Imaging of cerebral blood flow and metabolism

Metabolic derangements are associated with impaired glucose delivery following traumatic brain injury

Metabolic derangements following traumatic brain injury are poorly characterized. In this single-centre observational cohort study we combined 18F-FDG and multi-tracer oxygen-15 PET to comprehensively characterize the extent and spatial pattern of metabolic derangements. Twenty-six patients requiring sedation and ventilation with intracranial pressure monitoring following head injury within a Neurosciences Critical Care Unit, and 47 healthy volunteers were recruited. Eighteen volunteers were excluded for age over 60 years (n = 11), movement-related artefact (n = 3) or physiological instability during imaging (n = 4). We measured cerebral blood flow, blood volume, oxygen extraction fraction, and 18F-FDG transport into the brain (K1) and its phosphorylation (k3). We calculated oxygen metabolism, 18F-FDG influx rate constant (Ki), glucose metabolism and the oxygen/glucose metabolic ratio. Lesion core, penumbra and peri-penumbra, and normal-appearing brain, ischaemic brain volume and k3 hotspot regions were compared with plasma and microdialysis glucose in patients. Twenty-six head injury patients, median age 40 years (22 male, four female) underwent 34 combined 18F-FDG and oxygen-15 PET at early, intermediate, and late time points (within 24 h, Days 2-5, and Days 6-12 post-injury; n = 12, 8, and 14, respectively), and were compared with 20 volunteers, median age 43 years (15 male, five female) who underwent oxygen-15, and nine volunteers, median age 56 years (three male, six female) who underwent 18F-FDG PET. Higher plasma glucose was associated with higher microdialysate glucose. Blood flow and K1 were decreased in the vicinity of lesions, and closely related when blood flow was <25 ml/100 ml/min. Within normal-appearing brain, K1 was maintained despite lower blood flow than volunteers. Glucose utilization was globally reduced in comparison with volunteers (P < 0.001). k3 was variable; highest within lesions with some patients showing increases with blood flow <25 ml/100 ml/min, but falling steeply with blood flow lower than 12 ml/100 ml/min. k3 hotspots were found distant from lesions, with k3 increases associated with lower plasma glucose (Rho -0.33, P < 0.001) and microdialysis glucose (Rho -0.73, P = 0.02). k3 hotspots showed similar K1 and glucose metabolism to volunteers despite lower blood flow and oxygen metabolism (P < 0.001, both comparisons); oxygen extraction fraction increases consistent with ischaemia were uncommon. We show that glucose delivery was dependent on plasma glucose and cerebral blood flow. Overall glucose utilization was low, but regional increases were associated with reductions in glucose availability, blood flow and oxygen metabolism in the absence of ischaemia. Clinical management should optimize blood flow and glucose delivery and could explore the use of alternative energy substrates.

Effects of video game playing on cerebral blood flow in young adults: a SPECT study

To study the impact of video game playing on the human brain, the effects of two video games playing on cerebral blood flow (CBF) in young adults were determined. Thirty healthy subjects comprising 18 males and 12 females who were familiar with video game playing were recruited. Each subject underwent three sessions of single photon emission computed tomography (SPECT) with a bolus injection of 20 mCi (99m)Tc ECD IV to measure their CBF. The first measurement was performed as baseline, the second and third measurements were performed after playing two different video games for 30 min, respectively. Statistic parametric mapping (SPM2) with Matlab 6.5 implemented on a personal computer was used for image analysis. CBF was significantly decreased in the prefrontal cortex and significantly increased in the temporal and occipital cortices after both video games playing. Furthermore, decreased CBF in the anterior cingulate cortex (ACC) which was significantly correlated with the number of killed characters was found after the violent game playing. The major finding of hypo-perfusion in prefrontal regions after video game playing is consistent with a previous study showing reduced or abnormal prefrontal cortex functions after video game playing. The second finding of decreased CBF in the ACC after playing the violent video game provides support for a previous hypothesis that the ACC might play a role in regulating violent behavior.

Imaging in the diagnosis and prognosis of traumatic brain injury

INTRODUCTION

Traumatic brain injury (TBI) is a major cause of death and disability worldwide. Improved understanding of the impact of head injury and the extent and development of neuronal loss and cognitive dysfunction could lead to improved therapy and outcome for patients.

