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DOI: 10.4103/2348-0548.174740
Near-infrared spectroscopy—current status
Publikationsverlauf
Publikationsdatum:
05. Mai 2018 (online)
INTRODUCTION
Near-infrared spectroscopy (NIRS) is a non-invasive technique for measuring regional oxygen saturation (rsO2). It provides real-time information of changes in rSO2 of cerebral and somatic tissues. It can provide an early warning of decreased oxygen delivery. Tissue ischaemia is a significant contributor to increased morbidity and mortality, and thus measurement of tissue oxygenation is of paramount importance in critical care settings. In 1977, Franz Jöbsis first observed that light in the near-infrared light spectrum (wavelength 700–950 nm) can traverse biological tissue because of the relative transparency of tissue to light in this wavelength range.[1] This discovery later led to the development of NIRS technique to measure tissue oxygen saturation.
NIRS relies on ‘Beer-Lambert law’ (i.e., measurement of a substance concentration based on its absorption of light). Thus, measurement is based on determining haemoglobin oxygenation according to the light absorbed by haemoglobin.[2] The absorption of light is proportional to the concentration of certain chromophore molecules, mainly iron in haemoglobin and copper in cytochrome. In the brain, the primary infra-red light absorbing molecules are metal complex chromophores, namely, oxyhaemoglobin, deoxyhaemoglobin and cytochrome-C oxidase. Because about 70% of the blood in the brain is in the veins and capillaries and 25% in the arteries, most of the haemoglobin is in the venous circulation. Thus, NIRS gives a venous-weighted relative oxygen index of tissue beneath the probe.[3] [4] Cerebral oximetry does not depend on pulsatility of blood flow, unlike pulse oximetry.
The NIRS sensors are applied on either side of the forehead on a clean, dirt-free, non-greasy and non-hairy part of the skin. Each NIRS sensor (optodes) has a light-emitting source and two photodetectors integrated into a self-adhesive rubber plate that is attached to the forehead. The emitter and detectors are situated 4–8 cm apart. Light is generated at specific wavelengths typically by light-emitting diodes, and is usually detected by silicon photodiodes. The light emitted from the emitter passes through the scalp, skull bone and brain tissue. The photodetectors capture the reflected light from the underlying tissue. The light detected by the photodetector close to the emitter passes through the scalp and skull bone, while the light detected by the photodetector farther from the emitter passes through the brain tissue [Figure 1]. Near field photodetection is then substracted from far field photodetection to provide a measurement of brain tissue oxygenation.
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In adults, bilateral frontal cerebral oximetry is used to monitor perfusion to at-risk areas of grey matter within cerebral cortex in the watershed areas between the anterior cerebral artery and middle cerebral artery. The smaller head circumference of neonates and children permits greater depth of penetration and assessment of subcortical tissue oxygenation. The sensors illuminate up to a volume of 10 ml of hemispherical tissue.
There have been several attempts to determine the normal and critical value of regional cerebral oxygen saturation (rScO2). However, data on cut-off values are still limited. The normal values of rScO2 are reported to be 60–80% in various studies. In an animal study, a decrease of the absolute value of rScO2 below 50% was associated with electroencephalogram abnormalities, and a further decrease in rScO2 below 40% lead to increased brain lactate levels.[5] Absolute rScO2 values below 50% have been repeatedly shown to be associated with an unfavourable clinical and/or neurological outcomes. Fischer et al. observed that a decrease in rScO2 below 60% absolute was associated with an increased complication rate in patients undergoing aortic arch surgery.[6] Tang et al. found the incidence of post-operative cognitive dysfunction increased in patients undergoing thoracic surgery even if rScO2 decreased below 65% for more than 5 min.[7] It is more appropriate to interpret the trend changes in rScO2, rather than absolute values.[8] A 20% decline in rScO2 from baseline is considered to be ischaemic threshold.[9] [10] In general, a decrease in rScO2 is reflective of an increase in oxygen extraction as a result of increased metabolism, decreased perfusion and/or stagnant perfusion. High rScO2 may be indicative of increased perfusion, decreased tissue bed metabolism and/or less oxygen extraction.
