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Seong-Gi Kim

Publications and source records attributed to Seong-Gi Kim.

30 records · Page 2Linked to original sources
PubMed2003

Spin-echo fMRI in humans using high spatial resolutions and high magnetic fields.

The Hahn spin-echo (HSE)-based BOLD effect at high magnetic fields is expected to provide functional images that originate exclusively from the microvasculature. The blood contribution that dominates HSE BOLD contrast at low magnetic fields (e.g., 1.5 T), and degrades specificity, is highly attenuated at high fields because the apparent T(2) of venous blood in an HSE experiment decreases quadratically with increasing magnetic field. In contrast, the HSE BOLD contrast is believed to arise from the microvasculature and increase supralinearly with the magnetic field strength. In this work we report the results of detailed and quantitative evaluations of HSE BOLD signal changes for functional imaging in the human visual cortex at 4 and 7 T. This study used high spatial resolution, afforded by the increased signal-to-noise ratio (SNR) of higher field strengths and surface coils, to avoid partial volume effects (PVEs), and demonstrated increased contrast-to-noise ratio (CNR) and spatial specificity at the higher field strengths. The HSE BOLD signal changes induced by visual stimulation were predominantly linearly dependent on the echo time (TE). They increased in magnitude almost quadratically in going from 4 to 7 T when the blood contribution was suppressed using Stejskal-Tanner gradients that suppress signals from the blood due to its inhomogeneous flow and higher diffusion constant relative to tissue. The HSE signal changes at 7 T were modeled accurately using a vascular volume of 1.5%, in agreement with the capillary volume of gray matter. Furthermore, high-resolution acquisitions indicate that CNR increased with voxel sizes < 1 mm(3) due to diminishing white matter or cerebrospinal fluid-space vs. gray matter PVEs. It was concluded that the high-field HSE functional MRI (fMRI) signals originated largely from the capillaries, and that the magnitude of the signal changes associated with brain function reached sufficiently high levels at 7 T to make it a useful approach for mapping on the millimeter to submillimeter spatial scale.

Adult↗
PubMed2003

Microvascular BOLD contribution at 4 and 7 T in the human brain: gradient-echo and spin-echo fMRI with suppression of blood effects.

The BOLD signal consists of an intravascular (IV) and an extravascular (EV) component from both small and large vessels. Their relative contributions are dependent on field strength, imaging technique, and echo time. The IV and EV contributions were investigated in the human visual cortex at 4 and 7 T using spin-echo and gradient-echo BOLD fMRI with and without suppression of blood effects. Spin-echo acquisition suppresses EV BOLD from large veins and reflects predominantly blood T(2) changes and EV BOLD signal from small blood vessels. At a short echo time (32 ms), diffusion gradient-based suppression of blood signals resulted in a 75% and 20% decrease in spin-echo BOLD changes at 4 T and 7 T, respectively. However, at echo times (55-65 ms) approximating tissue T(2) typically used for optimal BOLD contrast, these gradients had much smaller effects at both fields, consistent with the decreasing blood T(2) with increasing field strength. Gradient-echo BOLD percent changes, with relatively long echo times at both fields, were virtually unaffected by gradients that attenuated the blood contribution because the EV BOLD surrounding both large and small vessels dominated. These results suggest that spin-echo BOLD fMRI at 4 and 7 T, with TE approximating tissue T(2), significantly reduces nonspecific mapping signals from large vessels and significantly accentuates microvasculature contributions.

Brain↗
PubMed2003

High-resolution functional magnetic resonance imaging of the animal brain.

To fully understand brain function, one must look beyond the level of a single neuron. By elucidating the spatial properties of the columnar and laminar functional architectures, information regarding the neural processing in the brain can be gained. To map these fine functional structures noninvasively and repeatedly, functional magnetic resonance imaging (fMRI) can be employed. In this article the basic principles of fMRI are introduced, including specific hardware requirements and the equipment necessary for animal magnetic resonance research. Since fMRI measures a change in secondary hemodynamic responses induced by neural activity, it is critical to understand the principles and potential pitfalls of fMRI techniques. Thus, the underlying physics of conventional blood oxygenation, cerebral blood flow, and cerebral blood volume-based fMRI techniques are extensively discussed. Tissue-specific signal change is close to the site of neural activity, while signals from large vessels can be distant from the actual active site. Thus, methods to minimize large vessel contributions and to maximize tissue signals are described. The fundamental limitation of fMRI spatial resolution is the intrinsic hemodynamic response. Based on our high-resolution fMRI studies, the hemodynamic response is regulated at submillimeter functional domains and thus spatial resolution can be achieved to an order of 100 microm. Since hemodynamic responses are sluggish, it is difficult to obtain very high temporal resolution. By using an approach with multiple experiments with different stimulus conditions, temporal resolution can be improved on the order of 100 ms. With current fMRI technologies, submillimeter columnar- and laminar-specific specific functional images can be obtained from animal brains.