AREAS COVERED

This paper reviews the currently available imaging techniques and defines their role in the diagnosis, management and prediction of outcome following traumatic brain injury. These imaging techniques provide delineation of the structural, physiological and functional derangements that result following acute injury, and map their development and association with late functional deficits. Imaging tools also have a role in defining the pathophysiological mechanisms responsible for further neuronal loss following the primary injury. Finally, this paper provides an overview of the role of functional imaging in classifying unresponsive coma and defining functional reorganisation of the brain following injury.

EXPERT OPINION

Brain imaging is of key importance in TBI management, enabling efficient and accurate diagnoses to be made, informing management decisions and contributing to prognostication. Developments in imaging techniques promise to improve understanding of the structural and functional derangements, improve management and guide the development and implementation of novel neuroprotective strategies following head injury.

In vivo measurement of CBF using ¹⁷O NMR signal of metabolically produced H₂¹⁷O as a perfusion tracer

The cerebral metabolic rate of oxygen of small animals can be reliably imaged using the in vivo (17) O magnetic resonance approach at high field. However, a separate measurement is required for imaging the cerebral blood flow in the same animal. In this study, we demonstrate that the (17) O NMR signal of metabolically produced H2 (17) O in the rat brain following an (17) O2 inhalation can serve as a perfusion tracer and its decay rate can be used to determine the absolute values of cerebral blood flow across a wide range of animal conditions. This finding suggests that the in vivo (17) O magnetic resonance approach is capable of imaging both cerebral metabolic rate of oxygen and cerebral blood flow simultaneously and noninvasively; and it provides new utilities for studying the cerebral oxygen metabolism and perfusion commonly associated with brain function and diseases.

Brain perfusion CT compared with15O-H2O-PET in healthy subjects

Background

Regional cerebral blood flow [rCBF] measurements are valuable for identifying angiogenically active tumours, and perfusion computed tomography [CT] has been suggested for that purpose. This study aimed to validate rCBF measurements by perfusion CT with positron-emission tomography [PET] and¹⁵O-labelled water [¹⁵O-H₂O] in healthy subjects.

Methods

RCBF was measured twice in 12 healthy subjects with¹⁵O-H₂O PET and once with perfusion CT performed over the basal ganglia. Matching rCBF values in regions of interest were compared.

Results

Measured with perfusion CT, rCBF was significantly and systematically overestimated. White matter rCBF values were 17.4 ± 2.0 (mean ± SD) mL min^-1 100 g^-1 for PET and 21.8 ± 3.4 mL min^-1 100 g^-1 for perfusion CT. Grey matter rCBF values were 48.7 ± 5.0 mL min^-1 100 g^-1 for PET and 71.8 ± 8.0 mL min^-1 100 g^-1 for perfusion CT. The overestimation of grey matter rCBF could be reduced from 47% to 20% after normalization to white matter rCBF, but the difference was still significant.

Conclusion

RCBF measured with perfusion CT does contain perfusion information, but neither quantitative nor relative values can substitute rCBF measured by¹⁵O-H₂O PET yet. This, however, does not necessarily preclude a useful role in patient management.

Cerebral blood flow modulation by transcutaneous cranial electrical stimulation with Limoge's current

OBJECTIVES

Transcutaneous cranial electrical stimulation (TCES) delivers a high-frequency (166 kHz) pulsed biphasic balanced current with a pulse repetition frequency of 100 Hz with 40% duty cycle through a negative electrode and two positive electrodes over the skull. TCES has a proven ability to potentiate anesthesia and analgesia, although the physiological mechanisms of this effect remain unclear. We hypothesized that the mechanism is a modulation of CBF in the central endogenous opioid system. This study aimed at determining the effects of TCES on CBF to elucidate its physiological mechanism.

METHODS

Thirty-six healthy volunteers were randomly assigned to active or placebo TCES, and all assessments were double blind. TCES was performed using the Anesthelec™ device. In the stimulated group, an active cable was used, and in the control group (sham), the cable was inactive. CBF was measured by XeCT™ before and after two hours of TCES.