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CLINICAL APPLICATIONS
An important application of NIRS is for perioperative assessment of cerebral oxygenation during cardiac surgeries. It is a useful monitor in patients on cardiopulmonary bypass (CPB) where stroke and post-operative cognitive dysfunction leading to poor neurological outcome is a concern for all clinicians. The use of NIRS has been found to decrease cerebral desaturation events during CPB, fewer incidences of strokes and less post-operative major organ morbidity (mechanical ventilation, myocardial infarction).[11] [12] Similarly, low rScO2 levels below 50% have been associated with increased risk of post-operative cognitive dysfunction and prolonged hospital stay by about three-fold.[13] [14]
NIRS has also been extensively used to monitor cerebral perfusion. It can be a valuable tool to detect cerebral ischaemia during carotid endarterectomy (CEA). Various cut-off values such as a decline of more than 20% from baseline have been recommended.[15] However, some studies recommend a decrease in rScO2 of more than 12% to be reliable, sensitive and reliable specific threshold for brain ischaemia.[16]
NIRS is increasingly being used in paediatric patients during cardiac surgery, neurosurgery and critical care settings for low birth weight infants and premature children at risk of apnoea. It may also be used to measure systemic perfusion via somatic channels.
NIRS monitoring can help predict cerebral desaturation events in high-risk surgeries such as shoulder surgery in beach chair position,[17] thoracic surgery with one-lung ventilation,[18] major abdominal surgery, hip surgery and laparoscopic surgeries with patients in reverse Trendelenburg position.[19] Its use can be extended to optimise cerebral oxygenation in patients at risk of perioperative stroke.
In traumatic brain injury (TBI), mortality, intracranial hypertension and compromised cerebral perfusion pressure have been found to be associated with rScO2 values below 60%.[20] NIRS changes precede changes in intracranial pressure in patients having delayed traumatic haematomas.[21] NIRS has also been used to test autoregulation of cerebral blood flow (CBF).[22]
NIRS is a valuable technique for monitoring the haemodynamic changes occurring in superficial regions of the cortex.[23] [24] [25] Over the last 20 years, NIRS has become an attractive alternative to functional magnetic resonance imaging (fMRI), with several clinical advantages.[26] Besides being continuous, non-invasive and portable, NIRS is less susceptible to movement artefacts enabling long-term monitoring of the haemodynamic activity at the bedside. In patients of subarachnoid haemorrhage having cerebral vasospasm, NIRS is particularly useful in detecting changes in cortical O2 saturation.[27] In stroke patients, studies have shown the usefulness of NIRS in detecting cerebral ischaemia.[28] Terborg et al. describes NIRS as non-invasive, rapid, repeatable, bedside method to detect reduction in cerebral perfusion in patients with acute ischaemic stroke without the need for transportation of critically ill patients.[29] However, the low penetration depth (upper 1 cm of the cerebral cortex) and less spatial resolution of NIRS than the resolution of standard fMRI scanners limits its sensitivity.[30]
Another application of near-infrared wave technique is the measurement of CBF. CBF measurement provides valuable information in the management of neurocritical care patients. However, currently available techniques for monitoring CBF have various limitations. The limitations include that they are invasive, do not provide continuous measurement, require exposure to ionising radiation, may require transportation of critically ill patients out of the neuro intensive care unit and expensive. Transcranial Doppler, though non-invasive, is limited to large vessel flow velocities, which do not necessarily reflect microvascular perfusion.[31] Diffuse correlation spectroscopy (DCS) is a novel non-invasive optical technique with potential to monitor bedside CBF. DCS uses near-infrared light (wavelength 650–950 nm) to provide a continuous measurement of transcranial blood flow. Changes in blood flow are determined from DCS by measuring the decay rate of the detected light intensity autocorrelation function.[32] The integration of DCS with NIRS in a monitor that can simultaneously monitor CBF and oxy- and deoxy-haemoglobin concentrations will facilitate monitoring both CBF and oxygen metabolism in neurocritical care patients. NIRS and DSC have been used to measure tissue perfusion and oxygenation and have shown changes in autoregulation in patients of ischaemic stroke.[33] The continuous and non-invasive nature of these optical techniques may lead to new clinical tools for use in the neurocritical patients. Like NIRS, DCS is also limited by the fact that it is a measure of local CBF (flow measurements limited to the area near the surface of the cerebral cortex).
The NIRS can measure haemodynamic changes associated with functional brain activity that arises from changes in blood oxygenation and blood volume in the area of activation.
