Photoplethysmogram

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Photoplethysmography
PPG.PNG
Representative PPG taken from an ear pulse oximeter. Variation in amplitude are from Respiratory Induced Variation.
MeSH D017156

A photoplethysmogram (PPG) is an optically obtained plethysmogram that can be used to detect blood volume changes in the microvascular bed of tissue. [1] [2] A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. [3] A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.

Contents

Finger pulse oximeter Pulse oximiter, Photoplethysmograph.jpg
Finger pulse oximeter

With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue. If the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus, as a small secondary peak.

The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. [4] Each cardiac cycle appears as a peak, as seen in the figure. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. [5] Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.

Although PPG sensors are in common use in a number of commercial and clinical applications, the exact mechanisms determining the shape of the PPG waveform are not yet fully understood. [6]

Sites for measuring PPG

While pulse oximeters are commonly used medical devices, the PPG signal they record is rarely displayed and is nominally only processed to determine blood oxygenation and heart rate. [2] The PPG can be obtained from transmissive absorption (as at the finger tip) or reflection (as on the forehead). [2]

In outpatient settings, pulse oximeters are commonly worn on the finger. However, in cases of shock, hypothermia, etc., blood flow to the periphery can be reduced, resulting in a PPG without a discernible cardiac pulse. [7] In this case, a PPG can be obtained from a pulse oximeter on the head, with the most common sites being the ear, nasal septum, and forehead. PPG can also be configured for multi-site photoplethysmography (MPPG), e.g. by making simultaneous measurements from the right and left ear lobes, index fingers and great toes, and offering further opportunities for the assessment of patients with suspected peripheral arterial disease, autonomic dysfunction, endothelial dysfunction, and arterial stiffness. MPPG also offers significant potential for data mining, e.g. using deep learning, as well as a range of other innovative pulse wave analysis techniques. [8] [9] [10] [11]

Motion artifacts are often a limiting factor preventing accurate readings during exercise and free living conditions. [6]

Uses

Monitoring heart rate and cardiac cycle

Premature Ventricular Contraction (PVC) can be seen in the PPG just as in the EKG and the Blood Pressure (BP). PVC detectionUsing PGG.png
Premature Ventricular Contraction (PVC) can be seen in the PPG just as in the EKG and the Blood Pressure (BP).
Venous pulsations can clearly be seen in this PPG. VenousPulsationAnd PGG.png
Venous pulsations can clearly be seen in this PPG.

Because the skin is so richly perfused, it is relatively easy to detect the pulsatile component of the cardiac cycle. The DC component of the signal is attributable to the bulk absorption of the skin tissue, while the AC component is directly attributable to variation in blood volume in the skin caused by the pressure pulse of the cardiac cycle.

The height of AC component of the photoplethysmogram is proportional to the pulse pressure, the difference between the systolic and diastolic pressure in the arteries. As seen in the figure showing premature ventricular contractions (PVCs), the PPG pulse for the cardiac cycle with the PVC results in lower amplitude blood pressure and a PPG. Ventricular tachycardia and ventricular fibrillation can also be detected. [12]

Monitoring respiration

The effects of sodium nitroprusside (Nipride), a peripheral vasodilator, on the finger PPG of a sedated subject. As expected, the PPG amplitude increases after infusion, and additionally, the Respiratory Induced Variation (RIV) becomes enhanced. NiprideEffectOnPPG-large.png
The effects of sodium nitroprusside (Nipride), a peripheral vasodilator, on the finger PPG of a sedated subject. As expected, the PPG amplitude increases after infusion, and additionally, the Respiratory Induced Variation (RIV) becomes enhanced.

Respiration affects the cardiac cycle by varying the intrapleural pressure, the pressure between the thoracic wall and the lungs. Since the heart resides in the thoracic cavity between the lungs, the partial pressure of inhaling and exhaling greatly influence the pressure on the vena cava and the filling of the right atrium.

