Standardized mean regional method for calculating global positron emission tomographic measurements

Tobacco smoking interferes with GABAA receptor neuroadaptations during prolonged alcohol withdrawal

Understanding the effects of tobacco smoking on neuroadaptations in GABAA receptor levels over alcohol withdrawal will provide critical insights for the treatment of comorbid alcohol and nicotine dependence. We conducted parallel studies in human subjects and nonhuman primates to investigate the differential effects of tobacco smoking and nicotine on changes in GABAA receptor availability during acute and prolonged alcohol withdrawal. We report that alcohol withdrawal with or without concurrent tobacco smoking/nicotine consumption resulted in significant and robust elevations in GABAA receptor levels over the first week of withdrawal. Over prolonged withdrawal, GABAA receptors returned to control levels in alcohol-dependent nonsmokers, but alcohol-dependent smokers had significant and sustained elevations in GABAA receptors that were associated with craving for alcohol and cigarettes. In nonhuman primates, GABAA receptor levels normalized by 1 mo of abstinence in both groups--that is, those that consumed alcohol alone or the combination of alcohol and nicotine. These data suggest that constituents in tobacco smoke other than nicotine block the recovery of GABAA receptor systems during sustained alcohol abstinence, contributing to alcohol relapse and the perpetuation of smoking.

Magnetic resonance imaging of resting OEF and CMRO₂ using a generalized calibration model for hypercapnia and hyperoxia

We present a method allowing determination of resting cerebral oxygen metabolism (CMRO₂) from MRI and end-tidal O₂ measurements acquired during a pair of respiratory manipulations producing different combinations of hypercapnia and hyperoxia. The approach is based on a recently introduced generalization of calibrated MRI signal models that is valid for arbitrary combinations of blood flow and oxygenation change. Application of this model to MRI and respiratory data during a predominantly hyperoxic gas manipulation yields a specific functional relationship between the resting BOLD signal M and the resting oxygen extraction fraction OEF₀. Repeating the procedure using a second, primarily hypercapnic, manipulation provides a different functional form of M vs. OEF₀. These two equations can be readily solved for the two unknowns M and OEF₀. The procedure also yields the resting arterial O₂ content, which when multiplied by resting cerebral blood flow provides the total oxygen delivery in absolute physical units. The resultant map of oxygen delivery can be multiplied by the map of OEF₀ to obtain a map of the resting cerebral metabolic rate of oxygen consumption (CMRO₂) in absolute physical units. Application of this procedure in a group of seven human subjects provided average values of 0.35 ± 0.04 and 6.0 ± 0.7% for OEF₀ and M, respectively in gray-matter (M valid for 30 ms echo-time at 3T). Multiplying OEF₀ estimates by the individual values of resting gray-matter CBF (mean 52 ± 5 ml/100 g/min) and the measured arterial O₂ content gave a group average resting CMRO₂ value of 145 ± 30 μmol/100 g/min. The method also allowed the generation of maps depicting resting OEF, BOLD signal, and CMRO₂.

Oxygen metabolism in ischemic stroke using magnetic resonance imaging

Detecting "at-risk" but potentially salvageable brain tissue, known as the ischemic penumbra, is of importance for identifying patients who may benefit from thrombolytic or other treatments beyond the currently FDA-approved short therapeutic window for tissue plasminogen activator. Since the magnetic resonance blood oxygenation level-dependent (BOLD) contrast may provide information concerning tissue oxygen metabolism, its utilization in ischemic stroke has been explored. The focus of this review is to provide an introduction of several BOLD-based methods, including susceptibility-weighted imaging, R2 BOLD, R2*, R2', MR_OEF, and MR_OMI approaches to assess cerebral oxygenation changes induced by ischemia. Specifically, we will review the underlying pathophysiological basis of the imaging approaches, followed by a brief introduction of BOLD contrast, and finally the applications of BOLD approaches in ischemic stroke. The advantages and disadvantages of each method are addressed. In summary, the BOLD-based methods are promising for imaging oxygenation in ischemic tissue. Future steps would include technical refinement and vigorous validation against another independent method, such as positron emission tomography.