The fMRI has become the gold standard for in vivo imaging of the human brain. However, NIRS has also become a popular technology for studying brain function (due to being more convenient and less expensive than fMRI). Cui et al. demonstrated that NIRS signals are often highly correlated with fMRI measurements. Their findings suggest that, while NIRS can be an appropriate substitute for fMRI for studying brain activity related to cognitive tasks, care should be taken to ensure that the spatial resolution is adequate, and the design accounts for weaker SNR, especially in brain regions more distal from the scalp.[34]
NIRS seems to be an important tool in clinician’s armamentarium for predicting cerebral and somatic tissue desaturation events. Despite its potential advantages over other neuromonitoring techniques such as being user-friendly, non-invasive and being capable of taking measurements over multiple regions of interest simultaneously with high temporal resolution; further investigation and technological advances are necessary before it can be introduced more widely into clinical practice.
Financial support and sponsorship
Nil.
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Conflicts of interest
There are no conflicts of interest.
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Die Autoren geben an, dass kein Interessenkonflikt besteht.
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REFERENCES
- 1 Jöbsis FF. Noninvasive, infrared monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters. Science 1977; 198: 1264-7
- 2 Ferrari M, Mottola L, Quaresima V. Principles, techniques, and limitations of near infrared spectroscopy. Can J Appl Physiol 2004; 29: 463-87
- 3 Gupta N, Dash HH. Neuromonitoring. In: Mehta Y. editor. Textbook of Critical Care. 1st ed.. New Delhi: Jaypee Publishers; 2015. p. 66-89
- 4 Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth 2009; 103 Suppl (Suppl. 01) i3-13
- 5 Kurth CD, Levy WJ, McCann J. Near-infrared spectroscopy cerebral oxygen saturation thresholds for hypoxia-ischemia in piglets. J Cereb Blood Flow Metab 2002; 22: 335-41
- 6 Fischer GW, Lin HM, Krol M, Galati MF, Di Luozzo G, Griepp RB. et al. Noninvasive cerebral oxygenation may predict outcome in patients undergoing aortic arch surgery. J Thorac Cardiovasc Surg 2011; 141: 815-21
- 7 Tang L, Kazan R, Taddei R, Zaouter C, Cyr S, Hemmerling TM. Reduced cerebral oxygen saturation during thoracic surgery predicts early postoperative cognitive dysfunction. Br J Anaesth 2012; 108: 623-9
- 8 Pollard V, Prough DS, DeMelo AE, Deyo DJ, Uchida T, Stoddart HF. Validation in volunteers of a near-infrared spectroscope for monitoring brain oxygenation in vivo . Anesth Analg 1996; 82: 269-77
- 9 Singer I, Edmonds HL. Tissue oximetry for the diagnosis of neurally mediated syncope. Pacing Clin Electrophysiol 2000; 23 (11 Pt 2) 2006-9
- 10 Moritz S, Kasprzak P, Arlt M, Taeger K, Metz C. Accuracy of cerebral monitoring in detecting cerebral ischemia during carotid endarterectomy: A comparison of transcranial Doppler sonography, near-infrared spectroscopy, stump pressure, and somatosensory evoked potentials. Anesthesiology 2007; 107: 563-9
- 11 Zheng F, Sheinberg R, Yee MS, Ono M, Zheng Y, Hogue CW. Cerebral near-infrared spectroscopy monitoring and neurologic outcomes in adult cardiac surgery patients: A systematic review. Anesth Analg 2013; 116: 663-76
- 12 Murkin JM, Adams SJ, Novick RJ, Quantz M, Bainbridge D, Iglesias I. et al. Monitoring brain oxygen saturation during coronary bypass surgery: A randomized, prospective study. Anesth Analg 2007; 104: 51-8
- 13 Slater JP, Guarino T, Stack J, Vinod K, Bustami RT, Brown 3rd JM. et al. Cerebral oxygen desaturation predicts cognitive decline and longer hospital stay after cardiac surgery. Ann Thorac Surg 2009; 87: 36-44
