G. Pawlik

Regional Kinetic Constants and Cerebral Metabolic Rate for Glucose in Normal Human Volunteers Determined by Dynamic Positron Emission Tomography of [18F]-2-Fluoro-2-Deoxy-D-Glucose

Using dynamic [18F]fluorodeoxyglucose (FDG) positron emission tomography with a high-resolution, seven-slice positron camera, the kinetic constants of the original three-compartment model of Sokoloff and co-workers (1977) were determined in 43 distinct topographic brain regions of seven healthy male volunteers aged 28–38 years. Regional averages of the cerebral metabolic rate for glucose (CMRglu) were calculated both from individually fitted rate constants (CMRglukinetic) and from activity maps recorded 30–40 min after FDG injection, employing a four-parameter operational equation with standard rate constants from the literature (CMRgluautoradiographic). Metabolic rates and kinetic constants varied significantly among regions and subjects, but not between hemispheres. k1 ranged between 0.0485 ± 0.00778 min−1 in the oval center and 0.0990 ± 0.01347 min−1 in the primary visual cortex. k2 ranged from 0.1198 ± 0.01533 min−1 in the temporal white matter to 0.1472 ± 0.01817 min−1 in the cerebellar dentate nucleus. k3 was lowest (0.0386 ± 0.01482 min−1) in temporal white matter and highest (0.0823 ± 0.02552 min−1) in the caudate nucleus. Maximum likelihood cluster analysis revealed four homogeneous groups of brain regions according to their respective kinetic constants: (1) white matter and mixed brainstem structures; (2) cerebellar gray matter and hippocampal formations; (3) basal ganglia and frontolateral and primary visual cortex; and (4) other cerebral cortex and thalamus. Across the entire brain, k1 and k2 were positively correlated (r = 0.79); k1 and k3 showed some correlation (r = 0.59); but no significant linear association was found between k2 and k3. A strong correlation with CMRglu could be demonstrated for k1 (r = 0.88) and k3 (r = 0.90), but k2 was loosely correlated (r = 0.56). CMRglu kinetic ranged from 17.0 ± 2.45 μmol/100 g/min in the occipital white matter to 41.1 ± 5.62 μmol/100 g/min in the frontolateral cortex. In most regions the mean values of CMRglu kinetic did not differ significantly from CMRglu autoradiographic. With few exceptions, however, within-region variance was significantly less for CMRglu kinetic than for CMRglu autoradiographic, suggesting greater individual reliability of results obtained by the kinetic approach.

Tissue at risk of infarction rescued by early reperfusion : A positron emission tomography study in systemic recombinant tissue plasminogen activator thrombolysis of acute Stroke

Thrombolytic therapy of acute ischemic stroke can be successful only as long as there is penumbral tissue perfused at rates between the thresholds of normal function and irreversible structural damage, respectively. To determine the proportion of tissue at risk of infarction, cerebral perfusion was studied in 12 patients with acute ischemic stroke who underwent treatment with systemic recombinant tissue plasminogen activator (0.9 mg/kg body weight according to National Institute of Neurological Disorders and Stroke protocol) within 3 hours of onset of symptoms, using [15O]-H2O positron emission tomography (PET) before or during, and repeatedly after thrombolysis. The size of the regions of critically hypoperfused gray matter were identified on the initial PET scans, and changes of perfusion in those areas were related to the clinical course (followed by the National Institutes of Health stroke scale) and to the volume of infarcted gray matter demarcated on magnetic resonance imaging 3 weeks after the stroke. Whereas the initial clinical score was unrelated to the size of the ischemic area, after 3 weeks there was a strong correlation between clinical deficit and volume size of infarcted gray matter (Spearmans rho, 0.96; P < 0.001). All patients with a severely hypoperfused (< 12 mL/100 g/min) gray matter region measuring less than 15 mL on first PET showed full morphologic and clinical recovery (n = 5), whereas those with ischemic areas larger than 20 mL developed infarction and experienced persistent neurologic deficits of varying degree. Infarct sizes, however, were smaller than expected from previous correlative PET and morphologic studies of patients with acute stroke: only 22.7% of the gray matter initially perfused at rates below the conventional threshold of critical ischemia became necrotic. Actually, the percentage of initially ischemic voxels that became reperfused at almost normal levels clearly predicted the degree of clinical improvement achieved within 3 weeks. These sequential blood flow PET studies demonstrate that critically hypoperfused tissue can be preserved by early reperfusion, perhaps related to thrombolytic therapy. The results correspond with experimental findings demonstrating the prevention of large infarcts by early reperfusion to misery perfused but viable tissue.

