Vascular Mean Transit Time Research Articles

Spatial coefficient of variation in pseudo-continuous arterial spin labeling cerebral blood flow images as a hemodynamic measure for cerebrovascular steno-occlusive disease: A comparative 15O positron emission tomography study

Pseudo-continuous arterial spin labeling (pCASL) is a completely non-invasive method of cerebral perfusion measurement. However, cerebral blood flow (CBF) quantification is hampered by arterial transit artifacts characterized by bright vascular signals surrounded by decreased signals in tissue regions, which commonly appear in patients with reduced cerebral perfusion pressure. The spatial coefficient of variation (CoV) of pCASL CBF images has been proposed as an alternative region-of-interest (ROI)-based hemodynamic measure to predict prolonged arterial transit time (ATT). This retrospective study investigates the utility of spatial CoV by comparison with 15O positron emission tomography (PET). For patients with cerebrovascular steno-occlusive disease (n = 17), spatial CoV was positively correlated with ATT independently measured by pulsed arterial spin labeling (r = 0.597, p < 0.001), confirming its role as an ATT-like hemodynamic measure. Comparisons with 15O PET demonstrated that spatial CoV was positively correlated with vascular mean transit time (r = 0.587, p < 0.001) and negatively correlated with both resting CBF (r = −0.541, p = 0.001) and CBF response to hypercapnia (r = −0.373, p = 0.030). ROI-based spatial CoV calculated from single time-point pCASL can potentially detect subtle perfusion abnormalities in clinical settings.

Simulation of contrast agent transport in arteries with multilayer arterial wall: impact of arterial transmural transport on the bolus delay and dispersion.

One assumption of DSC-MRI is that the injected contrast agent is kept totally intravascular and the arterial wall is impermeable to contrast agent. The assumption is unreal for such small contrast agent as Gd-DTPA can leak into the arterial wall. To investigate whether the unreal assumption is valid for the estimation of the delay and dispersion of the contrast agent bolus, we simulated flow and Gd-DTPA transport in a model with multilayer arterial wall and analyzed the bolus delay and dispersion qualified by mean vascular transit time (MVTT) and the variance of the vascular transport function. Factors that may affect Gd-DTPA transport hence the delay and dispersion were further investigated, such as integrity of endothelium and disturbed flow. The results revealed that arterial transmural transport would slightly affect MVTT and moderately increase the variance. In addition, although the integrity of endothelium can significantly affect the accumulation of contrast agent in the arterial wall, it had small effects on the bolus delay and dispersion. However, the disturbed flow would significantly increase both MVTT and the variance. In conclusion, arterial transmural transport may have a small effect on the bolus delay and dispersion when compared to the flow pattern in the artery.

The vascular mean transit time: a surrogate for the penumbra flow threshold?

Depicting the salvageable tissue is increasingly used in the clinical setting following stroke. As absolute cerebral blood flow (CBF) is difficult to measure using perfusion magnetic resonance or computed tomography and has limitations as a penumbral marker, time-based variables, particularly the mean transit time (MTT), are routinely used as surrogates. However, a direct validation of MTT as a predictor of the penumbra threshold using gold-standard positron emission tomography (PET) is lacking. Using (15)O-PET data sets obtained from two independent acute stroke samples (N=7 and N=30, respectively), we derived areas under the curve (AUCs), optimal thresholds (OTs), and 90%-specificity thresholds (90%-Ts) from receiver operating characteristic curves for absolute MTT, MTT delay, and MTT ratio to predict three penumbra thresholds ('classic': CBF <20 mL/100 g per min; 'normalized': CBF ratio <0.5; and 'stringent': both CBF <20 mL/100 g per min and oxygen extraction fraction >0.55). In sample 1, AUCs ranged from 0.79 to 0.92, indicating good validity; OTs ranged from 7.8 to 8.3 seconds, 2.8 to 4.7 seconds, and 151% to 267% for absolute MTT, MTT delay, and MTT ratio, respectively, while as expected, 90%-Ts were longer. There was no significant difference between sample 1 and sample 2 for any of the above measurements, save for a single MTT parameter with a single penumbra threshold. These consistent findings from gold-standard PET obtained in two independent cohorts document that MTT is a very good surrogate to CBF for depicting the penumbra threshold.