- 14 de Tournay-Jetté E, Dupuis G, Bherer L, Deschamps A, Cartier R, Denault A. The relationship between cerebral oxygen saturation changes and postoperative cognitive dysfunction in elderly patients after coronary artery bypass graft surgery. J Cardiothorac Vasc Anesth 2011; 25: 95-104
- 15 Samra SK, Dy EA, Welch K, Dorje P, Zelenock GB, Stanley JC. Evaluation of a cerebral oximeter as a monitor of cerebral ischemia during carotid endarterectomy. Anesthesiology 2000; 93: 964-70
- 16 Mille T, Tachimiri ME, Klersy C, Ticozzelli G, Bellinzona G, Blangetti I. et al. Near infrared spectroscopy monitoring during carotid endarterectomy: Which threshold value is critical?. Eur J Vasc Endovasc Surg 2004; 27: 646-50
- 17 Murphy GS, Szokol JW, Marymont JH, Greenberg SB, Avram MJ, Vender JS. et al. Cerebral oxygen desaturation events assessed by near-infrared spectroscopy during shoulder arthroscopy in the beach chair and lateral decubitus positions. Anesth Analg 2010; 111: 496-505
- 18 Hemmerling TM, Bluteau MC, Kazan R, Bracco D. Significant decrease of cerebral oxygen saturation during single-lung ventilation measured using absolute oximetry. Br J Anaesth 2008; 101: 870-5
- 19 Nielsen HB. Systematic review of near-infrared spectroscopy determined cerebral oxygenation during non-cardiac surgery. Front Physiol 2014; 5: 93
- 20 Dunham CM, Ransom KJ, Flowers LL, Siegal JD, Kohli CM. Cerebral hypoxia in severely brain-injured patients is associated with admission Glasgow Coma Scale score, computed tomographic severity, cerebral perfusion pressure, and survival. J Trauma 2004; 56: 482-9
- 21 Gopinath SP, Robertson CS, Contant CF, Narayan RK, Grossman RG, Chance B. Early detection of delayed traumatic intracranial hematomas using near-infrared spectroscopy. J Neurosurg 1995; 83: 438-44
- 22 Steiner LA, Pfister D, Strebel SP, Radolovich D, Smielewski P, Czosnyka M. Near-infrared spectroscopy can monitor dynamic cerebral autoregulation in adults. Neurocrit Care 2009; 10: 122-8
- 23 Villringer A, Planck J, Hock C, Schleinkofer L, Dirnagl U. Near infrared spectroscopy (NIRS): A new tool to study hemodynamic changes during activation of brain function in human adults. Neurosci Lett 1993; 154: 101-4
- 24 Hoshi Y. Functional near-infrared spectroscopy: Current status and future prospects. J Biomed Opt 2007; 12: 062106
- 25 Hillman EM. Optical brain imaging in vivo: Techniques and applications from animal to man. J Biomed Opt. 2007 12: 051402
- 26 Irani F, Platek SM, Bunce S, Ruocco AC, Chute D. Functional near infrared spectroscopy (fNIRS): Anemerging neuroimaging technology with important applications for the study of brain disorders. Clin Neuropsychol 2010; 21: 9-37
- 27 Yokose N, Sakatani K, Murata Y, Awano T, Igarashi T, Nakamura S. et al. Bedside monitoring of cerebral blood oxygenation and hemodynamics after aneurysmal subarachnoid hemorrhage by quantitative time-resolved near-infrared spectroscopy. World Neurosurg 2010; 73: 508-13
- 28 Liebert A, Wabnitz H, Steinbrink J, Möller M, Macdonald R, Rinneberg H. et al. Bed-side assessment of cerebral perfusion in stroke patients based on optical monitoring of a dye bolus by time-resolved diffuse reflectance. Neuroimage 2005; 24: 426-35
- 29 Terborg C, Bramer S, Harscher S, Simon M, Witte OW. Bedside assessment of cerebral perfusion reductions in patients with acute ischaemic stroke by near-infrared spectroscopy and indocyanine green. J Neurol Neurosurg Psychiatry 2004; 75: 38-42
- 30 Boas DA, Dale AM, Franceschini MA. Diffuse optical imaging of brain activation: Approaches to optimizing image sensitivity, resolution, and accuracy. Neuroimage 2004; 23 Suppl (Suppl. 01) S275-88
