METHOD AND APPARATUS FOR VITAL SIGNS MEASUREMENT

• US 20180153455A1
• Filed: 06/02/2016
• Published: 06/07/2018
• Est. Priority Date: 06/05/2015
• Status: Active Grant

• Current Assignee: Oxehealth Limited
• Original Assignee: Oxford University Innovation Limited (University of Oxford)
• Inventors: GUAZZI, Alessandro, MONTOYA, Mauricio Villarroel, TARASSENKO, Lionel
• Granted Patent: US 10,939,852 B2
• Time in Patent Office: Days
• CPC Class Codes: A61B 5/0077 Devices for viewing the surface of the body, e.g. camera, magnifying lens
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/0816 Measuring devices for examining respiratory frequency measuring frequency of electric signals G01R23/00
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7221 Determining signal validity, reliability or quality A61B5/7207 and A61B5/7217 take precedence
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: G06T 2207/30076 Plethysmography
• CPC Class Codes: G06T 2207/30104 Vascular flow; Blood flow; Perfusion
• CPC Class Codes: G06T 7/0012 Biomedical image inspection

Abstract

A method of monitoring changes in oxygen saturation of a subject by analysing a three colour channel video image of the exposed skin of the subject. Within each colour channel a normalised signal obtained by dividing the intensity signal by its mean value, and the normalised signals are averaged across plural regions of interest within the exposed skin area image of the subject. Regions of interest are selected on the basis of the signal-to-noise ratios for the heart rate and breathing rate components. A single representative waveform for each colour channel is obtained by signal averaging and the ratio of the amplitudes of the representative waveforms from two different colour channels, e.g. blue and red, is taken. The changes in the ratio of amplitudes is output as a measure of changes in blood oxygen saturation.

Claims

1. A method of determining changes in blood oxygen saturation of a subject comprising the steps of:

obtaining a video image of an area of exposed skin of the subject, the video image comprising signals representing intensity in at least two different colour channels;

defining in the image a plurality of regions of interest in said area of exposed skin of the subject;

determining a signal to noise ratio of a heart rate or breathing rate frequency component of one of the colour channel signals or both for each region of interest;

determining whether to reject or not reject regions of interest based on the determined signal to noise ratio of a heart rate or breathing rate frequency component or both;

processing the colour channel signals from non-rejected regions of interest by;

normalising the signal in each of the colour channels by dividing each of the signals by its baseline component;

determining the ratio of the amplitudes of the normalised signals from two of the colour channel signals; and

outputting changes over time in the ratio as representing changes in blood oxygen saturation of the subject.

2. The method according to claim 1 wherein the baseline component is the average value of the signal over a predetermined period.

3. The method according to claim 1 wherein the ratio of the amplitudes of the averaged normalised signals from blue and red colour channel signals are determined.

4. The method according to claim 1 further comprising the step of averaging each the normalised signals within each colour channel before calculating the ratio of amplitudes.

5. The method according to claim 4 wherein the step of averaging each of the normalised signals within each colour channel comprises averaging together the signals for that colour channel from each of a plurality of regions of interest within said area of skin of the subject.

6. The method according to claim 5 wherein in averaging each of the normalised signals within each colour channel the signals from each region of interest are weighted according to the strength of the signal to noise ratio for a heart rate frequency component of one of the colour channel signals from that region of interest.

7. The method according to claim 4 wherein the step of averaging each of the normalised signals within each colour channel comprises signal averaging to determine a representative waveform for a predetermined time period of said signal.

8. The method according to claim 7 wherein said signal averaging comprises detecting the times of peaks in one of the colour channel signals, then within each colour channel selecting sections of the normalised signal extending a predetermined time either side of the detected peak times and averaging together the selected sections of the normalised signal.

9. The method according to claim 1 wherein the area of exposed skin of the subject is selected by detecting areas in the image for which a signal to noise ratio function for the heart rate is maximised.

10. The method according to claim 1 wherein the regions of interest are formed by dividing the selected area into plural contiguous regions.

11. The method according to claim 1 wherein regions of interest are rejected if a signal to noise ratio for a heart rate frequency component of a colour channel signal from the region of interest is below a predetermined threshold.

12. The method according to claim 1 wherein regions of interest are rejected if a signal to noise ratio for a breathing rate frequency component of a colour channel signal from the region of interest is above a predetermined threshold.

13. The method according to claim 1 wherein regions of interest are rejected if the phase of a heart rate frequency component of a colour channel signal from the region of interest is outside a predetermined threshold of the phase of a heart rate frequency component of the colour channel signal averaged over a plurality of the regions of interest.

14. The method according to claim 1 wherein the signal in each colour channel is windowed into temporal windows, and the processing steps of normalising and determining are performed for each time window to determine and output a ratio for each time window.

15. A system for determining changes in blood oxygen saturation of a subject comprising:

a video camera for obtaining a video image of an area of exposed skin of the subject, the video image comprising signals representing intensity in at least two different colour channels;

a signal processor adapted to receive the signals from the video camera and process them by;

normalising the signal in each of the colour channels by dividing each of the signals by its baseline component;

defining in the image a plurality of regions of interest in said area of exposed skin of the subject;

determining a signal to noise ratio of a heart rate or breathing rate frequency component of one of the colour channel signals or both for each region of interest;

determining whether to reject or not reject regions of interest based on the determined signal to noise ratio of a heart rate or breathing rate frequency component or both;

determining the ratio of the amplitudes of the normalised signals from two of the colour channel signals for non-rejected regions of interest; and

outputting changes over time in the ratio as representing changes in blood oxygen saturation of the subject; and

a display adapted to display the changes over time in the ratio.