During inspiration, intrapleural pressure decreases by up to 4 mm Hg, which distends the right atrium, allowing for faster filling from the vena cava, increasing ventricular preload, but decreasing stroke volume. Conversely during expiration, the heart is compressed, decreasing cardiac efficiency and increasing stroke volume. When the frequency and depth of respiration increases, the venous return increases, leading to increased cardiac output. [14]

Much research has focused on estimating respiratory rate from the photoplethysmogram, [15] as well as more detailed respiratory measurements such as inspiratory time. [16]

Monitoring depth of anesthesia

Effects of an incision on a subject under general anesthesia on the photoplethysmograph (PPG) and blood pressure (BP). EffectsOfIncisionOnPPG-Large.png
Effects of an incision on a subject under general anesthesia on the photoplethysmograph (PPG) and blood pressure (BP).

Anesthesiologists must often judge subjectively whether a patient is sufficiently anesthetized for surgery. As seen in the figure, if a patient is not sufficiently anesthetized, the sympathetic nervous system response to an incision can generate an immediate response in the amplitude of the PPG. [13]

Monitoring hypo- and hypervolemia

Shamir, Eidelman, et al. studied the interaction between inspiration and removal of 10% of a patient’s blood volume for blood banking before surgery. [17] They found that blood loss could be detected both from the photoplethysmogram from a pulse oximeter and an arterial catheter. Patients showed a decrease in the cardiac pulse amplitude caused by reduced cardiac preload during exhalation when the heart is being compressed.

Monitoring blood pressure

The FDA reportedly provided clearance to a photoplethysmography-based cuffless blood pressure monitor in August 2019. [18]

Remote photoplethysmography

Conventional imaging

While photoplethysmography commonly requires some form of contact with the human skin (e.g., ear, finger), remote photoplethysmography allows physiological processes such as blood flow to be determined without skin contact. This is achieved by using face video to analyze subtle momentary changes in the subject's skin color which are not detectable to the human eye. [19] [20] Such camera-based measurement of blood oxygen levels provides a contactless alternative to conventional photoplethysmography. For instance, it can be used to monitor the heart rate of newborn babies, [21] or analyzed with deep neural networks to quantify stress levels. [11]

Digital holography

Photoplethysmography of the thumb by off-axis digital holography. Final 5dff67f59d8d9300145440a4 3.gif
Photoplethysmography of the thumb by off-axis digital holography.
pulsatile waves on the back of a frog measured by off-axis holographic photoplethysmography Final 5ddec7a2afcb84001426276a 982750.gif
pulsatile waves on the back of a frog measured by off-axis holographic photoplethysmography

Remote photoplethysmography can also be performed by digital holography, which is sensitive to the phase of light waves, and hence can reveal sub-micron out-of-plane motion. In particular, wide-field imaging of pulsatile motion induced by blood flow can be measured on the thumb by digital holography. The results are comparable to blood pulse monitored by plethysmography during an occlusion-reperfusion experiment. [22] A major advantage of this system is that no physical contact with the studied tissue surface area is required. The two major limitations of this approach are (i) the off-axis interferometric configuration that reduces the available spatial bandwidth of the sensor array, and (ii) the use of short-time Fourier transform (via discrete Fourier transform) analysis that filters-off physiological signals.

laser Doppler imaging of pulse waves on the surface of the hand by holographic photoplethysmography from on-axis digital interferometry. Hand Emmanuel gain6 color 9 35.gif
laser Doppler imaging of pulse waves on the surface of the hand by holographic photoplethysmography from on-axis digital interferometry.

Principal component analysis of digital holograms [23] reconstructed from digitized interferograms acquired at rates beyond ~1000 frames per second reveals surface waves on the hand. This method is an efficient way of performing digital holography from on-axis interferograms, which alleviates both the spatial bandwidth reduction of the off-axis configuration and the filtering of physiological signals. A higher spatial bandwidth is crucial for larger image field of view.

