Functional Neuroimaging Modalities Research Articles

A novel method for integrating MEG and BOLD fMRI signals with the linear convolution model in human primary somatosensory cortex

Characterizing the neurovascular coupling between hemodynamic signals and their neural origins is crucial to functional neuroimaging research, even more so as new methods become available for integrating results from different functional neuroimaging modalities. We present a novel method to relate magnetoencephalography (MEG) and BOLD fMRI data from primary somatosensory cortex within the context of the linear convolution model. This model, which relates neural activity to BOLD signal change, has been widely used to predict BOLD signals but typically lacks experimentally derived measurements of neural activity. In this study, an fMRI experiment is performed using variable-duration (< or =1 s) vibrotactile stimuli applied at 22 Hz, analogous to a previously published MEG study (Nangini et al., [2006]: Neuroimage 33:252-262), testing whether MEG source waveforms from the previous study can inform the convolution model and improve BOLD signal estimates across all stimulus durations. The typical formulation of the convolution model in which the input is given by the stimulus profile is referred to as Model 1. Model 2 is based on an energy argument relating metabolic demand to the postsynaptic currents largely responsible for the MEG current dipoles, and uses the energy density of the estimated MEG source waveforms as input to the convolution model. It is shown that Model 2 improves the BOLD signal estimates compared to Model 1 under the experimental conditions implemented, suggesting that MEG energy density can be a useful index of hemodynamic activity.

A Curious Consensus: “Brain Scans Prove Disease”?

Presumably in recognition of the fact that there is no chemical test that can be used to diagnose any of the alleged mental illnesses, journalists and medical opinion leaders have proclaimed that brain scans--particularly, functional imaging studies--can be used to confirm the presence of psychiatric disease. Although the scientific record contradicts these assertions, the news media have ignored a critical evaluation of what, exactly, the new technologies purport to explain. The article that follows presents a critical analysis of the theoretical, practical, and philosophical limitations of the functional neuroimaging modalities (fMRI, PET, SPECT), and why these methods are not used outside of the research setting in the clinical practice of psychiatry today.

Advances in neurosurgical technique in the current management of brain tumors

Despite significant advances in anatomical and functional neuroimaging modalities (eg, magnetic resonance [MR] imaging [MRI], MR spectroscopy [MRS], diffusion and perfusion MR, functional MRI [fMRI], magnetic-source imaging [MSI], diffusion tensor imaging [DTI]) and neuronavigation techniques, intraoperatively obtained functional information remains of crucial importance to the neurosurgeon, especially when operating on tumors that are located in or adjacent to functional cortical sites and subcortical pathways. This article focuses on recent advances in the surgical management of of intracerebral tumors with special emphasis on intraoperative cortical and subcortical stimulation mapping methods, and the prognostic significance of surgery on patient outcome.

Is central benzodiazepine receptor imaging useful for the identification of epileptogenic foci in localization-related epilepsies?

In the presurgical evaluation of patients with partial epilepsies, the most extensively studied functional neuro-imaging modality to define the origin of seizure onset is fluorine-18 fluorodeoxyglucose positron emission tomography (FDG PET). Generally, this technique reveals a widespread zone of interictal glucose hypometabolism in the region of the epileptogenic focus. However, the technique may miss the epileptogenic region and FDG PET abnormalities may extend beyond the seizure onset zone. Consequently, for the precise identification of epileptogenic regions more specific imaging probes than FDG are warranted. This review considers the clinical utility of iomazenil (IMZ) SPET and flumazenil (FMZ) PET for the precise localization of epileptogenic foci in partial epilepsy syndromes.

Functional neuroimaging in Alzheimer's type dementia

This article aims to review the role of the functional neuroimaging modality of positron emission tomography (PET) in the early diagnosis of Alzheimer's disease (AD). Clinical diagnosis in the early disease stages is difficult and treatments are emerging which rather than reversing structural damage are likely to slow or halt the disease process. While currently no routine diagnostic test confirms AD presence, imaging techniques are an important expanding field in biological neuropsychiatry. The challenge for neuroimaging methods is to achieve high specificity and sensitivity in early disease stages. Glucose metabolic PET imaging with fluorodeoxyglucose (FDG) has the potential to detect very early neocortical dysfunction before even abnormal neuropsychological testing is obtainable. The implications are for the identification of minimally symptomatic patients that could benefit most from treatment strategies, as well as the monitoring of treatment response and possible therapeutic deceleration of the disease. FDG PET correlates with AD neuropathology and is able to indicate disease progression or severity, meeting both functional neuroimaging prerequisites in diagnosing AD. A combination of functional neuroimaging with different techniques should be able to provide highest diagnostic specificity in diagnosing dementia. This may even lead to a new classification of dementias according to differences in the causative aetiology.

