Measurement Of Cerebral Metabolism Research Articles

Relationships Between Resting Energy Expenditure and Transcranial Doppler Measurements in Patients With and Without Brain Death.

Introduction Brain metabolism deteriorates during brain death, suggesting that cerebral metabolic measurements could serve as a prognostic factor. The application of transcranial Doppler can be useful in evaluating patients evolving to brain death. Resting energy expenditure is lower than expected in patients with brain death, and this is caused by the decrease in cerebral blood flow and consequently lower oxygen supply. The primary aim of this retrospective study is to investigate the early metabolic changes in patients with clinical criteria of brain death and examine if these changes are related to a gradual decrease in blood flow velocities in the middle cerebral artery. Methods All consecutive patients from 1st June 2018 to 30th April 2022, admitted to the ICU with brain injury and a GCS ≤8, were included retrospectively in the study. Patients were allocated into two groups: Group A, patients without clinical signs of brain death (n = 32), and Group B, patients with brain death (n = 34). In each group, three sets of metabolic measurements were performed concomitantly with cerebral blood flow velocities using transcranial Doppler (a) upon admission to the ICU, (b) once hemodynamic stabilization was obtained, and (c) 48 hours after their hemodynamic stabilization or when brain death was confirmed by clinical criteria. Resting energy expenditure (REE) measurements were performed using a metabolic computer. Cerebral blood flow velocities were measured after a period of 30 min using a 2-MHZ 2D ultrasound probe. Results Brain-dead patients had a significant decrease in their metabolic parameters as the cerebral blood flow velocities recorded with the transcranial Doppler deteriorated, (REE Group A = 1667.65 ± 597 vs Group B = 1376.12 ± 615, p = 0.05 and REE predictedGroup A = 113.19 ± 44.9 vs Group B = 93.29 ± 41.5, p = 0.066 for measurement 1;REE Group A = 1844 ± 530.9 vs Group B = 1219.97 ± 489, p < 0.001 and REE predictedGroup A = 124.38 ± 39 vs Group B = 81.35 ± 30.4, p < 0.001 for measurement 2; REE Group A = 1750.97 ± 414, p < 0.001 and REE predicted Group A = 116.38 ± 19.2 vs Group B = 56.09 ± 19.6, p < 0.001 for measurement 3).Multiple stepwise regression analysis revealed a strong relationship between age, the worsening of the blood flow velocities pattern, and the decrease in REE (multiple R = 0.264, F = 5.55, p = 0.009).Furthermore, a statistically significant correlation was found between temperature and REE (correlation coefficient = 0.500, 0.674, 0.784 for measurements 1, 2, and 3, respectively, and p < 0.001 for all measures). Conclusions In brain-dead patients, the gradual decrease in cerebral blood flow leads to a decrease in REE as well as thermogenetic control. These changes can be detected early after the patient's admission to the ICU.

Functional recovery after accidental deep hypothermic cardiac arrest: Comparison of different cardiopulmonary bypass rewarming strategies.

Introduction: Using a porcine model of accidental immersion hypothermia and hypothermic cardiac arrest (HCA), the aim of the present study was to compare effects of different rewarming strategies on CPB on need for vascular fluid supply, level of cardiac restitution, and cerebral metabolism and pressures. Materials and Methods: Totally sixteen healthy, anesthetized castrated male pigs were immersion cooled to 20°C to induce HCA, maintained for 75 min and then randomized into two groups: 1) animals receiving CPB rewarming to 30°C followed by immersion rewarming to 36°C (CPB30, n = 8), or 2) animals receiving CPB rewarming to 36°C (CPB36, n = 8). Measurements of cerebral metabolism were collected using a microdialysis catheter. After rewarming to 36°C, surviving animals in both groups were further warmed by immersion to 38°C and observed for 2 h. Results: Survival rate at 2 h after rewarming was 5 out of 8 animals in the CPB30 group, and 8 out of 8 in the CPB36 group. All surviving animals displayed significant acute cardiac dysfunction irrespective of rewarming method. Differences between groups in CPB exposure time or rewarming rate created no differences in need for vascular volume supply, in variables of cerebral metabolism, or in cerebral pressures and blood flow. Conclusion: As 3 out of 8 animals did not survive weaning from CPB at 30°C, early weaning gave no advantages over weaning at 36°C. Further, in surviving animals, the results showed no differences between groups in the need for vascular volume replacement, nor any differences in cerebral blood flow or pressures. Most prominent, after weaning from CPB, was the existence of acute cardiac failure which was responsible for the inability to create an adequate perfusion irrespective of rewarming strategy.