Improved fMRI calibration: precisely controlled hyperoxic versus hypercapnic stimuli

The calibration of functional magnetic resonance imaging (fMRI) for the estimation of neuronal activation-induced changes in cerebral metabolic rate of oxygen (CMRO(2)) has been achieved through hypercapnic-induced iso-metabolic increases in cerebral blood flow (CBF). Hypercapnia (HC) has been traditionally implemented through alterations in the fixed inspired fractional concentrations of carbon dioxide (F(I)CO(2)) without otherwise controlling end-tidal partial pressures of carbon dioxide (P(ET)CO(2)) or oxygen (P(ET)O(2)). There are several shortcomings to the use of this manual HC method that may be improved by using precise targeting of P(ET)CO(2) while maintaining iso-oxia. Similarly, precise control of blood gases can be used to induce isocapnic hyperoxia (HO) to reduce venous deoxyhaemoglobin (dHb) and thus increase BOLD signals, without appreciably altering CMRO(2) or CBF. The aim of our study was to use precise end-tidal targeting to compare the calibration of BOLD signals under an isocapnic hyperoxic protocol (HOP) (rises in P(ET)O(2) to 140, 240 and 340 mm Hg from baseline) to that of an iso-oxic hypercapnic protocol (HCP) (rises in P(ET)CO(2) of 3, 5, 7 and 9 mm Hg from baseline). Nine healthy volunteers were imaged at 3T while monitoring end-tidal gas concentrations and simultaneously measuring BOLD and CBF signals, via arterial spin labeling (ASL), during graded HCP and HOP, alternating with normocapnic states in a blocked experimental design. The variability of the calibration constant obtained under HOP (M(HOP)) was 0.3-0.5 that of the HCP one (M(HCP)). In addition, M-variances with precise gas targeting (M(HCP) and M(HOP)) were less than those reported in studies using traditional F(I)CO(2) and F(I)O(2) methods (M(HC) and M(HO), respectively). We conclude that precise controlled gas delivery markedly improves BOLD-calibration for fMRI studies of oxygen metabolism with both the HCP and the more precise HOP-alternative.

A calibration method for quantitative BOLD fMRI based on hyperoxia

The estimation of changes in CMR(O2) using functional MRI involves an essential calibration step using a vasoactive agent to induce an isometabolic change in CBF. This calibration procedure is performed most commonly using hypercapnia as the isometabolic stimulus. However, hypercapnia possesses a number of detrimental side effects. Here, a new method is presented using hyperoxia to perform the same calibration step. This procedure requires independent measurement of Pa(O2), the BOLD signal, and CBF. We demonstrate that this method yields results that are comparable to those derived using other methods. Further, the hyperoxia technique is able to provide an estimate of the calibration constant that has lower overall intersubject and intersession variability compared to the hypercapnia approach.

Asymmetric regional cerebral blood flow in sedated baboons measured by positron emission tomography (PET)

The analysis of structural brain asymmetry has been a focal point in anthropological theories of human brain evolution and the development of lateralized behaviors. While physiological brain asymmetries have been documented for humans and animals presenting with pathological conditions or under certain activation tasks, published studies on baseline asymmetries in healthy individuals have produced conflicting results. We tested for the presence of cerebral blood flow asymmetries in 7 healthy, sedated baboons using positron emission tomography, a method of in vivo autoradiography. Five of the 7 baboons exhibited hemispheric asymmetries in which left-sided flow was significantly greater than right-sided flow. Furthermore, the degree of asymmetry in 8 of 24 brain regions was found to be significantly correlated with age; older individuals exhibited a higher degree of asymmetry than younger individuals. Cerebral blood flow itself was uncorrelated with age, and differences between males and females were not significant.

A default mode of brain function

A baseline or control state is fundamental to the understanding of most complex systems. Defining a baseline state in the human brain, arguably our most complex system, poses a particular challenge. Many suspect that left unconstrained, its activity will vary unpredictably. Despite this prediction we identify a baseline state of the normal adult human brain in terms of the brain oxygen extraction fraction or OEF. The OEF is defined as the ratio of oxygen used by the brain to oxygen delivered by flowing blood and is remarkably uniform in the awake but resting state (e.g., lying quietly with eyes closed). Local deviations in the OEF represent the physiological basis of signals of changes in neuronal activity obtained with functional MRI during a wide variety of human behaviors. We used quantitative metabolic and circulatory measurements from positron-emission tomography to obtain the OEF regionally throughout the brain. Areas of activation were conspicuous by their absence. All significant deviations from the mean hemisphere OEF were increases, signifying deactivations, and resided almost exclusively in the visual system. Defining the baseline state of an area in this manner attaches meaning to a group of areas that consistently exhibit decreases from this baseline, during a wide variety of goal-directed behaviors monitored with positron-emission tomography and functional MRI. These decreases suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors.