A refinement of holographic photoplethysmography, holographic laser Doppler imaging, enables non-invasive blood flow pulse wave monitoring in blood vessels of the retina, choroid, conjunctiva, and iris. [24] In particular, laser Doppler holography of the eye fundus, the choroid constitutes the predominant contribution to the high frequency laser Doppler signal. It is however possible to circumvent its influence by subtracting the spatially averaged baseline signal, and achieve high temporal resolution and full-field imaging capability of pulsatile blood flow.

See also

Related Research Articles

In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the neck, wrist, at the groin, behind the knee, near the ankle joint, and on foot. The radial pulse is commonly measured using three fingers. This has a reason: the finger closest to the heart is used to occlude the pulse pressure, the middle finger is used get a crude estimate of the blood pressure, and the finger most distal to the heart is used to nullify the effect of the ulnar pulse as the two arteries are connected via the palmar arches. The study of the pulse is known as sphygmology.

<span class="mw-page-title-main">Blood pressure</span> Pressure exerted by circulating blood upon the walls of arteries

Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" refers to the pressure in a brachial artery, where it is most commonly measured. Blood pressure is usually expressed in terms of the systolic pressure over diastolic pressure in the cardiac cycle. It is measured in millimeters of mercury (mmHg) above the surrounding atmospheric pressure, or in kilopascals (kPa). The difference between the systolic and diastolic pressures is known as pulse pressure, while the average pressure during a cardiac cycle is known as mean arterial pressure.

<span class="mw-page-title-main">Cardiac output</span> Measurement of blood pumped by the heart

In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols , , or , is the volumetric flow rate of the heart's pumping output: that is, the volume of blood being pumped by a single ventricle of the heart, per unit time. Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:

Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously monitors and adjusts to conditions in the body and its environment. Hemodynamics explains the physical laws that govern the flow of blood in the blood vessels.

<span class="mw-page-title-main">Pulse oximetry</span> Measurement of blood oxygen saturation

Pulse oximetry is a noninvasive method for monitoring blood oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy of the more accurate reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis.

<span class="mw-page-title-main">Mean arterial pressure</span> Average blood pressure in an individual during a single cardiac cycle

In medicine, the mean arterial pressure (MAP) is an average calculated blood pressure in an individual during a single cardiac cycle. Although methods of estimating MAP vary, a common calculation is to take one-third of the pulse pressure, and add that amount to the diastolic pressure. A normal MAP is about 90 mmHg.

<span class="mw-page-title-main">Capnography</span> Monitoring of the concentration of carbon dioxide in respiratory gases

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO
2) in the respiratory gases. Its main development has been as a monitoring tool for use during anesthesia and intensive care. It is usually presented as a graph of CO
2 (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). The plot may also show the inspired CO
2, which is of interest when rebreathing systems are being used. When the measurement is taken at the end of a breath (exhaling), it is called "end tidal" CO
2 (PETCO2).

<span class="mw-page-title-main">Transcranial Doppler</span>

Transcranial Doppler (TCD) and transcranial color Doppler (TCCD) are types of Doppler ultrasonography that measure the velocity of blood flow through the brain's blood vessels by measuring the echoes of ultrasound waves moving transcranially. These modes of medical imaging conduct a spectral analysis of the acoustic signals they receive and can therefore be classified as methods of active acoustocerebrography. They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage, and other problems. These relatively quick and inexpensive tests are growing in popularity. The tests are effective for detecting sickle cell disease, ischemic cerebrovascular disease, subarachnoid hemorrhage, arteriovenous malformations, and cerebral circulatory arrest. The tests are possibly useful for perioperative monitoring and meningeal infection. The equipment used for these tests is becoming increasingly portable, making it possible for a clinician to travel to a hospital, to a doctor's office, or to a nursing home for both inpatient and outpatient studies. The tests are often used in conjunction with other tests such as MRI, MRA, carotid duplex ultrasound and CT scans. The tests are also used for research in cognitive neuroscience.

