Biomedical subjects

K Ugurbil

Publications and source records attributed to K Ugurbil.

At least 37 records · Page 2Linked to original sources

PubMed2000

A functional magnetic resonance imaging study of the role of left posterior superior temporal gyrus in speech production: implications for the explanation of conduction aphasia.

Conduction aphasia, characterized by good auditory comprehension and fluent but disordered speech production, is classically viewed as a disconnection syndrome. We review recent evidence which suggests that at least one form of conduction aphasia results from damage to cortical fields in the left posterior superior temporal gyrus which participate not only in speech perception, but also in phonemic aspects of speech production. As a test of this hypothesis, we carried out a 4T functional magnetic resonance imaging study in which subjects named visually presented objects sub-vocally. Group-based analyses showed that a majority of participants showed activation in two regions on the dorsal portion of the left posterior superior temporal gyrus.

Adult↗

PubMed2000

Subchronic in vivo effects of a high static magnetic field (9.4 T) in rats.

The potential adverse biologic effects of sub chronic (cumulatively 10 weeks) exposure to a high magnetic field (9.4 T) were evaluated in young adult male and female Fischer rats as well as in their progeny. Biologic end points in adult rats included changes in daily clinical observations; spatial memory tests; weekly heart rates, body weights, food and water consumption, and the feed efficiency ratio; terminal hematologic, blood biochemical and urinary parameter changes; gross pathologic findings; and major organ weights. In offspring, biologic end points included the gestation period, number of live births, number of pups, ratio of male to female pups/litter; postnatal time period of eye opening; birth and weekly body weights, behavioral changes, central nervous system responses, as well as hematologic, blood biochemistry, and urinary parameter changes; and gross pathologic findings. Findings from this study showed that there were no adverse biologic effects in male and female adult rats or their progeny that could be attributed to 10-week exposure to a 9.4-T static magnetic field.

Age Factors↗

PubMed2000

Functional magnetic resonance imaging with intermolecular multiple-quantum coherences.

For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold.

Brain↗

PubMed2000

Motor area activity during mental rotation studied by time-resolved single-trial fMRI.

The functional equivalence of overt movements and dynamic imagery is of fundamental importance in neuroscience. Here, we investigated the participation of the neocortical motor areas in a classic task of dynamic imagery, Shepard and Metzler's mental rotation task, by time-resolved single-trial functional Magnetic Resonance Imaging (fMRI). The subjects performed the mental-rotation task 16 times, each time with different object pairs. Functional images were acquired for each pair separately, and the onset times and widths of the activation peaks in each area of interest were compared to the response times. We found a bilateral involvement of the superior parietal lobule, lateral premotor area, and supplementary motor area in all subjects; we found, furthermore, that those areas likely participate in the very act of mental rotation. We also found an activation in the left primary motor cortex, which seemed to be associated with the right-hand button press at the end of the task period.

Brain↗

PubMed2000

Use of magnetic resonance spectroscopy for in vivo evaluation of high-energy phosphate metabolism in normal and abnormal myocardium.

31P- and 1H-nuclear magnetic resonance spectroscopy (MRS) are powerful tools for studying myocardial energy metabolism. The purpose of this review is to illustrate how these MRS techniques can be used to study complex bioenergetic issues in normal and abnormal in vivo myocardium. The results provide insight into the energetic alterations present in remodeled and hypertrophied myocardium. A detailed understanding of energy metabolism in normal and abnormal myocardium may point the way to improved preventive, diagnostic, and therapeutic modalities for left ventricular dysfunction.

Coronary Circulation↗

PubMed1999

Retinotopic mapping of lateral geniculate nucleus in humans using functional magnetic resonance imaging.

Subcortical nuclei in the thalamus, which play an important role in many functions of the human brain, provide challenging targets for functional mapping with neuroimaging techniques because of their small sizes and deep locations. In this study, we explore the capability of high-resolution functional magnetic resonance imaging at 4 Tesla for mapping the retinotopic organization in the lateral geniculate nucleus (LGN). Our results show that the hemifield visual stimulation only activates LGN in the contralateral hemisphere, and the lower-field and upper-field visual stimulations activate the superior and inferior portion of LGN, respectively. These results reveal a similar retinotopic organization between the human and nonhuman primate LGN and between LGN and the primary visual cortex. We conclude that high-resolution functional magnetic resonance imaging is capable of functional mapping of suborganizations in small nuclei together with cortical activation. This will have an impact for studying the thalamocortical networks in the human brain.