RESULTS

Globally, CBF was unchanged by TCES. However, locally, TCES induced a significant CBF decrease in the brainstem and thalamus, which are structures involved in pain and anxiety (TCES and control CBF decrease were 18.5 and 11.9 mL/100g brain tissue/min, respectively).

CONCLUSION

TCES can modulate local CBF but it has no effect on overall CBF. [Clinical Trials. gov number: NCT00273663].

Estimating cerebral oxygen metabolism from fMRI with a dynamic multicompartment Windkessel model

Stimulus evoked changes in cerebral blood flow, volume, and oxygenation arise from responses to underlying neuronally mediated changes in vascular tone and cerebral oxygen metabolism. There is increasing evidence that the magnitude and temporal characteristics of these evoked hemodynamic changes are additionally influenced by the local properties of the vasculature including the levels of baseline cerebral blood flow, volume, and blood oxygenation. In this work, we utilize a physiologically motivated vascular model to describe the temporal characteristics of evoked hemodynamic responses and their expected relationships to the structural and biomechanical properties of the underlying vasculature. We use this model in a temporal curve-fitting analysis of the high-temporal resolution functional MRI data to estimate the underlying cerebral vascular and metabolic responses in the brain. We present evidence for the feasibility of our model-based analysis to estimate transient changes in the cerebral metabolic rate of oxygen (CMRO(2)) in the human motor cortex from combined pulsed arterial spin labeling (ASL) and blood oxygen level dependent (BOLD) MRI. We examine both the numerical characteristics of this model and present experimental evidence to support this model by examining concurrently measured ASL, BOLD, and near-infrared spectroscopy to validate the calculated changes in underlying CMRO(2).

Acute Traumatic Brain Injury: A Review of Recent Advances in Imaging and Management

Acute traumatic brain injury (TBI) is a major cause of death and disability in young persons worldwide, producing a substantial economic burden on health services. New technology in computed tomography and magnetic resonance imaging is allowing the acquisition of more accurate and detailed information on cerebral pathology post-TBI. This has greatly improved prognostic ability in TBI and enables earlier identification of pathology, making it potentially amenable to therapeutic intervention. Recent advances in the management of TBI have been hampered by a lack of class I evidence arising from difficulties in applying strict study protocols to a patient subset as heterogeneous as post-TBI patients. The most definite benefits in terms of survival after TBI come from admission to a specialist neurosurgical centre, with goal-targeted therapy and intensive care services. Some traditional therapies for the treatment of acute TBI have been proven to be harmful and should be avoided. A number of management strategies have proved potentially beneficial post-TBI, but there is insufficient evidence to make definitive recommendations at present. Future therapies that are currently under investigation include decompressive craniectomy, progesterone therapy, and possibly therapeutic hypothermia.

Imaging after brain injury

Head injury remains an important cause of death and disability in young adults. This review will discuss the role of structural imaging using computed tomography (CT) and magnetic resonance imaging (MRI) and physiological imaging using CT perfusion, 131Xe CT, MRI and spectroscopy (MRS), single photon emission computed tomography, and positron emission tomography (PET) in the assessment, management, and prediction of outcome after head injury. CT allows rapid assessment of brain pathology which ensures patients who require urgent surgical intervention receive appropriate care. Although MRI provides greater spatial resolution, particularly within the posterior fossa and deep white matter, a complete assessment of the burden of injury requires imaging of cerebral physiology. Physiological imaging techniques can only provide 'snap shots' of physiology within the injured brain, but they can be repeated, and such data can be used to assess the impact of therapeutic interventions. Perfusion imaging based on CT techniques (xenon CT and CT perfusion) can be implemented easily in most hospital centres, and provide quantitative perfusion data in addition to structural images. PET imaging provides unparalleled insights into cerebral physiology and pathophysiology, but is not widely available and is primarily a research tool. MR technology continues to develop and is becoming generally available. Using a complex variety of sequences, MR can provide data concerning both structural and physiological derangements. Future developments with such imaging techniques should improve understanding of the pathophysiology of brain injury and provide data that should improve management and prediction of functional outcome.