Remote Cardiorespiratory Monitoring (Sensing) with RADAR and LIDAR

 

 

Introduction

 

Remote or contactless monitoring of heart rate and respiratory rate, termed remote cardiorespiratory monitoring, can be performed with Doppler RADAR or other RADAR techniques as well as with Doppler LIDAR. It can be used in patient care in hospitals or at home in order to monitor conditions such as sleep apnea or to calculate energy expenditure. Using specific algorithms it can be used to infer mood state. Also, the Pentagon is reported to have a Doppler LIDAR that can detect heart rate signatures at a distance of 200 meters.

 

Remote cardiorespiratory monitoring consists of the transmission of an electromagnetic signal which is reflected off the thorax wall and is received with the purpose of analysis via demodulation. During respiration, the thoracic wall performs a periodic motion which ranges from 4 mm to 12 mm peak-to-peak in adults, while due to the heartbeat the peak-to-peak motion is approximately 0.5 mm. The reflected and received signal is similar to the transmitted signal with its phase modulated by the position of the thorax. In other words, the reflected signal is modulated by the thoracic motion.

 

 

Remote Cardiorespiratory Monitoring (Sensing) with RADAR 

 

Continuous wave (CW) (unmodulated) microwave Doppler RADAR 

 

Remote cardiorespiratory monitoring can be performed with Doppler RADAR. Use of continuous wave (CW) unmodulated microwave Doppler RADAR at 2.4 GHz and 1.6 GHz is reported by Droitcour A.D. (Droitcour A.D. 2006 dissertation - Stanford) to detect accurately the heart beat from a distance of 1 m and the respiratory rate from a distance of 1.5 m, as proven by measurements from 22 patients using standard techniques as control (thoracic impedance and inductive plethysmography) (Droitcour A.D. et al 2015). Instead of using bulky equipment, a chip Doppler RADAR transceiver (transmitter and receiver) is created, which functions as a heart rate and respiration rate sensor/monitor (Droitcour A.D. et al 2009).

 

 

 

Figure 1: From Droitcour A.D. 2006 (dissertation) Continuous wave Doppler RADAR for cardiorespiratory monitoring (cf. legend above).

 

 

Additionally, a Doppler-RADAR phased array approach with beamforming architecture to generate two simultaneous beams has been used (Nosrati 2019).

 

An informal analogy to the Doppler RADAR would be an automatic tennis ball machine which throws balls onto a wall with a velocity of one meter per second. If the ball bounces off the wall and return to us in two seconds, we can infer that it travelled for a distance of two meters towards the wall and back in two seconds and therefore that the wall is at a range/position of one meter. If the wall started to move towards us after the return of the first ball, then we would receive the first ball in two seconds, the next in less than two seconds and the next in even shorter than that time. In general, we would receive the balls back more frequently. By analyzing the shift in frequency, we can determine the motion and position of the wall.

 


 

Continuous wave (CW) frequency modulated (FM) microwave RADAR (FMCW)

 

The “Vital-Radio” by the Katabi MIT lab uses continuous wave (CW) frequency modulated (FM) microwave RADAR (FMCW) (5.46 GHz to 7.25 GHz) to measure respiratory rate and heart rate from different subjects from a distance of 8 meters or from another room (Adib et al 2015, http://witrack.csail.mit.edu/vitalradio/, https://youtu.be/MH6vA1fkUgY). It is noted that this approach does not use the Doppler effect. It was suggested that the system could be incorporated in a Wi-Fi router in order to be used for vital sign monitoring in a residency (“Smart Homes that Monitor Breathing and Heart Rate” by Adib et al 2015). An additional reference for the technique can be found at this weblink.

 

The same lab for their “EQ-Radio” project processed the FMCW microwave RADAR data from thorax reflection with an algorithm that predicts with accuracy the heartbeat from the relevant small amplitude RADAR signal (heartbeat segmentation algorithm) and used both the respiration and the heartbeat data to compute emotion-dependent features and to infer emotional state (Zhao et al 2016, http://news.mit.edu/2016/detecting-emotions-with-wireless-signals-0920). “EQ-Radio” uses a 2D emotion model whose axes are valence and arousal and classifies between four basic emotional states (in accordance with literature on categorizing human emotions): sadness (negative valence and negative arousal), anger (negative valence and positive arousal), pleasure (positive valence and negative arousal), and joy (positive valence and positive arousal).

 

 

 

Figure 2: The "EQ-radio" by the Katabi MIT lab uses FMCW RADAR (cf. horn antennas) for cardiorespiratory monitoring and detection of emotional state (screen capture from https://youtu.be/nmcDnEhZTJM

 

 

 

 

Figure 3: Facebook post by MIT News presenting the "EQ-radio" (https://www.facebook.com/MITnews/posts/1176795985697364)

 

 

 

Researchers at the Georgia Tech Research Institute (GRTI) created several versions of a RADAR vital sign monitor (RVSM) (homodyne/FMCW) among which one was used for Olympic athletes at the 1996 Altlanta Olympics with the purpose of measuring heartbeat at 10 meters and respiration rate at 20 meters (Greneker at al 1997). The same system was studied for biometric identification based on the heartbeat signature and also for stress determination.

 

It has been mentioned (Droitcour A.D. 2006) that at 100m, the limit was moving background clutter and not the system sensitivity.

 

A spin-off of these RVSP systems termed “RADAR Flashlight” was used for different applications, including security and law enforcement (ref.1),(ref.2),(ref.3)(ref.4).