Comparative Regional Analysis of 2-Fluorodeoxyglucose and Methylglucose Uptake in Brain of Four Stroke Patients. With Special Reference to the Regional Estimation of the Lumped Constant

The glucose metabolic rate of the human brain can be measured with labeled deoxyglucose, using positron emission tomography, provided certain conditions are fulfilled. The original method assumed irreversible trapping of deoxyglucose metabolites in brain during the experimental period, and it further requires that a conversion factor between deoxyglucose and glucose, the “lumped constant,” be known for the brain regions of interest. We examined the assumption of irreversible trapping of fluorodeoxyglucose metabolites in brain of four patients in 365 normal and 4 recently infarcted regions. The average net, steady-state rate of fluorodeoxyglucose (KD) accumulation in normal regions of the four patients was 0.025 ml g−1 min−1. We also examined the variability of the lumped constant. We first confirmed that methylglucose is not phosphorylated in the human brain. We then estimated the lumped constant from the regional distribution of labeled methylglucose in brain. The average (virtual) volume of distribution of labeled methylglucose in the normal regions was 0.46 ml g−1 and was the same in both gray and white matter structures. The average brain glucose content corresponding to this value was 1.3 μmol g−1, assuming a Michaelis constant (Kt) of 3.7 mM for glucose transport across the blood–brain barrier. The lumped constant varied insignificantly between 0.4 and 0.5 in most regions, with an overall average of 0.44. It did not vary significantly between the patients and was the same in gray and white matter structures, but was inversely related to the calculated metabolic rate. This observation indicates that metabolic rates calculated with a fixed lumped constant (e.g., 0.40) would be slightly underestimated at high metabolic rates and slightly overestimated at low metabolic rates. The average glucose metabolic rates of the 365 normal regions, in which gray matter regions prevailed by 20:1, was 32 μmol 100 g−1 min−1. The average glucose phosphorylation rate in white matter was 20 μmol 100 g−1 min−1 with a lumped constant of 0.45. In the recently infarcted areas, the lumped constants varied from 0.37 to 2.83, corresponding to glucose metabolic rates varying from 2 to 18 μmol 100 g−1 min−1. Two infarct types were identified. In one type, the phosphorylation-limited type, glucose content and the lumped constant were close to normal (1 μmol g−1 and 0.40, respectively). In the other, the transport/flow-limited type, the glucose content was low (0.2 μmol g−1), and the lumped constant in excess of unity. The evidence from the present study upholds the model of Sokoloff et al. in every detail.

Estimation of Local Cerebral Glucose Utilization by Positron Emission Tomography of [18F]2-Fluoro-2-deoxy-D-glucose: A Critical Appraisal of Optimization Procedures

Various approaches estimating local cerebral glucose utilization by positron emission tomography of labeled deoxyglucose are compared. Autoradiographic methods that predict the glucose utilization rate from a single scan are unreliable in pathologic tissue because of abnormal values of the model rate constants. A normalization procedure using the ratio of measured tissue activity to activity calculated with standard rate constants is proposed to readjust the values of the rate constants. Reliable estimates of metabolic rates can be obtained from dynamic recordings of tracer uptake. In the graphic approach, metabolic rate can be derived from the slope of a segment of a transformed uptake curve, which becomes linear at 15–20 min after intravenous tracer injection, with an accuracy comparable with that in complete dynamic studies. However, by recording and analyzing full-length uptake curves, in addition to metabolic rate, the model rate constants can be determined regionally. The physiological significance of those parameters is demonstrated in crossed cerebellar deactivation in 30 patients with supratentorial infarcts. Mild hypometabolism both within the ischemic lesion and in the morphologically intact cerebellum is accompanied by a reduction of the phosphorylation rate only. Severe metabolic depression, by contrast, affects both cerebellar transport and phosphorylation processes, whereas in the cerebrum, only the rate constant k1 is significantly correlated with the degree of metabolic disturbance.

