Imaging the epileptic brain with positron emission tomography

The time-evolving epileptic brain network: concepts, definitions, accomplishments, perspectives

Epilepsy is now considered a network disease that affects the brain across multiple levels of spatial and temporal scales. The paradigm shift from an epileptic focus-a discrete cortical area from which seizures originate-to a widespread epileptic network-spanning lobes and hemispheres-considerably advanced our understanding of epilepsy and continues to influence both research and clinical treatment of this multi-faceted high-impact neurological disorder. The epileptic network, however, is not static but evolves in time which requires novel approaches for an in-depth characterization. In this review, we discuss conceptual basics of network theory and critically examine state-of-the-art recording techniques and analysis tools used to assess and characterize a time-evolving human epileptic brain network. We give an account on current shortcomings and highlight potential developments towards an improved clinical management of epilepsy.

Role of Interictal Arterial Spin Labeling Magnetic Resonance Perfusion in Mesial Temporal Lobe Epilepsy

Context:

Electrophysiological and hemodynamic data can be integrated to accurately identify the generators of abnormal electrical activity in drug-resistant focal epilepsy. Arterial Spin Labeling (ASL), a magnetic resonance imaging (MRI) technique for quantitative noninvasive measurement of cerebral blood flow (CBF), can provide a direct measure of variations in cerebral perfusion associated with the epileptogenic zone.

Aims:

1. To evaluate usefulness of ASL for detecting interictal temporal hypoperfusion to localize the epileptogenic zone in patients of drug resistant mesial temporal lobe epilepsy (MTLE). 2. Correlation of localization of epileptogenic zone on ASL MR perfusion with structural MRI and EEG.

Methods and Materials:

30 patients with MTLE and10 age and gender matched normal controls were studied. All patients underwent ictal video EEG monitoring non-invasively, MR imaging with epilepsy protocol and pseudocontinuous ASL (PCASL) perfusion study. Relative CBF (rCBF) values in bilateral mesial temporal lobes were measured utilizing quantitative analysis of perfusion images. A perfusion asymmetry index (AI) was calculated for each region.

Results:

In patients, ipsilateral mesial temporal rCBF was significantly decreased compared with contralateral mesial temporal rCBF (p = 0.021). Mesial temporal blood flow was more asymmetric in patients than in normal control participants (p = 0.000). Clear perfusion asymmetry on PCASL-MRI was identified despite normal structural-MRI in 5 cases, agreeing with EEG laterality.

Conclusions:

Pseudo-continuous ASL offers a promising approach to detect interictal hypoperfusion in TLE and as a clinical alternative to SPECT and PET due to non-invasiveness and easy accessibility. Incorporation of ASL into routine pre-surgical evaluation protocols can help to localize epileptogenic zone in surgical candidates.

The treatment value of IL-1β monoclonal antibody under the targeting location of alpha-methyl-L-tryptophan and superparamagnetic iron oxide nanoparticles in an acute temporal lobe epilepsy model

BACKGROUND

Temporal lobe epilepsy (TLE) is a common and often refractory brain disease that is closely correlated with inflammation. Alpha-methyl-L-tryptophan (AMT) is recognized as a surrogate marker for epilepsy, characterized by high uptake in the epileptic focus. There are many advantages of using the magnetic targeting drug delivery system of superparamagnetic iron oxide nanoparticles (SPIONs) to treat many diseases, including epilepsy. We hypothesized that AMT and an IL-1β monoclonal antibody (anti-IL-1β mAb) chelated to SPIONs would utilize the unique advantages of SPIONs and AMT to deliver the anti-IL-1β mAb across the blood-brain barrier (BBB) as a targeted therapy.

METHODS

Acute TLE was induced in 30 rats via treatment with lithium-chloride pilocarpine. The effects of plain-SPIONs, anti-IL-1β-mAb-SPIONs, or AMT-anti-IL-1β-mAb-SPIONs on seizure onset were assessed 48 h later. Perl's iron staining, Nissl staining, immunofluorescence staining and western blotting were performed after magnetic resonance imaging examination.