Comparison of transcranial Doppler ultrasonography and positron emission tomography using a three-dimensional template of the middle cerebral artery

Objective: Transcranial Doppler (TCD) measures blood flow velocities (BFV) and is an indirect method of assessing cerebral blood flow (CBF). Positron emission tomography (PET) is a direct method to measure CBF. This study evaluates the correlations between TCD and PET findingsMethods: Nine patients with a symptomatic carotid artery stenosis, who underwent CEA, were studied pre- and post-operatively on the ipsi- and contralateral sides. Measurements of the BFV, CO2 reactivity, CBF, cerebral blood volume (CBV) and mean vascular transit time (MVTT) were performed using a three-dimensional volume of interest (VOI) for the middle cerebral artery (MCA).Results: CBF in the MCA region, as measured with PET, shows a good correlation with BFV, as measured with TCD, with similar pattern for total, gray and white matter MCA territory (Pearson's correlation coefficients: 0.751, 0.748 and 0.748, respectively). This correlation was found in the pre-operative as well as the post-operative state. No association could be demonstrated between CO2 reactivity and CBV or (Pearson's correlation coefficients: 0.051 and 0.166, respectively).Conclusion: With PET, it is possible to create three-dimensional VOI of arterial territories. CBF measured in these VOI seems to correlate with BFV before and after CEA on ipsi- and contralateral sides, while CBV shows no association with pre-operative CO2 reactivity.

Mapping the Zonal Organization of Tumor Perfusion and Permeability in a Rat Glioma Model by Using Dynamic Contrast-enhanced Synchrotron Radiation CT

To depict and analyze in vivo the tumor zone organization of C6 gliomas depicted on quantitative parametric maps obtained with dynamic contrast material-enhanced synchrotron radiation computed tomography (CT) in a tightly controlled data-processing protocol. Animal use was compliant with official French guidelines and was assessed by the local Internal Evaluation Committee for Animal Welfare and Rights. Fifteen Wistar rats with orthotopically implanted gliomas were studied at monochromatic synchrotron radiation CT after receiving a bolus injection of contrast material. The iodine concentration maps were analyzed by using a compartmental model selected from among a package of models. Choice of model and assessment of the relevance of the model were guided by quality criteria. Tissue blood flow (F(T)), tissue blood volume fraction (V(T)), permeability-surface area product (PS), artery-to-tissue delay (D(A-T)), and vascular mean transit time (MTT) maps were obtained. Parametric map findings were compared with histologic findings. Local regions of interest were selected in the contralateral hemisphere and in several tumor structures to characterize the tumor microvasculature. Differences in parameter values between regions were assessed with the Wilcoxon method. Whole-tumor parameters were expressed as means +/- standard errors of the mean: Mean F(T), V(T), PS, and D(A-T) values and MTT were 61.4 mL/min/100 mL +/- 15.3, 2.4% +/- 0.4, 0.37 mL/min/100 mL +/- 0.11, 0.24 second +/- 0.06; and 3.9 seconds +/- 0.83, respectively. MTT and mean PS were significantly lower (P < .01) in the normal contralateral tissue: 1.10 seconds +/- 0.06 and < or = 10(-5) mL/min/100 mL, respectively. Tumor regions were characterized by significantly different (P < .05) F(T) and V(T) pairs: 108 mL/min/100 mL and 3.66%, respectively, at the periphery; 45.9 mL/min/100 mL and 1.91%, respectively, in the intermediate zone; 5.1 mL/min/100 mL and 0.42%, respectively, in the center; and 210 mL/min/100 mL and 6.82%, respectively, in the maximal value region. Fine mapping of the glioma microcirculation is feasible with dynamic contrast-enhanced synchrotron radiation CT performed with well-controlled analytic protocols. http://radiology.rsnajnls.org/cgi/content/full/2501071929/DC1.

Peroperative Neuromonitoring during Carotid Endarterectomy in Relation to Preoperative Positron Emission Tomography Findings

Objectives: To compare stump pressure (SP), transcranial Doppler (TCD), electroencephalography (EEG) and selective shunting during carotid endarterectomy (CEA) with preoperative positron emission tomography (PET) parameters. Materials and methods: Preoperative PET measurements and preoperative neuromonitoring were performed in ten patients undergoing CEA for symptomatic carotid artery disease. PET parameters measured were cerebral blood flow (CBF), oxygen extraction fraction (OEF), cerebral oxygen metabolism (CMRO2), cerebral blood volume (CBV), mean vascular transit time (MVTT) and cerebral perfusion pressure (CPP). Results of these measurements in ipsilateral medial cerebral artery (MCA), ipsilateral hemisphere and total cerebrum were compared with absolute mean SP, mean SP < 40 mmHg, TCD, EEG changes and selective shunting. Results: None of the PET parameters showed any significant correlations with peroperative neuromonitoring findings. There were only trends for correlations of CBF and MVTT with TCD changes and of CPP and CMRO2 with selective shunting. Conclusions: Preoperative PET examinations are not usefull for predicting the need for shunting during CEA.