- 31 Reinhard M, Wehrle-Wieland E, Grabiak D, Roth M, Guschlbauer B, Timmer J. et al. Oscillatory cerebral hemodynamics – The macro- vs. microvascular level. J Neurol Sci 2006; 250: 103-9
- 32 Pine DJ, Weitz DA, Chaikin PM, Herbolzheimer E. Diffusing wave spectroscopy. Phys Rev Lett 1988; 60: 1134-7
- 33 Durduran T, Zhou C, Edlow BL, Yu G, Choe R, Kim MN. et al. Transcranial optical monitoring of cerebrovascular hemodynamics in acute stroke patients. Opt Express 2009; 17: 3884-902
- 34 Cui X, Bray S, Bryant DM, Glover GH, Reiss AL. A quantitative comparison of NIRS and fMRI across multiple cognitive tasks. Neuroimage 2011; 54: 2808-21
-
REFERENCES
- 1 Jöbsis FF. Noninvasive, infrared monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters. Science 1977; 198: 1264-7
- 2 Ferrari M, Mottola L, Quaresima V. Principles, techniques, and limitations of near infrared spectroscopy. Can J Appl Physiol 2004; 29: 463-87
- 3 Gupta N, Dash HH. Neuromonitoring. In: Mehta Y. editor. Textbook of Critical Care. 1st ed.. New Delhi: Jaypee Publishers; 2015. p. 66-89
- 4 Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth 2009; 103 Suppl (Suppl. 01) i3-13
- 5 Kurth CD, Levy WJ, McCann J. Near-infrared spectroscopy cerebral oxygen saturation thresholds for hypoxia-ischemia in piglets. J Cereb Blood Flow Metab 2002; 22: 335-41
- 6 Fischer GW, Lin HM, Krol M, Galati MF, Di Luozzo G, Griepp RB. et al. Noninvasive cerebral oxygenation may predict outcome in patients undergoing aortic arch surgery. J Thorac Cardiovasc Surg 2011; 141: 815-21
- 7 Tang L, Kazan R, Taddei R, Zaouter C, Cyr S, Hemmerling TM. Reduced cerebral oxygen saturation during thoracic surgery predicts early postoperative cognitive dysfunction. Br J Anaesth 2012; 108: 623-9
- 8 Pollard V, Prough DS, DeMelo AE, Deyo DJ, Uchida T, Stoddart HF. Validation in volunteers of a near-infrared spectroscope for monitoring brain oxygenation in vivo . Anesth Analg 1996; 82: 269-77
- 9 Singer I, Edmonds HL. Tissue oximetry for the diagnosis of neurally mediated syncope. Pacing Clin Electrophysiol 2000; 23 (11 Pt 2) 2006-9
- 10 Moritz S, Kasprzak P, Arlt M, Taeger K, Metz C. Accuracy of cerebral monitoring in detecting cerebral ischemia during carotid endarterectomy: A comparison of transcranial Doppler sonography, near-infrared spectroscopy, stump pressure, and somatosensory evoked potentials. Anesthesiology 2007; 107: 563-9
- 11 Zheng F, Sheinberg R, Yee MS, Ono M, Zheng Y, Hogue CW. Cerebral near-infrared spectroscopy monitoring and neurologic outcomes in adult cardiac surgery patients: A systematic review. Anesth Analg 2013; 116: 663-76
- 12 Murkin JM, Adams SJ, Novick RJ, Quantz M, Bainbridge D, Iglesias I. et al. Monitoring brain oxygen saturation during coronary bypass surgery: A randomized, prospective study. Anesth Analg 2007; 104: 51-8
- 13 Slater JP, Guarino T, Stack J, Vinod K, Bustami RT, Brown 3rd JM. et al. Cerebral oxygen desaturation predicts cognitive decline and longer hospital stay after cardiac surgery. Ann Thorac Surg 2009; 87: 36-44
- 14 de Tournay-Jetté E, Dupuis G, Bherer L, Deschamps A, Cartier R, Denault A. The relationship between cerebral oxygen saturation changes and postoperative cognitive dysfunction in elderly patients after coronary artery bypass graft surgery. J Cardiothorac Vasc Anesth 2011; 25: 95-104
- 15 Samra SK, Dy EA, Welch K, Dorje P, Zelenock GB, Stanley JC. Evaluation of a cerebral oximeter as a monitor of cerebral ischemia during carotid endarterectomy. Anesthesiology 2000; 93: 964-70
- 16 Mille T, Tachimiri ME, Klersy C, Ticozzelli G, Bellinzona G, Blangetti I. et al. Near infrared spectroscopy monitoring during carotid endarterectomy: Which threshold value is critical?. Eur J Vasc Endovasc Surg 2004; 27: 646-50