RESULTS

The imaging and histopathology in combination with the molecular biology findings showed that AMT-anti-IL-1β-mAb-SPIONs were more likely to penetrate the BBB in the acute TLE model to reach the targeting location and deliver a therapeutic effect than plain-SPIONs and anti-IL-1β-mAb-SPIONs.

CONCLUSIONS

This study demonstrated the significance of anti-IL-1β-mAb treatment in acute TLE with respect to the unique advantages of SPIONs and the active location-targeting characteristic of AMT.

Intracranial EEG potentials estimated from MEG sources: A new approach to correlate MEG and iEEG data in epilepsy

Detection of epileptic spikes in MagnetoEncephaloGraphy (MEG) requires synchronized neuronal activity over a minimum of 4cm2. We previously validated the Maximum Entropy on the Mean (MEM) as a source localization able to recover the spatial extent of the epileptic spike generators. The purpose of this study was to evaluate quantitatively, using intracranial EEG (iEEG), the spatial extent recovered from MEG sources by estimating iEEG potentials generated by these MEG sources. We evaluated five patients with focal epilepsy who had a pre-operative MEG acquisition and iEEG with MRI-compatible electrodes. Individual MEG epileptic spikes were localized along the cortical surface segmented from a pre-operative MRI, which was co-registered with the MRI obtained with iEEG electrodes in place for identification of iEEG contacts. An iEEG forward model estimated the influence of every dipolar source of the cortical surface on each iEEG contact. This iEEG forward model was applied to MEG sources to estimate iEEG potentials that would have been generated by these sources. MEG-estimated iEEG potentials were compared with measured iEEG potentials using four source localization methods: two variants of MEM and two standard methods equivalent to minimum norm and LORETA estimates. Our results demonstrated an excellent MEG/iEEG correspondence in the presumed focus for four out of five patients. In one patient, the deep generator identified in iEEG could not be localized in MEG. MEG-estimated iEEG potentials is a promising method to evaluate which MEG sources could be retrieved and validated with iEEG data, providing accurate results especially when applied to MEM localizations. Hum Brain Mapp 37:1661-1683, 2016. © 2016 Wiley Periodicals, Inc.

Patient-specific detection of cerebral blood flow alterations as assessed by arterial spin labeling in drug-resistant epileptic patients

Electrophysiological and hemodynamic data can be integrated to accurately and precisely identify the generators of abnormal electrical activity in drug-resistant focal epilepsy. Arterial Spin Labeling (ASL), a magnetic resonance imaging (MRI) technique for quantitative noninvasive measurement of cerebral blood flow (CBF), can provide a direct measure of variations in cerebral perfusion associated with the epileptic focus. In this study, we aimed to confirm the ASL diagnostic value in the identification of the epileptogenic zone, as compared to electrical source imaging (ESI) results, and to apply a template-based approach to depict statistically significant CBF alterations. Standard video-electroencephalography (EEG), high-density EEG, and ASL were performed to identify clinical seizure semiology and noninvasively localize the epileptic focus in 12 drug-resistant focal epilepsy patients. The same ASL protocol was applied to a control group of 17 healthy volunteers from which a normal perfusion template was constructed using a mixed-effect approach. CBF maps of each patient were then statistically compared to the reference template to identify perfusion alterations. Significant hypo- and hyperperfused areas were identified in all cases, showing good agreement between ASL and ESI results. Interictal hypoperfusion was observed at the site of the seizure in 10/12 patients and early postictal hyperperfusion in 2/12. The epileptic focus was correctly identified within the surgical resection margins in the 5 patients who underwent lobectomy, all of which had good postsurgical outcomes. The combined use of ESI and ASL can aid in the noninvasive evaluation of drug-resistant epileptic patients.

