Cerebral glucose metabolism as a function of age in man: influence of the rate constants in the fluorodeoxyglucose method

Lower brain glucose metabolism in normal ageing is predominantly frontal and temporal: A systematic review and pooled effect size and activation likelihood estimates meta-analyses

This review provides a qualitative and quantitative analysis of cerebral glucose metabolism in ageing. We undertook a systematic literature review followed by pooled effect size and activation likelihood estimates (ALE) meta-analyses. Studies were retrieved from PubMed following the PRISMA guidelines. After reviewing 635 records, 21 studies with 22 independent samples (n = 911 participants) were included in the pooled effect size analyses. Eight studies with eleven separate samples (n = 713 participants) were included in the ALE analyses. Pooled effect sizes showed significantly lower cerebral metabolic rates of glucose for older versus younger adults for the whole brain, as well as for the frontal, temporal, parietal, and occipital lobes. Among the sub-cortical structures, the caudate showed a lower metabolic rate among older adults. In sub-group analyses controlling for changes in brain volume or partial volume effects, the lower glucose metabolism among older adults in the frontal lobe remained significant, whereas confidence intervals crossed zero for the other lobes and structures. The ALE identified nine clusters of lower glucose metabolism among older adults, ranging from 200 to 2640 mm³ . The two largest clusters were in the left and right inferior frontal and superior temporal gyri and the insula. Clusters were also found in the inferior temporal junction, the anterior cingulate and caudate. Taken together, the results are consistent with research showing less efficient glucose metabolism in the ageing brain. The findings are discussed in the context of theories of cognitive ageing and are compared to those found in neurodegenerative disease.

Electroconvulsive therapy changes the regional resting state function measured by regional homogeneity (ReHo) and amplitude of low frequency fluctuations (ALFF) in elderly major depressive disorder patients: An exploratory study

Electroconvulsive therapy (ECT) is the most effective and rapid treatment for severe major depressive disorder (MDD) in elderly patients. The mechanism of ECT is unclear, and studies on ECT in elderly MDD patients by resting-state functional magnetic resonance imaging are rare. Thirteen elderly MDD patients were scanned before and after ECT using a 3.0T MRI scanner. Regional homogeneity (ReHo) and amplitude of low-frequency fluctuations (ALFF) were processed to compare resting-state function before and after treatment. Depression and anxiety symptoms of all patients abated after ECT. Decreased ReHo values in the bilateral superior frontal gyrus (SFG) were observed after ECT, and the values of right SFG significantly correlated with an altered Hamilton depression rating scale score. Increased ALFF values in the left middle frontal gyrus, right middle frontal gyrus, orbital part, and decreased ALFF values in the left midcingulate area, left precentral gyrus, right SFG/middle frontal gyrus after ECT were also observed. These results support the hypothesis that ECT may affect the regional resting state brain function in geriatric MDD patients.

Data-driven identification of intensity normalization region based on longitudinal coherency of 18F-FDG metabolism in the healthy brain

OBJECTIVES

In brain ¹⁸F-FDG PET data intensity normalization is usually applied to control for unwanted factors confounding brain metabolism. However, it can be difficult to determine a proper intensity normalization region as a reference for the identification of abnormal metabolism in diseased brains. In neurodegenerative disorders, differentiating disease-related changes in brain metabolism from age-associated natural changes remains challenging. This study proposes a new data-driven method to identify proper intensity normalization regions in order to improve separation of age-associated natural changes from disease related changes in brain metabolism.

METHODS

127 female and 128 male healthy subjects (age: 20 to 79) with brain¹⁸F-FDG PET/CT in the course of a whole body cancer screening were included. Brain PET images were processed using SPM8 and were parcellated into 116 anatomical regions according to the AAL template. It is assumed that normal brain ¹⁸F-FDG metabolism has longitudinal coherency and this coherency leads to better model fitting. The coefficient of determination R² was proposed as the coherence coefficient, and the total coherence coefficient (overall fitting quality) was employed as an index to assess proper intensity normalization strategies on single subjects and age-cohort averaged data. Age-associated longitudinal changes of normal subjects were derived using the identified intensity normalization method correspondingly. In addition, 15 subjects with clinically diagnosed Parkinson's disease were assessed to evaluate the clinical potential of the proposed new method.

RESULTS

Intensity normalizations by paracentral lobule and cerebellar tonsil, both regions derived from the new data-driven coherency method, showed significantly better coherence coefficients than other intensity normalization regions, and especially better than the most widely used global mean normalization. Intensity normalization by paracentral lobule was the most consistent method within both analysis strategies (subject-based and age-cohort averaging). In addition, the proposed new intensity normalization method using the paracentral lobule generates significantly higher differentiation from the age-associated changes than other intensity normalization methods.

CONCLUSION

Proper intensity normalization can enhance the longitudinal coherency of normal brain glucose metabolism. The paracentral lobule followed by the cerebellar tonsil are shown to be the two most stable intensity normalization regions concerning age-dependent brain metabolism. This may provide the potential to better differentiate disease-related changes from age-related changes in brain metabolism, which is of relevance in the diagnosis of neurodegenerative disorders.

Different partial volume correction methods lead to different conclusions: An (18)F-FDG-PET study of aging

A cross-sectional group study of the effects of aging on brain metabolism as measured with (18)F-FDG-PET was performed using several different partial volume correction (PVC) methods: no correction (NoPVC), Meltzer (MZ), Müller-Gärtner (MG), and the symmetric geometric transfer matrix (SGTM) using 99 subjects aged 65-87years from the Harvard Aging Brain study. Sensitivity to parameter selection was tested for MZ and MG. The various methods and parameter settings resulted in an extremely wide range of conclusions as to the effects of age on metabolism, from almost no changes to virtually all of cortical regions showing a decrease with age. Simulations showed that NoPVC had significant bias that made the age effect on metabolism appear to be much larger and more significant than it is. MZ was found to be the same as NoPVC for liberal brain masks; for conservative brain masks, MZ showed few areas correlated with age. MG and SGTM were found to be similar; however, MG was sensitive to a thresholding parameter that can result in data loss. CSF uptake was surprisingly high at about 15% of that in gray matter. The exclusion of CSF from SGTM and MG models, which is almost universally done, caused a substantial loss in the power to detect age-related changes. This diversity of results reflects the literature on the metabolism of aging and suggests that extreme care should be taken when applying PVC or interpreting results that have been corrected for partial volume effects. Using the SGTM, significant age-related changes of about 7% per decade were found in frontal and cingulate cortices as well as primary visual and insular cortices.

