Time of flight camera (TOF) for contact-less and marker-less 3D respiratory motion detection

Objectives External detector systems currently used for respiratory motion tracking require contact with the patient. A TOF camera (Swiss Ranger SR4000) providing 3D surface images without any patient contact has been evaluated for respiratory motion synchronization.

Methods A TOF camera yields 3D surface maps (acquisition frequency of >30Hz), where each point of the map corresponds to the Euclidean distance between the camera and the object. A moving phantom was build based on a plane in plywood (35cm x 45cm) attached on a moving platform. Measures were made under static and moving conditions (pseudo-sinusoidal motion of 13mm and 25mm in amplitude). The TOF camera was mounted at 1m and at different angles (90° and 45°) with respect to the moving surface plane. Finally, measurements were performed on a normal volunteer in comparison to the use of a pressure gauge based belt. A total of 200 frames (30frames/second) were acquired for each measurement.

Results The error (at the level of the pixel) was 1.5mm and 2.5mm obtained with the camera at 90° and 45° with respect to the moving plane. Considering a moving averaging of 2 subsequent frames, the accuracy improved to 1.2mm and 2.1mm at 90° and 45° respectively. This error was constant considering static or moving target acquisitions and independent of the motion magnitude. The normal volunteer measurements showed a high level of accuracy at the level of the pixel and perfect agreement considering the regional movement captured by the pressure belt.

Conclusions A TOF camera provides an accurate 3D respiratory motion measure without any contact with the patient. The depth-resolution is sufficient to detect with high precision the respiratory signal at the level of the pixel.