[Neuroimaging in pediatric epilepsy]

The main causes of epilepsy in children are cortical malformations (hemimegalencephaly, cortical dysplasia, lissencephaly, etc.) and phakomatosis (tuberous sclerosis, Sturge-Weber disease, neurofibromatosis type 1, etc.), perinatal ischemia, traumatisms, infections, mesial temporal sclerosis, metabolic diseases, and tumors. Imaging indications are precise, including partial seizures and a pathological electroencephalogram. Twenty-five percent of these epilepsy cases are pharmacoresistant. Indeed, MRI is essential to consider surgical treatment, allowing one to localize potential epileptogenic anatomic lesions. The protocol includes sequences in three planes of space, weighted in T1, T2, Flair, T1 inversion-recovery, and T1 after gadolinium injection. MRI findings are characteristic for some tumors, but most malformations are subtle. Consequently recent techniques (spectroscopy, diffusion, etc.) are crucial when conventional MRI is not sufficient. The aim of this article is to illustrate, with a substantive image revue, this wide diversity of etiologies in pediatric epilepsy, in order to help the attendee recognize MRI findings, also discussing the role of newer imaging modalities in this field.

Applications of positron emission tomography in the newborn nursery

Positron emission tomography (PET) is a relatively noninvasive imaging test that is able to detect abnormalities in different organs based on derangements in the chemical functions and/or receptor expression at the cellular level. PET imaging of the brain has been shown to be a powerful diagnostic tool for detecting neurochemical abnormalities associated with various neurologic disorders as well as to study normal brain development. Although its use in detecting neurological abnormalities has been well described in adults and pediatrics, its application in the newborn nursery has not been explored adequately. Early detection of brain injury secondary to intrauterine and perinatal insults using PET imaging can provide new insight in prognosis and in instituting early therapy. In this review, the authors describe applications of PET imaging in the newborn nursery specifically related to the detection of metabolic changes seen in hypoxic ischemic encephalopathy, neonatal seizures, and neuroinflammation in the neonatal period.

Side matters: diffusion tensor imaging tractography in left and right temporal lobe epilepsy

BACKGROUND AND PURPOSE

Noninvasive imaging plays a pivotal role in lateralization of the seizure focus in presurgical patients with temporal lobe epilepsy (TLE). Our goal was to evaluate the utility of diffusion tensor imaging (DTI) tractography in TLE.

MATERIALS AND METHODS

Twenty-one patients with TLE (11 right, 10 left TLE) and 21 controls were enrolled. A 1.5T MR imaging scanner was used to obtain 51 diffusion-gradient-direction images per subject. Eight pairs of white matter fiber tracts were traced, and fiber tract fractional anisotropy (FA) was calculated and compared with controls. Fiber tract FA asymmetry and discriminant function analysis were evaluated in all subjects and fiber tracts respectively.

RESULTS

Compared with controls, patients with TLE demonstrated decreased FA in 5 ipsilateral fiber tracts. Patients with left TLE had 6 ipsilateral and 4 contralateral fiber tracts with decreased FA. Patients with right TLE had 4 ipsilateral but no contralateral tracts with decreased FA compared with controls. Right-sided FA asymmetry was demonstrated in patients with right TLE for 5 fiber tracts, and left-sided asymmetry, for patients with left TLE for 1 fiber tract. Discriminant function analysis correctly categorized patients into left-versus-right TLE in 90% of all cases (100% correct in all patients without hippocampal sclerosis) by using uncinate fasciculus and parahippocampal fiber tracts.

CONCLUSIONS

We found widespread reductions in fiber tract FA in patients with TLE, which were most pronounced ipsilateral to the seizure focus. Patients with left TLE had greater, more diffuse changes, whereas patients with right TLE showed changes that were primarily ipsilateral. Disease was lateralized to a high degree independent of identifiable hippocampal pathology noted on conventional MR imaging.