Animals↗
PubMed2003

The size of corpus callosum correlates with functional activation of medial motor cortical areas in bimanual and unimanual movements.

Effects of the size of corpus callosum measured from in vivo magnetic resonance imaging (MRI) recordings on cortical activations evaluated using functional MRI (fMRI) were analyzed during motor tasks. Twelve right-handed men performed unilateral finger movements and bilateral movements either with or without a temporal delay between left and right fingers. The size of the rostral part of corpus callosum and the anterior and posterior callosal truncus explained 11.9 and 15.2% of activation in the mesial frontal cortex in unimanual left and right finger movements, respectively. In bimanual simultaneous movements, 34.2% of the activated voxels in the mesial frontal cortex were related to the size of corpus callosum. In bimanual movements in which left finger movement preceded the onset of the right finger movement, the callosal size accounted for 88.7% of activation in the mesial frontal cortex. In contrast, when the right finger movement preceded the left, callosal size accounted for only 31.3% of the mesial frontal cortex activation. The correlations between callosal parameters and activation over the lateral cortex were sparse and occurred only in bimanual movements. The results suggest that corpus callosum modulates the activity of the supplementary motor and cingulate cortical areas depending on temporal complexity of bimanual movements.

Adult↗
PubMed2003

Cerebellar activation during copying geometrical shapes.

We studied functional MRI activation in the cerebellum during copying 9 geometrical shapes (equilateral triangle, isosceles triangle, square, diamond, vertical trapezoid, pentagon, hexagon, circle, and vertical lemniscate). Twenty subjects were imaged during 3 consecutive 45-s periods (rest, visual presentation, and copying). First, there was a positive relation between cerebellar activation and the peak speed of individual movements. This effect was strongest in the lateral and posterior ipsilateral cerebellum but it was also present in the paramedian zones of both cerebellar hemispheres and in the vermis. A finer grain analysis of the relations between the time course of the blood oxygenation level-dependent activation and movement parameters revealed a significant relation to hand position and speed but not to acceleration. Second, there was a significant relation between the intensity of voxel activation during visual presentation and the speed of the upcoming movement. The spatial distribution of these voxels was very similar to that of the voxels activated during copying, indicating that the cerebellum might be involved in motor rehearsal, in addition to its role during movement execution. Finally, a factor analysis of the intensity of activated voxels in the ipsilateral cerebellum during copying (adjusted for the speed effect) extracted 3 shape factors. Factor 1 reflected "roundness," factor 2 "upward pointing," and factor 3 "pointing (up or down) and elongation." These results link cerebellar activation to more global, spatial aspects of copying.

Adult↗
PubMed2002

Comparison of diffusion-weighted high-resolution CBF and spin-echo BOLD fMRI at 9.4 T.

The quantification of blood oxygenation-level dependent (BOLD) functional MRI (fMRI) signals is closely related to cerebral blood flow (CBF) change; therefore, understanding the exact relationship between BOLD and CBF changes on a pixel-by-pixel basis is fundamental. In this study, quantitative CBF changes induced by neural activity were used to quantify BOLD signal changes during somatosensory stimulation in alpha-chloralose-anesthetized rats. To examine the influence of fast-moving vascular spins in quantifying CBF, bipolar gradients were employed. Our data show no significant difference in relative CBF changes obtained with and without bipolar gradients. To compare BOLD and CBF signal changes induced by neural stimulation, a spin-echo (SE) sequence with long SE time of 40 ms at 9.4 T was used in conjunction with an arterial spin labeling technique. SE BOLD changes were quantitatively correlated to CBF changes on a pixel-by-pixel and animal-by-animal basis. Thus, SE BOLD-based fMRI at high magnetic fields allows a quantitative comparison of functional brain activities across brain regions and subjects.

Animals↗
PubMed2002

High-resolution, spin-echo BOLD, and CBF fMRI at 4 and 7 T.

With growing interest in noninvasive mapping of columnar organization and other small functional structures in the brain, achieving high spatial resolution and specificity in fMRI is of critical importance. We implemented a simple method for BOLD and perfusion fMRI with high spatial resolution and specificity. Increased spatial resolution was achieved by selectively exciting a slab of interest along the phase-encoding direction for EPI, resulting in a reduced FOV and number of phase-encoding steps. Improved spatial specificity was achieved by using SE EPI acquisition at high fields, where it is predominantly sensitive to signal changes in the microvasculature. Robust SE BOLD and perfusion fMRI were obtained with a nominal in-plane resolution up to 0.5 x 0.5 mm(2) at 7 and 4 Tesla, and were highly reproducible under repeated measures. This methodology enables high-resolution and high-specificity studies of functional topography in the millimeter to submillimeter spatial scales of the human brain.

Adult↗
PubMed2002

Insights into new techniques for high resolution functional MRI.

