Understanding functional neuroimaging methods based on neurovascular coupling

Imaging in pediatric disorders of the gut-brain interactions: current best practice and future directions

INTRODUCTION

Disorders of Gut-Brain Interactions (DGBI) are a common clinical problem in children and pose significant challenges to the attending pediatrician. Radiological investigations are commonly ordered to evaluate these children.

AREA COVERED

This review focuses on the current best practice of using radiological investigations in DGBIs and how novel radiological investigations could revolutionize the assessment and therapeutic approach of DGBI in children.

EXPERT OPINION

We believe imaging in DGBI is still in its early stages, but it has the potential to revolutionize how we diagnose and treat children with DGBI. As the understanding of the gut-brain axis continues to grow, we can expect to see the disappearance of conventional imaging techniques and the emergence of more sophisticated imaging techniques with less radiation exposure in the future which provide more clinically meaningful information regarding the gut-brain axis and its influence on intestinal function. Some of the novel imaging modalities will be able to broaden our horizon of understanding DGBI in children providing more useful therapeutic options to minimize their suffering.

Prefrontal NIRS signal is unaffected by forehead Doppler flux during incremental cycling exercise

Near-infrared spectroscopy (NIRS) is used to measure tissue concentrations of oxyhemoglobin (O2 Hb) and deoxyhemoglobin (HHb). In the context of exercise, NIRS confers a higher signal-to-noise ratio than other neuroimaging techniques. However, part of the signal may be influenced by thermoregulatory hyperemia in the superficial cutaneous capillaries of the forehead. The degree to which NIRS signals during exercise reflect cerebral or extracerebral hemodynamic changes is a continuing source of controversy. However, the influence of skin blood flow may be attenuated depending on the NIRS technique (e.g., frequency domain machines with maximal optode separation distances >3.5 cm). The purpose of this study was to compare the changes in forehead skin blood flow and cerebral hemoglobin concentration during incremental exercise versus direct vasodilation of the forehead skin induced by gradual local heating. Thirty participants (12 females, 18 males; age: 20.8 ± 3.2 years; body mass index: 23.8 ± 3.7 kg·m-2 ) participated in the study. Forehead skin blood flow was quantified laser Doppler flux and absolute concentrations of cerebral O2 Hb and HHb were measured by NIRS. Local heating significantly increased the Doppler flux signal across time and these changes were significantly correlated with skin temperature. During incremental exercise, skin temperature, Doppler flux, O2 Hb and HHb increased however, the only significant change that was consistently correlated with Doppler flux was skin temperature. Therefore, a significant change in forehead skin blood flow may not significantly the NIRS hemoglobin data, depending on the type of NIRS device used.

Re-thinking the Etiological Framework of Neurodegeneration

Neurodegenerative diseases are among the leading causes of disability and death worldwide. The disease-related socioeconomic burden is expected to increase with the steadily increasing life expectancy. In spite of decades of clinical and basic research, most strategies designed to manage degenerative brain diseases are palliative. This is not surprising as neurodegeneration progresses "silently" for decades before symptoms are noticed. Importantly, conceptual models with heuristic value used to study neurodegeneration have been constructed retrospectively, based on signs and symptoms already present in affected patients; a circumstance that may confound causes and consequences. Hence, innovative, paradigm-shifting views of the etiology of these diseases are necessary to enable their timely prevention and treatment. Here, we outline four alternative views, not mutually exclusive, on different etiological paths toward neurodegeneration. First, we propose neurodegeneration as being a secondary outcome of a primary cardiovascular cause with vascular pathology disrupting the vital homeostatic interactions between the vasculature and the brain, resulting in cognitive impairment, dementia, and cerebrovascular events such as stroke. Second, we suggest that the persistence of senescent cells in neuronal circuits may favor, together with systemic metabolic diseases, neurodegeneration to occur. Third, we argue that neurodegeneration may start in response to altered body and brain trophic interactions established via the hardwire that connects peripheral targets with central neuronal structures or by means of extracellular vesicle (EV)-mediated communication. Lastly, we elaborate on how lifespan body dysbiosis may be linked to the origin of neurodegeneration. We highlight the existence of bacterial products that modulate the gut-brain axis causing neuroinflammation and neuronal dysfunction. As a concluding section, we end by recommending research avenues to investigate these etiological paths in the future. We think that this requires an integrated, interdisciplinary conceptual research approach based on the investigation of the multimodal aspects of physiology and pathophysiology. It involves utilizing proper conceptual models, experimental animal units, and identifying currently unused opportunities derived from human data. Overall, the proposed etiological paths and experimental recommendations will be important guidelines for future cross-discipline research to overcome the translational roadblock and to develop causative treatments for neurodegenerative diseases.

