This review describes the diffusion model for light transport in tissues and the medical applications of diffuse light. Diffuse optics is particularly useful for measurement of tissue hemodynamics, wherein quantitative assessment of oxy- and deoxy-hemoglobin concentrations and blood flow are desired. The theoretical basis for near-infrared or diffuse optical spectroscopy is developed, and the basic elements of diffuse optical tomography are outlined. We also discuss diffuse correlation spectroscopy, a technique whereby temporal correlation functions of diffusing light are transported through tissue and are used to measure blood flow. Essential instrumentation is described, and representative brain and breast functional imaging and monitoring results illustrate the workings of these new tissue diagnostics.

/pdf/diffuse-optics-for-tissue-monitoring-and-tomography-curjv2s4p6.pdf

Diffuse optics for tissue monitoring and tomography

Measurement of intracranial arterial blood flow velocity is a new technique with potentially a number of very useful applications. This study validates the technique by comparing it to cerebral blood flow (CBF) measured using intravenous Xenon133 and extracranial clearance recording. We have measured the middle cerebral artery (MCA) blood flow velocity in 17 symptomatic patients with the EME TC 264 transcranial Doppler velocimeter and compared these measurements to the ipsilateral hemispheric cerebral blood flow measured with an intravenous Xenon133 technique (Novo Cerebrograph 10A). Measurements were made at rest and during hypercapnia. The absolute measurement of MCA velocity and hemispheric CBF showed a poor correlation (r = 0.424, p less than 0.01) due to wide between-patient variations at rest but the blood flow response to hypercapnia, expressed as a reactivity index, showed a good correlation (r = 0.849, p less than 0.001). Thus changes in MCA velocity reliably correlate with changes in cerebral blood flow but the absolute velocity cannot be used as an indicator of CBF.

Transcranial Doppler measurement of middle cerebral artery blood flow velocity: a validation study.

During the past decade, technological advances have made it possible to measure regional cerebral hemodynamics in individual patients. Studies performed with these techniques have demonstrated that the degree of carotid stenosis correlates poorly with the hemodynamic status of the ipsilateral cerebral circulation. The primary determinant of cerebral perfusion pressure and blood flow under these circumstances is the adequacy of collateral circulatory pathways. Since collateral circulation varies from patient to patient, there is no critical degree of carotid stenosis that consistently produces hemodynamic compromise of the cerebral circulation. It is, thus, time to abandon the concept of the hemodynamically significant carotid stenosis as it relates to the pathogenesis and treatment of cerebrovascular disease. Measurement of regional cerebral hemodynamics have provided new insight into the pathogenesis of tranisent ischemic attacks and generated some preliminary data on the prognostic and therapeutic importance of chronic reductions in regional cerebral perfusion pressure. Further investigations into the importance of hemodynamic factors in ischemic stroke can now be based on accurate assessment of cerebral (not carotid or vertebrobasilar) hemodynamics in the context of other coexisting epidemiological, clinical, hematological, and angiographic risk factors.

Cerebral hemodynamics in ischemic cerebrovascular disease.

Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function.

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

https://iopscience.iop.org/article/10.1088/0967-3334/19/3/001/pdf

Assessment of cerebral pressure autoregulation in humans - a review of measurement methods

Effect of internal carotid artery occlusion on middle cerebral artery blood flow at rest and in response to hypercapnia

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Transcranial Doppler measurement of middle cerebral artery blood flow velocity: a validation study.

Effect of internal carotid artery occlusion on middle cerebral artery blood flow at rest and in response to hypercapnia