Stable Xenon Computed Tomography Cerebral Blood Flow Measurement In Neurological Disease: Review And Protocols

摘要

Introduction Abnormalities in cerebral blood flow (CBF) are implicated in the pathophysiology of increasingly large number of neurological and neurosurgical disorders 1. The most obvious evidence of this awareness is the importance now given to maintaining an adequate cerebral perfusion pressure (CPP) in a wide variety of conditions. It is surprising that in spite of this knowledge, measurement of CBF has not become a routine tool in the management of patients with neurological illnesses. While CBF studies are increasingly the subject of research and publication, the vast majority of institutions caring for these patients do not have the equipment to perform this measurement, and only a small number of centers that have the capability use it in routine management. This is due to many factors including cost, ease of performance, concerns about safety and the current lack of ability to integrate the results into a standard patient care protocol.The list of diseases in which CBF abnormalities are implicated is growing as our knowledge of the underlying processes increases. Ischemia, besides the being the obvious pathology in stroke, is now also known to have a significant effect on outcome in head injuries 2, 3, 4, subarachnoid hemorrhage 5, 6 and in raised intracranial pressure (ICP) due to various causes. Hyperemia can have equally deleterious effects in head injuries 7, and in patients who have had endovascular or surgical treatment for carotid artery disease or arteriovenous malformations 8. Besides these primarily neurological diseases, abnormalities in CBF have been detected in psychiatric disorders 9, 10 and implicated in several systemic illnesses with neurological manifestations such as hepatic and renal failure 11.The awareness that CBF abnormalities are an important cause of pathophysiology has resulted in the need for an ability to measure the flow. Measurement will enable the physician to decide whether the cause of a deficit is abnormal flow, whether ischemic or hyperemic. It can also be used to assess the effectiveness of therapy aimed at modifying the abnormal state and therefore as a guide to further intervention. Measurement of cerebral blood flow Numerous techniques exist which are currently used clinically to estimate or measure CBF. These can be roughly classified into direct and indirect methods: Direct 1) Kety and Schmidt performed the first measurements of CBF in humans using nitrous oxide and the Fick principle 12. Since nitrous oxide is inert and diffuses rapidly into the brain, CBF could be calculated if its concentration could be simultaneously measured in the arterial blood entering the head and the venous blood leaving it, using a previously established blood-brain partition coefficient. This method is difficult to perform, as it requires multiple samples of arterial and internal jugular venous blood. It does not yield regional measurements but is presumed to be a measurement of global CBF; this also can be questioned because the variation in the dural sinuses and the torcula call into question the reliability of a sample taken from one jugular vein. A major advantage of this technique is that simultaneous measurement of blood gases and metabolites can be performed to assess cerebral metabolism. 2) 133Xe: the use of radioisotopes to measure cerebral blood flow was first reported by Ingvar et al 13 in 1961, who used 85Kr to measure CBF in exposed brain. Later 133Xe was found to be more effective because it is a gamma emitter and can be measured extracranially. It is also safer because of a shorter half-life. The initial techniques were complicated involved injecting the isotope directly into the carotid artery, but the process was considerably simplified in 1967 by Obrist et al 14, 15 who developed a technique of inhaling the isotope as a gas, and even further simplified now by administering it as a solution in an intravenous injection. The procedure involves knowledge of the arterial concentration of