Adult↗

PubMed1999

Myocardial oxygenation during high work states in hearts with postinfarction remodeling.

BACKGROUND: Postinfarction left ventricular remodeling (LVR) is associated with reductions in myocardial high-energy phosphate (HEP) levels, which are more severe in animals that develop overt congestive heart failure (CHF). During high work states, further HEP loss occurs, which suggests demand-induced ischemia. This study tested the hypothesis that inadequate myocyte oxygen availability is the basis for these HEP abnormalities. METHODS AND RESULTS: Myocardial infarction was produced by left circumflex coronary artery ligation in swine. Studies were performed in 20 normal animals, 14 animals with compensated LVR, and 9 animals with CHF. Phosphocreatine (PCr)/ATP was determined with 31P NMR and deoxymyoglobin (Mb-delta) with 1H NMR in myocardium remote from the infarct. Basal PCr/ATP tended to be decreased in postinfarct hearts, and this was significant in animals with CHF. Infusion of dobutamine (20 microg x kg-1 x min-1 IV) caused doubling of the rate-pressure product in both normal and LVR hearts and resulted in comparable significant decreases of PCr/ATP in both groups. This decrease in PCr/ATP was not associated with detectable Mb-delta. In CHF hearts, rate-pressure product increased only 40% in response to dobutamine; this attenuated response also was not associated with detectable Mb-delta. CONCLUSIONS: Thus, the decrease of PCr/ATP during dobutamine infusion is not the result of insufficient myocardial oxygen availability. Furthermore, in CHF hearts, the low basal PCr/ATP and the attenuated response to dobutamine occurred in the absence of myocardial hypoxia, indicating that the HEP and contractile abnormalities were not the result of insufficient oxygen availability.

Adenosine Triphosphate↗

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

Localized in vivo 1H NMR detection of neurotransmitter labeling in rat brain during infusion of [1-13C] D-glucose.

Resolved localized nuclear magnetic resonance (NMR) signals of 1H bound to 13C label in the carbon positions of glutamate C4, C3 and glutamine C4, C3, as well as in aspartate C3, lactate C3, alanine C3, gamma-aminobutyric acid C3, and glucose C1 were simultaneously observed in spectra obtained from rat brain in vivo. Time-resolved label incorporation was measured with a new adiabatic carbon editing and decoupling (ACED) single-voxel stimulated echo acquisition mode (STEAM) sequence. Adiabatic carbon broadband decoupling of 12 kHz bandwidth was achieved in vivo, which decoupled the entire 13C spectrum at 9.4 T. Resonances from N-acetyl-aspartate and creatine were also detected, consistent with natural-abundance 13C levels. These results emphasize the potential of 1H NMR for following complex biochemical pathways in localized areas of resting rat brain as well as during focal activation using infusions of 13C-labeled glucose.

Animals↗

PubMed1999

In vitro and in vivo studies of 1H NMR visibility to detect deoxyhemoglobin and deoxymyoglobin signals in myocardium.

1H nuclear magnetic resonance (NMR) spectroscopy can be used noninvasively to detect the proximal histidyl N delta proton signals of deoxymyoglobin in the myocardium. However, the quantification of deoxymyoglobin is based on the assumption that the deoxymyoglobin signal detected is not contaminated by the deoxyhemoglobin signals contributed from the blood. The purpose of this study was to conduct in vitro and in vivo 1H NMR studies to examine the in vivo NMR visibility of deoxyhemoglobin in the myocardium. The results demonstrate that the NMR visibility of alpha and beta subunits of deoxyhemoglobin is sensitive to the pulse width for spin excitation because of short T2 relaxation times, and they are not NMR visible in the canine myocardium in vivo at 4.7 T when a 0.5-1.0 msec long Gaussian excitation pulse is used. Therefore, the resonance peak detected at approximately 72 ppm (relative to the water resonance) in the ischemic canine myocardium in vivo is dominated by deoxymyoglobin.

Animals↗

PubMed1999

Diffusion-weighted spin-echo fMRI at 9.4 T: microvascular/tissue contribution to BOLD signal changes.

