Medical applications of shortwave FM radar: remote monitoring of cardiac and respiratory motion

Abstract

Purpose: This article introduces the use of low power continuous wave frequency modulated radar for medical applications, specifically for remote monitoring of vital signs in patients.

Methods: Gigahertz frequency radar measures the electromagnetic wave signal reflected from the surface of a human body and from tissue boundaries. Time series analysis of the measured signal provides simultaneous information on range, size, and reflective properties of multiple targets in the field of view of the radar. This information is used to extract the respiratory and cardiac rates of the patient in real time.

Results: The results from several preliminary human subject experiments are provided. The heart and respiration rate frequencies extracted from the radar signal match those measured independently for all the experiments, including a case when additional targets are simultaneously resolved in the field of view and a case when only the patient's extremity is visible to the radar antennas.

Conclusions: Micropower continuous wave FM radar is a reliable, robust, inexpensive, and harmless tool for real-time monitoring of the cardiac and respiratory rates. Additionally, it opens a range of new and exciting opportunities in diagnostic and critical care medicine. Differences between the presented approach and other types of radars used for biomedical applications are discussed.

Figure 1

Frequency modulation function. The solid line denotes the instantaneous frequency of the emitted signal 1 and the dashed line that of a signal reflected by a target with time of flight τ.

Figure 2

Setup of the human subject experiments. (a) Measuring (constant) breathing and cardiac rates from signal reflected by the patient’s chest. (b) Measuring two targets simultaneously: A small ceramic pendulum is placed halfway between the radar and the subject. (c) Monitoring uneven respiration using signal reflected by the patient’s hand. The rest of the patient’s body is shielded by a 1 mm fine copper mesh screen.

Figure 3

Amplitude and phase shift of effective radar cross section of a human subject, proportional units. Left: time-domain plots. Right: Frequency-domain plots. (a) Respiration-filtered signal. 0.25 Hz peaks in both amplitude and phase frequency curves correspond to 15 breaths per minute. (b) Cardiac-filtered radar data. Frequency peaks at 1.3 Hz correspond to the heart rate of 78 bpm. Note the shorter time-domain display window: 0–4 s.

Figure 4

All plots use arbitrary units. (a) MFMR used to measure the patient’s vital signs and the oscillation frequency of a pendulum. Top: Denoised pendulum signal, A₁₀ (frequencies above 6 Hz cut off). Bottom: Human subject signal A₂₀ filtered to amplify respiration (0.01–0.6 Hz band filter) and cardiac motion (0.6–6 Hz band filter). (b) Radar reflection off the subject’s hand vs pressure belt signal. Coached breathing pattern: five deep breaths at 5 s∕cycle, a 18 s breathhold, six breaths at 3 s∕cycle. The signals derived from the radar and from the respiration monitor belt clearly mirror each other.