 

 

Figure 4: GTRI Radar Vital Sign Monitor for Olympic Athletes and RADAR Cardiogram taken at 15 Feet (From Greneker 2006).

 


 

Ultrawide-band RADAR (UWB)

 

A large number of studies for remote cardiorespiratory monitoring use Ultrawide-band RADAR (UWB) (Liang et al 2018). It is reported that the use of circularly polarized waves with UWB RADAR provides improved accuracy (Chan et al 2013). It is noted that UWB radar has been successfully combined with MRI for medical navigation/guidance purposes (Thiel et al 2010), (ref).

 

 

Pulsed RADAR - Wi-Fi


For completeness purposes, it is mentioned that pulsed RADAR represented by Wi-Fi which is a pulsed 2.4 GHz signal, has been used for motion detection (Adib et al 2013), (Pu et al 2013).

 

 

 

Remote Cardiorespiratory Monitoring (Sensing) with LIDAR

 

 

A laser monitor has been used in a approach that would be similar to LIDAR (Kondo et al 1997).

 

In June 2019, the MIT Tech review reported that the Pentagon (the Combatting Terrorism Technical Support Office or CTTSO of the U.S. Special Forces) has a laser for identification of cardiac signatures from a distance of 200 m and over typical clothing. The device called “Jetson” developed by Ideal Innovations Inc. uses an infrared laser to detect the movement of the body surface caused by the heartbeat using laser vibrometry. It is noted that the device was adapted from an off-the-shelf device for remote measurement of vibration for structures such as wind turbines. Adaptation included a special gimbal to stabilize the invisible laser spot (having the size of a quarter) on the remote target. A time interval of 30 seconds is currently required for signal return, therefore the subject should be either sitting or standing.

 

 

CTTCO reference citing "Jetson" (pdf - page 49) cited in Techspot article

 

Figure 1: MIT Tech Review post refering to Jetson 

https://www.facebook.com/technologyreview/posts/10157412456464798

 

 

 

Laser monitoring of chest wall displacement - GaAs laser

(~ 870 nm)

 

(Future studies for infant ventillation)

 

T. Kondo, T. Uhlig, P. Pemberton, and P.D. Sly, “Laser monitoring of chest wall displacement, “European Respiratory Journal, vol. 10, pp. 1865-1869, 1997 http://erj.ersjournals.com/content/10/8/1865.long

 

Excerpt from Amy Diane Droitcour’s thesis (2006):

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.84.7516&rep=rep1&type=pdf

 

In the study by T.Kondo et al “a laser sensor was used to measure anterioposterior chest wall motion. This is a non-contact measurement, offering no resistance to respiration and no tactile stimuli, which should ensure a noninvasive measurement of respiration that does not alter the respiratory pattern. The laser monitor measures the distance between the chest wall and the sensor, and obtains a respiratory waveform by plotting the change in distance over time. The laser monitor can track rapid changes in lung volume with almost no lag. They propose a monitor with multiple laser sensors so that they can monitor multiple points on the chest, and better model the volumes of respiration.”

 

Helpful resource on Lasers: http://www.explainthatstuff.com/lasers.html

 

Laser Therapy – Physiotherapy cf. http://www2.information-book.com/physics/lasers/

 

@infobook tweet

 

 

 

 

 

Calculating tidal volume (volume of air displaced during respiration) by magnetometers measuring displacement of body surfaces (sternum, abdomen etc)

 

Excerpt from Amy Diane Droitcour’s thesis (2006):

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.84.7516&rep=rep1&type=pdf

 

“In magnetometer measurements, one coil is driven by an oscillator to produce a weak magnetic field, while other coils are attached to the skin on the thorax and abdomen. The coils on the skin pick up the magnetic field and can determine their position in the field. Magnetometers are susceptible to rotational movement, which creates artifacts [98]. Magnetometers were used to measure the anterio-posterior motion of the rib cage and abdomen [98], and to measure displacement between the abdomen and the sternum [110]. In [110], two transmitter coils operating at two different frequencies are placed near the spine at the sternal level and on the abdomen. Two receiving coils are also placed on the body: one tuned to both frequencies is placed on the sternum to measure the sternal-umbilical displacement and the rib cage anterio-posterior displacement. The other is tuned only to the frequency of the abdominal transmitter and measures the anterior-posterior abdominal displacement. With these three measurements, after calibration, respiratory volume can be estimated using a three-degree-of-freedom model.”

 

References below  (both available upon subscription)

[98] J Med Eng Technol. 1983 Sep-Oct;7(5):217-23.

Using body surface movements to study breathing.

Gribbin HR.

http://www.tandfonline.com/doi/abs/10.3109/03091908309032586?journalCode=ijmt20

 

[110] Chest. 2002 Aug;122(2):684-91.

Tidal volume and respiratory timing derived from a portable ventilation monitor.

McCool FD1, Wang JEbi KL.

http://www.sciencedirect.com/science/article/pii/S0012369215514074

 

Excerpts from [110]: The device consists  of  a  flowmeter,  transmitter,  and receiver  circuitry. Schematic of the magnetometer device depicting coil placement is shown below. Two transmitter coils (yellow and black) oscillate at 8.97 KHz  and  7.0  KHz,  respectively.  The  red  receiver  coil  is tuned to both frequencies, and the green receiver coil is tuned only to 7.0 KHz. The received signals are processed as three channels by three detection circuits. Dual functionality of the second receiver coil  eliminates  the  need  for  a  third  pair  of  coils,  thereby simplifying design and decreasing power needs.

 

 

 

 

Figure (modified) from reference [110] mentioned above.