Peroperative Neuromonitoring during Carotid Endarterectomy in Relation to Preoperative Positron Emission Tomography Findings

To compare stump pressure (SP), transcranial Doppler (TCD), electroencephalography (EEG) and selective shunting during carotid endarterectomy (CEA) with preoperative positron emission tomography (PET) parameters. Preoperative PET measurements and peroperative neuromonitoring were performed in ten patients undergoing CEA for symptomatic carotid artery disease. PET parameters measured were cerebral blood flow (CBF), oxygen extraction fraction (OEF), cerebral oxygen metabolism (CMRO(2)), cerebral blood volume (CBV), mean vascular transit time (MVTT) and cerebral perfusion pressure (CPP). Results of these measurements in ipsilateral medial cerebral artery (MCA), ipsilateral hemisphere and total cerebrum were compared with absolute mean SP, mean SP<40mmHg, TCD, EEG changes and selective shunting. None of the PET parameters showed any significant correlations with peroperative neuromonitoring findings. There were only trends for correlations of CBF and MVTT with TCD changes and of CPP and CMRO(2) with selective shunting. Preoperative PET examinations are not useful for predicting the need for shunting during CEA.

Cerebral Vascular Mean Transit Time in Healthy Humans: A Comparative Study with PET and Dynamic Susceptibility Contrast-Enhanced MRI

Cerebral vascular mean transit time (MTT), defined as the ratio of cerebral blood volume to cerebral blood flow (CBV/CBF), is a valuable indicator of the cerebral circulation. Positron emission tomography (PET) and dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSC-MRI) are useful for the quantitative determination of MTT in the clinical setting. The aim of this study was to establish a normal value set of MTT as determined by PET and by DSC-MRI and to identify differences between these methods. Seven healthy volunteers were studied with (15)O-PET (H(2)(15)O and C(15)O) and gradient-echo echo-planar DSC-MRI at 1.5 T. In the DSC-MRI study with bolus injection of contrast agent, deconvolution analysis was performed. Comparison of gray-to-white matter ratios showed fairly good agreement between PET and DSC-MRI for all parameters (relative CBV, relative CBF, and relative MTT), confirming the validity of relative measurements with DSC-MRI. However, quantitative MTT measured by DSC-MRI was significantly shorter than that measured by PET in cerebral cortical regions (2.8 to 3.0 secs for DSC-MRI versus 3.9 to 4.3 secs for PET) and the centrum semiovale (3.5 secs for DSC-MRI versus 4.8 secs for PET). These discrepancies may be because of the differences in the intrinsic sensitivity of each imaging modality to vascular components; whereas PET measurement of CBV is equally sensitive to all vascular components, measurement with DSC-MRI originates from the microvasculature in the vicinity of the brain parenchyma. This underlying difference may influence interpretation of MTT determined by PET or by DSC-MRI for patients with cerebrovascular disease.

Cerebral vascular mean transit time determined by PET and DSC-MRI

Cerebral vascular mean transit time determined by PET and DSC-MRI

Tracer Delay Correction of Cerebral Blood Flow with Dynamic Susceptibility Contrast-Enhanced MRI

Cerebral blood flow (CBF) and vascular mean transit time (MTT) can be determined by dynamic susceptibility contrast-enhanced magnetic resonance imaging and deconvolution with an arterial input function. However, deconvolution by a singular value decomposition (SVD) method is sensitive to the tracer delay that often occurs in patients with cerebrovascular disease. We investigated the effect of tracer delay on CBF determined by SVD deconvolution. Simulation study showed that underestimation of CBF due to tracer delay was larger for shorter MTTs. We developed a delay correction method that determines tracer delay by means of least-squares fitting pixel-by-pixel. The corrected CBF was determined by SVD deconvolution after time-shifting of the measured concentration curve. The simulations showed that the corrected CBF was insensitive to tracer delay irrespective of the vascular model, although CBF fluctuation increased slightly. We applied the delay correction to the CBF and MTT images acquired for nine patients with hyperacute stroke and unilateral occlusion of the middle cerebral artery. We found in some patients that the delay correction modulated the contrast of CBF and MTT images. For hyperacute stroke patients, tracer delay correction is essential to obtain reliable perfusion image when SVD deconvolution is used.