Holographic interferometry (HI) is a technique which enables the measurements of static and dynamic displacements of objects with optically rough surfaces at optical interferometric precision. These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing and radiation dosimetry. It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed. It can also be used to generate contours representing the form of the surface.

<span class="mw-page-title-main">Arterial resistivity index</span>

The arterial resistivity index, developed by Léandre Pourcelot, is a measure of pulsatile blood flow that reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement.

The photoplethysmogram (PPG) measurement made at a peripheral site, such as the finger, ear or forehead represents the volume of blood in the vessel at the site of measurement. The PPG signal consists of pulses that reflect the change in vascular blood volume with each cardiac beat. Beat-to-beat fluctuations, known as photoplethysmogram variability (PPGV) are found in the signal baseline and amplitude which reflects various physiological influences such as respiration and regulation of vascular tone by the sympathetic nervous system.

<span class="mw-page-title-main">Monitoring (medicine)</span> Observation of a disease, condition or one or several medical parameters over time

In medicine, monitoring is the observation of a disease, condition or one or several medical parameters over time.

<span class="mw-page-title-main">Laser Doppler imaging</span>

Laser Doppler imaging (LDI) is an imaging method that uses a laser beam to image live tissue. When the laser light reaches the tissue, the moving blood cells generate Doppler components in the reflected (backscattered) light. The light that comes back is detected using a photodiode that converts it into an electrical signal. Then the signal is processed to calculate a signal that is proportional to the tissue perfusion in the imaged area. When the process is completed, the signal is processed to generate an image that shows the perfusion on a screen.

Continuous noninvasive arterial pressure (CNAP) is the method of measuring beat-to-beat arterial blood pressure in real-time without any interruptions (continuously) and without cannulating the human body (noninvasive).

The article reviews the evolution of continuous noninvasive arterial pressure measurement (CNAP). The historical gap between ease of use, but intermittent upper arm instruments and bulky, but continuous “pulse writers” (sphygmographs) is discussed starting with the first efforts to measure pulse, published by Jules Harrison in 1835. Such sphygmographs led a shadowy existence in the past, while Riva Rocci's upper arm blood pressure measurement started its triumphant success over 100 years ago. In recent times, CNAP measurement introduced by Jan Penáz in 1973 enabled the first recording of noninvasive beat-to-beat blood pressure resulting in marketed products such as the Finapres™ device and its successors. Recently, a novel method for CNAP monitoring has been designed for patient monitoring in perioperative, critical and emergency care, where blood pressure needs to be measured repeatedly or even continuously to facilitate the best care for patients.

<span class="mw-page-title-main">Doppler ultrasonography</span> Ultrasound imaging of the movement of tissues and body fluids using the Doppler effect

Doppler ultrasonography is medical ultrasonography that employs the Doppler effect to perform imaging of the movement of tissues and body fluids, and their relative velocity to the probe. By calculating the frequency shift of a particular sample volume, for example, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized.

<span class="mw-page-title-main">Blood pressure measurement</span> Techniques for determining blood pressure

Arterial blood pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure. Blood pressure values are generally reported in millimetres of mercury (mmHg), though modern aneroid and electronic devices do not contain mercury.

quantium Medical Cardiac Output (qCO) uses impedance cardiography in a simple, continuous, and non-invasive way to estimate the cardiac output (CO) and other hemodynamic parameters such as the stroke volume (SV) and cardiac index (CI). The CO estimated by the qCO monitor is referred to as the "qCO". The impedance plethysmography allows determining changes in volume of the body tissues based on the measurement of the electric impedance at the body surface.

Bioinstrumentation or Biomedical Instrumentation is an application of biomedical engineering which focuses on development of devices and mechanics used to measure, evaluate, and treat biological systems. The goal of biomedical instrumentation focuses on the use of multiple sensors to monitor physiological characteristics of a human or animal for diagnostic and disease treatment purposes. Such instrumentation originated as a necessity to constantly monitor vital signs of Astronauts during NASA's Mercury, Gemini, and Apollo missions.