A study of verbal and spatial information processing using event-related potentials and positron emission tomography

The activated cerebral regions and the timing of information processing in the hemispheres was investigated using event-related potentials (ERP) and regional cerebral blood flow (rCBF) as the neurophysiological indicators. Seven men and one woman (age 19-27 years) were asked to categorize two-syllable Japanese nouns (verbal condition) and to judge the difference between pairs of rectangles (spatial condition), both tests presented on a monochrome display. In the electroencephalogram (EEG) session, EEG were recorded from 16 electrode sites, with linked earlobe electrodes as reference. In the positron emission tomography (PET) session, rCBF were measured by the 15O-labeled H2O bolus injection method. Regions of interest were the frontal, temporal, parietal, occipital and central lobes, and the entire cerebral hemispheres. When the subtracted voltages of the ERP in homologous scalp sites were compared for the verbal and spatial conditions, the significant differences were at F7.F8 and T5.T6 (the 10-20 system). The latencies of the differences at T5.T6 were around 200, 250 and 320 ms. A significant difference in rCBF between the verbal and spatial conditions was found only in the temporal region. It was concluded that early processing of information, that is, registration and simple recognition, may be performed mainly in the left temporal lobe for verbal information and in the right for spatial information.

Functional-lesion investigation of developmental stuttering with positron emission tomography

Positron emission tomographic (PET) H2(15)O measurements of resting-state regional cerebral blood flow (CBF) were obtained in 29 right-handed men, 10 of whom stuttered and 19 of whom did not. PET images were analyzed by sampling 74 regions of interest (ROIs), 37 per hemisphere. ROI placement was guided both physiologically and anatomically. Physiological ROI placement was based on speech motor activations. Anatomical ROIs were positioned by reference to a stereotactic, neurosurgical atlas with positions confirmed and finely adjusted by co-registered magnetic-resonance images (MRIs). For all subjects, PET and MR images were normal to visual inspection. Highly significant (p < 0.0001) between-region and between-hemisphere effects were found for both groups, as have been previously reported for normal subjects, but no significant between-group differences were found for any regional CBF values. Analysis by a laterality index found a weakly significant between-groups effect (p = 0.04) that was isolated to five regions, four of which are implicated in speech or hearing. However, these regional laterality effects showed no consistent directionality, nor did these regions have absolute differences in regional blood flow between groups. Present findings do not support recent suggestions that developmental stuttering is associated with abnormalities of brain blood flow at rest. Rather, our findings indicate an essentially normal functional brain terrain with a small number of minor differences in hemispheric symmetry.

Changes in local cerebral blood flow by neuroactivation and vasoactivation in patients with impaired cognitive function

Imaging of local cerebral blood flow (lCBF) may serve as an important supplementary tool in the aetiological assessment of dementias. In early or preclinical disease, however, there are less characteristic changes in lCBF. In the present study it was investigated whether vasoactivation or neuroactivation may produce more pronounced local lCBF deficits. Local CBF was investigated by using technetium-99m hexamethylpropylene amine oxime and single-photon emission tomography (SPET) in 80 patients (50 with mild cognitive impairment and 30 with dementia of Alzheimer type (DAT), all without evidence of cerebrovascular disease) at rest (baseline) and during activation. In 31 studies patients underwent vasomotor activation with acetazolamide, while 62 studies were performed under cognitive challenge (neuroactivation by labyrinth task). Cortical activity relative to that of cerebellum increased significantly in a right temporal region and tended to increase in other cortical regions upon vasoactivation. In contrast, neuroactivation reduced cortical activity relative to that of cerebellum in several left and right temporal and in left parietal regions. Visual classification of SPET images of patients with probable DAT by three observers resulted in a reduction of the number of definitely abnormal patterns from 9/12 to 4/12 by vasoactivation and an increase from 10/18 to 15/18 by neuroactivation. Correspondingly, abnormal ratings in patients with mild cognitive dysfunction were reduced from 7/19 to 5/19 by vasoactivation and were increased from 12/21 to 18/21 by neuroactivation. In conclusion, vasoactivation does not enhance local relative perfusion deficits in patients with cognitive impairment of non-vascular aetiology, whereas neuroactivation by labyrinth task produces more pronounced local flow differences and enhances abnormal patterns in lCBF imaging.