Regional distribution of human cerebral vascular mean transit time measured by positron emission tomography

Cerebral vascular mean transit time (MTT) characterizes the cerebral circulation. MTT has been measured in humans by carotid angiography, x-ray computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET). However, regional distribution of MTT has not been investigated in detail. Thus, we investigated regional distribution of normal human MTT. Regional cerebral blood flow (CBF) and cerebral blood volume (CBV) were measured by PET with H 2 15O and 11CO, respectively, in each of 10 normal subjects. MTT was calculated as MTT = CBV/CBF. MTT for cerebral cortical regions was 3.2 to 4.4 s. These values were in accord with MTT measured by carotid angiography, CT, and MRI. Considered regionally, MTT was longest in the temporooccipital cortex, and shorter in the cerebellum, thalamus, and putamen, than in all other regions. These regional differences in MTT that are inversely proportional to cerebral perfusion pressure might relate to regional differences in cerebral vascular tone. Simulation studies showed that errors in CBF and thus MTT caused by regional differences in regional tracer appearance time, distribution volume, and gray–white matter mixing were negligible.

Evaluation of flow characteristics of soft-tissue vascular malformations using technetium-99m labelled red blood cells

The estimation of intralesional haemodynamics is crucial in determining appropriate treatment for soft-tissue vascular malformations. The aim of this study was to develop a method to evaluate the flow characteristics of soft-tissue vascular malformations using technetium-99m labelled red blood cells (99mTc-RBCs). Seventy-nine soft-tissue vascular malformations, including 20 arteriovenous malformations and 59 venous malformations, in 57 patients were examined. Following the intravenous injection of 99mTc-RBCs, dynamic imaging was performed for 30 min with the lesion in the field of view (99mTc-RBC flow study). A time-activity curve was generated for the lesion, and the lesion was categorized as a high-flow or low-flow lesion by visual inspection of the curve. In low-flow lesions, mean vascular transit time (MTT) was calculated by curve fitting based on a two-compartment model. Twenty-nine lesions in 19 patients were examined twice, and reproducibility was assessed. In 23 venous malformations in 16 patients, 99mTc-Sn colloid was percutaneously injected into the intravascular space of the lesion, and dynamic data of 5-min duration were acquired (direct puncture scintigraphy). MTT was estimated from the washout curve and compared with MTT estimated by 99mTc-RBC flow study. 99mTc-RBC flow study classified all 20 arteriovenous malformations as high-flow lesions and all 59 venous malformations as low-flow lesions. In the low-flow lesions, MTT estimated by 99mTc-RBC flow study ranged from 61.2 to 2174.9 s. In the reproducibility study, complete concordance in classification and high correlation in MTT were shown between the first and second examinations. MTT estimated by 99mTc-RBC flow study was significantly correlated with that estimated by direct puncture scintigraphy. In summary, 99mTc-RBC flow study provides a quantitative indicator of intralesional haemodynamics in low-flow lesions in addition to accurate distinction between high-flow and low-flow lesions. The results of this study suggest the feasibility of detailed evaluation of flow characteristics in soft-tissue vascular malformations using 99mTc-RBCs.