The nature of vascular contribution to blood oxygenation level dependent (BOLD) contrast used in functional MRI (fMRI) is poorly understood. To investigate vascular contributions at an ultrahigh magnetic field of 9.4 T, diffusion-weighted fMRI techniques were used in a rat forepaw stimulation model. Tissue and blood T(2) values were measured to optimize the echo time for fMRI. The T(2) of arterial blood was 40.8 +/- 3.4 msec (mean +/- SD; n = 5), similar to the tissue T(2) of 38.6 +/- 2.1 msec (n = 16). In comparison, the T(2) of venous blood at an oxygenation level of 79.6 +/- 6.1% was 9. 2 +/- 2.3 msec (n = 11). The optimal spin-echo time of 40 msec was confirmed from echo-time dependency fMRI studies. The intravascular contribution was examined using a graded diffusion-weighted spin-echo echo-planar imaging technique with diffusion weighting factor (b) values of up to 1200 sec/mm(2). Relative BOLD signal changes induced by forepaw stimulation showed no dependence on the strength or direction of the diffusion-sensitizing gradients, suggesting that the large vessel contribution to the BOLD signal is negligible at 9.4 T. However, gradient-echo fMRI performed with bipolar diffusion sensitizing gradients, which suppress intravascular components from large vessels, showed higher percent signal changes in the surface of the brain. This effect was attributed to the extravascular contribution from large vessels. These findings demonstrate that caution should be exercised when interpreting that higher percent changes obtained with gradient-echo BOLD fMRI are related to stronger neural activation. Magn Reson Med 42:919-928, 1999.

Animals↗

PubMed1999

Myocardial oxygenation at high workstates in hearts with left ventricular hypertrophy.

BACKGROUND: High cardiac workloads produced by catecholamine infusion result in loss of myocardial phosphocreatine (PCr) and accumulation of inorganic phosphate (Pi) which are more prominent in heart with left ventricular hypertrophy (LVH) than in normal hearts. Since ischemia can cause changes in phosphorylated compounds similar to those during catecholamine stimulation, this study tested the hypothesis that the exaggerated depletion of PCr and accumulation of Pi during high workloads in LVH is the result of impaired myocyte oxygenation. METHODS AND RESULTS: 31P- and 1H-NMR spectroscopy were used to determine myocardial high energy phosphate levels and myoglobin desaturation, respectively, in eight normal dogs and nine dogs with LVH produced by ascending aortic banding. The mean LV weight/body weight ratio was approximately twice normal in the LVH group. Infusion of dobutamine (15 and 30 micrograms/kg/min), and dobutamine + dopamine (each 20 micrograms/kg/min) caused progressive similar increases in the heart rate x systolic LV pressure product to a maximum of 57.4 +/- 3.3 x 10(3) in normal and 63.9 +/- 2.7 x 10(3) in LVH animals, while myocardial oxygen consumption increased from 0.09 +/- 0.01 to 0.24 +/- 0.04 in normals and from 0.10 +/- 0.02 to 0.25 +/- 0.03 ml/min/g in LVH. PCr/ATP ratios during basal conditions were lower in LVH hearts (1.73 +/- 0.10, 1.61 +/- 0.09 and 1.51 +/- 0.09 in subepicardium, midwall and subendocardium, respectively) as compared with normals (2.24 +/- 0.09, 2.01 +/- 0.08 and 1.89 +/- 0.07; each p < 0.01 normal vs. LVH). Catecholamine infusions caused dose-related decreases in PCr/ATP and appearance of Pi which was more marked in LVH than in normal hearts. 1H-NMR spectroscopy did not detect deoxymyoglobin in either normal or LVH hearts even during the highest workloads. In contrast, occlusion of the anterior descending coronary artery resulted in a large deoxymyoglobin signal. CONCLUSIONS: Increases of cardiac work produced by catecholamine stimulation resulted in greater decreases of PCr and greater increases of Pi in hypertrophied than in normal hearts. These abnormalities were not the result of inadequate intracellular oxygen availability and consequently cannot be ascribed to demand ischemia.

Adenosine Triphosphate↗

PubMed1999

Noninvasive measurements of [1-(13)C]glycogen concentrations and metabolism in rat brain in vivo.

Using a specific 13C NMR localization method, 13C label incorporation into the glycogen C1 resonance was measured while infusing [1-(13)C]glucose in intact rats. The maximal concentration of [1-(13)C]glycogen was 5.1 +/- 0.6 micromol g(-1) (mean +/- SE, n = 8). During the first 60 min of acute hyperglycemia, the rate of 13C label incorporation (synthase flux) was 2.3 +/- 0.7 micromol g(-1) h(-1) (mean +/- SE, n = 9 rats), which was higher (p < 0.01) than the rate of 0.49 +/- 0.14 micromol g(-1) h(-1) measured > or = 2 h later. To assess whether the incorporation of 13C label was due to turnover or net synthesis, the infusion was continued in seven rats with unlabeled glucose. The rate of 13C label decline (phosphorylase flux) was lower (0.33 +/- 0.10 micromol g(-1) h(-1)) than the initial rate of label incorporation (p < 0.01) and appeared to be independent of the duration of the preceding infusion of [1-(13)C]glucose (p > 0.05 for correlation). The results implied that net glycogen synthesis of approximately 3 micromol g(-1) had occurred, similar to previous reports. When infusing unlabeled glucose before [1-(13)C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 +/- 0.08 micromol g(-1) h(-1). The results suggest that steady-state glycogen turnover rates during hyperglycemia are approximately 1% of glucose consumption.