Theory of Mind in normal ageing and neurodegenerative pathologies

This paper reviews findings in three subcomponents of social cognition (i.e., Theory of Mind, facial emotion recognition, empathy) during ageing. Changes over time in social cognition were evaluated in normal ageing and in patients with various neurodegenerative pathologies, such as Alzheimer's disease, mild cognitive impairment, frontal and temporal variants of frontotemporal lobar degeneration and Parkinson's disease. Findings suggest a decline in social cognition with normal ageing, a decline that is at least partially independent of a more general cognitive or executive decline. The investigation of neurodegenerative pathologies showing specific deficits in Theory of Mind in relation to damage to specific cerebral regions led us to suggest a neural network involved in Theory of Mind processes, namely a fronto-subcortical loop linking the basal ganglia to the regions of the frontal lobes.

Structural and Functional Imaging Correlates for Age-Related Changes in the Brain

In recent years, investigators have made significant progress in documenting brain structure and function as it relates to aging by using positron emission tomography, conventional magnetic resonance (MR) imaging, advanced MR techniques, and functional MR imaging. This review summarizes the latest advances in understanding physiologic maturation and aging as detected by these neuroimaging modalities. We also present our experience with MR volumetric and positron emission tomography analysis in separate cohorts of healthy subjects in the pediatric and adult age groups respectively. Our results are consistent with previous studies and include the following: total brain volume was found to increase with age (up to 20 years of age). Whole brain metabolism and frontal lobe metabolism both decrease significantly with age (38% and 42%, respectively), whereas cerebellar metabolism does not show a significant decline with age. Defining normal alterations in brain function and structure allows early detection of disorders such as Alzheimer's and Parkinson's diseases, which are commonly associated with normal aging.

Brain FDG PET study of normal aging in Japanese: effect of atrophy correction

PURPOSE

The aim of this study was to investigate the effects of atrophy correction on the results of 18F-fluorodeoxyglucose positron emission tomography (FDG PET) in the context of normal aging.

METHODS

Before the human study was performed, a Hoffman 3D brain phantom experiment was carried out in order to validate a newly developed correction method for partial volume effects (PVEs). Brain FDG PET was then performed in 139 healthy Japanese volunteers (71 men, 68 women; age 24-81 years). PET images were corrected for PVEs using grey matter volume, which was segmented from co-registered magnetic resonance images and convoluted with the spatial resolution of the PET scanner. We investigated the correlation between advancing age and relative regional FDG activity, which was normalised to the global activity before and after PVE correction using Statistical Parametric Mapping 99.

RESULTS

The PET image, when corrected for PVEs, provided more homogeneous tracer distribution in the whole phantom than in the original PET image. The human PET study of both sexes revealed significant negative correlations between age and relative FDG activity in the bilateral perisylvian and medial frontal areas before PVE correction. However, these negative correlations were largely resolved after PVE correction.

CONCLUSION

Correction for PVEs was effective in our FDG PET study. The reduction in FDG uptake with advancing age that was detected by FDG PET without PVE correction could be accounted for largely by an age-related cerebral volume loss in the bilateral perisylvian and medial frontal areas.

Cerebral perfusion and metabolism in resuscitated patients with severe post-hypoxic encephalopathy

Positron emission tomography (PET) was used for the study of regional cerebral perfusion and metabolism in eight patients with severe post-hypoxic encephalopathy, caused by cardiac arrest and resulting in a coma lasting for at least 24 h. Using this method, we aimed to identify regional vulnerability, which was hypothesized to provide (i) insight in pathogenic mechanisms and (ii) early prognostic parameters. On day 1 post-resuscitation, 18-Fluor deoxyglucose ([F18]-FDG) indicated a marked decrease of cerebral metabolic activity. Gray matter glucose consumption was 54% of normal values, whereas white matter uptake was 70% of normal. Regional differences followed a pattern of neuronal density rather than specific patterns of functionally or biochemically defined regions or of vascular territories. In contrast to [F18]-FDG, the distribution of 15-oxygen labeled water ([O-15]-water) showed a better demarcation between gray and white matter, whereas focal deficit was not observed. In some patients, hyperperfusion relative to regional glucose consumption was observed in the occipital poles and basal ganglia. This suggests loss of vascular tone, i.e. vascular paralysis, in the basilar artery territory. CT and MRI scanning did not show any major change with respect to the hypoxic injury. In the small group studied, all patients had a poor outcome. The comparison between survivors and nonsurvivors did not reveal obvious differences in PET data, suggesting that this technique does not provide major prognostic clues adding to the prognostic information derived from serial neurological assessment in the restricted patient group characterized by prolonged coma.

Normal patterns and variants in single-photon emission computed tomography and positron emission tomography brain imaging

One of the most important issues in evaluating functional brain scans for research or clinical purposes is to be able to identify normal variants. Determining the baseline "normal" state of the brain is not easy to characterize since many normal brain functions and mental processes affect brain activity. This article reviews issues pertaining to the technical and neurophysiological aspects of functional brain imaging that might alter "normal" activity and will also consider how normal brain activity changes throughout the lifespan.