Wearable brain imaging with multimodal physiological monitoring

The brain is a central component of cognitive and physical human performance. Measures, including functional brain activation, cerebral perfusion, cerebral oxygenation, evoked electrical responses, and resting hemodynamic and electrical activity are all related to, or can predict, health status or performance decrements. However, measuring brain physiology typically requires large, stationary machines that are not suitable for mobile or self-monitoring. Moreover, when individuals are ambulatory, systemic physiological fluctuations—e.g., in heart rate, blood pressure, skin perfusion, and more—can interfere with noninvasive brain measurements. In efforts to address the physiological monitoring and performance assessment needs for astronauts during spaceflight, we have developed easy-to-use, wearable prototypes, such as NINscan, for near-infrared scanning, which can collect synchronized multimodal physiology data, including hemodynamic deep-tissue imaging (including brain and muscles), electroencephalography, electrocardiography, electromyography, electrooculography, accelerometry, gyroscopy, pressure, respiration, and temperature measurements. Given their self-contained and portable nature, these devices can be deployed in a much broader range of settings—including austere environments—thereby, enabling a wider range of novel medical and research physiology applications. We review these, including high-altitude assessments, self-deployable multimodal e.g., (polysomnographic) recordings in remote or low-resource environments, fluid shifts in variable-gravity, or spaceflight analog environments, intracranial brain motion during high-impact sports, and long-duration monitoring for clinical symptom-capture in various clinical conditions. In addition to further enhancing sensitivity and miniaturization, advanced computational algorithms could help support real-time feedback and alerts regarding performance and health.

The blood-brain barrier-gatekeeper to neuronal homeostasis: clinical implications in the setting of stroke

The blood-brain barrier is part of the neurovascular unit and serves as a functional and anatomical barrier between the blood and the extracellular space. It controls the flow of solutes in and out of the brain thereby providing an optimal environment for neuronal functioning. Paracellular transport between endothelial cells is restricted by tight junctions and transendothelial transport is reduced and more selective compared to capillaries of other organs. Further, the blood-brain barrier is involved in controlling blood flow and it is the site for signaling damage of the nervous system to the peripheral immune system. As an important player in brain homeostasis, blood-brain barrier dysfunction has been implicated in the pathophysiology of many brain diseases including stroke, traumatic brain injury, brain tumors, epilepsy and neurodegenerative disorders. In this article - highlighting recent advances in basic science - we review the features of the blood-brain barrier and their significance for neuronal homeostasis to discuss clinical implications for neurological complications following cerebral ischemia.

