. 4D-CT data of 15 lung cancer patients were acquired by the RPM system with the “Air-bag System”. On each phase of the 4D data, correlations between the lung volume and the body volume were evaluated. 4D-CT data of another 3 lung cancer patients were acquired by the RPM system recording with two different respiratory motions. One is an abdominal respiratory motion using an infrared reflective marker placed on the patient's abdomen, the other one is a thorax respiratory volume shift using the air bag set on the patient's chest. The respiratory phases (8 phases) at each instant in time based on modeling two different motion amplitudes were compared. There was a strong correlation (r2 = 0.976) between the lung volume and the body volume on each respiratory phase. Comparing the reconstructed 4D-CT images on each respiratory phase recorded by using an infrared reflective marker to those recorded by using the “Air-bag System” resulted in no difference on all respiratory phases. The novel respiratory gating device which monitors the patient's body volume as an external respiration signal was developed. Recording with this “Air-bag System”, applicability of RPM system is not limited owing to enough amplitude and more accurate determination of the magnitude of respiratory motion. Moreover, abdominal compression with the air bag reduces the movement of diaphragm which can decrease the intra-thoracic organ motion without loading additional stress on the patient.