Monitoring respiratory motion using continuous wave Doppler radar in a near field multi antenna approach

To avoid motion artifacts in medical imaging or to minimize exposure of healthy tissues in radiation therapy medical devices are often synchronized with the patient´s respiration. Today´s respiratory motion monitors require additional effort in preparing the patient, such as mounting of a motion belt or the placement of an optical reflector on the patient breast, and they are not able to measure internal organ motion without implanting markers. An interesting alternative to assess the person´s respiratory motion is a continuous wave Doppler radar. By placing the antennas close to the body, the radar waves propagate into the body and are reflected on boundaries between body tissues, for example between muscle and adipose tissue or on the outline of organs. We designed, implemented and evaluated such a system operating at 869 MHz. To avoid the requirement for exact positioning of the persons, an array of one receive and four transmitting antennas is used. The hardware and the signal processing software were tested with 10 test persons. As a reference the respiratory signal of a state-of-the-art external respiratory gating system was recorded simultaneously. Comparing the radar data with the signals from the external respiratory motion monitor shows a very good correlation regarding the prediction of respiratory motion phases (p = 0.917). Our concept of using an array of transmit antennas turned out to be widely insensitive to the positioning of the test persons. A time shift between the respiratory motion curves recorded with the radar system and the motion curves from the external respiratory monitor was observed which indicates a slight difference between internal organ motion and motion detected by the external respiratory monitor.