Contributions and complexities from the use of in vivo animal models to improve understanding of human neuroimaging signals

Many of the major advances in our understanding of how functional brain imaging signals relate to neuronal activity over the previous two decades have arisen from physiological research studies involving experimental animal models. This approach has been successful partly because it provides opportunities to measure both the hemodynamic changes that underpin many human functional brain imaging techniques and the neuronal activity about which we wish to make inferences. Although research into the coupling of neuronal and hemodynamic responses using animal models has provided a general validation of the correspondence of neuroimaging signals to specific types of neuronal activity, it is also highlighting the key complexities and uncertainties in estimating neural signals from hemodynamic markers. This review will detail how research in animal models is contributing to our rapidly evolving understanding of what human neuroimaging techniques tell us about neuronal activity. It will highlight emerging issues in the interpretation of neuroimaging data that arise from in vivo research studies, for example spatial and temporal constraints to neuroimaging signal interpretation, or the effects of disease and modulatory neurotransmitters upon neurovascular coupling. We will also give critical consideration to the limitations and possible complexities of translating data acquired in the typical animals models used in this area to the arena of human fMRI. These include the commonplace use of anesthesia in animal research studies and the fact that many neuropsychological questions that are being actively explored in humans have limited homologs within current animal models for neuroimaging research. Finally we will highlighting approaches, both in experimental animals models (e.g. imaging in conscious, behaving animals) and human studies (e.g. combined fMRI-EEG), that mitigate against these challenges.

Connectomics: a new direction in research to understand the mechanism of acupuncture

Acupuncture has been used to treat various disorders in China and some other eastern countries for thousands of years. Nowadays, acupuncture is gradually accepted as an alternative and complementary method in western countries for its undeniable therapeutic effects. However, its central mechanism is still unclear. It is especially difficult to reveal how different regions in the brain influence one another and how the relationship is among these regions responding to acupuncture treatment. Recently, by applying neuroimaging techniques and network theory, acupuncture studies can make further efforts to investigate the influence of acupuncture on regional cerebral functional connectivity (FC) and the modulation on “acupuncture-related” networks. Connectomics appears to be a new direction in research to further understand the central mechanism underlying acupuncture. In this paper, an overview of connectomics application in acupuncture research will be discussed, with special emphasis on present findings of acupuncture and its influence on cerebral FC. Firstly, the connectomics concept and its significance on acupuncture will be outlined. Secondly, the commonly used brain imaging techniques will be briefly introduced. Thirdly, the influence of acupuncture on FC will be discussed in greater detail. Finally, the possible direction in forthcoming research will be reviewed by analyzing the limitation of present studies.

Ethanol modulates the neurovascular coupling

Despite some evidence of the underlying molecular mechanisms the neuronal basis of ethanol-induced effects on the neurovascular coupling that forms the BOLD (blood oxygenation level dependent) signal is poorly understood. In a recent fMRI (functional magnetic resonance imaging) study monitoring ethanol-induced changes of the BOLD signal a reduction of the amplitude and a prolongation of the BOLD signal were observed. However, the BOLD signal is assumed to consist of a complex superposition of different underlying signals. To gain insight how ethanol influences stimulus efficacy, oxygen extraction, transit time and vessel-related parameters the fMRI time series from the sensori-motor and the visual cortex were analyzed using the balloon model. The results show a region-dependent decrease of the stimulus efficacy to trigger a post-stimulus neurovascular response as well as a prolongation of the transit time through the venous compartment. Oxygen extraction, feedback mechanisms and other vessel-related parameters were not affected. The results may be interpreted as follows: the overall mechanisms of the neurovascular coupling are still acting well at the moderate ethanol level of about 0.8‰ (in particular the vessel-related parts), but the potency to evoke a neurovascular response is already compromised most obviously in the supplementary motor area responsible for complex synchronizing and planning processes.

Fast optical signal in visual cortex: Improving detection by General Linear Convolution Model

In this study we applied the General Linear Convolution Model to fast optical signals (FOS). We modeled the Impulse Response Function (IRF) as a rectangular function lasting 30ms, with variable time delay with respect to the stimulus onset. Simulated data confirmed the feasibility of this approach and its capability of detecting simulated activations in case of very unfavorable Signal to Noise Ratio (SNR), providing better results than the grand average method. The model was tested in a cohort of 10 healthy volunteers who underwent to hemi-field visual stimulation. Experimental data quantified the IRF time delay at 80-100ms after the stimulus onset, in agreement with classical visual evoked potential literature and previous optical imaging studies based on grand average approach and a larger number of trails. FOS confirmed the expected contralateral activation in the occipital region. Correlational analysis between hemodynamic intensity signal, phase and intensity FOS supports diffusive rather than optical absorption changes associated with neuronal activity in the activated cortical volume. Our study provides a feasible method for detecting fast cortical activations by means of FOS.