Moderate hypothermia with remote ischaemic preconditioning improves cerebral protection compared to deep hypothermia: a study using a surviving porcine model.

The optimal temperature management of hypothermic circulatory arrest is still controversial. Moderate hypothermia preserves cerebral autoregulation and shortens cardiopulmonary bypass (CPB) duration. However, moderate hypothermia alone has inferior organ protection to deep hypothermia, so adjuncts that increase the ischaemic tolerance are needed. Thus, we hypothesized that a combination of remote ischaemic preconditioning (RIPC) and moderate hypothermia would be superior to deep hypothermia alone. Sixteen pigs were randomized to either RIPC or control groups (8 + 8). The RIPC group underwent 4 cycles of transient hind limb ischaemia. The RIPC group underwent cooling with CPB to 24°C, and the control group underwent cooling with CPB to 18°C, followed by a 30-min arrest period and subsequent rewarming to 36°C. Measurements of cerebral metabolism were made from sagittal sinus blood samples and common carotid artery blood flow. The permissible periods of hypothermic circulatory arrest were calculated based on these measurements. Neurological recovery was evaluated daily during a 7-day follow-up, and the brain was harvested for histopathological analysis. Six pigs in the RIPC group reached normal neurological function, but none in the control group reached normal neurological function (P = 0.007). The composite neurological score of all postoperative days was higher in the RIPC group than in the control group [55 (52-58) vs 45 (39-51), P = 0.026]. At 24°C, the estimated permissible periods of hypothermic circulatory arrest were 21 (17-25) min in the RIPC group and 11 (9-13) min in the control group (P = 0.007). RIPC combined with moderate hypothermia provides superior cerebral protection.

Vulnerability of multiple large-scale brain networks in dementia with Lewy bodies.

Aberrations of large-scale brain networks are found in the majority of neurodegenerative disorders. The brain connectivity alterations underlying dementia with Lewy bodies (DLB) remain, however, still elusive, with contrasting results possibly due to the pathological and clinical heterogeneity characterizing this disorder. Here, we provide a molecular assessment of brain network alterations, based on cerebral metabolic measurements as proxies of synaptic activity and density, in a large cohort of DLB patients (N = 72). We applied a seed-based interregional correlation analysis approach (p < .01, false discovery rate corrected) to evaluate large-scale resting-state networks' integrity and their interactions. We found both local and long-distance metabolic connectivity alterations, affecting the posterior cortical networks, that is, primary visual and the posterior default mode network, as well as the limbic and attention networks, suggesting a widespread derangement of the brain connectome. Notably, patients with the lowest visual and attention cognitive scores showed the most severe connectivity derangement in regions of the primary visual network. In addition, network-level alterations were differentially associated with the core clinical manifestations, namely, hallucinations with more severe metabolic dysfunction of the attention and visual networks, and rapid eye movement sleep behavior disorder with alterations of connectivity of attention and subcortical networks. These multiple network-level vulnerabilities may modulate the core clinical and cognitive features of DLB and suggest that DLB should be considered as a complex multinetwork disorder.

Regional association of pCASL-MRI with FDG-PET and PiB-PET in people at risk for autosomal dominant Alzheimer's disease.

Autosomal dominant Alzheimer's disease (ADAD) is a small subset of Alzheimer's disease that is genetically determined with 100% penetrance. It provides a valuable window into studying the course of pathologic processes that leads to dementia. Arterial spin labeling (ASL) MRI is a potential AD imaging marker that non-invasively measures cerebral perfusion. In this study, we investigated the relationship of cerebral blood flow measured by pseudo-continuous ASL (pCASL) MRI with measures of cerebral metabolism (FDG PET) and amyloid deposition (Pittsburgh Compound B (PiB) PET). Thirty-one participants at risk for ADAD (age 39 ± 13 years, 19 females) were recruited into this study, and 21 of them received both MRI and FDG and PiB PET scans. Considerable variability was observed in regional correlations between ASL-CBF and FDG across subjects. Both regional hypo-perfusion and hypo-metabolism were associated with amyloid deposition. Cross-sectional analyses of each biomarker as a function of the estimated years to expected dementia diagnosis indicated an inverse relationship of both perfusion and glucose metabolism with amyloid deposition during AD development. These findings indicate that neurovascular dysfunction is associated with amyloid pathology, and also indicate that ASL CBF may serve as a sensitive early biomarker for AD. The direct comparison among the three biomarkers provides complementary information for understanding the pathophysiological process of AD.