Comparison of regional cerebral blood flow and glucose metabolism in the normal brain: effect of aging

The regional cerebral blood flow (rCBF) and metabolic rate for glucose (rCMRGlc) are associated with functional activity of the neural cells. The present work reports a comparison study between rCBF and rCMRGlc in a normal population as a function of age. 10 young (25.9+/-5.6 years) and 10 old (65.4+/-6.1 years) volunteers were similarly studied at rest. In each subject, rCBF and rCMRGlc were measured in sequence, during the same session. Both rCBF and rCMRGlc values were found to decrease from young (mean rCBF=43.7 ml/100 g per min; mean rCMRGlc=40.6 micromol/100 g per min) to old age (mean rCBF=37.3 ml/100 g per min; mean rCMRGlc=35.2 micromol/100 g per min), resulting in a drop over 40 years of 14.8% (0.37%/year) and 13.3% (0.34%/year), respectively. On a regional basis, the frontal and the visual cortices were observed to have, respectively, the highest and the lowest reduction in rCBF, while, for rCMRGlc, these extremes were observed in striatum and cerebellum. Despite these differences, the ratio of rCBF to rCMRGlc was found to have a similar behavior in all brain regions for young and old subjects as shown by a correlation coefficient of 88%. This comparative study indicates a decline in rCBF and rCMRGlc values and a coupling between CBF and CMRGlc as a function of age.

Decreased brain glucose metabolism in microvessels from patients with Alzheimer's disease

We studied brain glucose metabolism in patients with Alzheimer's disease and age-matched controls in vivo by PET and assessed brain glucose utilization and the phosphorylation constant K3 for hexokinase. In addition we determined in vitro the binding of 2DG and measured its phosphorylation to 2DG-phosphate in cerebral microvessels obtained at autopsy from subjects with Alzheimer's disease and age-matched controls. In patients with Alzheimer's disease we found a marked decrease in the kinetic constant K3 for the hexokinase, and a marked decrease in the overall metabolism of glucose in our PET studies; in microvessels there was a marked decrease in the affinity of 2DG and a decrease in hexokinase activity. Alzheimer's disease may be related to a complex alteration in brain glucose metabolism.

PET studies of cerebral glucose metabolism in conscious rhesus macaques

A growing body of evidence suggests that rhesus macaques may be a good model of human brain aging. We used positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG) to measure regional cerebral metabolic rates for glucose (rCMRglc) in young and aged rhesus macaques to determine if age-related decreases, such as those reported in humans, also occur in macaques. Whereas the aged animals had lower metabolic rates in every brain region studied, the largest differences were in left temporal cortex. The largest differences were also observed in left temporal cortex when relative rCMRglc values were used. Both rCMRglc and relative rCMRglc were marked by substantial individual variation within the aged group. This variation may parallel the variation observed in behavioral studies. Future studies that include both PET and behavioral measures should help determine if there is a relationship between age-related changes in rCMRglc and behavior.

Reduced temporal lobe glucose metabolism in aging

The results of a positron emission tomography study of regional cerebral metabolic rates of glucose are reported for 8 healthy old subjects (mean age, 66 yr; standard deviation [SD], 5) and 9 young subjects (mean age, 27 yr; SD, 4.6) using a high-resolution positron emission tomograph and the glucose metabolic tracer 18F-fluorodeoxyglucose. Older subjects showed significantly lower cerebral metabolic rates than did the young subjects, in anterior, middle, and posterior temporal neocortex and in mesial temporal cortex, with the largest differences occurring in anterior temporal cortex (temporal pole). The current findings may reflect either decreases in regional cerebral metabolic rates for glucose that occur with normal aging, or early indications of cognitive dysfunction that is associated with age-related disorders.

Errors introduced by tissue heterogeneity in estimation of local cerebral glucose utilization with current kinetic models of the [18F]fluorodeoxyglucose method

The effects of tissue heterogeneity on the estimation of regional cerebral glucose utilization (rCMRglc) in normal humans with [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET) were compared with respect to the various kinetic models of the [18F]FDG method. The kinetic models were conventional homogeneous tissue models of the [18F]FDG method, with (4K Model) and without (3K Model) a rate constant to account for an apparent loss of [18F]2-fluoro-2-deoxy-D-glucose-6-phosphate ([18F]FDG-6-P), and a tissue heterogeneity model (TH Model). When either of the kinetic models designed for homogeneous tissues was applied to heterogeneous tissues, estimates of the rate constant for efflux of [18F]FDG from the tissue (k2*) and of the rate constant for phosphorylation of [18F]FDG (k3*) decreased as the duration of the experimental period was increased. When the 4K Model was used, estimates of the rate constant for the apparent dephosphorylation of [18F]FDG-6-P (k4*) were significantly greater than zero and fell with increasing duration of the experimental period. Although the TH Model included no term to describe an apparent dephosphorylation of [18F]FDG-6-P, the fit of the TH Model to the time course of total tissue radioactivity was at least as good as and often better than the fit of the 4K Model in the 120-min period following the pulse of [18F]FDG. Hence, the high estimates of k4* found in PET studies of less than or equal to 120 min can be explained as the consequence of measuring radioactivity in a heterogeneous tissue and applying a model designed for a homogeneous tissue; there remains no evidence of significant dephosphorylation of [18F]FDG-6-P in this time period. Furthermore, use of the 4K Model led to an overestimation of rCMRglc; whole-brain glucose utilization calculated with the 4K Model was greater than 20% higher than values usually obtained in normal humans by the model-independent Kety-Schmidt technique. rCMRglc was accurately estimated by the TH Model and, in experimental periods sufficiently long to minimize the effects of tissue heterogeneity, also by the original 3K Model of the deoxyglucose method.

Verbal fluency and positron emission tomographic mapping of regional cerebral glucose metabolism

Impairment in verbal fluency (VF) has been a consistently reported clinical feature of focal cerebral deficits in frontal and temporal regions. More recent behavioral activation studies with healthy control subjects using positron emission tomography (PET), however, have noted a negative correlation between performance on verbal fluency tasks and regional cortical activity. To see if this negative relationship extends to steady-state non-activation PET measures, thirty-three healthy adults were given a VF task within a day of their 18F-2-fluoro-2-deoxy-D-glucose PET scan. VF was found to correlate positively with left temporal cortical region metabolic activity but to correlate negatively with right and left frontal activity. VF was not correlated significantly with right temporal cortical metabolic activity. Some previous studies with normals using behavioral activation paradigms and PET have reported negative correlations between metabolic activity and cognitive performance similar to that reported here. An explanation for the disparate relationships that were observed between frontal and temporal brain areas and VF might be found in the mediation of different task demands by these separate locations, i.e., task planning and/or initiation by frontal regions and verbal memory by the left temporal area.