Functional neuroimaging studies of human somatosensory cortex

Two studies were carried out to assess the applicability of echoplanar fMRI at 3.0 T to the analysis of somatosensory mechanisms in humans. Vibrotactile stimulation of the tips of digits two and five reliably generated significant clusters of activation in primary (SI) and secondary (SII) somatosensory cortex, area 43, the pre-central gyrus, posterior insula, posterior parietal cortex and posterior cingulate. Separation of these responses by digit in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study employed microneurographic techniques in which individual median-nerve mechanoreceptive afferents were isolated, physiologically characterized, and microstimulated in conjunction with fMRI. Hemodynamic responses, observed in every case, were robust, focal, and physiologically orderly. These techniques will enable more detailed studies of the representation of the body surface in human somatosensory cortex, the relationship of that organization to short-term plasticity in responses to natural tactile stimuli, and effects of stimulus patterning and unimodal/cross-modal attentional manipulations. They also present unique opportunities to investigate the basic physiology of the BOLD effect, and to optimize the operating characteristics of two important human functional neuroimaging modalities—high-field fMRI and high-resolution EEG-in an unusually specific and well-characterized neurophysiological setting.

Functional activation of peri-infarct tissue for prediction of recovery after focal ischemia

In models of focal cerebral ischemia and in patients with ischemic stroke, functional disturbance and unresponsiveness to activation by specific tasks can be caused by irreversible morphologic destruction of the tissue by functional impairment in the penumbra, or by disconnection from the network by subcortical lesions. These three compartments can be distinguished by electrophysiological measures in the experimental models and by functional neuroimaging modalities in patients after having a stroke. The ability to (re)activate and (re)integrate important portions of a functional network can be tested by activation studies and has an impact on prognosis. Only if centers with a high hierarchy can be involved in a special task recovery will be satisfactory. The predominant activation of the secondary regions, which are usually not involved in task performance, permits only partial improvement, and the outcome is at a lower level. For the involvement of regions close to the damaged primary centers, neuronal plasticity may be responsible, and remote intrahemispheric activation and an interhemispheric shift of the activity to the contralateral network, might be due to the deficiency of collateral and transcallosal inhibition. The reviewed results prove that electrophysiological and neuroimaging studies can be used to assess the functional state of tissue after ischemia and may be of value for prediction of outcome in stroke patients.

Cortical Responses to Single Mechanoreceptive Afferent Microstimulation Revealed with fMRI

The technique of intraneural microneurography/microstimulation has been used extensively to study contributions of single, physiologically characterized mechanoreceptive afferents (MRAs) to properties of somatosensory experience in awake human subjects. Its power as a tool for sensory neurophysiology can be greatly enhanced, however, by combining it with functional neuroimaging techniques that permit simultaneous measurement of the associated CNS responses. Here we report its successful adaptation to the environment of a high-field MR scanner. Eight median-nerve MRAs were isolated and characterized in three subjects and microstimulated in conjunction with fMRI at 3.0 T. Hemodynamic responses were observed in every case, and these responses were robust, focal, and physiologically orderly. The combination of fMRI with microstimulation will enable more detailed studies of the representation of the body surface in human somatosensory cortex and further studies of the relationship of that organization to short-term plasticity in the human SI cortical response to natural tactile stimuli. It can also be used to study many additional topics in sensory neurophysiology, such as CNS responses to additional classes of afferents and the effects of stimulus patterning and unimodal/crossmodal attentional manipulations. Finally, it presents unique opportunities to investigate the basic physiology of the BOLD effect and to compare the operating characteristics of fMRI and EEG as human functional neuroimaging modalities in an unusually specific and well-characterized neurophysiological setting.

Electroencephalographic imaging of higher brain function.

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity that underlie mental function. Electroencephalography (EEG) provides temporal resolution in the millisecond range. However, traditional EEG technology and practice provide insufficient spatial detail to identify relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging or positron emission tomography. Recent advances help to overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical images, and by correcting the distortion caused by volume conduction of EEG signals through the skull and scalp. In addition, statistical measurements of sub-second interdependences between EEG time-series recorded from different locations can help to generate hypotheses about the instantaneous functional networks that form between different cortical regions during perception, thought and action. Example applications are presented from studies of language, attention and working memory. Along with its unique ability to monitor brain function as people perform everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities.