Epilepsy surgery and tuberous sclerosis complex: special considerations

Epilepsy surgery for medically refractory seizures among patients with tuberous sclerosis complex (TSC) is a well-accepted treatment option. Many epilepsy centers around the world have published their experience over the past several years, supporting the idea that the best seizure control is obtained when a single tuber and associated epileptogenic zone is documented and targeted surgically. Recent advances in imaging and physiological techniques that reveal the epileptogenic zone have been used successfully in children with TSC who are being evaluated for surgery. As a result, a number of different surgical strategies have emerged, each reflecting the experience, strengths, and referral biases of the individual treating teams. Experience suggests that some patients with TSC who present with seizures that are difficult to localize and do not meet the classic selection criteria for epilepsy surgery may, nevertheless, benefit from surgery. Tuberectomy alone is often not sufficient for obtaining seizure control. Intracranial electrode recordings performed in a large number of children with TSC undergoing epilepsy surgery have raised new questions about the relationship of the cortical tuber to the epileptogenic zone in TSC. A careful assessment of the risks and benefits of any surgical strategy, compared with those associated with continued refractory epilepsy, should be considered by the treating team in conjunction with the patient's family. Epilepsy surgery has not only benefited many children with TSC, but it also facilitates the understanding of epileptogenesis in TSC.

Neuroimaging in pediatric epilepsy: a multimodality approach

Pediatric patients with intractable epilepsy represent a challenging clinical population. However, recent advances in neuroimaging with a multimodality imaging approach that combines fluorine 18 fluorodeoxyglucose positron emission tomography, magnetoencephalography, diffusion tensor imaging, and magnetic source imaging with conventional magnetic resonance imaging continue to improve diagnosis and treatment in affected patients. These advances are increasing the understanding of the underlying disease process and improving the ability to noninvasively detect epileptogenic foci that in the past went undetected and whose accurate localization is crucial for a good outcome following surgical resection.

Progress in epilepsy: reducing the treatment gap and the promise of biomarkers

PURPOSE OF REVIEW

Since the antiquities, the history of epilepsy has been characterized by ignorance and human suffering. People with epilepsy have benefited substantially from results of modern basic and clinical research; however, serious challenges remain. Two programs begun in the past decade offer the promise of even greater progress in the future.

RECENT FINDINGS

The International League against Epilepsy, the International Bureau for Epilepsy and the WHO launched the Global Campaign against Epilepsy in 1997, which is using socioepidemiologic approaches to reduce the treatment gap and improve quality of life for people with epilepsy in the developing world. The US National Institute of Neurological Disorders and Stroke, and the American Epilepsy Society began a national neurobiological effort to move from targeting control of symptoms to strategies of prevention and cure, and in 2001 established benchmarks for future epilepsy research. The first of these benchmarks is to develop reliable biomarkers of epileptogenesis and epileptogenicity that could revolutionize our approach to diagnosis, treatment, prevention, and cure.

SUMMARY

Epilepsy continues to be a major health burden worldwide. The Global Campaign against Epilepsy is leading the way towards universal acceptability of epilepsy and access to medical care. Among the many current neurobiological research objectives, development of reliable epilepsy biomarkers would be a major step toward realizing a world where no one is compromised by epilepsy.

Functionalized magnetonanoparticles for MRI diagnosis and localization in epilepsy

PURPOSE

The development of nonradioactive and targeted magnetonanoparticles (MNP) capable of crossing the blood-brain barrier (BBB) and of concentrating in the epileptogenic tissues of acute and chronic animal models of temporal lobe epilepsy to render these tissues visible on magnetic resonance imaging (MRI).

METHODS

Nonradioactive alpha methyl tryptophan (AMT) was covalently attached to MNP composed of iron oxide and dextran. A rodent model of temporal lobe epilepsy was prepared by injecting kainic acid into the right hippocampus. AMT-MNP or plain MNP was injected in the tail-vein of two animals during the acute stage 3 days after status epilepticus, and AMT-MNP in five animals during the chronic stage. MRIs were obtained before and after particle injection in all animals. Intracranial EEGs were obtained in all chronic animals after completion of MRI studies.

