Correlation of regional cerebral blood flow with regional oxygen uptake using 15 O method

Imaging methods for evaluating brain function in man

Imaging of brain function and neurotransmission is an important bridge between basic and clinical research. Regional cerebral energy metabolism and blood flow are normally coupled to regional cerebral function. Positron tomography (PET) studies of cerebral glucose metabolism and blood flow, single photon tomography (SPECT) and MRI studies of cerebral perfusion, have been used to image cerebral development and aging in man. The sensitivity, temporal resolution, spatial resolution and lack of radiation have led to the widespread utilization of blood oxygen level dependent (BOLD) and MRI perfusion techniques. PET and SPECT methods for studying cerebral neurotransmission include studies of dopaminergic, serotonergic, cholinergic, opiate and GABAergic neurotransmission in man. Studies of cerebral neurotransmission in man have helped to delineate the mechanisms of action of antipsychotic and antidepressant drugs, the diagnosis and progression of Parkinson's disease, and to evaluate neuroprotective drugs. The strengths, limitations, and application of these modalities are reviewed. The application of these methods to cerebral development and aging are briefly discussed.

Radioactive oxygen-15 in the study of cerebral blood flow, blood volume, and oxygen metabolism

The short half-life of 15O led early observers to believe that it was unsuitable for use as a biological tracer. However, initial studies with this nuclide demonstrated its potential usefulness for in vivo, regional physiologic measurements. Subsequently, techniques were developed to measure cerebral blood flow (CBF), blood volume, and oxygen metabolism using intracarotid injection of 15O-labeled radiopharmaceuticals and highly collimated scintillation probes to record the time course of radioactivity in the brain. The development of positron emission tomography (PET) made possible the in vivo, noninvasive measurement of the absolute concentration of positron-emitting nuclides. A variety of tracer kinetic models were formulated to obtain physiologic measurements from tomographic images of the distribution of 15O-labeled radiopharmaceuticals in the brain. 15O-labeled carbon monoxide, administered by inhalation, binds to hemoglobin in RBCs, and therefore can be used as a intravascular tracer to measure regional cerebral blood volume (rCBV). Several strategies have been developed to measure regional CBF using 15O-labeled water as an inert, diffusible flow tracer. Regional cerebral oxygen metabolism is measured using scan data obtained following the inhalation of 15O-labeled oxygen; independent determinations of local blood flow and blood volume are also required for this measurement. The tracer kinetic models used to measure rCBV, blood flow, and oxygen metabolism will be described and their relative advantages and limitations discussed. Several examples of the use of 15O tracer methods will be reviewed to demonstrate their widespread applicability to the study of cerebral physiology and pathophysiology.

Regional cerebral blood flow and oxygen utilization in superficial temporal-middle cerebral artery anastomosis patients: an exploratory definition of clinical problems

Regional cerebral blood flow (rCBF) and regional cerebral oxygen utilization (rCMRO2) were measured before and after surgery in nine patients undergoing a superficial temporal artery-middle cerebral artery (STA-MCA) anastomosis to bypass cerebrovascular lesions not amenable to extracranial operative procedures. The objective of these studies was to determine whether measurements of this type could provide objective criteria for surgery as well as assess the effect of surgery. The preliminary data, although limited, suggest that measurements of regional cerebral hemodynamics and metabolism in these patients before and after surgery provide valuable data upon which to develop criteria and assess results. For example, 1) a significant depression of rCBF and rCMRO2 in patients in whom a major cerebral infarction has not occurred, or 2) relative preservation of rCMRO2 despite depressed rCBF seem to be favorable indications for establishing a functioning STA-MCA anastomosis. In such patients, STA-MCA anastomosis can be followed by a return of rCBJ and rCMRO2 to virtually normal levels. Relatively normal rCBF and rCMRO2 in the presence of an occluded internal carotid artery in asymptomatic patients indicates satisfactory collateral circulation and is probably a contraindication to surgery.

Cerebral hemodynamics and metabolism in pseudotumor cerebri

A comprehensive analysis of cerebral hemodynamics and metabolism was carried out in 14 patients with pseudotumor cerebri. Tracer techniques were employed to measure cerebral blood flow (CBF) and vascular reactivity to acute changes in arterial carbon dioxide tension and blood pressure, cerebral blood volume (CBV), and the cerebral metabolic rate for oxygen and glucose. There was a small reduction (p less than 0.01) in CBF (44 +/- 7 ml/100 gm/min; normal, 54 +/- 9) with normal vascular reactivity; an increase (p less than 0.005) in CBV (4.8 +/- 0.8 ml/100 gm; normal, 3.6 +/- 0.5), and normal cerebral metabolism. We conclude that an abnormality of the cerebral microvasculature is responsible for an elevation in CBV, but the intracranial hypertension can be explained only by tissue swelling due to an increase in water content. The relationship between the vascular abnormality and the tissue swelling remains to be defined.