Clinical implications of skeletal muscle blood-oxygenation-level-dependent (BOLD) MRI

Blood-oxygenation-level-dependent (BOLD) contrast in magnetic resonance (MR) imaging of skeletal muscle mainly depends on changes of oxygen saturation in the microcirculation. In recent years, an increasing number of studies have evaluated the clinical relevance of skeletal muscle BOLD MR imaging in vascular diseases, such as peripheral arterial occlusive disease, diabetes mellitus, and chronic compartment syndrome. BOLD imaging combines the advantages of MR imaging, i.e., high spatial resolution, no exposure to ionizing radiation, with functional information of local microvascular perfusion. Due to intrinsic contrast provoked via changes in hemoglobin oxygen saturation, it is a safe and easy applicable procedure on standard whole-body MR devices. Therefore, BOLD MR imaging of skeletal muscle is a potential new diagnostic tool in the clinical evaluation of vascular, inflammatory, and muscular pathologies. Our review focuses on the current evidence concerning the use of BOLD MR imaging of skeletal muscle under pathological conditions and highlights ways for future clinical and scientific applications.

Neurovascular coupling and functional neuroimaging in epilepsy

INTRODUCTION: The neural regulation of the microcirculation is done by the functional neurovascular unit that is composed of vascular, astroglial and neuronal cells. The neurovascular unit represents the interface between the Central Nervous System and the Vascular System. OBJECTIVE: This paper reviews the literature on functional neuroimaging with a particular focus on the mechanisms of the neurovascular coupling. CONCLUSIONS: Functional neuroimaging techniques as functional MRI, SPECT and PET distinguish metabolic and physiological processes underlying normal and abnormal events, based on neurovascular coupling. Although these techniques still have limitations in temporal and spatial resolution, they have considerably reduced the need for intracranial electrodes or invasive functional tests in the presurgical evaluation for intractable epilepsy. Recently, new techniques as optical approaches (measurement of intrinsic optical signals and near infrared spectroscopy) have increased both temporal and spatial resolutions. The use of such techniques in animal models has yielded experimental evidence for a neurovascular coupling in normal and epileptic conditions.

Observer-independent analysis of high-resolution MR images of the human cerebral cortex: in vivo delineation of cortical areas

Using high-resolution MRI, it is now possible to examine the living human cortex down to a resolution of less than 300 mum. Thus, in vivo imaging is now approaching the resolution that has been successfully used in histological analysis of the cerebral cortex for many years, e.g., low-magnification light microscopy. This allows unprecedented views of cortical microstructure that reflect defined histological features, specifically, individual cortical layers. As in histological brain mapping, it is possible to use the changes in the cortical lamination patterns to define individual cortical areas. This allows in vivo neuroanatomical maps to be generated for individual subjects and precise correlation of the results from functional imaging studies in these subjects with their own microanatomical information. To this end, we adapted the well-established observer-independent cytoarchitectonic mapping techniques for defining cortical borders based on changes in cortical lamination for in vivo parcellation of high-resolution structural MR images.