RESULTS

AMT-MNP crossed the BBB and intraparenchymal uptake was visible on MRI. In the acute condition, AMT-MNP appeared to localize to both hippocampi, whereas plain MNP only identified unilateral, presumably inflammatory, changes. In the chronic condition, AMT-MNP uptake correlated with the occurrence of spontaneous seizures, and the location of uptake appeared to agree with bilateral or unilateral epileptogenicity confirmed by subsequent intracranial EEG.

DISCUSSION

Nonradioactive AMT-MNP can cross the BBB and may accurately localize epileptogenic cerebral regions. The MNP-MRI approach is potentially applicable to the use of any bioactive molecules as ligands for imaging normal and abnormal localized cerebral functions, accurately, safely, and inexpensively.

Reduced GABAA benzodiazepine receptor binding in veterans with post-traumatic stress disorder

Gamma-aminobutyric acid (GABA(A)) receptors are thought to play an important role in modulating the central nervous system in response to stress. Animal data have shown alterations in the GABA(A) receptor complex by uncontrollable stressors. SPECT imaging with benzodiazepine ligands showed lower distribution volumes of the benzodiazepine-GABA(A) receptor in the prefrontal cortex of patients with post-traumatic stress disorder (PTSD) in one, but not in another study. The objective of the present study was to assess differences in the benzodiazepine-GABA(A) receptor complex in veterans with and without PTSD using [(11)C]flumazenil and positron emission tomography (PET). Nine drug naive male Dutch veterans with deployment related PTSD and seven male Dutch veterans without PTSD were recruited, and matched for age, region and year of deployment. Each subject received a [(11)C]flumazenil PET scan and a structural magnetic resonance imaging scan. Dynamic 3D PET scans with a total duration of 60 min were acquired, and binding in template based and manually defined regions of interest (ROI) was quantified using validated plasma input and reference tissue models. In addition, parametric binding potential images were compared on a voxel-by-voxel basis using statistical parametric mapping (SPM2). ROI analyses using both template based and manual ROIs showed significantly reduced [(11)C]flumazenil binding in PTSD subjects throughout the cortex, hippocampus and thalamus. SPM analysis confirmed these results. The observed global reduction of [(11)C]flumazenil binding in patients with PTSD provides circumstantial evidence for the role of the benzodiazepine-GABA(A) receptor in the pathophysiology of PTSD and is consistent with previous animal research and clinical psychopharmacological studies.

Molecular Neuroimaging: From Conventional to Emerging Techniques

The use of molecular imaging techniques in the central nervous system (CNS) has a rich history. Most of the important developments in imaging-such as computed tomography, magnetic resonance imaging, single photon emission computed tomography, and positron emission tomography-began with neuropsychiatric applications. These techniques and modalities were then found to be useful for imaging other organs involved with various disease processes. Molecular imaging of the CNS has enabled scientists and researchers to understand better the basic biology of brain function and the way in which various disease processes affect the brain. Unlike other organs, the brain is not easily accessible, and it has a highly selective barrier at the endothelial cell level known as the blood-brain barrier. Furthermore, the brain is the most complex cellular network known to exist. Various neurotransmitters act in either an excitatory or an inhibitory fashion on adjacent neurons through a multitude of mechanisms. The various neuronal systems and the myriad of neurotransmitter systems become altered in many diseases. Some of the most devastating diseases, including Alzheimer disease, Parkinson disease, brain tumors, psychiatric disease, and numerous degenerative neurologic diseases, affect only the brain. Molecular neuroimaging will be critical to the future understanding and treatment of these diseases. Molecular neuroimaging of the brain shows tremendous promise for clinical application. In this article, the current state and clinical applications of molecular neuroimaging will be reviewed.