<span class="mw-page-title-main">CardiacSense Ltd</span> Israeli medical products company

CardiacSense is a developer of a wearable technology for continuous cardiac arrhythmia detection and vital signs monitoring. CardiacSense is based in Caesarea, Israel.

References

  1. ↑ Allen J (March 2007). "Photoplethysmography and its application in clinical physiological measurement". Physiological Measurement. 28 (3): R1–39. doi:10.1088/0967-3334/28/3/R01. PMID   17322588. S2CID   11450080.
  2. 1 2 3 Kyriacou PA, Allen J, eds. (2021). Photoplethysmography: Technology, Signal Analysis and Applications. Elsevier.
  3. ↑ Shelley K, Shelley S, Lake C (2001). "Pulse Oximeter Waveform: Photoelectric Plethysmography". In Lake C, Hines R, Blitt C (eds.). Clinical Monitoring. W.B. Saunders Company. pp. 420–428.
  4. ↑ Pelaez EA, Villegas ER (2007). "LED power reduction trade-offs for ambulatory pulse oximetry". 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society. Vol. 2007. pp. 2296–2299. doi:10.1109/IEMBS.2007.4352784. ISBN   978-1-4244-0787-3. PMID   18002450. S2CID   34626885.
  5. ↑ Reisner A, Shaltis PA, McCombie D, Asada HH (May 2008). "Utility of the photoplethysmogram in circulatory monitoring". Anesthesiology. 108 (5): 950–958. doi: 10.1097/ALN.0b013e31816c89e1 . PMID   18431132.
  6. 1 2 Charlton PH, Kyriaco PA, Mant J, Marozas V, Chowienczyk P, Alastruey J (March 2022). "Wearable Photoplethysmography for Cardiovascular Monitoring". Proceedings of the IEEE. Institute of Electrical and Electronics Engineers. 110 (3): 355–381. doi:10.1109/JPROC.2022.3149785. PMC   7612541 . PMID   35356509.
  7. ↑ Budidha K, Kyriacou PA (2015). "Investigation of photoplethysmography and arterial blood oxygen saturation from the ear-canal and the finger under conditions of artificially induced hypothermia" (PDF). 2015 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). Vol. 2015. pp. 7954–7957. doi:10.1109/EMBC.2015.7320237. ISBN   978-1-4244-9271-8. PMID   26738137. S2CID   4574235.
  8. ↑ Allen J, Overbeck K, Nath AF, Murray A, Stansby G (April 2008). "A prospective comparison of bilateral photoplethysmography versus the ankle-brachial pressure index for detecting and quantifying lower limb peripheral arterial disease". Journal of Vascular Surgery. 47 (4): 794–802. doi: 10.1016/j.jvs.2007.11.057 . PMID   18381141.
  9. ↑ McKay ND, Griffiths B, Di Maria C, Hedley S, Murray A, Allen J (October 2014). "Novel photoplethysmography cardiovascular assessments in patients with Raynaud's phenomenon and systemic sclerosis: a pilot study". Rheumatology. 53 (10): 1855–1863. doi:10.1093/rheumatology/keu196. PMID   24850874.
  10. ↑ Mizeva I, Di Maria C, Frick P, Podtaev S, Allen J (March 2015). "Quantifying the correlation between photoplethysmography and laser Doppler flowmetry microvascular low-frequency oscillations". Journal of Biomedical Optics. 20 (3): 037007. Bibcode:2015JBO....20c7007M. doi: 10.1117/1.JBO.20.3.037007 . PMID   25764202. S2CID   206437523.
  11. 1 2 Al-Jebrni AH, Chwyl B, Wang XY, Wong A, Saab BJ (2020-05-01). "AI-enabled remote and objective quantification of stress at scale". Biomedical Signal Processing and Control. 59: 101929. doi: 10.1016/j.bspc.2020.101929 . ISSN   1746-8094.