Uses and limitations of positron emission tomography in clinical pharmacokinetics/dynamics (Part II)

Positron emission tomography (PET) involves imaging the biodistribution and tissue localisation of small amounts of radiolabelled biomolecules or drugs. In Part I of this article, which appeared in the previous issue of the Journal, the applications of pharmacokinetics in PET were discussed in order to derive quantitative measures of physiological function. Part II examines the use of PET imaging as a tool to study the pharmacokinetics and pharmacodynamics of specific drugs.

The metabolic landscape of cortico-basal ganglionic degeneration: regional asymmetries studied with positron emission tomography

Regional metabolic rate for glucose (rCMRGlc) was estimated using [18F]fluorodeoxyglucose (FDG) and positron emission tomography (PET) in five patients (four men, one woman; mean age 68; mean disease duration 2.4 years) with clinical findings consistent with the syndrome of cortico-basal ganglionic degeneration (CBGD). Left-right rCMRGlc asymmetry, (L-R)/(L + R) x 100, was calculated for 13 grey matter regions and compared with regional metabolic data from 18 normal volunteers and nine patients with asymmetrical Parkinson's disease (PD). In the CBGD group mean metabolic asymmetry values in the thalamus, inferior parietal lobule and hippocampus were greater than those measured in normal control subjects and patients with asymmetrical PD (p less than 0.02). Parietal lobe asymmetry of 5% or more was evident in all CBGD patients, whereas in PD patients and normal controls, all regional asymmetry measures were less than 5% in absolute value. Measures of frontal, parietal and hemispheric metabolic asymmetry were found to be positively correlated with asymmetries in thalamic rCMRGlc (p less than 0.05). The presence of cortico-thalamic metabolic asymmetry is consistent with the focal neuropathological changes reported in CBGD brains. Our findings suggest that metabolic asymmetries detected with FDG/PET may support a diagnosis of CBGD in life.

Brain-behavior relationships in aphasia studied by positron emission tomography

Positron emission tomography allows for the study of human brain physiology and chemistry including cerebral blood flow, oxygen or glucose metabolism. We applied PET to study glucose metabolism using aphasia as a model of neurobehavior. The most striking observation was that the extent of cerebral glucose metabolic changes in aphasic patients consistently involve brain regions that are not structurally damaged. The remote metabolic effects can be predicted depending on the location and extent of structural damage. Two observations were made: (1) In our experience, all right-handed aphasic patients with left hemisphere structural lesions have metabolic abnormalities in the left temporoparietal region, and (2) metabolic abnormalities are variably found in undamaged, left prefrontal lobe, basal ganglia, and thalamus. Variations in clinical aphasic syndromes were found to relate to these frontal metabolic changes, suggesting that aspects of the aphasia result from differences in prefrontal function rather than directly from structural damage to perisylvian or deep structures.

Cerebellar and frontal hypometabolism in alcoholic cerebellar degeneration studied with positron emission tomography

Local cerebral metabolic rate for glucose was studied utilizing 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography (PET) in 14 chronically alcohol-dependent patients and 8 normal control subjects of similar age and sex. Nine of the 14 patients (Group A) had clinical signs of alcoholic cerebellar degeneration, and the remaining 5 (Group B) did not have signs of alcoholic cerebellar degeneration. PET studies of Group A revealed significantly decreased local cerebral metabolic rates for glucose in the superior cerebellar vermis in comparison with the normal control subjects. Group B did not show decreased rates in the cerebellum. Both Groups A and B showed decreased local cerebral metabolic rates for glucose bilaterally in the medial frontal area of the cerebral cortex in comparison with the normal control subjects. The severity of the clinical neurological impairment was significantly correlated with the degree of hypometabolism in both the superior cerebellar vermis and the medial frontal region of the cerebral cortex. The degree of atrophy detected in computed tomography scans was significantly correlated with local cerebral metabolic rates in the medial frontal area of the cerebral cortex, but not in the cerebellum. The data indicate that hypometabolism in the superior cerebellar vermis closely follows clinical symptomatology in patients with alcoholic cerebellar degeneration, and does not occur in alcohol-dependent patients without clinical evidence of cerebellar dysfunction. Hypometabolism in the medial frontal region of the cerebral cortex is a prominent finding in alcohol-dependent patients with or without alcoholic cerebellar degeneration.

Anxiety and cerebral cortical metabolism in normal persons

The State-Trait Anxiety Inventory (STAI) was administered to 43 normal volunteers immediately before and after a positron emission tomography (PET) procedure with [18F]-2-fluoro-2-deoxy-D-glucose (18F-FDG). High trait-anxious individuals had significantly higher state (situational) anxiety associated with the PET scan procedure than did low trait-anxious persons. State anxiety decreased significantly for all respondents following the PET scan procedure. No significant relationships between global or regional cortical metabolic rates and state anxiety were observed. The direct cortical metabolic effects of heightened anxiety in the scan setting, should they exist, are likely obscured in the normal variance of the 18F-FDG method.

