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Results for “functional MRI”

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At least 883 records · Page 49Linked to original sources
PubMed1998

Dynamic knee-extensor and cycle exercise: functional MRI of muscular activity.

Repeated studies using human dynamic knee-extensor exercise have reported high mass specific blood flows. These studies suggest that the high perfusion-to-muscle mass ratio can approach 400 ml(-1) x min x 100 g(-1) in the human quadriceps. However, in these studies mass specific blood flows were calculated based on the assumption that the quadriceps are the only muscles involved in the knee-extensor exercise, which is difficult to verify in an in vivo human model. Previous validations of this assumption have been performed using electromyography (EMG) and assessments of strain gauge tracings, but neither has been able to completely assess the involvement of all thigh muscles in this exercise. To address this issue four subjects exercised at 90% of their work rate maximum for 2.0-2.5 minutes (45-100 watts) and then a transverse section of the thigh (20 cm proximal to the knee) was studied using proton (1H) transverse relaxation time (T2) weighted magnetic resonance (MR) imaging to distinguish active from non-active muscles by the increased signal intensity (SI). On a separate occasion, measurements following 2.0-2.5 minutes of conventional two legged cycle ergometry at 90% of maximum work rate (150-400 watts) were made in the same subjects to contrast this traditional "whole leg" exercise with the unique muscle recruitment in dynamic knee-extension. Following knee-extensor exercise there was a clearly visible change in SI and a significant increase in T2 only in the four muscles of the quadriceps (P<0.05). After bicycle exercise SI changes and T2 revealed a varied muscle use across all muscles. From these MR data it can be concluded that unlike cycle exercise, in which all muscles are recruited to varying extents, single leg knee-extensor exercise is limited to the four muscles of the quadriceps. Thus, the common practice of normalizing blood flow and metabolic data to the quadriceps muscle mass in human knee-extensor exercise studies appears appropriate.

Adult↗
PubMed2005

Perfusion functional MRI reveals cerebral blood flow pattern under psychological stress.

Despite the prevalence of stress in everyday life and its impact on happiness, health, and cognition, little is known about the neural substrate of the experience of everyday stress in humans. We use a quantitative and noninvasive neuroimaging technique, arterial spin-labeling perfusion MRI, to measure cerebral blood flow (CBF) changes associated with mild to moderate stress induced by a mental arithmetic task with performance monitoring. Elicitation of stress was verified by self-report of stress and emotional state and measures of heart rate and salivary-cortisol level. The change in CBF induced by the stress task was positively correlated with subjective stress rating in the ventral right prefrontal cortex (RPFC) and left insula/putamen area. The ventral RPFC along with right insula/putamen and anterior cingulate showed sustained activation after task completion in subjects reporting a high stress level during arithmetic tasks. Additionally, variations of baseline CBF in the ventral RPFC and right orbitofrontal cortex were found to correlate with changes in salivary-cortisol level and heart rate caused by undergoing stress tasks. We further demonstrated that the observed right prefrontal activation could not be attributed to increased cognitive demand accompanying stress tasks and extended beyond neural pathways associated with negative emotions. Our results provide neuroimaging evidence that psychological stress induces negative emotion and vigilance and that the ventral RPFC plays a key role in the central stress response.

Adult↗
PubMed2006

Discrete and analogue quantity processing in the parietal lobe: a functional MRI study.

The human intraparietal sulcus (IPS) is implicated in processing symbolic number information and possibly in nonsymbolic number information. Specific IPS activity for discrete quantities (numerosities) as compared with continuous, analogue quantity has not been demonstrated. Here we use a stimulus-driven paradigm to distinguish automatic estimation of "how many things" from "how much" and "how long." The discrete analogue response task (DART) uses the perception of hues which can change either abruptly (discrete, numerous stimuli) or smoothly (analogue, nonnumerous stimuli) in space or in time. Subjects decide whether they saw more green or more blue. A conjunction analysis of spatial and temporal conditions revealed that bilateral IPS was significantly more active during the processing of discrete stimuli than during analogue stimuli, as was a parietal-occipital transition zone. We suggest that processing numerosity is a distinct process from processing analogue quantity, whether extended in space or time, and that an intraparietal network connects objects' segmentation to the estimation of their numerosity.

