Remote physiological sensing or monitoring

Remote sensing with RADAR, LIDAR or SONAR

Sensing is the detection of physical, chemical or biological quantities. The devices performing this function are called sensors and are distinguished in different types. For instance, we have electric field sensors, magnetic field sensors and electromagnetic field or light sensors such as those of a camera. 

Sensing is distinguished in active and passive. When we take a picture with our camera, we may use a flash or not. When a flash is not used, the sensor registers the ambient light as it is reflected from the objects. When a flash is used, we send light towards the object, or in other words, we illuminate the object. A certain amount of the sent light is reflected towards the sensor and is registered by it. When we illuminate an object, we conduct active sensing as opposed to passive.

When the detection is conducted from a distance, we refer to remote sensing. A well-known remote sensing technique uses radio waves to illuminate an object and to detect its presence and its range or distance; it is termed “Radio Detection and Ranging” and is best known by the acronym "RADAR" (Figure 1). It was originally used to detect ships and their range in order to avoid collisions. A similar technique, “Light Detection and Ranging” (LIDAR) uses lasers, while “Sound Detection and Ranging” (SONAR) uses sound waves.

In the simplest RADAR form, in order to detect an object and determine its range, a transmitter emits an electromagnetic wave of a certain radiofrequency which travels a distance equal to its range before reaching the object (Figure 1). The wave is then reflected by the object and travels back the same distance before reaching the RADAR receiver. The velocity of electromagnetic waves is known: they travel a distance of 300.000 Km per one second. Therefore, if we measure the time required for a wave round-trip (time-of-flight) corresponding to a distance which is the double of the range, we can determine the range of the object. For instance, if the time-of-flight is 0.001 second (1 ms), the range is 150 Km. 

Figure 1: On the left, a RADAR emits an electromagnetic pulse which allows to calculate the range (distance) of an object. On the right, an object moving towards the RADAR pushes back the waves, leading to an increased frequency at reception.

Two types of RADAR that are well-known are: (a) the police RADAR guns, which detect the speed of vehicles, and (b) the weather RADAR which detect the speed of rain droplets, among other (Figure 1, left). In this context, when a car moves towards the RADAR, the waves emitted by the transmitter are reflected while they are being pushed back by the car (e.g. the car windshield) towards the RADAR. As a result, they arrive in more frequent intervals towards the receiver (Figure 1, right). In other words, the receiver registers an increased electromagnetic wave frequency or a higher frequency. The observed frequency shift is known as the Doppler shift, from the name of the scientist who postulated it. By analyzing this shift, it is possible to calculate the speed.

Physiological sensing including remote techniques

Physiological sensing or monitoring is defined as the measurement of physiological parameters, such as the vital signs which include body temperature, blood pressure, heart rate (pulse), and respiratory rate. It can be classified based on the measured parameter, in thermal, hemodynamic, cardiorespiratory and also neural sensing or monitoring, among other.

Physiological sensing may include non-contact or remote techniques which use RADAR or LIDAR systems, that is radio wave detection or laser detection. 

Remote cardiorespiratory sensing with RADAR and LIDAR

Principle of operation

There has been significant progress in remote cardiorespiratory sensing using RADAR systems, including Doppler, as described in the reference text book "Doppler Radar Physiological Sensing" [Wiley] (Figure 3). During respiration, which consists of inhalation and exhalation, the lungs are first filled with air and then they are emptied. As a result, the thorax expands and contracts and thereby moves back and forth by marking a displacement of a few millimeters to a few centimeters, depending on the person (Figure 2). Electromagnetic waves sent by a RADAR towards the thorax will be reflected and returned towards the RADAR receiver. By analyzing the return signal, it is possible to measure the miniscule changes in the distance of the thorax from the RADAR and therefore capture the thorax movement due to respiration, as shown in Figure 2. Even the micro-movements of the thorax due to the heartbeat can be registered as dents in the electromagnetic signal, thus providing a subtle uneven wavy pattern as shown in Figure 2 (blue wavy line).

Figure 2. Left: Illustration of the breathing cycle as measured by a RADAR placed in front of a person. R0 is the static distance to the radar, T is the period of a one respiration cycle and AB the peak-to-peak amplitude of the thorax expansion (Trange A, 2021 p.24). Right: The top graphs plots the radiofrequency (RF) signal reflected off the thorax of a person. The envelop of the RF signals follows the inhale-exhale motion, while small dents are due to the heartbeat. The bottom graph plots the ECG of the subject measured concurrently with the RF signal. The numbers report the beat-length in seconds. Note the small variations in consecutive beat lengths (modified from the MIT EQ-Radio publication [Zhao M. et al 2018]).

The MIT “Vital-Radio” and “EQ-Radio”

The approach was made popular initially in 2015 by the MIT "Vital-Radio" [CSAIL], [New Scientist], [TechXplore] which monitored with RADAR the vital signs of respiratory and cardiac rate. In 2016, the same MIT team presented the “EQ-Radio" which processed the RADAR cardiorespiratory measurements with an algorithm for emotion inference and could therefore determine, as mentioned, if a person was excited, happy, angry or sad [MIT-EQ-Radio]. A demonstration of the “Vital-Radio” system has been provided by the lead MIT researcher [TED Talk] (Figure 3). The device has been used for monitoring of COVID19 patients [MIT CSAIL]. Microsoft Research has also shown interest in physiological sensing [Microsoft].

Figure 3: Reference textbook for Doppler RADAR physiological sensing on the left and demonstration of cardiorespiratory modelling on the right. 

The Georgia Tech RADAR vital sign monitor (RVSM) used in the 1996 Atlanta Olympics

A most notable prior development in the field of RADAR cardiorespiratory sensing has been provided by the Georgia Tech Research Institute (GRTI). Researchers at the GRTI had developed a RADAR vital sign monitor (RVSM) in a form of a parabolic antenna dish which was used for monitoring of Olympic athletes at the 1996 Atlanta Olympics (Figure 4). The system could measure heartbeat at 10 meters and respiration rate at 20 meters [Greneker et al 1998], while it has been reported [Droitcour A.D. 2006] that at 100 meters, the limit was moving background clutter and not the system sensitivity. A spin-off system termed the “RADAR flashlight” has been used for through-the-wall detection of humans [Greneker et al 1998], [Science Daily], [NCJRS]. The same system reportedly could be used: (a) for biometric identification of personnel of secured environments based on the heartbeat signature or (b) for determining the stress level being experienced by an individual on the basis of respiration and heartbeat rates [Greneker E. 1997].

Figure 4: GTRI Radar vital sign monitor for Olympic athletes and RADAR cardiogram taken at 15 feet (4.5 meters)[Greneker E. 2006]. 

The LIDAR cardiac biometric sensor of the U.S. military

A similar LIDAR system of the U.S. military performing remote cardiac sensing and biometrics was described by the MIT Tech Review in 2019 [MIT Tech Review]. This is an infrared laser sensor, termed the Jetson prototype, which detects the unique cardiac signature of a person at a distance of 200 meters. A coin-size laser spot is directed on a subject in order to measure the surface movement caused by the heartbeat. This constitutes a unique feature allowing biometric identification of a person. As a duration of about 30 seconds is required at present in order to get a good return, the sensor can be effective when the subject is sitting or standing. It can work through typical clothing (shirt or jacket) but not thicker clothing such as a winter coat. The sensor is based on the laser vibrometry technique and is adapted from an off-the-shelf device that is used to check vibration from a distance in structures such as wind turbines. The sensor has been developed for the Pentagon after the request of the U.S. Special Forces.