The added value of [18F]-fluoro-D-deoxyglucose positron emission tomography in screening for temporal lobe epilepsy surgery

PURPOSE

[18F]-Fluoro-d-deoxyglucose positron emission tomography (FDG-PET) is an expensive, invasive, and not widely available technique used in the presurgical evaluation of temporal lobe epilepsy. We assessed its added value to the decision-making process in relation to other commonly used tests.

METHODS

In a retrospective study of a large series of consecutive patients referred to the national Dutch epilepsy surgery program between 1996 and 2002, the contribution of FDG-PET, magnetic resonance imaging (MRI), and video-electroencephalogram (video-EEG) monitoring findings, alone or in combination, to the decision whether to perform surgery was investigated. The impact of FDG-PET was quantified by comparing documented decisions concerning surgery before and after FDG-PET results.

RESULTS

Of 469 included patients, 110 (23%) underwent FDG-PET. In 78 of these patients (71%), FDG-PET findings led clinicians to change the decision they had made based on MRI and video-EEG monitoring findings. In 17% of all referred patients, the decision regarding surgical candidacy was based on FDG-PET findings. FDG-PET was most useful when previous MRI results were normal (p < 0.0001) or did not show unilateral temporal abnormalities (p < 0.0001), or when ictal EEG results were not consistent with MRI findings (p < 0.0001) or videotaped seizure semiology (p = 0.027). The positive and negative predictive values for MRI and video-EEG monitoring, which ranged from 0.48 to 0.67, were improved to 0.62 to 0.86 in combination with FDG-PET.

CONCLUSIONS

In patients referred for TLE surgery, FDG-PET findings can form the basis for deciding whether a patient is eligible for surgery, and especially when MRI or video-EEG monitoring are nonlocalizing.

Positron emission tomography of the brain

Positron emission tomography (PTE) is a technique that allows imaging of the temporal and spatial distribution of positron-emitting radionuclides. The purpose of this article is to outline the current clinical use for PET imaging in the brain and other radiopharmaceutical used for assessing various physiologic parameters pertaining to tumor metabolism.

Neuroimaging of the child with developmental delay

Developmental delay (DD) affects approximately 1% to 3% of all children in the United States. This diagnosis significantly impedes quality of life and full participation in the life of the family, school, and community. In this setting, the clinician's ability to detect, diagnose, and possibly treat the cause for DD in a timely manner depends on a multimodality approach to neuroimaging and a robust understanding of the various imaging algorithms aimed at determining the etiology of disease, structural and/or anatomic defects, functional activity, metabolic profiles, and genetic characteristics. Taken separately and in combination, these features are effectively depicted and analyzed using an array of brain imaging modalities: ultrasound, computed tomography, nuclear medicine, magnetic resonance (MR) spectroscopy, and a growing mix of sophisticated MR imaging (MRI) techniques, including diffusion-weighted imaging, diffusion tensor imaging, perfusion MRI, and functional MRI. Thus, equipped with these advanced imaging capabilities, pediatric neurologists and neuroradiologists are now positioned to diagnose with greater accuracy and speed; this, in turn, results in more effective treatment plans and improved patient outcomes as measured by progress in reaching developmental milestones and in ameliorating secondary conditions such as seizures, poor motor control, incontinence, and impulsivity. The purpose of this article is to present the numerous causes of pediatric DD, describe their respective neuroimaging findings, discuss various neuroimaging approaches for elucidating etiology, and offer specific guidelines for optimizing imaging results in the setting of multimodality imaging capabilities.

Positron Emission Tomography of the Brain

Positron emission tomography (PET) is a technique that allows imaging of the temporal and spatial distribution of positron-emitting radionuclides. The purpose of this article is to outline the current clinical use for PET imaging in the brain and other radiopharmaceuticals used for assessing various physiologic parameters pertaining to tumor metabolism.