Pathophysiology and the Monitoring Methods for Cardiac Arrest Associated Brain Injury.

Cardiac arrest (CA) is a well-known cause of global brain ischemia. After CA and subsequent loss of consciousness, oxygen tension starts to decline and leads to a series of cellular changes that will lead to cellular death, if not reversed immediately, with brain edema as a result. The electroencephalographic activity starts to change as well. Although increased intracranial pressure (ICP) is not a direct result of cardiac arrest, it can still occur due to hypoxic-ischemic encephalopathy induced changes in brain tissue, and is a measure of brain edema after CA and ischemic brain injury. In this review, we will discuss the pathophysiology of brain edema after CA, some available techniques, and methods to monitor brain oxygen, electroencephalography (EEG), ICP (intracranial pressure), and microdialysis on its measurement of cerebral metabolism and its usefulness both in clinical practice and possible basic science research in development. With this review, we hope to gain knowledge of the more personalized information about patient status and specifics of their brain injury, and thus facilitating the physicians’ decision making in terms of which treatments to pursue.

The correlation between brain near-infrared spectroscopy and cerebral blood flow in piglets with intracranial hypertension.

Cerebral perfusion pressure (CPP) is used as a surrogate for measurement of cerebral blood flow (CBF) but its determination requires that intracranial pressure be directly measured. Near-infrared spectroscopy (NIRS) can noninvasively measure tissue oxygenation. We hypothesized that NIRS would correlate well with CBF, with cerebral metabolism of oxygen (CMRO2) and glucose and with lactate production as CPP was reduced. Seven anesthetized piglets were subjected to reductions in CPP to 60, 50, 40, 30, and 20 mmHg by infusing an artificial cerebral spinal fluid into the lateral ventricle of the brain. After a period of equilibration, NIRS over the left temporal cortex and regional CBF (microspheres) were measured at each CPP level as well as arterial and internal jugular PaO2 , glucose, and lactate. CMRO2 and glucose consumption and lactate production were calculated by standard formulae. NIRS correlated very well (P < 0.05) with CBF in the left temporal cortex [mean r (95% CI) = 0.95 (0.91-0.99)] and with left hemispheric CMRO2 [0.94 (0.90-0.98)], glucose consumption [0.87 (0.76-0.97)], and lactate production [0.89 (0.81-0.97)]. The correlation of NIRS with CBF was slightly better (P < 0.05) than that of CPP with CBF [0.89 (0.84-0.94)]. In this model of global cerebral hypertension, NIRS correlated well with CBF and measures of cerebral metabolism, and might be useful as a surrogate for CPP. Further studies are warranted to determine if NIRS is associated with these variables in focal cerebral injury.

Metabolic imaging of ischemic stroke: the present and future.

Measures of cerebral metabolism may be useful in the selection of patients for reperfusion therapies and as end points in clinical trials. However, there are currently no clinically routine techniques that provide such data directly. We review how imaging modalities in current clinical use may provide surrogate markers of metabolic activity. Promising techniques for metabolic imaging that are currently in the pipeline are reviewed.

March 2013: This Month in JoVE

Here are some highlights from the March 2013 issue of Journal of Visualized Experiments (JoVE). This month, JoVE features several video articles in the Clinical and Translational Medicine section. In the ophthalmology department, Connors et al. of Massachusetts Eye & Ear and Harvard Medical School have developed an audio-based virtual environment simulator to help blind people improve their navigation skills. The simulator allows users to build cognitive maps of three-dimensional spaces in the context of a video game-using only audio cues. The users were not aware that the virtual building in the game represents an actual building; furthermore, the users were never asked to recall the building layout while playing. Nonetheless it appears that the game can help them navigate through unfamiliar spaces. At Massachusetts General Hospital and Harvard Medical School, Lin et al. are developing an improved method to noninvasively measure brain metabolism and blood flow in newborns. This method combines measurements of hemoglobin oxygen saturation (SO2) with cerebral blood flow index (CBFi), which provides accurate measurements of cerebral metabolism and hemodynamics. This method also complies with neonatal ICU policies, making it potentially useful for assessing brain health, development, and response to therapy in newborns. Also in this section, Lee et al. of UC San Francisco/VA Medical Center demonstrate how a biopolymer gel, derived from polysaccharides found in brown algae, can help patients with heart failure. The alginate-based biopolymer is injected into the area between the base and apex of the left ventricle free wall. This reduces left ventricle size and thickens its free wall; it also reduces left ventricular wall stress, as assessed by mathematical modeling. This therapy, called Algisyl-LVR, is currently in clinical development for treating patients with dilated cardiomyopathy. In JoVE Immunology and Infection, Robinson et al. of University of Texas Health Science Center-Houston demonstrate a noninvasive technique for imaging lymphatic vessels, which are involved in many physiological and disease processes. A near-infrared fluorescent dye, such as indocyanine green (ICG), is injected intradermally-allowing the lymphatic vasculature to be visualized using a near-infrared fluorescence imaging system. The JoVE Applied Physics section features a method in the field of fluid mechanics, developed at Brigham Young University in collaboration with the Naval Undersea Warfare Center of Newport, Rhode Island. Truscott et al. describe a novel technique for quantitative three-dimensional imaging of fluid flow fields, such as the airflow passing over a set of synthetic vocal folds, or bubbly flow fields. This method can help resolve a range of problems in the area of fluid mechanics. This summary provides a preview of the cutting-edge video publications appearing this month in JoVE. Visit the website and check out the full-length articles, plus many more, in JoVE: The Journal of Visualized Experiments.