Non-invasive functional magnetic resonance imaging (fMRI) has opened a unique window into human and animal brain function, with a spatial resolution of a few millimeters and a temporal resolution of a few seconds. To further improve the current technical limitations of fMRI, various post-processing and data acquisition schemes were developed. Improved fMRI methods include variations of a conventional fMRI technique, mapping a single physiological parameter such as cerebral blood flow or cerebral blood volume, and direct mapping of neural activity. Advances in fMRI techniques allow scientists to map submillimeter columnar and laminar functional structures and to detect tens of millisecond neural activity in certain specific tasks.

Animals↗
PubMed2002

Origin of negative blood oxygenation level-dependent fMRI signals.

Functional magnetic resonance imaging (fMRI) techniques are based on the assumption that changes in spike activity are accompanied by modulation in the blood oxygenation level-dependent (BOLD) signal. In addition to conventional increases in BOLD signals, sustained negative BOLD signal changes are occasionally observed and are thought to reflect a decrease in neural activity. In this study, the source of the negative BOLD signal was investigated using T2*-weighted BOLD and cerebral blood volume (CBV) techniques in isoflurane-anesthetized cats. A positive BOLD signal change was observed in the primary visual cortex (area 18) during visual stimulation, while a prolonged negative BOLD change was detected in the adjacent suprasylvian gyrus containing higher-order visual areas. However, in both regions neurons are known to increase spike activity during visual stimulation. The positive and negative BOLD amplitudes obtained at six spatial-frequency stimuli were highly correlated, and negative BOLD percent changes were approximately one third of the positive changes. Area 18 with positive BOLD signals experienced an increase in CBV, while regions exhibiting the prolonged negative BOLD signal underwent a decrease in CBV. The CBV changes in area 18 were faster than the BOLD signals from the same corresponding region and the CBV changes in the suprasylvian gyrus. The results support the notion that reallocation of cortical blood resources could overcome a local demand for increased cerebral blood flow induced by increased neural activity. The findings of this study imply that caution should be taken when interpreting the negative BOLD signals as a decrease in neuronal activity.

Animals↗
PubMed2002

Effect of basal conditions on the magnitude and dynamics of the blood oxygenation level-dependent fMRI response.

The effect of the basal cerebral blood flow (CBF) on both the magnitude and dynamics of the functional hemodynamic response in humans has not been fully investigated. Thus, the hemodynamic response to visual stimulation was measured using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in human subjects in a 7-T magnetic field under different basal conditions: hypocapnia, normocapnia, and hypercapnia. Hypercapnia was induced by inhalation of a 5% carbon dioxide gas mixture and hypocapnia was produced by hyperventilation. As the fMRI baseline signal increased linearly with expired CO2 from hypocapnic to hypercapnic levels, the magnitude of the BOLD response to visual stimulation decreased linearly. Measures of the dynamics of the visually evoked BOLD response (onset time, full-width-at-half-maximum, and time-to-peak) increased linearly with the basal fMRI signal and the end-tidal CO2 level. The basal CBF level, modulated by the arterial partial pressure of CO2, significantly affects both the magnitude and dynamics of the BOLD response induced by neural activity. These results suggest that caution should be exercised when comparing stimulus-induced fMRI responses under different physiologic or pharmacologic states.

Brain Mapping↗
PubMed2002

Functional magnetic resonance imaging of the retina.

PURPOSE: This study explored the feasibility of mapping the retina's responses to visual stimuli noninvasively, by using functional magnetic resonance imaging (fMRI). METHODS: fMRI was performed on a 9.4-Tesla scanner to map activity-evoked signal changes of the retina-choroid complex associated with visual stimulation in anesthetized cats (n = 6). Three to 12 1-mm slices were acquired in a single shot using inversion-recovery, echo-planar imaging with a nominal in-plane resolution of 468 x 468 microm(2). Visual stimuli were presented to the full visual field and to the upper and lower visual fields. The stimuli were drifting or stationary gratings, which were compared with the dark condition. Activation maps were computed using cross-correlation analysis and overlaid on anatomic images. Multislice activation maps were reconstructed and flattened onto a two-dimensional surface. RESULTS: fMRI activation maps showed robust increased activity in the retina-choroid complex after visual stimulation. The average stimulus-evoked fMRI signal increase associated with drifting-grating stimulus was 1.7% +/- 0.5% (P < 10(-4), n = 6) compared with dark. Multislice functional images of the retina flattened onto a two-dimensional surface showed relatively uniform activation. No statistically significant activation was observed in and around the optic nerve head. Hemifield stimulation studies demonstrated that stimuli presented to the upper half of the visual field activated the lower part of the retina, and stimuli presented to the lower half of the visual field activated the upper part of the retina, as expected. Signal changes evoked by the stationary gratings compared with the dark basal condition were positive but were approximately half that evoked by the drifting gratings (1.0% +/- 0.1% versus 2.1% +/- 0.3%, P < 10(-4)). CONCLUSIONS: To the best of our knowledge, this is the first fMRI study of the retina, demonstrating its feasibility in imaging retinal function dynamically in a noninvasive manner and at relatively high spatial resolution.

Animals↗