The metabolic anatomy of Parkinson's disease: complementary [18F]fluorodeoxyglucose and [18F]fluorodopa positron emission tomographic studies

We studied the metabolic anatomy of typical Parkinson's disease (PD) using [18F]fluorodeoxyglucose (FDG) and [18F]fluorodopa (FDOPA) and positron emission tomography (PET). Fourteen PD patients (mean age 49 years) had FDG/PET scans, of which 11 were scanned with both FDOPA and FDG. After the injection of FDOPA, brain uptake and arterial plasma radioactivity were monitored for 2 h. Striatal FDOPA uptake was analyzed with regard to a two-compartment model, and target-to-background ratios (TBRs) and TBR-versus-time slopes were also calculated. Regional patterns of metabolic covariation were extracted from FDG/PET data using the Scaled Subprofile Model (SSM). SSM pattern weights, FDOPA uptake constants (Ki), TBRs, and TBR slopes were correlated with clinical measures for bradykinesia, rigidity, tremor, gait disturbance, left-right asymmetry, dementia, and overall disease severity. In PD patients, rate constants for FDOPA uptake correlated with individual measures of bradykinesia (p = 0.001) and gait disability (p less than 0.05). SSM analysis revealed a distinct pattern of regional metabolic asymmetries, which correlated with motor asymmetries (p less than 0.001) and left-right differences in Ki (p less than 0.01). Our data suggest that in PD patients, FDG/PET and FDOPA/PET may provide unique and complementary information about underlying disease processes.

Potassium-induced increases in oxygen consumption are diminished by age in rat hippocampal slices

Previous reports showed that cytochrome alpha,alpha 3 responded to heightened brain activity with shifts toward oxidation in adult rats but toward reduction in aged animals. To determine whether this change indicates an age-associated limitation in mitochondrial respiratory capacity, the present study compared oxygen consumption in hippocampal slices of young adult (6 month, control) and aged (26 month) rats. Slices were used to insure that results were independent of cerebrovascular factors. Age was without effect on oxygen consumption under 'resting' conditions (i.e. with slices bathed at 3.5 mM K+), but oxygen consumption was not increased as much in hippocampal slices from aged rats under heightened energy demands (produced by raising the extracellular potassium ion activity [( K+]o to 50 mM). This lesser oxygen consumption response to enhanced metabolic demand suggests that there are age-associated limits to the brain's ability to increase its metabolic rate.

Altered glucose metabolism in microvessels from patients with Alzheimer's disease

Microvessels isolated from temporal cortex of patients with Alzheimer's disease showed decreased uptake of glucose when compared with vessels from age-matched or young control subjects. This was due to decreased hexokinase activity in the Alzheimer samples, as determined by ion exchange chromatography. This finding was confirmed independently by determination of the phosphorylation constant for hexokinase, K3, using positron emission tomography. The results suggest that Alzheimer's disease may result from a global defect in brain energy metabolism.

Regional brain function in hallucinations: a study of regional cerebral blood flow with 99m-Tc-HMPAO-SPECT in patients with auditory hallucinations, tactile hallucinations, and normal controls

From the supposition that there exists a possible connection between certain psychopathological symptoms, and/or syndromes (e.g., hallucinations) and regional cerebral dysfunction, patients suffering from auditory and tactile hallucinations were investigated, in a symptom-oriented study, using the method of technetium-99m-Hexamethyl-propylenamine Oxime (99m-Tc-HMPAO)-Single Photon Emission Computed Tomography (SPECT) and compared with normal controls. The results support Jackson's hypothesis to the effect that hallucinatory phenomena will primarily occur when the normally inhibitive influence of the upper cortical centers over the lower brain structures diminishes, resulting in relative hyperactivity in the basal regions. In addition to the brain activity-changes generally observed in hallucinating patients, it was possible to identify regional cerebral blood flow (rCBF)-distribution patterns characteristic in certain forms of hallucinatory phenomena, i.e., a significant increase of activity in the hippocampal regions (including hippocampus, parahippocampus, and amygdala) only in patients with auditory hallucinations, and a significant reduction of rCBF in the inferior temporal regions in patients with tactile hallucinations.

A relationship between metabolism in frontal lobes and cerebellum in normal subjects studied with PET

Lesions of one cerebral hemisphere are associated with decreased glucose metabolism, oxygen metabolism, and blood flow in the contralateral cerebellar hemisphere. We used positron emission tomography to look for a functional relationship in cerebral metabolism between the cerebral cortex and the contralateral cerebellum in normal human subjects. Twenty-four normal subjects were scanned with [18F]fluoro-2-deoxy-D-glucose while in a resting state. Asymmetry in local CMRglu (LCMRglu) in the frontal cortex was strongly correlated with asymmetry in LCMRglu in the opposite direction in the cerebellar hemispheres (r = -0.60, p less than 0.001). Widespread subregions of the frontal cortex were found to contribute to this relationship. Considering these results together with previous studies demonstrating that frontal lesions are associated with decreased metabolism in the contralateral cerebellum, we conclude that the frontal cortex exerts a strong modulating influence on metabolism in the contralateral cerebellum in normal subjects, and that this influence may be asymmetrical.

Sensitivity of cerebral glucose metabolism to age, gender, brain volume, brain atrophy, and cerebrovascular risk factors

In 76 normal volunteers studied by positron emission tomography, with [18F]fluorodeoxyglucose, CMRglu was significantly lower in the elderly as compared with young subjects and significantly higher in females relative to males. However, in 58 of these subjects who also had magnetic resonance imaging scans, age and gender were found to be unrelated to CMRglu, when the effects of brain volume and brain atrophy on CMRglu were partialed out using covariate analyses. Individually, brain volume was found to have a significant effect on CMRglu, explaining approximately 17% of the variability in CMRglu measures and brain atrophy explaining approximately 8% of the variance in CMRglu. Together these two measures accounted for approximately 21% of the variance. Cerebrovascular risk factors in normal subjects were not found to affect mean CMRglu or the variability of CMRglu measures. In this study almost 80% of the variance in CMRglu could not be explained by any of the factors that had been considered. This implies a lack of sensitivity of absolute values of global CMRglu to the mild effects of brain dysfunction. Although some of the unexplained variance is probably methodological in origin, physiological factors that are difficult to quantify, such as the state of arousal, are likely to be contributory as well.