PO10-TU-15 PET measurements of cerebral metabolism correlate with magnetic resonance spectroscopy in early multiple sclerosis

PO10-TU-15 PET measurements of cerebral metabolism correlate with magnetic resonance spectroscopy in early multiple sclerosis

Cerebral blood flow and metabolism.

To date, regional measurements of cerebral blood flow in humans have not been especially helpful in predicting the outcome of acute brain injury. The recent appearance within the medical environment of biologically significant pharmaceuticals labelled with the positron-emitting isotopes 15O, 13N and 11C has made possible the quantitative regional measurement of cerebral metabolism in humans. This advance, coupled with the development of radically new detection systems, will allow the safe measurement of truly regional blood flow and metabolism in humans in the near future. Such information is basic to an understanding of the pathophysiology of acute injury to the brain and the rational prediction of outcome in the individual case.

Comparison of local measurement of cerebral metabolism and to cerebral PvO2 during alterations in intracranial pressure, PaCO2 and arterial pressure--an experimental study in goat.

to assess the changes in local brain PO2, PCO2, pH (PO2br, PCO2br, pHbr) measured by a intraparenchymal probe (Neurotrend, Codeman) and compare them to simultaneous recording of cerebral PvO2 and blood flow (CBF). Arterial, venous longitudinal sinus blood samples and CBF were analyzed in 8 adult anesthetized, ventilated goats. Three step increase of intracranial pressure (ICP) (16, 22, 29 mm Hg) were performed by inflation of an epidural balloon. At each ICP level, similar changes in MAP and in PaCO2 were performed. At constant PaCO2 and MAP, balloon inflation induced a fast response: a decrease of PO2br, PCO2br and pHbr (starting 14 +/- 12 sec, 45 +/- 23 sec and 55 +/- 19 sec after the peak ICP, respectively). Since the second inflation level, PO2br decreased (p < 0.05) despite an ICP returning at 22 mm Hg and a cerebral perfusion pressure (CPP) larger than 90 mmHg. During changes in PaCO2, PO2br paralleled CBF and PvO2 before the second balloon inflation but diverged at higher ICP. In the same time pH-pHbr gradient rose. Hypotension did not induce sizeable changes. The direct metabolic monitoring of cerebral tissue locally compressed show fast response. At steady state, it shows alterations which are not detected by the measurement of the oxygen saturation in the longitudinal sinus or that of CBF. It confirms that the threshold for ICP which may require therapy in presence of focal brain compression is around 20 mm Hg even in the presence of a CPP > 90 mm Hg.

Clinical significance of reduced cerebral metabolism in multiple sclerosis: a combined PET and MRI study.

