Remote plethysmographic imaging using ambient light

Abstract

Plethysmographic signals were measured remotely (> 1m) using ambient light and a simple consumer level digital camera in movie mode. Heart and respiration rates could be quantified up to several harmonics. Although the green channel featuring the strongest plethysmographic signal, corresponding to an absorption peak by (oxy-) hemoglobin, the red and blue channels also contained plethysmographic information. The results show that ambient light photo-plethysmography may be useful for medical purposes such as characterization of vascular skin lesions (e.g., port wine stains) and remote sensing of vital signs (e.g., heart and respiration rates) for triage or sports purposes.

Fig. 1

(a): PV_raw(t) signals for G and B channels as indicated and a ROI on the forehead (ROI I in Fig. 2(b)). Movie recording started 5 s after the subject finished physical exercise. Boxed areas in (a) are shown as insert graphs. In the left insert (t = 30–50s), HR can be observed to decrease. Impact of voluntary hyperventilation on RR and HR is best seen in the right (t = 185 – 205 s). (b) and (c) Joint time-frequency diagrams (10 s time window) for G and B channels, respectively. HR decreases from 1.7 to 1.1 Hz during recuperation from t = 0 – 120 s. In (c), the band associated with RR initially decreases gradually during recuperation and abruptly increases to 0.3 Hz during hyperventilation.

Fig. 2

(a) PV_AC(t) signals (G channel) for four ROI (I–IV) indicated in (b). (c) Corresponding power spectra. Signals for ROI’s II and III are reduced (× 0.1) for clarity. The bar in (a) represents 10 pixel values for II and III and 1 for I and IV. The power spectrum for ROI IV is displayed ×5.

Fig. 3

(a–c) A movie excerpt (frame 179, t = 6 s, (Media1)), selected to demonstrate a low signal for PV_AC(t), (b) and high signal for PV_BP(t) (c). (d) PV_AC (t) for the ROI indicated in (a), for the R, G and B channels, displayed up to t = 30 s (media 1 shows up to t = 15 s). (e) Power spectrum for the G channel indicating amplitude modulation of RR (≈ 0.12 Hz) and HR (≈ 1.12 Hz). (f) PV_BP (t) signals, displaying an amplitude modulated HR signal for the G channel. Vertical dashed lines in (d) and (f) indicate the time of the movie excerpt.

Fig. 4

(a) Still (G channel only) from a movie. (b) Corresponding power map (at HR = 1.06 Hz) including artifactual high powers in areas with high contrast. (c) The movement artifact map consisting of average powers for bandwidths (0.80 – 0.95 and 1.17– 1.33 Hz). (d) Artifact corrected map: map (b) minus map (c).

Fig. 5

(a) Treated PWS area (dashed line) and 2 ROI (PWS and normal skin). (b–c) Frames 9 and 22, respectively, (Media 2). Intensities in (b–c) and Media 2 are linearly proportional to PV_BP(t) (BP filter: 0.8 – 6 Hz). (d) Phase map (computed for 1.43 Hz) showing clear contrast between PWS and normal skin. (e) A fragment of the PV_BP(t) signals in the ROI’s, dashed lines indicate the lowest points for the PWS signal occurring prior to those for the normal skin signal. (f,g) Lissajous presentations of these signals. The circle and triangle in (e,f) indicate the phases for the images in (b) and (c), respectively. (h) Power map, arrows illustrate areas with relatively low pulsatility surrounded by areas with high pulsatility.

Fig. 6

(a–c) PV_BP(t) for R, G and B channels, respectively and (d) the original movie, a screenshot of (Media 3) (t = 3.3 s.). (e–g) Corresponding power maps for R, G and B for the HR frequency (1.06 Hz) and the full 30 s. movie. Arrows in (b,e) indicate a structure which may be the right carotid artery. Arrows in (f,g) indicate displacement artifacts caused by the left carotid artery. (h) Phase map for the G channel, the arrow indicates a gradient of the phase.