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The measurement of cerebral blood flow and metabolism in human
subjects
Source: 1997;.
Author: Mazziotta JC, Cohen M, Toga AW.
Abstract:
The ability to measure the cerebral metabolism and blood flow of the human brain provides both clinicians and neuroscientists a valuable array of observations with regard to normal brain function and the course of hemodynamic and pathophysiologic events that occur during cerebral ischemia and infarction. This chapter addresses the salient features of such measurements using modern functional imaging techniques, both invasive and noninvasive. Since only a short synopsis of these methods and observations can be provided here, the reader is referred to more comprehensive sources for further information. Determinations of human cerebral blood flow and metabolism can be obtained using either intrinsic signals that are the result of the natural neurochemistry and physiology of the brain or by the introduction of exogenous agents that reveal this underlying state. Those methods that use intrinsic signals to measure cerebral perfusion and aspects of metabolism include optical intrinsic signal imaging, an intraoperative technique using light reflected off the surface of the brain, and certain forms of MRI. POSITRON EMISSION TOMOGRAPHY: Currently, PET is used to measure cerebral metabolism, blood flow and volume, oxygen utilization, amino acid incorporation, neurotransmitter synthesis, and receptor binding. Measurements of tissue concentrations of the PET tracer are obtained by imaging with the PET tomography, resulting in radioisotope distribution maps. SPECT: SPECT uses radiopharmaceuticals administered intravenously or by inhalation to evaluate function in the human brain. SPECT provides estimates of cerebral perfusion and blood volume.Cerebral blood volume is measured with single photon emitting isotopes bound to large compounds that do not escape from the cerebral vasculature. MRI: MRI has demonstrated efficacy in evaluation of areas of chronic and subacute infarct. Recently MRI methods based on functional measures and other contrast mechanisms have been developed with the potential to revolutionize the applications of MRI in the acute evaluation of stroke. In one form of perfusion imaging, local perfusion is detected by measuring the passage of an injected tracer agent. XENON-CT: Estimates of cerebral perfusion can be obtained by having subjects inhale nonradioactive xenon gas during CT imaging. Subtraction of CT scans obtained prior to xenon inhalation from those acquired after inhalation give estimates of regional brain perfusion. The basic principle is that xenon gas is freely diffusible from the cerebral vasculature into the brain parenchyma. By continuous inhalation of the gas, a steady-state condition is established where the concentration of xenon remains stable in each brain region and is proportional to local lperfusion. Xenon gas increases the attenuation of x-rays, thereby resulting in a change in regional x-ray attenation that is then measured by the CT scanner's detectors. OIS IMAGING: Optical properties of the brain determine the physical interactions between electromagnetic radiation and the tissue being imaged. In the visible spectrum, optical reflectance methods provide rapid, repeatable, noncontact measurements of cortical functional activity. Optical reflectance studies of humans have been performed with direct electrical stimulation of the brain and in response to peripheral somesthetic stimulation. Optical signals have been observed intraoperatively iwth direct electrocortical induced epileptiform activity.
Source: 1997;.
Author: Mazziotta JC, Cohen M, Toga AW.
Abstract:
The ability to measure the cerebral metabolism and blood flow of the human brain provides both clinicians and neuroscientists a valuable array of observations with regard to normal brain function and the course of hemodynamic and pathophysiologic events that occur during cerebral ischemia and infarction. This chapter addresses the salient features of such measurements using modern functional imaging techniques, both invasive and noninvasive. Since only a short synopsis of these methods and observations can be provided here, the reader is referred to more comprehensive sources for further information. Determinations of human cerebral blood flow and metabolism can be obtained using either intrinsic signals that are the result of the natural neurochemistry and physiology of the brain or by the introduction of exogenous agents that reveal this underlying state. Those methods that use intrinsic signals to measure cerebral perfusion and aspects of metabolism include optical intrinsic signal imaging, an intraoperative technique using light reflected off the surface of the brain, and certain forms of MRI. POSITRON EMISSION TOMOGRAPHY: Currently, PET is used to measure cerebral metabolism, blood flow and volume, oxygen utilization, amino acid incorporation, neurotransmitter synthesis, and receptor binding. Measurements of tissue concentrations of the PET tracer are obtained by imaging with the PET tomography, resulting in radioisotope distribution maps. SPECT: SPECT uses radiopharmaceuticals administered intravenously or by inhalation to evaluate function in the human brain. SPECT provides estimates of cerebral perfusion and blood volume.Cerebral blood volume is measured with single photon emitting isotopes bound to large compounds that do not escape from the cerebral vasculature. MRI: MRI has demonstrated efficacy in evaluation of areas of chronic and subacute infarct. Recently MRI methods based on functional measures and other contrast mechanisms have been developed with the potential to revolutionize the applications of MRI in the acute evaluation of stroke. In one form of perfusion imaging, local perfusion is detected by measuring the passage of an injected tracer agent. XENON-CT: Estimates of cerebral perfusion can be obtained by having subjects inhale nonradioactive xenon gas during CT imaging. Subtraction of CT scans obtained prior to xenon inhalation from those acquired after inhalation give estimates of regional brain perfusion. The basic principle is that xenon gas is freely diffusible from the cerebral vasculature into the brain parenchyma. By continuous inhalation of the gas, a steady-state condition is established where the concentration of xenon remains stable in each brain region and is proportional to local lperfusion. Xenon gas increases the attenuation of x-rays, thereby resulting in a change in regional x-ray attenation that is then measured by the CT scanner's detectors. OIS IMAGING: Optical properties of the brain determine the physical interactions between electromagnetic radiation and the tissue being imaged. In the visible spectrum, optical reflectance methods provide rapid, repeatable, noncontact measurements of cortical functional activity. Optical reflectance studies of humans have been performed with direct electrical stimulation of the brain and in response to peripheral somesthetic stimulation. Optical signals have been observed intraoperatively iwth direct electrocortical induced epileptiform activity.