Early [11C]Flumazenil/H2O Positron Emission Tomography Predicts Irreversible Ischemic Cortical Damage in Stroke Patients Receiving Acute Thrombolytic Therapy

BACKGROUND AND PURPOSE Central benzodiazepine receptor ligands, such as [(11)C]flumazenil (FMZ), are markers of neuronal integrity and therefore might be useful in the differentiation of functionally and morphologically damaged tissue early in ischemic stroke. We sought to assess the value of a benzodiazepine receptor ligand for the early identification of irreversible ischemic damage to cortical areas that cannot benefit from reperfusion. METHODS Eleven patients (7 male, 4 female, aged 52 to 75 years) with acute, hemispheric ischemic stroke were treated with alteplase (recombinant tissue plasminogen activator; 0.9 mg/kg according to National Institute of Neurological Disorders and Stroke protocol) within 3 hours of onset of symptoms. At the beginning of thrombolysis, cortical cerebral blood flow ([(15)O]H(2)O) and FMZ binding were assessed by positron emission tomography (PET). Those early PET findings were related to the change in neurological deficit (National Institutes of Health Stroke Scale) and to the extent of cortical damage on MRI or CT 3 weeks after the stroke. RESULTS Hypoperfusion was observed in all cases, and in 8 patients the values were below critical thresholds estimated at 12 mL/100 g per minute, comprising 1 to 174 cm(3) of cortical tissue. Substantial reperfusion was seen in most of these regions 24 hours after thrombolysis. In 4 cases, distinct areas of decreased FMZ binding were detected. Those patients suffered permanent lesions in cortical areas corresponding to their FMZ defects (112 versus 146, 3 versus 3, 2 versus 1, and 128 versus 136 cm(3)). In the other patients no morphological defects were detected on MRI or CT, although blood flow was critically decreased in areas ranging in size up to 78 cm(3) before thrombolysis. CONCLUSIONS These findings suggest that imaging of benzodiazepine receptors by FMZ PET distinguishes between irreversibly damaged and viable penumbra tissue early after acute stroke.

Speech-induced cerebral metabolic activation reflects recovery from aphasia.

Six stroke patients with clinically significant aphasia were studied 4 weeks and again 12-18 months after their first left hemispheric ictus. The regional cerebral metabolic rate of glucose (rCMRglc) was measured repeatedly by PET at rest and during word repetition, and severity of speech impairment was assessed by a neuropsychologic test battery. The patterns of speech-associated activation of glucose metabolism were related to improvement in language performance as measured by the Token test. Three patients experienced significant recovery from aphasia (Token test: 47 to 3, 45 to 12, and 37 to 5 points, respectively), whereas 3 patients had poor outcome (Token test from 48 to 45, and from 47 to 39 and 24, respectively). Good recovery was related to activation of left hemispheric speech areas surrounding the infarct, especially left superior temporal gyrus. In contrast, the 3 patients with persistent aphasia showed rCMRglc recruitment in right hemispheric regions and were unable to activate left hemispheric speech areas on follow-up. These results indicate that favorable outcome is related to partial sparing of speech areas of the dominant hemisphere that can be (re-) activated. Predominant recruitment of contralateral areas is not efficacious for a considerable recovery from aphasia. It rather indicates unspecific involvement of widespread networks in the effort to perform a complex task.

Regional cerebral glucose metabolism in man during wakefulness, sleep, and dreaming

Regional cerebral glucose metabolism was measured by positron emission tomography in 4 healthy male volunteers, both during wakefulness and sleep, using the 2-deoxy-D-[2-18F]glucose method. While 3 of the subjects did not report dreaming and, during sleep stages I-IV of variable duration, exhibited a non-selective decrease in metabolic rates averaging 12.6 +/- 4.73% (mean +/- S.D.) for the entire brain, the fourth volunteer who experienced an extended nightmare during his sleep examination showed a generalized increase in cerebral glucose utilization ranging from 2.1% in lentiform nucleus to 30.0% in the superior frontal cortex, with a weighted whole brain average of 16.4%. These findings suggest that energy metabolism in the human brain is generally depressed during slow-wave sleep as opposed to a--possibly differential--activation during dreaming.