Mechanisms of working memory dysfunction after mild and moderate TBI: evidence from functional MRI and neurogenetics

Cognitive complaints are a frequent source of distress and disability after mild and moderate traumatic brain injury (TBI). While there are deficits in several cognitive domains, many aspects of these complaints and deficits suggest that problems in working memory (WM) play an important role. Functional imaging studies in healthy individuals have outlined the neural substrate of WM and have shown that regions important in WM circuitry overlap with regions commonly vulnerable to damage in TBI. Use of functional MRI (fMRI) in individuals with mild and moderate TBI suggests that they can have problems in the activation and allocation of WM, and several lines of evidence suggest that subtle alterations in central catecholaminergic sensitivity may underlie these problems. We review the evidence from fMRI and neurogenetic studies that support the role of catecholaminergic dysregulation in the etiology of WM complaints and deficits after mild and moderate TBI.

Blood Oxygenation Level–Dependent Magnetic Resonance Imaging of the Skeletal Muscle in Patients With Peripheral Arterial Occlusive Disease

Background— Blood oxygenation level–dependent (BOLD) magnetic resonance imaging (MRI) has been used to measure T2* changes in skeletal muscle tissue of healthy volunteers. The BOLD effect is assumed to primarily reflect changes in blood oxygenation at the tissue level. We compared the calf muscle BOLD response of patients with peripheral arterial occlusive disease (PAOD) to that of an age-matched non-PAOD group during postischemic reactive hyperemia.

Methods and Results— PAOD patients (n=17) with symptoms of intermittent calf claudication and an age-matched non-PAOD group (n=11) underwent T2*-weighted single-shot multiecho planar imaging on a whole-body magnetic resonance scanner at 1.5 T. Muscle BOLD MRI of the calf was performed during reactive hyperemia provoked by a cuff-compression paradigm. T2* maps were generated with an automated fitting procedure. Maximal T2* change (ΔT2* max ) and time to peak to reach ΔT2* max for gastrocnemius, soleus, tibial anterior, and peroneal muscle were evaluated. Compared with the non-PAOD group, patients revealed significantly lower ΔT2* max -values, with a mean of 7.3±5.3% versus 13.1±5.6% ( P <0.001), and significantly delayed time-to-peak values, with a mean of 109.3±79.3 versus 32.2±13.3 seconds ( P <0.001).

Conclusions— T2* time courses of the muscle BOLD MRI signal during postocclusive reactive hyperemia revealed statistically significant differences in the key parameters (ΔT2* max ; time to peak) in PAOD patients compared with age-matched non-PAOD controls.

Relationship between Cerebral Activity and Movement Frequency of Maximal Finger Tapping

To examine the cerebral activity of the motor cortex during maximum movement, we measured regional cerebral blood flow (rCBF) in twelve normal volunteers, using near infrared spectroscopy (NIRS). Repetitive tapping of the right index finger was performed at 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 Hz, and during maximum effort (ME). The relative increase rate of rCBF during movement beginning with a resting condition was calculated for each movement condition. The left primary sensorimotor cortex showed significant activation during ME compared to the other frequencies. The rapid increase of rCBF was seen immediately after the initiation of finger tapping at all the tested frequencies but showed no increase following that. However, the rCBF during ME continued to increase until the end of the task. Change of the integrated electromyogram (iEMG) for the frequency and change of rCBF for the frequency at all the tested frequencies showed similar tendencies.