Motor dysfunction in olivopontocerebellar atrophy is related to cerebral metabolic rate studied with positron emission tomography

We compared the severity of motor dysfunction with local cerebral metabolic rates for glucose (lCMRGlc) and the degree of tissue atrophy in 30 patients with olivopontocerebellar atrophy (OPCA). We devised a scale to quantitate the degree of ataxia in the neurological examinations. lCMRGlc was measured with 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography (PET). Tissue atrophy was assessed by visual rating of computed tomographic scans. PET studies revealed marked hypometabolism in the cerebellar vermis, cerebellar hemispheres, and brainstem of OPCA patients compared with 30 control subjects. A significant correlation was found between severity of motor impairment and lCMRGlc within the cerebellar vermis, both cerebellar hemispheres, and the brainstem. A significant but weaker relationship was noted between the degree of tissue atrophy in these regions and clinical severity. Partial correlation analysis revealed that motor dysfunction in OPCA correlated more strongly with lCMRGlc than with the amount of tissue atrophy. These results suggest that the clinical manifestations of OPCA are more closely related to the metabolic state of the tissue than to the structural changes in the cerebellum.

Cerebral hypometabolism in progressive supranuclear palsy studied with positron emission tomography

Progressive supranuclear palsy (PSP) is characterized by supranuclear palsy of gaze, axial dystonia, bradykinesia, rigidity, and a progressive dementia. Pathological changes in this disorder are generally restricted to subcortical structures, yet the type and range of cognitive deficits suggest the involvement of many cerebral regions. We examined the extent of functional impairment to cerebral cortical and subcortical structures as measured by the level of glucose metabolic activity at rest. Fourteen patients with PSP were compared to 21 normal volunteers of similar age using 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography. Glucose metabolism was reduced in the caudate nucleus, putamen, thalamus, pons, and cerebral cortex, but not in the cerebellum in the patients with PSP as compared to the normal subjects. Analysis of individual brain regions revealed significant declines in cerebral glucose utilization in most regions throughout the cerebral cortex, particularly those in the superior half of the frontal lobe. Declines in the most affected regions of cerebral cortex were greater than those in any single subcortical structure. Although using conventional neuropathological techniques the cerebral cortex appears to be unaffected in PSP, significant and pervasive functional impairments in both cortical and subcortical structures are present. These observations help to account for the constellation of cognitive symptoms in individual patients with PSP and the difficulty encountered in identifying a characteristic psychometric profile for this group of patients.

PET in clinical oncology

Positron Emission Tomography (PET) is an imaging technique that produces cross sectional images based on tissue biochemical and physiological processes. PET complements other anatomic imaging techniques such as x-ray CT and magnetic resonance imaging (MRI). Fundamental processes such as glucose metabolism, oxygen metabolism, and blood flow can be imaged and quantified with PET, in addition to many other processes of both clinical and investigative interest. PET is now emerging as a clinical tool in oncology and is useful in noninvasively grading tumors, in determining tumor activity and recurrence, and in monitoring the effects of a variety of therapeutic interventions with tumors. While most of the applications of PET in oncology to date have been in brain tumors, the technique is now being applied in tumor evaluations outside of the central nervous system.

Speech disorders in olivopontocerebellar atrophy correlate with positron emission tomography findings

We compared the severity of ataxic and spastic dysarthria with local cerebral metabolic rates for glucose (lCMRGlc) in 30 patients with olivopontocerebellar atrophy (OPCA). Perceptual analysis was used to examine the speech disorders, and rating scales were devised to quantitate the degree of ataxia and spasticity in the speech of each patient. lCMRGlc was measured with 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography (PET). PET studies revealed marked hypometabolism in the cerebellar hemispheres, cerebellar vermis, and brainstem of OPCA patients compared with 30 control subjects. With data normalized to the cerebral cortex, a significant inverse correlation was found between the severity of ataxia in speech and the lCMRGlc within the cerebellar vermis, cerebellar hemispheres, and brainstem, but not within the thalamus. No significant correlation was found between the severity of spasticity in speech and lCMRGlc in any of these structures. The findings support the view that the severity of ataxia in speech in OPCA is related to the functional activity of the cerebellum and its connections in the brainstem.

Cerebellar and brainstem hypometabolism in olivopontocerebellar atrophy detected with positron emission tomography

We studied local cerebral metabolic rates for glucose (1CMRglc) with 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography (PET) in 30 patients with olivopontocerebellar atrophy (OPCA) and 30 age-matched control subjects without neurological disease. The diagnosis of OPCA was based on the history and physical findings and on the exclusion of other causes of cerebellar ataxia by means of laboratory investigations. Computed tomographic scans revealed some degree of atrophy of the cerebellum in most patients with OPCA, and many also had atrophy of the brainstem. PET studies in these patients revealed significant hypometabolism in the cerebellar hemispheres, cerebellar vermis, and brainstem in comparison with the normal control subjects. A significant relationship was found between the degree of atrophy and the level of 1CMRglc in the cerebellum and brainstem. Nevertheless, several patients had minimal atrophy and substantially reduced 1CMRglc, suggesting that atrophy does not fully account for the finding of hypometabolism. 1CMRglc was within normal limits for the thalamus and cerebral cortex. The data suggest that PET/1CMRglc may be useful as a diagnostic test in patients with the adult onset of cerebellar ataxia.

Positron emission tomography with the deoxyglucose technique and the diagnosis of Alzheimer's disease

Riege and Metter provide a useful review of the application of PET in the evaluation of Alzheimer's disease (AD). We share their enthusiasm for continued support and development of tools to image metabolic processes. Our commentary focuses on neuroimaging and the diagnosis of AD and introduces some new data that directly impacts on the interpretation of PET-2-deoxyglucose (2DG) data.

Positron emission tomography studies of normal aging: a replication of PET III and 18-FDG using PET VI and 11-CDG

Using PET VI and 11-CDG we replicated our earlier PET III and 18-FDG normal aging findings. Examination of young and old normal volunteers revealed the absence of any absolute regional age-related changes in glucose utilization. For the combined sample (N = 81) we did find evidence to suggest a relative hypofrontal change with increasing age. A strong relationship between age and ventricular size (CT) was also found. These findings suggest the preserved glucose metabolism of the resting aging brain in the presence of structural atrophic changes.