Assessment of regional myocardial blood flow and regional fractional oxygen extraction in dogs, using 15O-water and 15O-hemoglobin

A new approach to the assessment of regional myocardial blood flow and fractional oxygen extraction has been developed using 15O-water (H2-15O) and 15O-hemoglobin (15O-Hb). Bolus doses (1 mCi) of H2-15O and 15O-Hb were injected 10 minutes apart into the left main coronary artery of 12 normal dogs. Sequential images of regional myocardial tracer clearance were obtained over 5 minutes with a positron camera. Myocardial blood flow calculated from the monoexponential washout of H2-15O after background correction was 78 +/- 6 (SE) ml/100 g per min. Functional images of regional blood flow in which the image of peak activity was divided by the integrated image of H2-15O washout were derived by computer processing. These images demonstrated homogeneous blood flow in the normal myocardium. Fractional myocardial O2 extraction was determined from an image of initial distribution of O2 used (obtained by extrapolating back to time zero the series of images obtained after 15O-Hb administration), divided by initial distribution of O2 delivered (obtained by back extrapolating H2-15O washout). These functional images showed uniform distribution of fractional O2 extraction in the normal myocardium. Thus, these studies show that regional myocardial blood flow and regional oxygen extraction can be measured simultaneously by sequential imaging after serial intracoronary injections of H2-15O and 15O-Hb.

Effects of subarachnoid hemorrhage on cerebral blood volume, blood flow, and oxygen utilization in humans

Forty-five studies of regional cerebral blood volume (rCBV), regional cerebral blood flow (rCBF), and regional cerebral oxygen utilization (rCMRO2) were performed in 30 patients undergoing diagnostic cerebral angiography for evaluation of a subarachnoid hemorrhage due to a ruptured intracranial aneurysm. Tracer methods employing radioactive oxygen-15 were used to measure rCBV, rCBF, and rCMRO2. The patient studies were divided into groups based on their neurological status and the presence or absence of cerebral vasospasm. Subarachnoid hemorrhage, with and without vasospasm, produced significant decreases in CBF and CMRO2. In general, patients with more severe neurological deficits, and patients with more severe degrees of vasospasm, had a more marked depression of CBF and CMRO2. The most striking finding was a significant (p less than 0.001) increase in CBV (to 58% above normal) in patients with severe neurological deficits associated with severe cerebral vasospasm. This large increase suggests that cerebral vasospasm consists of constriction of the large, radiographically visible extraparenchymal vessels accompained by a massive dilation of intraparenchymal vessels.

The continuous inhalation of oxygen-15 for assessing regional oxygen extraction in the brain of man

A non-invasive steady-state method for studying the regional accumulation of oxygen in the brain by continuously inhaling Oxygen-15 has been investigated. Oxygen respiration by tissue results in the formation of water of metabolism which may be considered as the "exhaust product" of respiration. In turn the steady-state distribution of this product may be related to that of oxygen utilization. It has been found in monkeys that an appreciable component of the signal, recorded over the head during the inhalation of 15O2, is attributable to the local production of 15O-labelled water of metabolism. In man the distribution of radioactivity recorded over the head during 15O2 inhalation clearly relates to active cerebral tissue. Theoretically the respiration product is linearly dependent on the oxygen extraction ratio of the tissue, and at normal cerebral perfusion it is less sensitive to changes in blood flow. At low rates of perfusion a more linear dependence on flow is shown. The dual dependence on blood flow and oxygen extraction limits the interpretation of the cerebral distribution obtained with this technique. Means for obtaining more definitive measurements with this approach are discussed.

[Neurological complications of heart surgery. Review of their pathogenesis and bases for their treatment]

São relatadas as complicações neurológicas assinaladas em 320 pacientes submetidos a cirurgias cardíacas, com uma incidência de 7,8%. As etiologias mais comuns encontradas foram a embolia aérea (4,0%) e isquemia cerebral após hipotensão sistêmica (2,4%). Os autores analisam a patogenia das complicações cerebrais e tentam correlacioná-las com o fluxo sangüíneo cerebral, com o metabolismo cerebral e com a dinâmica dos pequenos vasos cerebrais. Uma revisão a respeito dos métodos para tratamento da isquemia cerebral é apresentada.