The effect of maximal finger tapping on cerebral activation

The purpose of the present study was to investigate the effect of the repetition rate of a simple movement on the magnitude of neuronal recruitment at maximal effort in humans. Nine right-handed healthy subjects [age: 27.4 +/- 4.8 yr, stature: 174.5 +/- 12.2 cm, body-weight 74.3 +/- 16.6 kg (Mean +/- SD)] participated in this study. We measured the regional cerebral hemodynamics using 24-channel near infrared spectroscopy (NIRS). An auditory-cued, repetitive flexion movement of the right index finger against a button was performed as the finger-tapping task at maximal effort (ME), at 25% of maximal effort (25% ME) and at 50% of maximal effort (50% ME). The increase of the left primary motor cortex hemodynamics during movement relative to the hemodynamics under the resting condition was calculated for each pair of movement conditions. The frequency of finger-tapping was 1.61 +/- 0.18 Hz (25% ME trial), 3.23 +/- 0.36 Hz (50% ME trial), and 6.46 +/- 0.72 Hz (ME trial). The left primary motor cortex showed significant activation under all conditions. The change in total hemoglobin ([tHb]) between the ME trial and the resting value (1.19 +/- 0.93 mmol.mm) was significantly higher than those between the resting value and the 25% ME trial (0.04 +/- 0.04 mmol.mm) or the 50% ME trial (0.08 +/- 0.11 mmol x mm) (p < 0.05). There was a 29.8-fold increase of the [tHb] value between the 50% ME trial and the ME trial, but only a 2-fold increase of the [tHb] value between the 25% ME trial and the 50% ME trial. These results demonstrated that the rate of change in regional cerebral hemoglobin at a maximal effort finger-tapping task was much higher than that at a low frequency finger-tapping task.

Successive depth variations in microvascular distribution of rat somatosensory cortex

Although hemodynamic-based functional brain imaging techniques are powerful tools to explore the brain functions noninvasively, hemodynamic-based signal is strongly affected by spatial configuration of microvessels. Understanding the quantitative relations between microvascular structure and functional activity is therefore significant to make a valid signal interpretation for the imaging techniques. In the present study, we evaluated depth profiles of microvascular distributions in rat somatosensory subfields (barrel field, forelimb region, trunk region and hindlimb region) and characterized depth variations in microvascular structures, such as locations, lengths and directions of microvessels, throughout the cortical layers (I-VI). To obtain the accurate microvascular structure, we made a customized casting method by using confocal laser scanning microscope. We observed that microvascular distribution successively varied throughout the cortical layers (I-VI) and that the maximum number density of microvessels was consistently found in middle layers (III-V). In addition, superficial layers had relatively long microvessels, almost perpendicular to the cortical surface, whereas middle layers had short microvessels propagating in all directions. These regional differences in microvascular structures were closely related to the somatosensory subfields, e.g., barrel field was the greatest number density of microvessels among the investigated subfields. Based on these observations, we compared microvascular profiles with previously reported distribution patterns of tissue partial pressure of oxygen (pO2). The results showed that tissue pO2 was correlated with microvascular distribution in some but not all of the subfields. This finding shows that detailed microvascular profiles are helpful to investigate causal relationships between microvascular structure and functional activities in cerebral cortex.

The spatial extent of the BOLD response

Functional magnetic resonance imaging is routinely used to localize brain function, with multiple brain scans averaged together to reveal activation volumes. In this study, we examine the seldom-studied effect of multiple scan averaging on the extent of activation volume. Using restricted visual field stimulation, we obtained a large number of scan repetitions and analyzed changes in activation volume with progressively increased averaging and across single scans. Activation volume increased monotonically with averaging and failed to asymptote when as many as 22 scans were averaged together. Expansions in the spatial extent of activation were not random; rather, they were centered about activation loci that appear with little or no averaging. Using empirical and simulated data, changes with averaging in activation volumes and cross correlation coefficient distributions revealed the presence of considerably more activated voxels than commonly surmised. Many voxels have low SNR and remain undetected without extensive averaging. The primary source of such voxels was not downstream venous drainage since there was no significant and consistent delay difference between voxels activated at different averaging levels. Voxels with low SNR may reflect a diffuse subthreshold activity centered about spiking neurons, dephasing gradients from distal veins, or simply a blood flow response extending beyond the locus of neuronal firing. Across single scans, as much as twofold changes in activation volume were observed. These changes were not correlated with the order of scan acquisition, subject task performance, or signal and noise properties of activated voxels. Instead, they may reflect subtle changes between overlapping noise and signal frequency components.

