Biomedical subjects

E Yacoub

Publications and source records attributed to E Yacoub.

9 recordsLinked to original sources

PubMed2002

Detunable transverse electromagnetic (TEM) volume coil for high-field NMR.

Most high-field MRI systems do not have the actively detuned body coils that are integral to clinical systems operating at 1.5T and lower field strengths. Therefore, many clinical applications requiring homogeneous volume excitation in combination with local surface coil reception are not easily implemented at high fields. To solve this problem for neuroimaging applications, actively detunable transverse electromagnetic (TEM) head coils were developed to be used with receive-only surface coils for signal-to-noise ratio (SNR) gains and improved spatial coverage from homogeneously excited regions. These SNR and field of view (FOV) gains were achieved by application of a detunable TEM volume coil to human brain imaging at 4T.

Brain↗

PubMed2001

Detection of the early decrease in fMRI signal in the motor area.

The initial decrease in BOLD signal at stimulus onset (i.e., the initial dip) has generated a great deal of interest because of its potential for providing more spatially specific mapping. Despite a number of experimental fMRI studies demonstrating its existence in the visual cortex, the initial dip has not been reported in other areas. The present work examined the initial dip in the motor area. Using a visually-guided finger-tapping paradigm, the dip was detected in both motor and visual areas simultaneously. The dip in the motor area was found to exhibit characteristics similar to those revealed by visual stimulation studies. The motor dip peaked approximately 2 sec after stimulus onset and reached an amplitude that was roughly 0.3 times of the positive amplitude. Furthermore, the dip in the motor area was more localized and less sensitive to large vessels, indicating an improvement in spatial specificity despite the relatively low spatial resolution used in this study. These data indicate that the initial dip is a general phenomenon that can be employed for more spatially specific functional mapping, although its full utility in humans remains to be further demonstrated. Magn Reson Med 45:184-190, 2001.

Humans↗

PubMed2001

Imaging brain function in humans at 7 Tesla.

This article describes experimental studies performed to demonstrate the feasibility of BOLD fMRI using echo-planar imaging (EPI) at 7 T and to characterize the BOLD response in humans at this ultrahigh magnetic field. Visual stimulation studies were performed in normal subjects using high-resolution multishot EPI sequences. Changes in R(*)(2) arising from visual stimulation were experimentally determined using fMRI measurements obtained at multiple echo times. The results obtained at 7 T were compared to those at 4 T. Experimental data indicate that fMRI can be reliably performed at 7 T and that at this field strength both the sensitivity and spatial specificity of the BOLD response are increased. This study suggests that ultrahigh field MR systems are advantageous for functional mapping in humans. Magn Reson Med 45:588-594, 2001.

Brain↗

PubMed2001

New strategy for reconstructing partial-Fourier imaging data in functional MRI.

Most partial Fourier (PF) approaches use a low-resolution phase estimate in the reconstruction to account for non-zero phases of the image. These methods may fail when there are large phase errors, a situation commonly encountered in T(*)(2)-weighted functional MRI (fMRI). To mitigate this problem, a method was developed based on the inversion of a matrix formulated according to a phase map derived from iterative reconstruction. To make this method computationally practical for fMRI, a strategy was introduced such that the matrix inversion is performed only once for each slice in the time series, assuming that the phase map remains constant in the time series. To ensure the temporal phase invariance, physiological noise correction and global phase correction were applied to the data before the reconstruction. This method was demonstrated to be robust and efficient for fMRI. Magn Reson Med 46:1045-1048, 2001.

Algorithms↗

PubMed2001

Nonadditive two-way ANOVA for event-related fMRI data analysis.

A significant recent development in functional magnetic resonance imaging (fMRI) is the introduction of event-related fMRI, also known as time-resolved fMRI. Because the exact shape of the MR response in an event-related fMRI experiment is often not known, traditional methods developed for block design experiments, such as t test and correlation analysis, are not well-suited for extracting activated pixels from the event-related data. In this work, a statistical technique based on nonadditive two-way analysis of variance is developed for use in event-related studies. Theoretical and experimental work were carried out for establishing a statistical threshold to determine pixel activation. Experimental studies were performed to demonstrate the utility of this approach.