Effect of vascular activity in the determination of rate constants for the uptake of 18F-labeled 2-fluoro-2-deoxy-D-glucose: error analysis and normal values in older subjects

Regional cerebral blood volume (CBV) can be calculated using data obtained during the kinetic analysis of 18F-labeled 2-fluoro-2-deoxy-D-glucose (FDG) uptake measured by positron emission tomography (PET). As a result the influence of vascular activity upon the determination of FDG rate constants can be minimized. The method is investigated by simulation experiments and by analysis of PET studies on seven older, healthy human volunteers aged 52-70 years. The accuracy of measured FDG rate constants k1, k2, and k3, obtained either by omitting the early portion of the uptake curve or by explicit inclusion of CBV as a fit parameter, is compared. The root mean square error in measured rate constant for the latter method is equivalent to that obtained by omitting the first 2.5-3 min of tissue data and neglecting the CBV term. Hence, added information about the physiological state of the tissue is obtained without compromising the accuracy of the (FDG) rate constant measurement. In hyperemic tissue the explicit determination of the vascular fraction results in more accurate estimates of the FDG rate constants. The ratio of CBV determined by this method to CBV obtained using C15O in six subjects with CBV in the normal range was 0.92 +/- 0.32. A comparison of the CBV image obtained by this method with that obtained using C15O in an arteriovenous malformation case demonstrates the accuracy of the approach over a wide range of CBV values. The mean value for CBV fraction in gray matter obtained by this method in the older control group was 0.040 +/- 0.014. Average gray matter rate constants obtained were k1 = 0.084 +/- 0.012, k2 = 0.150 +/- 0.071, and k3 = 0.099 +/- 0.045 min-1.

Positron emission tomography imaging of regional cerebral glucose metabolism

The (F-18) fluorodeoxyglucose (FDG) technique to measure local cerebral metabolic rate for glucose (LCMRglu) is well accepted and widely used by many institutions around the world. A large number of studies has been carried out in normal volunteers and patients with a variety of CNS disorders. Several investigators have noted that no significant age-related changes in cerebral glucose use occur with normal aging. Some important and interesting findings have been revealed following sensory, motor, visual, and auditory stimulations. Functional imaging with FDG in certain neurologic disorders has dramatically improved our understanding of their underlying pathophysiologic phenomena. Some abnormalities detected on the positron emission tomography (PET) images have no corresponding changes on either x-ray computed tomograms (XCT) or magnetic resonance images (MRI). In patients with Alzheimer's disease, primary sensorimotor, visual, and cerebellar metabolic activity appears relatively preserved. In contrast, parietal, temporal, and to some degree, frontal glucose metabolism is significantly diminished even in the early stages of the disease. Patients with Huntington's disease and those at risk of developing this disorder have a typical pattern of diminished CMRglu in the caudate nuclei and putamen. In patients with stroke, PET images with FDG have demonstrated abnormal findings earlier than either XCT or MRI and with a wider topographic distribution. FDG scans have revealed interictal zones of decreased LCMRglu in approximately 70% of patients with partial epilepsy. The location of the area of hypometabolism corresponds to the site of the epileptic focus as determined by electroencephalography and microscopic examination of the resected tissue. Ictal scans during partial seizures demonstrate areas of hypermetabolism corresponding to the sites of seizure onset and spread. Several investigators have reported relative hypofrontal CMRglu in patients with schizophrenia. In our center, FDG scans from patients with schizophrenia were successfully differentiated from those obtained in normal controls. Finally, our preliminary data (using PET, XCT, and MRI) in patients with CNS disorders indicate that MRI provides excellent delineation of the structural abnormalities. It may prove to be superior to XCT in the evaluation of certain diseases such as cerebral ischemia and infarcts, head injury, tumors, and white matter lesions. Metabolic imaging with FDG provides functional information not obtainable with either MRI or NMR spectroscopy. Therefore, PET studies will play a complementary role to the anatomic imaging in the management of patients with CNS disorders.

Senile dementia and Alzheimer's disease. Brain blood flow and metabolism

Dementia of Alzheimer type is not form of accelerated aging. Blood flow, oxygen consumption and glucose utilization of the normally aged brain are maintained unchanged from the 3rd to the 7th decade of life. Thereafter, these parameters may decrease. Brain blood flow and oxidative metabolism is reduced in dementia of Alzheimer type and thus is different from the aged-matched mentally healthy subjects. There is evidence that the predominant impairment among these parameters may occur in cerebral glucose metabolism. This disturbance may precede changes in cerebral oxygen consumption and blood flow. Cerebral hypometabolism of glucose is accentuated in the temporo-parietal cortex. This finding may be helpful in diagnosing dementia of Alzheimer type.

The effect of age on glucose and energy metabolism in brain cortex of rats

It is well documented that the mature human brain oxidizes only glucose to obtain energy under physiological, nonstarved conditions. Through adulthood to the beginning of senescence, the balance between oxygen and glucose consumption of the brain was found to be unchanged as the basis for energy production. Beyond the age of 70 yr, however, cerebral glucose consumption appears to decrease. In the present study, the effect of advancing age on glucose and energy metabolism in brain cortex of rats was investigated. The study was carried out in male Wistar rats, 6 (young adult), 12 (adult), 24 and 30 (both aged) mth of age. Male Wistar rats may be designated as being 'aged' from 24 mth of life onwards. Intermediates of glycolysis, tricarboxylic acid cycle and energy-rich compounds were measured by means of sensitive standard enzymatic methods under steady-state conditions of arterial normotension, normoxemia, normocapnia and normothermia in anesthesia with 0.5 vol% halothane and nitrous oxide/oxygen 70:30. The 12-mth-old adult rats served as controls. The glucose concentration in brain cortex was found to be about 1.5 times higher in 6-mth-old than in 12-mth-old animals but did not differ in the 12-, 24-, and 30-mth-old rats. Besides glucose, fructose-1,6-phosphate and ATP decreased from young adult to adult rats while pyruvate, malate and creatine phosphate diminish with advancing age. A tendency to reduction with aging was also evident in glucose-6-phosphate, fructose-1, 6-diphosphate, and lactate. The fall in substrate concentrations may be attributed to the reduced activity of enzymes acting in glucose breakdown. It is concluded that glucose and energy metabolism may diminish with the process of normal aging, but that the reduction is of only moderate extent.