PET measurement of D2 and S2 receptor binding of 3-N-([2′-18F]fluoroethyl)spiperone in baboon brain

The regional pharmacokinetic behavior in baboon brain of 18F-fluoroethyl-and 18F-fluoropropylspiperone (18FESP, 18FPSP) at specific activities≥1000 Ci/mmol was studied with PET. Four hours after injection of 5–10 mCi 18FESP, uptake in striatum was 0.048%±0.005% of injected dose per cm3, which is almost the same as with 18F-and 11C-methylspiperone. While 18FPSP was taken up in much smaller amounts than 18FESP, striatum to cerebellum activity ratios were quite similar for both ligands (about 9 to 10 at 4 h p.i.). Because of its higher striatal uptake, 18FESP seems to be better suited for PET. Furthermore, relative binding to S2 receptors was much smaller for FESP: competing cold S2 antagonists (ritanserin, ketanserin) did not alter 18FESP binding to striatum, concurrently reducing uptake in frontal cortex by only 15%–20%. With coninjection of increasing amounts of cold FESP, saturation of 18FESP binding to striatum occurred at doses exceeding 10 μg per kg. Quantitative analysis of radiolabelled ligand in arterial plasma (decrease to 8% at 4 h p.i.) demonstrated identical metabolic turnover for both ligands. Direct use of binding fractions from the saturation curve resulted in overestimation of the receptor density in striatum. Using the 18FESP plasma concentration time curve and the dynamic uptake data, k3 of a three compartment model could be determined by non linear regression. However, dramatic changes of the dependence of k3 on the specifically bound ligand concentration were observed even at small loading doses of FESP. Estimation of Bmax yielded a D2 receptor density of only 6 pmol per cm3 in baboon striatum.

MRI-guided flumazenil- and FDG-PET in temporal lobe epilepsy.

In temporal lobe epilepsy (TLE) patients without lesions, major hippocampal sclerosis, or atrophy on magnetic resonance imaging (MRI), the localizing power of [11C]flumazenil (FMZ) and 2-[18F]fluoro-2-deoxy-D-glucose (FDG) was compared using high-resolution positron emission tomography (PET) studies and individually coregistered MRI scans. Following complete clinical, neuropsychological, and electrophysiological evaluation, benzodiazepine receptor density was assessed using the FMZ equilibrium method. Thirty minutes later, interictal FDG-PET was performed under resting conditions. PET images were matched to three-dimensionally coregistered, T1-weighted MRI. Each temporal lobe (TL) was divided into 12 volumes of interest. The regional FMZ data were normalized with respect to average cortical values. For each patient the right-left asymmetries of rCMRGlc and normalized FMZ data were calculated. In 7 to 10 patients, mesial TL structures showed reduced FMZ binding, with a decrease by at least 10% in the affected TL. Reductions of 10% or more of rCMRGlc usually were more widespread than FMZ reductions and often involved lateral temporal cortex. The regions of most pronounced disturbances are not necessarily identical in both methods. Three patients had a complex correspondence of lateralization with PET, neuropsychological, and EEG data. In 4 patients, lateralization was less clear from EEG or neuropsychological results but was still consistent with lateralization by PET. In 3 of 10 patients, however, major discrepancies were found. These data suggest that the combination of neuropsychological testing, EEG, and MRI-guided FMZ- and FDG-PET will help to select patients with clearly defined epileptogenic foci especially in mesial TLE. Even in cases without MRI lesions, TL epileptic foci can be lateralized with consistency across the methods; FMZ-PET shows the pathologic focus more circumscribed than FDG-PET.

Effect of Piracetam on Cerebral Glucose Metabolism in Alzheimer's Disease as Measured by Positron Emission Tomography

The effect of piracetam (a putative enhancer of cerebral metabolism) on regional CMRGlu was studied by positron emission tomography of 2[18F]-fluoro-2-deoxy-D-glucose in nine patients with Alzheimers disease, and in seven cases with multiinfarct dementia or unclassified dementia. In Alzheimers disease, i.v. administration of piracetam, 6 g b.i.d. for 2 weeks, significantly improved rCMRGlu in most cortical areas, whereas no effect on CMRGlu of the drug was observed in the multiinfarct dementia/unclassified dementia groups. These results lend further support to the notion that adjuvant piracetam treatment is of benefit in Alzheimers disease. They may also indicate that the typical metabolic depression in Alzheimers disease is caused by complex interaction of disturbed transmitter and cellular function rather than by a specific deficit in the cholinergic system alone.