Vibration-induced regional cerebral blood flow responses in normal aging

Task-induced changes in regional CBF (rCBF) can be measured with positron emission tomography (PET) and provide a powerful tool to map brain function. Many studies using these techniques have investigated responses in healthy young subjects. Since many pathological conditions occur more commonly in older subjects, it is necessary to compare blood flow responses in these patients with appropriately age-matched controls. Furthermore, the effects of normal aging on such blood flow responses remain unknown. For both reasons, we designed this study to determine whether vibration-induced CBF responses change with advancing age in normals. CBF was measured with PET and bolus-administered H2(15)O in 26 subjects from 20 to 72 years old (mean = 39; SD = 19). Regional responses were identified by subtraction-image analysis. Left and right hand vibration produced consistent responses in contralateral primary sensorimotor area (PSA) and supplementary motor area (SMA). Response magnitudes were compared to age by linear regression. There were no substantial relationships between age and responses to vibration for PSA or SMA (PSA r = -0.28, p = 0.054; SMA r = -0.33, p = 0.13). Power analysis demonstrates a high degree of confidence (99.7% for PSA and 87% for SMA) for detecting at least a moderate correlation (r = 0.6) between response magnitude and age. We conclude that the rCBF responses to vibrotactile hand stimulation do not change with normal aging.

Statistical issues in the analysis of neuroimages

Statistical issues in the analysis of neuroimages are reviewed. These include biological questions of interest, basic problems of measurement and experimental design, normalisation, standardisation and transformation, and statistical methodology. Finally, three data sets are reviewed to illustrate some of the issues raised in the report.

Brain structure localization in positron emission tomography: comparison of magnetic resonance imaging and a stereotactic method

A method commonly used for localization of brain regions on positron emission tomographic (PET) images is direct visualization and designation of structure on the image itself. This technique, however, is limited to portions of the brain having sufficient differential radionuclide uptake to permit the recognition of structure by observers familiar with brain anatomy. Two other methods commonly used instead of direct visualization are magnetic resonance imaging (MRI) and stereotactic methods of localization. This report compares the accuracy of a MRI method versus a stereotactic method of brain structure localization of PET. The three localization methods were analyzed for sixteen different brain regions in 5 different subjects by two independent observers. The results were converted to PET pixel size (1 pixel = .127 cm) for comparison. MRI localization differed from direct visualization by a mean and standard deviation of approximately 2 +/- 1 pixels in both the X and Y dimensions. Stereotactic localization differed from direct visualization by approximately 2 +/- 1 pixels in the X dimension and by approximately 6 +/- 2 pixels in the Y dimension. This larger variation seen with the stereotactic method may be attributed to the questionable assumption of linearity of structure location with respect to size of the inner table of the calvarium.

Nonoxidative glucose consumption during focal physiologic neural activity

Brain glucose uptake, oxygen metabolism, and blood flow in humans were measured with positron emission tomography, and a resting-state molar ratio of oxygen to glucose consumption of 4.1:1 was obtained. Physiological neural activity, however, increased glucose uptake and blood flow much more (51 and 50 percent, respectively) than oxygen consumption (5 percent) and produced a molar ratio for the increases of 0.4:1. Transient increases in neural activity cause a tissue uptake of glucose in excess of that consumed by oxidative metabolism, acutely consume much less energy than previously believed, and regulate local blood flow for purposes other than oxidative metabolism.

Brain blood volume, flow, and oxygen utilization measured with 15O radiotracers and positron emission tomography: revised metabolic computations

We have revised our methods for calculating regional blood volume, flow, oxygen extraction, and oxygen utilization from positron emission tomography data obtained using 15O-labeled radiotracers. These revisions include radioactive decay explicitly within the model equations instead of requiring all measured activity to be corrected for decay prior to incorporation in the equations. The revised equations yield small but significant differences in the computed values.