Adolescent↗
PubMed2002

Laminar specificity of functional MRI onset times during somatosensory stimulation in rat.

The blood oxygenation level-dependent (BOLD) response to somatosensory stimulation was measured in alpha-chloralose-anesthetized rats. BOLD fMRI was obtained at 40-ms temporal resolution and spatial resolution of 200 x 200 x 2,000 microm(3) by using a gated activation paradigm in an 11.7 T MRI. Results show a consistent heterogeneity of fMRI onset times and amplitudes. The earliest onset time (0.59 +/- 0.17 s, n = 9) corresponded anatomically to layer IV, with superficial and deeper layers starting significantly later (1.27 +/- 0.43 s in layers I-III, and 1.11 +/- 0.45 s in layer VI). The amplitude of BOLD signal changes also varied with the cortical depth from the pial surface. Changes in the supragranular layers (8.3%) were 44% bigger than changes in the intermediate layers (5.5%), located only approximately 700 microm below, and 144% larger than the bottom layer (3.5%), located approximately 1.4 mm below the pial surface. The data presented demonstrate that BOLD signal changes have distinct amplitude and temporal characteristics, which vary spatially across cortical layers.

Animals↗
PubMed2002

The role of functional MRI in the presurgical investigation of temporal lobe epilepsy patients: a clinical perspective and review.

Speculation regarding the role that fMRI might play in the presurgical assessment of temporal lobe epilepsy patients began almost as soon as the technology was introduced. Much of this speculation centred on hopes that fMRI might eventually supersede the invasive intracarotid amobarbital procedure (IAP). This review examines the progress that has been made to date towards this aim. A decade after its inception, fMRI paradigms that reliably lateralize language dominance have been established. However, fMRI has yet to achieve its full potential in the mapping of clinically relevant language skills in presurgical epilepsy patients. The role of fMRI in the assessment of postoperative amnesic risk is more complex. The results from recent fMRI memory studies in people with epilepsy have been encouraging. However, as yet, fMRI techniques cannot be used alone to confidently assess the risk of a postsurgical amnesic syndrome. It seems probable that these techniques will continue to be combined with traditional neuropsychological assessment and information from structural MRI studies and the other presurgical investigations to assess postoperative amnesic risk.

Amobarbital↗
PubMed2000

Heroin-induced neuronal activation in rat brain assessed by functional MRI.

The present study demonstrates the application of fMRI technology to neuropharmacology and the interaction of drug/receptor in the rat brain. Specifically, we have observed two different types of fMRI signal changes induced by acute i.v. heroin administration in rat brains under conditions of spontaneous and artificial respiration. Under spontaneous respiration, a global decrease in fMRI signal was observed; under artificial respiration, a region-specific increase in fMRI signal was identified and the activation sites are consistent with the distribution of opiate mu-receptors in rat brain as previously reported by autoradiography. Both heroin-induced fMRI signal changes were suppressed by pretreatment of naloxone, an opiate mu-receptor antagonist, and reversed by injection of naloxone following heroin infusion. These results suggest that fMRI has specific advantages in spatial and temporal resolution for studies of neuropharmacology and drugs of abuse.

Analgesics, Opioid↗
PubMed2001

Impact of signal-to-noise on functional MRI of the human amygdala.