Hemodynamic Effects of Middle Cerebral Artery Stenosis and Occlusion

BACKGROUND AND PURPOSE Middle cerebral artery (MCA) stenosis and occlusion may cause ischemic symptoms through both hemodynamic and embolic mechanisms. The purpose of this investigation was to determine the hemodynamic effects of these lesions. METHODS Ten patients with angiographically confirmed symptomatic occlusion (n = 5) or stenosis (n = 5) of the M1 segment of the MCA were studied by clinical examination, arteriography, and positron emission tomography (PET). Arterial supply to the distal MCA territory was classified from a review of the angiogram as being through the stenosis or from pial collaterals from anterior or posterior cerebral arteries. Regional measurements of cerebral blood flow, cerebral blood volume, cerebral rate of oxygen metabolism, oxygen extraction fraction, and ratio of cerebral blood volume/cerebral blood flow (mean vascular transit time, MTT) were obtained using PET. Hemodynamic status was categorized from PET scans as stage 0, normal hemodynamics; stage 1, autoregulatory vasodilatation (increased MTT); or stage 2, increased oxygen extraction fraction. RESULTS Of five patients with MCA occlusion, three had autoregulatory vasodilatation (stage 1) and two had increased oxygen extraction fraction distal to the lesion (stage 2). The MCA territory was supplied solely by pial collaterals in all five patients. Four of the five patients with focal MCA stenosis had normal hemodynamics (stage 0). One patient had stage 1 hemodynamic status. Blood flow to the MCA territory was through the stenosis in all patients; no pial collaterals were identified. CONCLUSION The frequency of hemodynamic compromise in patients with MCA occlusion is high. Pial collateralization is not a specific sign of increased oxygen extraction fraction in patients with MCA occlusion.

Measurement of an Index of Change in Mean Vascular Transit Time from a Single Pair of Dynamic H215O PET Studies

Measurement of an Index of Change in Mean Vascular Transit Time from a Single Pair of Dynamic H215O PET Studies

Flow characteristics of soft-tissue vascular anomalies evaluated by direct puncture scintigraphy.

Soft-tissue vascular anomalies such as haemangioma and vascular malformation are treated by surgical resection, arterial embolization or sclerotherapy. Because the effect of sclerotherapy, i.e. the percutaneous injection of sclerosing agents, depends on intralesional haemodynamics, estimation of flow characteristics of soft-tissue vascular anomalies is essential when determining appropriate patient management. However, lesions are at present divided into only two groups: high flow and low flow. We have developed a new method, direct puncture scintigraphy, to evaluate in detail the haemodynamics of vascular anomalies under conditions simulating sclerotherapy. Twenty-six soft-tissue vascular anomalies in 21 patients were studied. After 30 MBq of technetium-99m Sn colloid was injected percutaneously into the intravascular space of the lesion, dynamic imaging was performed for 5 min. A time-activity curve for the lesion was generated, with the infiltrated activity on injection subtracted. A monoexponential curve was fitted to the declining phase of the time-activity curve, and mean vascular transit time (MTT) was obtained. The lesions were classified into high-flow and low-flow lesions based on radionuclide angiography with intravenous injection of 99mTc-labelled red blood cells, and estimates of MTT in the two groups were compared. The imaging procedures were carried out with no major complications, and broad intralesional diffusion of 99mTc-Sn colloid was achieved in most lesions. The high-flow lesions (six lesions) had a short MTT, ranging from 1.6 to 3.4 s, while the low-flow lesions (20 lesions) had a longer MTT, with no overlap between the groups. MTT showed a wide range in low-flow lesions: it was less than 30 s in six lesions and more than 10 min in five other lesions. Direct puncture scintigraphy provides a quantitative indicator of the flow characteristics of soft-tissue vascular anomalies, and may aid in determining treatment strategies for patients with vascular anomalies.

Effects of cerebral angiomas on perifocal and remote tissue: a multivariate positron emission tomography study.

Using multitracer positron emission tomography, I investigated regional hemodynamic and metabolic changes in both perifocal and remote tissues of cerebral angiomas, with special reference to steal phenomena. In 22 patients (14 with arteriovenous malformations and eight with cavernomas) cerebral blood flow, cerebral blood volume, mean vascular transit time, cerebral metabolic rate for oxygen, oxygen extraction fraction, cerebral metabolic rate for glucose, and glucose extraction fraction were measured using standard positron emission tomographic methods. Twelve patients also had their cerebral glucose metabolism assessed during psychophysical activation. Regions of interest representing the angioma, perifocal and remote tissues, contralateral mirror regions, and standard brain regions were analyzed. There were no significant changes in hemodynamic variables or oxygen metabolism in the ipsilateral cerebral hemisphere, but ipsilateral glucose metabolism was reduced both at rest (p less than 0.01) and during activation (p less than 0.05). Glucose (p less than 0.001) and oxygen (p less than 0.001) metabolism in regions of perifocal tissue with low blood flow were decreased, with substrate extraction fractions showing no increase to compensate for insufficient blood flow. Functional recruitment of the cortex overlying the angioma beyond its periphery and supplied by the same arterial branches was subnormal (p less than 0.05) despite relatively unchanged hemodynamics in this tissue compartment. These data suggest that dysfunction of the cortex supplied by arterial branches also feeding the vascular malformation is related to neuronal deafferentation, while the proportionate decrease in blood flow and metabolism of perifocal tissue may be ascribed to neuronal loss in chronically hypoperfused areas, rather than to persistent hemodynamic steal effects.