Functional magnetic resonance imaging: emerging clinical applications

Functional magnetic resonance imaging (fMRI) is a relatively new and noninvasive method of functional brain mapping. Functional MRI is increasingly being applied to the study of neuropsychiatric disorders, including schizophrenia, Alzheimer's disease, traumatic brain injury, and others. Particularly noteworthy are findings related to plasticity in the adult human brain. Despite the promise of fMRI for improving the conceptualization, assessment, and treatment of neuropsychiatric disorders, important technical and scientific issues remain. Future research will address integrating fMRI with other emerging neuroimaging techniques.

Simultaneous measurements of cerebral oxygenation changes during brain activation by near-infrared spectroscopy and functional magnetic resonance imaging in healthy young and elderly subjects

Near infrared spectroscopy (NIRS) and functional magnetic resonance imaging (fMRI) both allow non-invasive monitoring of cerebral cortical oxygenation responses to various stimuli. To compare these methods in elderly subjects and to determine the effect of age on cortical oxygenation responses, we determined motor-task-related changes in deoxyhemoglobin concentration ([HHb]) over the left motor cortex in six healthy young subjects (age 35 +/- 9 years, mean +/- SD) and five healthy elderly subjects (age 73 +/- 3 years) by NIRS and blood-oxygen-level-dependent (BOLD) fMRI simultaneously. The motor-task consisted of seven cycles of 20-sec periods of contralateral finger-tapping at a rate as fast as possible alternated with 40-sec periods of rest. Time-locked averages over the seven cycles were used for further analysis. Task-related decreases in [HHb] over the motor cortex were measured by NIRS, with maximum changes of -0.83 +/- 0.38 mumol/L (P < 0.01) for the young and -0.32 +/- 0.17 mumol/L (P < 0.05) for the elderly subjects. The BOLD-fMRI signal increased over the cortex volume under investigation with NIRS, with maximum changes of 2.11 +/- 0.72% (P < 0.01) for the young and 1.75 +/- 0.71% (P < 0.01) for the elderly subjects. NIRS and BOLD-fMRI measurements showed good correlation in the young (r = -0.70, r(2) = 0.48, P < 0.001) and elderly subjects (r = -0.82, r(2) = 0.67, P < 0.001). Additionally, NIRS measurements demonstrated age-dependent decreases in task-related cerebral oxygenation responses (P < 0.05), whereas fMRI measurements demonstrated smaller areas of cortical activation in the elderly subjects (P < 0.05). These findings demonstrate that NIRS and fMRI similarly assess cortical oxygenation changes in young subjects and also in elderly subjects. In addition, cortical oxygenation responses to brain activation alter with aging.

Neurophysiologic basis of functional neuroimaging: animal studies

Functional neuroimaging adds metabolic or biochemical information to that obtained with anatomic imaging, allowing localization of a neural function. Positron emission tomography and single photon emission tomography make use of radioactive tracers tagged to a molecule which can indicate glucose metabolism, oxygen consumption, or blood flow. Functional magnetic resonance imaging uses the different magnetic properties of oxyhemoglobin and deoxyhemoglobin to identify areas of increased blood flow, which, in turn, reflects neuronal activation. Magnetic resonance spectroscopic imaging, with magnetically labeled molecules, can be used to follow biochemical pathways. Functional neuroimaging is based on the experimental data that neuronal activation leads to increased metabolism. Uptake of glucose and oxygen increases to meet increased energy needs. The fractionally increased glucose appears to be taken up mostly by glia, which metabolize it through glycolysis. The end product, lactate, is released for neuronal uptake and subsequent oxidative phosphorylation. To meet these metabolic needs, blood flow increases to such an extent that overall capillary oxyhemoglobin concentration increases. This changes the magnetic signal in the region and permits functional magnetic resonance imaging studies. Recent data suggest that there is an initial decrease in the concentration of oxyhemoglobin which may be more spatially specific to the area of neuronal activation. Further refinements in functional neuroimaging will lead to improved understanding of the normal functional anatomy of the brain and will shed further light on the pathophysiology of many neurologic disorders.