Adult↗

PubMed1999

Further evaluation of the initial negative response in functional magnetic resonance imaging.

The initial negative response (i.e., the dip) in the functional MR signal at stimulus onset has aroused much interest. Such a response is consistent with optical imaging data and can be potentially mapped to generate spatially more specific maps. However, there are still controversies regarding the exact origin of the initial response. In particular, experimental reports of its echo-time dependence have been inconsistent. Furthermore, several investigators have suggested the possibility of an apparent dip that may arise artifactually when the interstimulus interval (ISI) is not sufficiently long. The present study investigates the echo-time dependence of the initial response and the effect of the ISI on the initial response. Experimental results obtained at TEs of 21, 30, and 45 msec demonstrate that the initial dip has a TE dependence that is in agreement with a T2* contrast, and thereby consistent with a blood oxygenation level-dependent origin. At an ISI of 90 sec, a statistically significant initial negative response was detected and shown to be indistinguishable from that observed at an ISI of 45 sec, which was used in our previous studies, indicating that the initial negative response observed at 4 T is not a consequence of short ISI.

Echo-Planar Imaging↗

PubMed1999

Detection of the early negative response in fMRI at 1.5 Tesla.

Recent experimental studies have revealed an initial decrease in magnetic resonance (MR) signal that is consistent with optical imaging results. This initial response, thought to arise from a transient increase in deoxyhemoglobin concentration, is probably more localized to the site of neuronal activation. However, with MR imaging, this early response has only been demonstrated at high fields. In this paper, the observation of the initial response at 1.5 T is reported. Compared with results obtained at 4 T, the present study reveals that the initial response grows more rapidly with the field strength than the late positive response, suggesting a more microvascular origin of the initial response.

Adult↗

PubMed1999

Neural correlates of visual form and visual spatial processing.

Cortico-cortical projections for visual processing that originate from the striate cortex are organized into two streams. The dorsal stream projects to the parietal region and the ventral stream to the inferior temporal region. One hypothesis is that the dorsal stream processes visual spatial information, and the ventral stream processes visual object information. Although recognition of human faces or common objects has been shown preferentially to activate the ventral stream, the issue of when such processing starts to engage the ventral or the dorsal stream is not clear. The question explored in this study is whether processing of visual form per se without evoking the brain mechanisms that are associated with recognition of human faces or common objects is sufficient to activate the ventral stream more significantly relative to the condition when only visual spatial processing is involved. Functional magnetic resonance images were acquired while subjects performed a delayed comparison task in which either visual spatial or visual form information was processed. Cortical areas that were preferentially activated in visual spatial or visual form processing showed not only ventral-dorsal segregation, but also hemispheric laterality. The results extended previous findings by showing that preferential activation in the ventral pathway is not contingent upon such powerful stimuli as faces and common objects. Processing of simple visual form information is cause enough for such activation to be observed. A strong left hemisphere dominance in visual form recognition was also revealed. The observed laterality may be a reflection that the left hemisphere is more important in symbolic and/or semantic coding of visual form information.

Adult↗

PubMedDate not supplied

Investigation of the initial dip in fMRI at 7 Tesla.

In agreement with optical imaging studies, previous fMRI studies have reported an initial decrease (i.e. the initial dip) in the BOLD response, which is believed to arise from an increase in oxygen consumption and to be mostly microvascular. To date, experimental studies of the initial dip in humans have been performed at fields up to 4 T, with relatively low spatial resolution. Because the sensitivity to microvascular contribution is increased at high magnetic fields, the present study investigated the initial dip at 7 T. In addition, to reduce the partial volume effect, the study is conducted at a high spatial resolution. The initial dip was detected in all subjects studied and was found to reside mostly in the gray matter. The relative amplitude of the early response was found to be 0.6, higher than that at 4 T (0.3) and 1.5 T (0.11). In addition, based on the assumption that the initial dip is a result of increased oxygen utilization, the fractional change in oxygen utilization was estimated to be 40% of that of the fractional change in cerebral blood flow. These results are in agreement with the notion that the initial dip arises from an increase in oxygen consumption.

Adult↗