METHOD AND SYSTEM FOR SIGNAL ANALYSIS

• US 20150379370A1
• Filed: 01/28/2014
• Published: 12/31/2015
• Est. Priority Date: 02/12/2013
• Status: Active Grant

• Current Assignee: Oxehealth Limited
• Original Assignee: Isis Innovation Limited
• Inventors: CLIFTON, David Andrew, MONTOYA, Mauricio Christian Villarroel, TARASSENKO, Lionel
• Granted Patent: US 9,659,229 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: A61B 2503/40 Animals
• CPC Class Codes: A61B 2576/00 Medical imaging apparatus involving image processing or analysis A61B1/00009, A61B6/52 and A61B8/52 take precedence; image processing per se G06T; details of algorithms for analysing medical images G06T7/0012
• CPC Class Codes: A61B 5/0013 Medical image data A61B1/00011, A61B6/56, A61B8/56 take precedence
• CPC Class Codes: A61B 5/0022 Monitoring a patient using a global network, e.g. telephone networks, internet
• CPC Class Codes: A61B 5/0075 by spectroscopy, i.e. measuring spectra, e.g. Raman spectroscopy, infrared absorption spectroscopy A61B5/0071 takes precedence
• CPC Class Codes: A61B 5/0077 Devices for viewing the surface of the body, e.g. camera, magnifying lens
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/02427 Details of sensor
• 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 5/743 Displaying an image simultaneously with additional graphical information, e.g. symbols, charts, function plots
• CPC Class Codes: G06T 11/001 Texturing; Colouring; Generation of texture or colour inpainting G06T5/77
• CPC Class Codes: G06T 11/206 Drawing of charts or graphs
• CPC Class Codes: G06T 2207/10016 Video; Image sequence
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 2207/20021 Dividing image into blocks, subimages or windows
• CPC Class Codes: G06T 2207/30076 Plethysmography
• CPC Class Codes: G06T 2207/30088 Skin; Dermal
• CPC Class Codes: G06T 2207/30104 : Vascular flow; Blood flow; Perfusion
• CPC Class Codes: G06T 2207/30201 : Face
• CPC Class Codes: G06T 7/0012 : Biomedical image inspection
• CPC Class Codes: G06T 7/10 : Segmentation; Edge detection motion-based segmentation G06T7/215
• CPC Class Codes: G06T 7/90 : Determination of colour characteristics
• CPC Class Codes: G16H 30/40 : for processing medical images, e.g. editing

Abstract

An image of a human, animal or machine subject, is analysed to detect regions which include strong periodic intensity variations, such as a photoplethysmogram (PPG) signal in a human or animal, or some periodic vibration in a machine. The image is divided into plural regions of fixed order is fitted to a representative intensity signal for that region. The poles of the fitted autoregressive model are thresholded by magnitude to select only the pole or poles with a magnitude greater than the threshold. The pole magnitude therefore acts as a signal quality index. The dominant pole is representative of the strongest periodic information and the frequency of that spectral component can be derived from the phase angle of the pole. The image may be redisplayed with image attributes, e.g. colour-coding, according to the pole magnitude in each region of interest and/or the dominant pole phase angle in each region of interest. In the case of a PPG image signal this can give maps of heart rate and breathing rate.

Claims

1. A method of analyzing an image of a subject to identify spatial areas which contain periodic intensity variations representing valid temporal information about the subject, comprising the steps of:

dividing said image into a plurality of spatial areas and obtaining respective signals representative of an image intensity for each spatial area;

spectrally analyzing the intensity variations of each representative signal by fitting respective autoregressive models to the representative signals, each of the fitted autoregressive models comprising a plurality of poles representing spectral components of the intensity variations of each representative signal, each pole having a magnitude dependent upon the strength of the spectral component and a phase angle dependent upon the frequency of the spectral component;

selecting those spatial areas whose fitted autoregressive model has a dominant pole whose magnitude is greater than a predetermined threshold and identifying those spatial areas as containing periodic intensity variations representing said valid temporal information.

2. A method according to claim 1 wherein the spatial areas are m by n pixels of the image where m and n are positive integers.

3. A method according to claim 1 wherein the representative signal is an average or modal intensity over the spatial area.

4. A method according to claim 1 wherein the image is a red-green-blue colour image and intensity variations in at least one of the three components are spectrally analysed.

5. A method according to claim 1 further comprising the step of identifying and ignoring poles representing spectral components outside an expected frequency range for valid temporal information.

6. A method according to claim 1 further comprising the step of identifying and ignoring poles representing ambient light intensity variations due to aliasing.

7. A method according to claim 1 further comprising the step of selecting the dominant pole as being either the pole of highest magnitude or the pole of smallest phase angle whose magnitude satisfies the threshold, and calculating from the phase angle the frequency of the intensity variation represented by the selected pole.

8. A method according to claim 7 further comprising identifying peaks in the frequency spectrum of the intensity variations and selecting as the dominant pole the pole which corresponds to the highest magnitude peak.

9. A method according to claim 1 further comprising defining a time window in each of said representative signals, performing the spectral analysis and outputting the temporal information identified as valid, advancing the window and repeating the spectral analysis and outputting the temporal information identified as valid.

10. A method according to claim 1 further comprising displaying the image with a display attribute of the spatial areas being set to represent visually the magnitude of a selected pole in the fitted autoregressive model for that spatial area.

11. A method according to claim 1 further comprising displaying the image with a display attribute of the spatial areas being set to represent visually the frequency of a pole of the fitted autoregressive model for that spatial area whose magnitude is greater than the predetermined threshold.

12. A method according to claim 10 wherein the display attribute set is the displayed colour thereby to provide colour coding representative of magnitude of the said poles.

13. A method according to claim 1 wherein the subject is human or animal and the image contains a photoplethysmogram image.

14. A method according to claim 13 wherein the temporal information is at least one of heart rate and breathing rate.

15. A method according to claim 1 further comprising segmenting the image on the basis of at least one of the magnitude and phase angle of at least one of the poles in each of said spatial areas.

16. A computer program comprising program code means for executing on a programmed computer system the method of claim 1.

17. An image analysis system adapted to execute the method of claim 1.

18. A vital sign monitor comprising an image analysis system according to claim 17.

19. A method according to claim 11 wherein the display attribute set is the displayed colour thereby to provide colour coding representative of frequency of the said poles.