The impact of signal-to-noise (SNR) on fMRI of the amygdala was investigated during a picture encoding task. The SNR value required to observe reliable activation was determined by computer simulations. Blood oxygen level-dependent (BOLD) sensitivity maps were generated to indicate brain regions with sufficient SNR to test the statistical hypotheses. The results showed that the medial aspect of the amygdala had insufficient SNR to detect a 1% peak BOLD signal change for a t-test comparison in a majority of subjects. None of these subjects showed activation in regions with unacceptable SNR values, indicating a low false positive rate. Furthermore, hemispheric asymmetries in the BOLD sensitivity maps mirrored asymmetries in the activation patterns. Impoverished SNR was also found in the basal forebrain and orbitofrontal cortex. These findings emphasize the importance of considering SNR when interpreting fMRI results in the limbic forebrain.

Adult↗
PubMed2001

Functional MRI of cortical activations induced by transcranial magnetic stimulation (TMS).

The effects of repetitive transcranial magnetic stimulation (rTMS) on human brain activity and associated hemodynamics were investigated by blood-oxygenation-level-dependent (BOLD) MRI using echo-planar imaging at 2.0 T. Apart from bilateral activation of the auditory cortex by the audible rTMS discharges (23 bursts, 1 s duration, 10 Hz, 10-20 s interstimulus intervals), BOLD responses were restricted to cortical representations of actual finger movements performed either voluntarily or evoked by suprathreshold rTMS of the motor cortex. Neither subthreshold rTMS of the motor cortex nor suprathreshold rTMS of the lateral premotor cortex induced a detectable BOLD response. These findings suggest that neuronal depolarization as induced by rTMS modulates the spiking output of a brain area but does not automatically alter cerebral blood flow and oxygenation. The observation of BOLD MRI activations probably reflects the afferent intracortical processing of real movements.

Adult↗
PubMed2001

Functional MRI changes during panic anticipation and imagery exposure.

While undergoing fMRI, six patients with DSM IV diagnosis of panic disorder and six normal controls performed directed imagery of neutral, moderate and high anxiety situations based on an individually determined behavioral hierarchy. Brain activity was compared during high vs neutral anxiety blocks for each group of subjects using SPM99b. Panic patients showed increased activity in inferior frontal cortex, hippocampus and throughout the cingulate both anterior and posterior, extending into the orbitofrontal cortex and encompassing both hemispheres. These areas may constitute the important circuit in the psychopathology of panic disorder. We propose that this pattern of activity may enhance the encoding and retrieval of strong emotional events, facilitating the recapitulation of traumatic experiences and leading to panic disorder in vulnerable individuals.

Adult↗
PubMed2002

Altered functional MRI responses in Huntington's disease.

This study examined the effects of Huntington's disease (HD) on neural activity during performance of the Porteus maze task. fMRI data were acquired from three HD patients and three controls. Reduced fMRI signal was observed in the patients relative to the controls in occipital, parietal and somato-motor cortex and in the caudate, while increased signal was found in HD in the left postcentral and right middle frontal gyri. The altered fMRI responses in HD patients may result from neural, metabolic, neurovascular coupling and/or hemodynamic differences associated with this disorder.

Adult↗
PubMed2004

Comparability of functional MRI response in young and old during inhibition.

When using fMRI to study age-related cognitive changes, it is important to establish the integrity of the hemodynamic response because, potentially, it can be affected by age and disease. However, there have been few attempts to document such integrity and no attempts using higher cognitive rather than perceptual or motor tasks. We used fMRI with 28 healthy young and older adults on an inhibitory control task. Although older and young adults differed in task performance and activation patterns, they had comparable hemodynamic responses. We conclude that activation during cognitive inhibition, which was predominantly increased in elders, was not due to vascular confounds or specific changes in hemodynamic coupling.

Adult↗
PubMed2001

A signal estimation approach to functional MRI.

In the last half decade, fast methods of magnetic resonance imaging have led to the possibility, for the first time, of non-invasive dynamic brain imaging. This has led to an explosion of work in the Neurosciences. From a signal processing viewpoint the problems are those of nonlinear spatio-temporal system identification. In this paper, we develop new methods of identification using novel spatial regularization. We also develop a new model comparison technique and use that to compare our method with existing techniques on some experimental data.

Brain↗