Exploring brain function with magnetic resonance imaging

Since its invention in the early 1990s, functional magnetic resonance imaging (fMRI) has rapidly assumed a leading role among the techniques used to localize brain activity. The spatial and temporal resolution provided by state-of-the-art MR technology and its non-invasive character, which allows multiple studies of the same subject, are some of the main advantages of fMRI over the other functional neuroimaging modalities that are based on changes in blood flow and cortical metabolism. This paper describes the basic principles and methodology of fMRI and some aspects of its application to functional activation studies. Attention is focused on the physiology of the blood oxygenation level-dependent (BOLD) contrast mechanism and on the acquisition of functional time-series with echo planar imaging (EPI). We also provide an introduction to the current strategies for the correction of signal artefacts and other image processing techniques. In order to convey an idea of the numerous applications of fMRI, we will review some of the recent results in the fields of cognitive and sensorimotor psychology and physiology.

Mapping cognitive brain function with modern high-resolution electroencephalography

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity that underlie mental function. While electroencephalography (EEG) provides temporal resolution in the millisecond range, which would seem to make it an ideal complement to other imaging modalities, traditional EEG technology and practice provides insufficient spatial detail to identify relationships between brain electrical events and structures and functions that are visualized by magnetic resonance imaging (MRI) or positron emission tomography (PET). Recent advances overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical information from each subject's MRI, and by correcting the distortion that is caused by volume conduction of EEG signals through the skull and scalp. Along with its ability to record how brains think when performing everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities.

Clinical neuroimaging in psychiatry.

The practical aspects of contemporary neuroimaging techniques relevant to clinical psychiatry are reviewed. In particular, the structural imaging modalities of computed axial tomography (CT) and magnetic resonance imaging (MRI) are described and compared. Likewise, functional neuroimaging modalities including the nuclear imaging techniques of positron emission tomography (PET) and single photon emission computed tomography (SPECT), as well as the newer magnetic resonance techniques of functional magnetic resonance imaging (fMRI) and magnetic resonance spectroscopy (MRS), are reviewed. Although structural imaging techniques are most useful for ruling out medical etiologies of mental status disturbances, functional neuroimaging techniques currently have an adjunctive role in the evaluation of dementia and seizure disorders and show promise for the evaluation of primary psychiatric disorders in the future. Specific guidelines are suggested regarding the use of these neuroimaging studies.

High resolution EEG: 124-channel recording, spatial deblurring and MRI integration methods

This paper describes a method for increasing the spatial detail of the EEG and for integrating physiological data with anatomical models based on magnetic resonance images (MRIs). This method includes techniques to efficiently record EEG data from up to 124 channels, to measure 3-D electrode positions for alignment with MRI-derived head models, and to estimate potentials near the outer convexity of the cortex using a spatial deblurring technique which uses a realistic model of the structure of the head and which makes no assumptions about the number or type of generator sources. The validity of this approach has been initially tested by comparing estimated cortical potentials with those measured with subdural grid recordings from two neurosurgical patients. The method is illustrated with somatosensory steady-state evoked potential data recorded from 5 healthy subjects. Results suggest that deblurred 124-channel topographic maps, registered with a subject's MRI and rendered in 3 dimensions, provide better spatial detail than has heretofore been obtained with scalp EEG recordings. The results also suggest that the potential for EEG as a functional neuroimaging modality has yet to be fully realized.

Neuroimaging of epilepsy, movement disorders, and degenerative diseases

Neuroimaging has improved the understanding, diagnosis, and management of several neurologic diseases and syndromes. Recent advances in the neuroimaging of epilepsy, movement disorders, and degenerative diseases of the nervous system are reviewed. Current research confirms that structural and functional neuroimages each provide unique, clinically useful information in these disorders. Quantification of images improves their diagnostic sensitivity and specificity. Presymptomatic or early neurochemical changes have been identified and followed longitudinally in several neurodegenerative diseases, providing a method for monitoring response to therapeutic intervention and pathophysiologic hypothesis testing. Functional activation studies and receptor-specific radioligands continue to advance our understanding of these disorders. Future methods will take increasing advantage of both the ability to measure neuropharmacological and neurochemical changes in vivo, and the ability to combine images and information obtained with distinct structural and functional neuroimaging modalities.