Borderzone hemodynamics in cerebrovascular disease

To investigate the possible existence of chronic selective hemodynamic impairment in the arterial borderzone regions of the brain, we used positron emission tomography (PET) to measure regional mean vascular transit time (rt, equal to the ratio of regional cerebral blood volume to cerebral blood flow) and regional oxygen extraction fraction (rOEF) in 32 patients with either severe internal carotid artery stenosis or occlusion and 11 normal controls. Twenty-four of the patients had had TIAs or amaurosis fugax from 1 to 60 days before PET; all had normal brain CT. We used a stereotactic localization method to locate the anterior and posterior borderzone regions of the middle cerebral artery (MCA) territory. We then calculated ratios of each borderzone to the ipsilateral MCA territory for both rt and rOEF. There was no significant difference from control ratios in any patient subgroup including those with greater than or equal to 75% stenosis or occlusion, those with or without contralateral greater than or equal to 50% stenosis, or those with abnormal hemodynamics in the MCA territory. We therefore found no evidence for selective borderzone hemodynamic impairment in this group of patients with severe carotid artery disease.

Hemodynamic and metabolic effects of cerebral arteriovenous malformations studied by positron emission tomography.

Seventeen patients with an intracranial arteriovenous malformation were studied with positron emission tomography. Cerebral blood flow, cerebral blood volume, oxygen extraction fraction, and glucose and oxygen metabolism were evaluated in both hemispheres, excluding the area of the malformation itself. Patients were divided into three groups according to the size of their malformation, and results obtained were compared with studies in healthy volunteers. The glucose metabolism was significantly (p less than 0.01) decreased in the ipsilateral hemisphere in all patients. The cerebral blood volume was significantly increased (p less than 0.001) ipsilaterally in the three groups, and contralaterally in patients with medium- and large-sized arteriovenous malformations. The cerebral blood volume to cerebral blood flow ration, an index of vascular mean transit time, was significantly increased (p less than 0.005) ipsilaterally in patients with medium- and large-sized malformations and contralaterally in patients with large ones. Cerebral blood flow, oxygen extraction fraction, and oxygen metabolism were within the normal range bilaterally in all three groups, but oxygen extraction fraction tended to be higher in patients with larger lesions. The lack of significant change in oxygen metabolism suggests that oxygen metabolism in cortical areas remote from the arteriovenous malformation has been maintained by compensatory hemodynamic mechanisms. These data reveal widespread metabolic and hemodynamic consequences of arteriovenous malformations and suggest that they are associated with impairment of glucose metabolism, both in ipsilateral regions remote from the lesion and in the contralateral hemisphere in patients with large lesions.

Renal perfusion and mean vascular transit time.

A method was developed to determine the mean vascular (arterio-venous) transit time (MVTT) of renal transplants. The renal transit times were calculated from perfusion curves obtained when grafts were examined with 99mTc-DTPA. About 400 examinations were performed during a 6-month period. As a result, influences of clinical complications on transit times were demonstrated. It was found that the individual MVTT is not a suitable parameter for assessment of renal grafts since individual transit times vary widely.

Physiological responses to focal cerebral ischemia in humans.

Positron emission tomography (PET) was used to investigate the regional hemodynamic and metabolic changes that accompany focal reductions in cerebral blood flow to ischemic but uninfarcted regions of the brain. Studies were performed on 7 patients chosen from a larger group of subjects with transient ischemic attacks and normal computed tomographic findings, specifically because their PET studies showed decreased blood flow to the symptomatic hemisphere rather than symmetrical flow to the two hemispheres. In regions with decreased blood flow, the cerebral metabolic rate for oxygen was also decreased, but to a lesser degree. Cerebral blood volume, vascular mean transit time, and fractional extraction of oxygen from blood were all increased. These findings indicate that cerebral oxygen metabolism was maintained in the face of decreased blood flow by local compensatory mechanisms that included dilation of intraparenchymal blood vessels and increased transfer of oxygen from blood to tissue. Such knowledge of the physiological characteristics of ischemic, uninfarcted brain is important if PET is to be used clinically to differentiate reversible cerebral ischemia from irreversible infarction.