Magnetic resonance imaging (MRI) in patients with multiple sclerosis (MS) has provided major insights into the disease's natural history, and many studies have focussed on possible correlations between MRI findings and the clinical manifestations of MS. In contrast, there are few reports on possible relationships between functional imaging data and cognitive function. The present study assessed the relationship between clinical presentation and combined anatomical and functional imaging data in MS. Twenty patients with definite MS underwent MRI and positron emission tomography (PET) to evaluate cerebral blood flow (rCBF) and oxygen metabolism (rCMRO2). The relationships between these neuroimaging findings and clinical data, including the Expanded Disability Status Scale (EDSS), Mini-mental status scale, Hasegawa Dementia Scale and relapse time, were evaluated with Spearman's rank correlation coefficients. A general reduction in rCBF and rCMRO2 in the gray and white matter were found in the MS patients. EDSS was correlated with the number and size of the lesions on MRI and was negatively correlated with rCMRO2. A correlation between the decrease in rCMRO2 and the level of cognitive impairment was also found. The severity of cerebral hypometabolism was also related to the number of relapses. Morphological and functional findings obtained by MRI and PET are closely related to the clinical status in MS. Our results suggest that measurement of cerebral metabolism in MS has the potential to be an objective marker for monitoring disease activity and to provide prognostic information.

Effect of hyperventilation on regional cerebral blood flow in head-injured children.

To study cerebral blood flow and cerebral oxygen consumption in severe head-injured children and also to assess the effect of hyperventilation on regional cerebral blood flow. Prospective cohort study. Pediatric intensive care unit at a tertiary-level university children's hospital. Twenty-three children with isolated severe brain injury, whose admission Glasgow Coma Scores were <8. PaCO2 was adjusted by altering minute ventilation. Cerebral metabolic measurements were made at three levels of PaCO2 (>35, 25 to 35, and <25 torr [>4.7, 3.3 to 4.7, and <3.3 kPa]) after allowing 15 mins for equilibrium. Thirty-eight studies (each study consisting of three sets of measurements at different levels of PaCO2) were performed on 23 patients. At each level of PaCO2, the following measurements were made: xenon-enhanced computed tomography scans; cerebral blood flow; intracranial pressure; jugular venous bulb oxygen saturation; mean arterial pressure; and arterial oxygen saturation. Derived variables included: cerebral oxygen consumption; cerebral perfusion pressure; and oxygen extraction ratio. Cerebral blood flow decreased below normal after head injury (mean 49.6 +/- 14.6 mL/min/100 g). Cerebral oxygen consumption decreased out of proportion to the decrease in cerebral blood flow; cerebral oxygen consumption was only a third of the normal range (mean 1.02 +/- 0.59 mL/min/100 g). Neither cerebral blood flow nor cerebral oxygen consumption showed any relationship to time after injury, Glasgow Coma Score at the time of presentation, or intracranial pressure. The frequency of one or more regions of ischemia (defined as cerebral blood flow of <18 mL/min/100 g) was 28.9% during normocapnia. This value increased to 73.1% for PaCO2 at <25 torr. Severe head injury in children produced a modest decrease in cerebral blood flow but a much larger decrease in cerebral oxygen consumption. Absolute hyperemia was uncommon at any time, but measured cerebral blood flow rates were still above the metabolic requirements of most children. The clear relationship between the frequency of cerebral ischemia and hypocarbia, combined with the rarity of hyperemia, suggests that hyperventilation should be used with caution and monitored carefully in children with severe head injuries.

Flow-related anaerobic metabolic changes in patients with severe stenosis of the internal carotid artery.

We sought to investigate whether the combination of blood flow measurements in the major cerebral arteries and measurements of cerebral metabolism can provide new insight into the hemodynamic effect of carotid lesions in patients with severe stenosis (> 70% reduction in diameter) of the internal carotid artery (ICA). Fifty-six patients with unilateral severe stenosis of the ICA and 14 control subjects underwent MR imaging, 1H MR spectroscopy, and MR angiography. Anaerobic metabolic changes were studied by assessing N-acetyl aspartate/choline and lactate/N-acetyl aspartate ratios in the symptomatic and asymptomatic hemispheres. Quantitative flow was measured in the common carotid arteries (CCAs), the ICAs, the basilar artery, and the middle cerebral arteries (MCAs). Blood flow was significantly decreased in the CCA, ICA, and MCA on the ipsilateral side compared with the contralateral side. Flow in the basilar artery was increased, whereas flow in the contralateral MCA was decreased compared with control subjects. We found a significant correlation between anaerobic metabolic changes and the reduction in blood flow in the CCA, ICA, and MCA on the ipsilateral side. This study shows that cerebral metabolism is less impaired in patients with relatively high flow in the major cerebral arteries on the ipsilateral side than in patients with relatively low flow on that side. The combination of MR spectroscopy and MR angiography can be of additional value in the understanding of cerebral hemodynamics and metabolism in patients with vascular disorders.

Regional hypometabolism in Alzheimer's disease as measured by positron emission tomography after correction for effects of partial volume averaging.