Functional neuroimaging in the preoperative evaluation of children with drug-resistant epilepsy

FUNCTIONAL NEUROIMAGING: Although the primary imaging modality in the management of epilepsy is magnetic resonance imaging MRI, functional neuroimaging with positron-emission tomography (PET) and single photon emission computed tomography (SPECT) often provides complementary information and, in a number of situations, provides unique information that cannot be obtained with MRI. The most commonly used PET tracers used for epilepsy evaluation are 2-deoxy-2-[(18)F]fluoro-D: -glucose (FDG) and [(11)C]flumazenil (FMZ). Recently, interictal PET with alpha-[(11)C]methyl-L: -tryptophan was found to be highly specific for the epileptic focus and can differentiate between epileptogenic and nonepileptogenic lesions in the same patient (e.g., in patients with tuberous sclerosis).

DISCUSSION

In this review, we discuss clinical applications of these three PET tracers in drug-resistant temporal and extratemporal lobe epilepsy, selected epilepsy syndromes of childhood, lesional and nonlesional epilepsy, and the challenges of imaging secondary epileptic foci. A brief discussion of SPECT applications in epilepsy is also included. With further development of new tracers highly sensitive and specific for epileptogenic brain regions, the presurgical evaluation of refractory epilepsy will be greatly facilitated. Approximately 0.5 to 1.0% of the population suffer from epilepsy, of which 15-20% are intractable. Infants and children, whose seizures have a focal onset are refractory to anticonvulsants and are prolonged, tend to have the worst cognitive outcome [Meador KJ, Neurology 58 (Suppl 5):S21-S26, 2002]. Seizures themselves affect the developing brain and contribute to an adverse neurologic outcome (Holmes, Pediatric Neurology 33:1-110, 2005).

CONCLUSION

Therefore, in treating children with intractable epilepsy, it is important to consider seizure control and to give allowance for normal cognitive development.

Functional imaging of vegetative state applying single photon emission tomography and positron emission tomography

Nuclear medicine techniques, such as single photon emission tomography (SPECT) and positron emission tomography (PET) have been applied in patients in a vegetative state to investigate brain function in a non-invasive manner. Parameters investigated include glucose metabolism, perfusion at rest, variations of regional perfusion after stimulation, and benzodiazepine receptor density. Compared to controls, patients in a vegetative state show a substantial reduction of glucose metabolism and perfusion. While patients post-anoxia exhibit a rather homogenous cortical reduction of glucose metabolism, patients after head trauma often show severe cortical and sub-cortical reductions at the site of primary trauma. To distinguish reduced glucose metabolism due to neuronal inactivation from neuronal loss, flumazenil-PET, an indicator of benzodiazepine receptor density, could add valuable information on the extent of brain damage. Activation studies focus on the evaluation of residual brain network, looking for processing in secondary projection fields. So far the predictive strength concerning possible recovery for the individual patient is limited, and PET and SPECT are not routine procedures in the assessment of patients in a vegetative state.

Neuroimaging and sleep medicine

In sleep medicine, patients with sleep disorders are evaluated and treated. The primary assessment tool of the field has traditionally been polysomnography. While polysomnography has been helpful in the evaluation of some sleep disorders, such as sleep apnea syndrome and periodic limb movement disorder, it has been less helpful in others, such as the insomnias, or sleep disorders secondary to mental disorders. These disorders are presumed to stem from some alteration in brain function that disrupts sleep. The development of functional neuroimaging methods provides a means to understand brain function in patients with sleep disorders in a manner not accessible to polysomnography. This paper summarizes functional neuroimaging findings during healthy sleep, then, reviews available studies in sleep disorders patients, and studies addressing the pharmacology of sleep and sleep disorders. Areas in which functional neuroimaging methods may be helpful in sleep medicine, and in which future development is advised, include: (1) clarification of pathophysiology; (2) aid in differential diagnosis; (3) assessment of treatment response; (4) guiding new drug development; and (5) monitoring treatment response.