Effects of age on dopamine and serotonin receptors measured by positron tomography in the living human brain

D2 dopamine and S2 serotonin receptors were imaged and measured in healthy human subjects by positron emission tomography after intravenous injection of 11C-labeled 3-N-methylspiperone. Levels of receptor in the caudate nucleus, putamen, and frontal cerebral cortex declined over the age span studied (19 to 73 years). The decline in D2 receptor in males was different from that in females.

The use and impact of positron computed tomography scanning in epilepsy

Through the effective combination of instrumentation, tracer kinetic principles, and radiopharmaceuticals, positron computed tomography (PET) allows for the analytic, noninvasive measurement of local tissue physiology in humans. A large number of studies have already been performed in patients with epilepsy using 18F-fluorodeoxyglucose (FDG) to measure local cerebral glucose utilization. In patients with complex partial epilepsy who are candidates for surgery, hypometabolic zones have been seen consistently (70%) in the interictal state. These areas of hypometabolism have been related to electroencephalographic findings, surgical pathology, and clinical symptomatology. The complex anatomical and pathophysiological investigation of these hypometabolic zones is discussed. Ictal studies of patients with partial seizures have demonstrated a much more variable metabolic pattern which usually consists of hypermetabolism relative to baseline or interictal studies. Generalized epilepsy produced by electroconvulsive shock and petit mal epilepsy have been studied using FDG to estimate glucose metabolism. These studies demonstrated hypermetabolism in the ictal state, relative to interictal or postictal scans, but with a more generalized pattern than ictal studies of partial seizures. Methodological problems in the study of epilepsy with PET are discussed in detail. The investigation of interictal hypometabolism through animal models of epilepsy and quantitative autoradiography is described as a means to understand the human PET results. The impact and future direction of PET studies in epileptic populations will probably employ the use of behavioral, pharmacological, and electrophysiological maneuvers to provide more specific details about the fundamental pathophysiological mechanisms of specific aspects of epilepsy. These techniques may allow for a truly pathophysiological classification system for the common and unusual types of epilepsy, and through this classification system improve the therapeutic and prognostic clinical approach to patients.

Cerebral metabolic relationships for selected brain regions in Alzheimer's, Huntington's, and Parkinson's diseases

Local CMRGlc values were determined for 13 regions in each hemisphere from tomographs of patients with Alzheimer's, Huntington's, and Parkinson's diseases who were studied using [18F]fluorodeoxyglucose with positron emission computed tomography. Intercorrelations among the 26 regional measures were calculated for each disease state and for normal controls, and were accepted as reliable at p less than 0.01, uncorrected for the number of comparisons. The number of reliable correlations was found to be decreased in Parkinson's and Huntington's diseases, two primarily subcortical disorders, and increased in Alzheimer's disease, a primarily cortical disorder. The changes suggest that one role of the basal ganglia involves coordinating or pacing the ability of cortical brain regions to function as a unit.

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 mumol/100 g/min in the occipital white matter to 41.1 +/- 5.62 mumol/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.

Patterns of local cerebral glucose utilization determined in Parkinson's disease by the [18F]fluorodeoxyglucose method

[18F]Fluorodeoxyglucose scans were performed on 9 patients with Parkinson's disease and 14 normal subjects. Five patients were restudied after an interval of 3 to 4 years. We found no selective metabolic change in striatum, where dopamine deficit is known to be greatest, in affected patients; cerebral glucose metabolism was reduced uniformly throughout the parkinsonian brain (average 18% decrease). With increased severity of bradykinesia and the development of mild to moderate dementia, global brain metabolism in Parkinson's disease decreased further. In one moderately demented patient with Parkinson's disease, severe parietal cortex hypometabolism was found, similar to that seen in Alzheimer's disease. In contrast, mildly to moderately demented patients with Huntington's disease have marked caudate hypometabolism, but cerebral glucose metabolism is normal elsewhere. It appears that in addition to the well-known neurotransmitter loss in the nigrostriatal system, there is an abnormal metabolic process involving neurons throughout the parkinsonian brain.

Correlations of glucose metabolism and structural damage to language function in aphasia

Studies of aphasic patients using [18F]fluorodeoxyglucose positron computed tomographs have shown areas of metabolic depression in the left hemisphere larger than the area of infarction noted on CT. To evaluate these metabolic differences in relationship to language abnormalities, 11 patients had metabolic scans, CT, and were administered the Boston Diagnostic Aphasia Battery and the Porch Index of Communicative Ability. Correlation analyses were computed between metabolic, CT, and language data. CT demonstrated reliable correlations of speaking, oral reading, and repetition to Wernicke' area, consistent with current anatomic language models, while metabolic data from areas posterior, inferior, and superior to the traditional Wernicke's area and the head of the caudate nucleus also had reliable correlations with aphasic language function. The utilization of both structural and metabolic brain measures may improve our understanding of the anatomy of language as related to aphasia.

Cerebral metabolic relationships for selected brain regions in healthy adults

The local cerebral metabolic rate for glucose was determined in 26 regions of the brain in 31 healthy subjects who underwent resting fluorodeoxyglucose positron emission tomography. Intercorrelations among the 26 regional measures were accepted as reliable at p less than 0.01 (r greater than 0.45), uncorrected for the number of measures. From the matrix two apparently separate functional metabolic systems were identified: (1) a superior system involving the superior and middle frontal gyri, the inferior parietal lobule, and the occipital cortex; and (2) an inferior system involving the inferior frontal, Broca's, and posterior temporal regions. Evidence is presented to suggest that the superior system is involved in visual processing, memory recognition, and decision making, while the inferior system seems to at least participate in language-related functions.