Mapping human somatosensory cortex with positron emission tomography

Positron emission tomography measurements of regional cerebral blood flow were used to detect focal neuronal activation in the first somatosensory cortex (SI) of humans induced by cutaneous vibratory stimulation. Intravenously administered water labeled with oxygen-15 (H2(15)O) was used as a blood flow tracer to obtain five stimulated-state and two resting-state blood flow images in each of eight normal volunteers. Three cutaneous surfaces were tested: lips, fingers, and toes. Intense, highly focal SI responses were seen during all 39 stimulated-state trials. The SI responses from the three stimulation sites were anatomically distinct and formed a medial-to-lateral homonculus in every subject. Response magnitudes (increase in local blood flow) and response locales (expressed as proportionately measured bicommissural stereotaxic coordinates) were highly consistent among subjects and on repeated trials for each subject. These findings suggest that eliciting cerebral blood flow responses by cutaneous vibration provides a safe, rapid, and reproducible tool for locating and assessing the functional status of somatosensory cortex, and offers potential clinical and research utility. This study has established normative values for future applications of this experimental paradigm.

Regional asymmetries of cerebral blood flow, blood volume, and oxygen utilization and extraction in normal subjects

Positron emission tomography (PET) and 15O-labeled radiotracers were used to measure regional CBF, cerebral blood volume (CBV), CMRO2, and oxygen extraction in 32 right-handed subjects at rest. Mean left hemispheric CBF (46.2 +/- 6.8 ml/100 g/min) and CMRO2 (2.60 +/- 0.59 ml/100 g/min) were significantly lower than right hemispheric values (47.4 +/- 7.2 and 2.66 +/- 0.61 ml/100 g/min, respectively; p less than 0.0001 for both), whereas left and right hemispheric CBV and oxygen extraction were not significantly different. We further investigated these asymmetries by comparing left- and right-sided values for specific cortical and subcortical regions. We found that left-sided CBF and CMRO2 were significantly lower than right-sided values for sensorimotor, occipital, and superior temporal regions, whereas only left-sided CBF values were lower for anterior cingulum. CBV was asymmetric for the anterior cingulate and mid-frontal regions, and oxygen extraction was asymmetric for the sensorimotor area. No asymmetries were observed in inferior parietal cortex, thalamus, putamen, or pallidum. Knowledge of these normal physiological asymmetries is essential for proper interpretation of PET studies of physiology and pathology. Furthermore, the ability to detect asymmetries with PET may lead to a better understanding of the lateralization of specific functions in the human brain.

Left globus pallidus abnormality in never-medicated patients with schizophrenia

Schizophrenia is a severe psychiatric disorder characterized by onset in young adulthood, the occurrence of hallucinations and delusions, and the development of enduring psychosocial disability. The pathophysiology of this disorder remains unknown. Studies of cerebral blood flow and metabolism designed to identify brain abnormalities in schizophrenia have been limited by inadequate methods of anatomical localization and the possibility of persistent medication effects. We have now used positron emission tomography and a validated method of anatomical localization in an attempt to identify abnormalities of regional cerebral blood flow in newly diagnosed never-medicated patients with schizophrenia. An exploratory study of 5 patients and 10 normal control subjects identified abnormally high blood flow in the left globus pallidus of patients with schizophrenia. A replication study of 5 additional patients and 10 additional control subjects confirmed this finding. No other abnormalities were found.

Focal physiological uncoupling of cerebral blood flow and oxidative metabolism during somatosensory stimulation in human subjects

Coupling between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) was studied using multiple sequential administrations of 15O-labeled radiotracers (half-life, 123 sec) and positron emission tomography. In the resting state an excellent correlation (mean r, 0.87) between CBF and CMRO2 was found when paired measurements of CBF and CMRO2 from multiple (30-48) brain regions were tested in each of 33 normal subjects. Regional uncoupling of CBF and CMRO2 was found, however, during neuronal activation induced by somatosensory stimulation. Stimulus-induced focal augmentation of cerebral blood flow (29% mean) far exceeded the concomitant local increase in tissue metabolic rate (mean, 5%), when resting-state and stimulated-state measurements were obtained in each of 9 subjects. Stimulus duration had no significant effect on response magnitude or on the degree of CBF-CMRO2 uncoupling observed. Dynamic, physiological regulation of CBF by a mechanism (neuronal or biochemical) dependent on neuronal firing per se, but independent of the cerebral metabolic rate of oxygen, is hypothesized.