Measurements of cerebral metabolism in patients with Alzheimer's disease (AD) using PET are artifactually depressed due to partial volume averaging of brain tissue activity with enlarged CSF spaces. To investigate the effects of correction for the expansion of CSF spaces on regional metabolic measures, as well as the correlations between neuropsychological test results and resting cerebral metabolism before and after partial volume correction, we applied an MRI-based method of partial volume correction to 18F-fluorodeoxyglucose (FDG)-PET data from eight patients diagnosed with probable AD and ten healthy elderly individuals. Before correction, the AD group had significantly lower cortex-to-cerebellum ratios in the posterior temporal, parietal, and frontal lobes in comparison to the control subjects. Partial volume correction of PET data resulted in 19 to 49% increases in regional activity in the AD group and 16 to 38% increases in the control group. The patients' persistence of significant hypometabolism in the frontal, posterior temporal, and parietal regions after partial volume correction suggests that a true reduction in regional cerebral glucose metabolism occurs in AD, even though its magnitude is a result of both metabolic reductions and the effects of atrophy. Partial volume correction of PET data in the AD group had a significant impact on the correlations between regional glucose metabolism and neuropsychological performance. These findings suggest that accounting for differential extent and distribution of cerebral atrophy in patients with AD and in healthy individuals may potentially improve our ability to interpret specific cognitive dysfunction in the context of the functional imaging data.

Progress in the measurement of laterality and implications for dyslexia research

Much of contemporary laterality research has been motivated by a need to increase the accuracy with which individuals can be classified as left- or right-hemisphere dominant for speech and language. Efforts to improve the classification accuracy of laterality methods have led not only to the refinement of laterality methods but also to the discovery of some of the mechanisms that contribute to asymmetric performance. Despite these advances, laterality methods still do not lead to definitive conclusions about hemispheric specialization in the individual case, and special caution must be used when interpreting results for individual dyslexic children. Event-related measurements of cerebral metabolism, still in the development phase, promise to complement but not replace behavioral laterality methods.

Stimulus rate dependence of regional cerebral blood flow in human striate cortex, demonstrated by positron emission tomography.

The purpose of this investigation was to determine the relationship between the repetition rate of a simple sensory stimulus and regional cerebral blood flow (rCBF) in the human brain. Positron emission tomography (PET), using intravenously administered H2(15)O as the diffusible blood-flow tracer, was employed for all CBF measurements. The use of H2(15)O with PET allowed eight CBF measurements to be made in rapid sequence under multiple stimulation conditions without removing the subject from the tomograph, thus minimizing changes in base-line CBF and in head position due to longer intervals between scans. Nine normal volunteers each underwent a series of eight H2(15)O PET measurements of CBF. Initial and final scans were made during visual deprivation. The six intervening scans were made during visual activation with patterned-flash stimuli given in random order at 1.0-, 3.9-, 7.8-, 15.5-, 33.1-, and 61-Hz repetition rates. In each subject the region of greatest rCBF increase was determined. Within this region the rCBF was determined for every test condition and then expressed as the percentage change from the value of the initial unstimulated scan (rCBF% delta). Anatomical localization of the region of greatest rCBF response was performed employing bony landmarks from a lateral skull radiograph, a template of the cranium created from a transmission attenuation scan and a stereotaxic atlas. In every subject, striate cortex rCBF% delta varied systematically with stimulus rate. Between 0 and 7.8 Hz, rCBF% delta was a linear function of stimulus repetition rate. The rCBF response peaked at 7.8 Hz and then declined. The rCBF% delta during visual stimulation was significantly greater than that during visual deprivation for every stimulus rate except 1.0 Hz. The anatomical localization of the region of peak rCBF response was determined for every subject to be the mesial occipital lobes along the calcarine fissure, primary visual cortex. We conclude that stimulus rate is a significant determinant of rCBF response in the visual cortex. Investigators of brain responses to selective activation procedures should be aware of the potential effects of stimulus rate on rCBF and other measurements of cerebral metabolism. For cerebral responses to selective activation to be meaningfully interpreted, the stimulus repetition rate must be taken into consideration. Response amplitude may be maximized by proper rate selection or be undetectable due to selection of too high or too low a repetition rate. Stimulus rate must be controlled for when responses to unlike stimuli or performance tasks are compared or ambiguities will be present as to whether response differences are

Quantitative measurements of cerebral metabolism with the searle positron camera

Quantitative measurements of cerebral metabolism with the searle positron camera