- 17 Murphy GS, Szokol JW, Marymont JH, Greenberg SB, Avram MJ, Vender JS. et al. Cerebral oxygen desaturation events assessed by near-infrared spectroscopy during shoulder arthroscopy in the beach chair and lateral decubitus positions. Anesth Analg 2010; 111: 496-505
- 18 Hemmerling TM, Bluteau MC, Kazan R, Bracco D. Significant decrease of cerebral oxygen saturation during single-lung ventilation measured using absolute oximetry. Br J Anaesth 2008; 101: 870-5
- 19 Nielsen HB. Systematic review of near-infrared spectroscopy determined cerebral oxygenation during non-cardiac surgery. Front Physiol 2014; 5: 93
- 20 Dunham CM, Ransom KJ, Flowers LL, Siegal JD, Kohli CM. Cerebral hypoxia in severely brain-injured patients is associated with admission Glasgow Coma Scale score, computed tomographic severity, cerebral perfusion pressure, and survival. J Trauma 2004; 56: 482-9
- 21 Gopinath SP, Robertson CS, Contant CF, Narayan RK, Grossman RG, Chance B. Early detection of delayed traumatic intracranial hematomas using near-infrared spectroscopy. J Neurosurg 1995; 83: 438-44
- 22 Steiner LA, Pfister D, Strebel SP, Radolovich D, Smielewski P, Czosnyka M. Near-infrared spectroscopy can monitor dynamic cerebral autoregulation in adults. Neurocrit Care 2009; 10: 122-8
- 23 Villringer A, Planck J, Hock C, Schleinkofer L, Dirnagl U. Near infrared spectroscopy (NIRS): A new tool to study hemodynamic changes during activation of brain function in human adults. Neurosci Lett 1993; 154: 101-4
- 24 Hoshi Y. Functional near-infrared spectroscopy: Current status and future prospects. J Biomed Opt 2007; 12: 062106
- 25 Hillman EM. Optical brain imaging in vivo: Techniques and applications from animal to man. J Biomed Opt. 2007 12: 051402
- 26 Irani F, Platek SM, Bunce S, Ruocco AC, Chute D. Functional near infrared spectroscopy (fNIRS): Anemerging neuroimaging technology with important applications for the study of brain disorders. Clin Neuropsychol 2010; 21: 9-37
- 27 Yokose N, Sakatani K, Murata Y, Awano T, Igarashi T, Nakamura S. et al. Bedside monitoring of cerebral blood oxygenation and hemodynamics after aneurysmal subarachnoid hemorrhage by quantitative time-resolved near-infrared spectroscopy. World Neurosurg 2010; 73: 508-13
- 28 Liebert A, Wabnitz H, Steinbrink J, Möller M, Macdonald R, Rinneberg H. et al. Bed-side assessment of cerebral perfusion in stroke patients based on optical monitoring of a dye bolus by time-resolved diffuse reflectance. Neuroimage 2005; 24: 426-35
- 29 Terborg C, Bramer S, Harscher S, Simon M, Witte OW. Bedside assessment of cerebral perfusion reductions in patients with acute ischaemic stroke by near-infrared spectroscopy and indocyanine green. J Neurol Neurosurg Psychiatry 2004; 75: 38-42
- 30 Boas DA, Dale AM, Franceschini MA. Diffuse optical imaging of brain activation: Approaches to optimizing image sensitivity, resolution, and accuracy. Neuroimage 2004; 23 Suppl (Suppl. 01) S275-88
- 31 Reinhard M, Wehrle-Wieland E, Grabiak D, Roth M, Guschlbauer B, Timmer J. et al. Oscillatory cerebral hemodynamics – The macro- vs. microvascular level. J Neurol Sci 2006; 250: 103-9
- 32 Pine DJ, Weitz DA, Chaikin PM, Herbolzheimer E. Diffusing wave spectroscopy. Phys Rev Lett 1988; 60: 1134-7
- 33 Durduran T, Zhou C, Edlow BL, Yu G, Choe R, Kim MN. et al. Transcranial optical monitoring of cerebrovascular hemodynamics in acute stroke patients. Opt Express 2009; 17: 3884-902
- 34 Cui X, Bray S, Bryant DM, Glover GH, Reiss AL. A quantitative comparison of NIRS and fMRI across multiple cognitive tasks. Neuroimage 2011; 54: 2808-21
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