  12. ↑ Alian AA, Shelley KH (December 2014). "Photoplethysmography". Best Practice & Research. Clinical Anaesthesiology. 28 (4): 395–406. doi:10.1016/j.bpa.2014.08.006. PMID   25480769.
  13. 1 2 Shelley KH (December 2007). "Photoplethysmography: beyond the calculation of arterial oxygen saturation and heart rate". Anesthesia and Analgesia. 105 (6 Suppl): S31–S36. doi: 10.1213/01.ane.0000269512.82836.c9 . PMID   18048895. S2CID   21556782.
  14. ↑ Shelley KH, Jablonka DH, Awad AA, Stout RG, Rezkanna H, Silverman DG (August 2006). "What is the best site for measuring the effect of ventilation on the pulse oximeter waveform?". Anesthesia and Analgesia. 103 (2): 372–7, table of contents. doi: 10.1213/01.ane.0000222477.67637.17 . PMID   16861419. S2CID   6926327.
  15. ↑ Charlton PH, Birrenkott DA, Bonnici T, Pimentel MA, Johnson AE, Alastruey J, et al. (2018). "Breathing Rate Estimation From the Electrocardiogram and Photoplethysmogram: A Review". IEEE Reviews in Biomedical Engineering. 11: 2–20. doi:10.1109/RBME.2017.2763681. PMC   7612521 . PMID   29990026.
  16. ↑ Davies HJ, Bachtiger P, Williams I, Molyneaux PL, Peters NS, Mandic DP (July 2022). "Wearable In-Ear PPG: Detailed Respiratory Variations Enable Classification of COPD". IEEE Transactions on Bio-Medical Engineering. 69 (7): 2390–2400. doi:10.1109/TBME.2022.3145688. hdl: 10044/1/96220 . PMID   35077352. S2CID   246287257.
  17. ↑ Shamir M, Eidelman LA, Floman Y, Kaplan L, Pizov R (February 1999). "Pulse oximetry plethysmographic waveform during changes in blood volume". British Journal of Anaesthesia. 82 (2): 178–81. doi: 10.1093/bja/82.2.178 . PMID   10364990.
  18. ↑ Wendling P (28 August 2019). "FDA Okays Biobeat's Cuffless Blood Pressure Monitor". Medscape. Retrieved 5 September 2019.
  19. ↑ Verkruysse W, Svaasand LO, Nelson JS (December 2008). "Remote plethysmographic imaging using ambient light". Optics Express. 16 (26): 21434–21445. Bibcode:2008OExpr..1621434V. doi:10.1364/OE.16.021434. PMC   2717852 . PMID   19104573.
  20. ↑ Rouast PV, Adam MT, Chiong R, Cornforth D, Lux E (2018). "Remote heart rate measurement using low-cost RGB face video: A technical literature review". Frontiers of Computer Science. 12 (5): 858–872. doi:10.1007/s11704-016-6243-6. S2CID   1483621.
  21. ↑ Example of contactless monitoring in action. YouTube . Archived from the original on 2021-12-11.
  22. ↑ Bencteux J, Pagnoux P, Kostas T, Bayat S, Atlan M (June 2015). "Holographic laser Doppler imaging of pulsatile blood flow". Journal of Biomedical Optics. 20 (6): 066006. arXiv: 1501.05776 . Bibcode:2015JBO....20f6006B. doi:10.1117/1.JBO.20.6.066006. PMID   26085180. S2CID   20234484.
  23. ↑ Puyo L, Bellonnet-Mottet L, Martin A, Te F, Paques M, Atlan M (2020). "Real-time digital holography of the retina by principal component analysis". arXiv: 2004.00923 [physics.med-ph].
  24. ↑ Puyo L, Paques M, Fink M, Sahel JA, Atlan M (September 2018). "In vivo laser Doppler holography of the human retina". Biomedical Optics Express. 9 (9): 4113–4129. arXiv: 1804.10066 . doi:10.1364/BOE.9.004113. PMC   6157768 . PMID   30615709.
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