Animals↗

PubMed1999

High spatial resolution functional magnetic resonance imaging at very-high-magnetic field.

Although neuroimaging methods have been used successfully to map large-scale neurocognitive networks distributed across the human cortex, functional mapping and differentiation of localized brain organization within a small structure has been limited by inadequate sensitivity for high spatial resolution imaging. Functional magnetic resonance imaging (fMRI) technique based on blood oxygenation level-dependent (BOLD) contrast has become one of the most useful neuroimaging techniques. It has been used extensively to study human brain function from sensory perception to cognitive performance. However, the majority of these studies used a relatively low spatial resolution (typically with a voxel size of 3.1 x 3.1 x 5.0 mm3), which is incapable of mapping on the millimeter and submillimeter spatial scale. In this article, we review the technical aspects of the high-resolution fMRI technique and the sensitivity and spatial specificity of BOLD-based fMRI. We demonstrate applications of high-resolution fMRI in studying the human visual pathway from the lateral geniculate nucleus in the thalamus to the ocular dominance columns in the primary visual cortex. Most results were obtained at very-high-magnetic fields (3.0 and 4.0 Tesla). They reveal that high-resolution fMRI at very-high-magnetic field is promising for functional mapping of brain organization from large cortical networks, small nuclei, and even to cellular layer structures.

Brain↗

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↗

PubMed1998

Human primary visual cortex and lateral geniculate nucleus activation during visual imagery.

The functional magnetic resonance (fMRI) technique can be robustly used to map functional activation of the visual pathway including the primary visual cortex (V1), the lateral geniculate nucleus (LGN), and other nuclei of humans during visual perception stimulation. One of the major controversies in visual neuroscience is whether lower-order visual areas involve the visual imagery process. This issue was examined using fMRI at high magnetic field. It was demonstrated for the first time that the LGN was activated during visual imagery process in the human brain together with V1 and other activation. There was a tight coupling of the activation between V1 and the LGN during visual imagery.

Adult↗

PubMed1998

Imaging of H217O distribution in the brain of a live rat by using proton-detected 17O MRI.

Imaging of H217O has a number of important applications. Mapping the distribution of H217O produced by oxidative metabolism of 17O-enriched oxygen gas may lead to a new method of metabolic functional imaging; regional cerebral blood flow also can be measured by measuring the H217O distribution after the injection of 17O-enriched physiological saline solution. Previous studies have proposed a method for indirect detection of 17O. The method is based on the shortening of the proton T2 in H217O solutions, caused by the residual 17O-1H scalar coupling and transferred to the bulk water via fast chemical exchange. It has been shown that the proton T2 of H217O solutions can be restored to that of H216O by irradiating the resonance frequency of the 17O nucleus. The indirect 17O image thus is obtained by taking the difference between two T2-weighted spin-echo images: one acquired after irradiation of the 17O resonance and one acquired without irradiation. It also has been established that, at relatively low concentrations of H217O, the indirect method yields an image that quantitatively reflects the H217O distribution in the sample. The method is referred to as PRIMO (proton imaging of oxygen). In this work, we show in vivo proton images of the H217O distribution in a rat brain after an i.v. injection of H217O-enriched physiological saline solution. Implementing the indirect detection method in an echo-planar imaging sequence enabled obtaining H217O images with good spatial and temporal resolution of few seconds.

Animals↗

PubMed1998

MR imaging contrast enhancement based on intermolecular zero quantum coherences.

A new method for magnetic resonance imaging (MRI) based on the detection of relatively strong signal from intermolecular zero-quantum coherences (iZQCs) is reported. Such a signal would not be observable in the conventional framework of magnetic resonance; it originates in long-range dipolar couplings (10 micrometers to 1 millimeter) that are traditionally ignored. Unlike conventional MRI, where image contrast is based on variations in spin density and relaxation times (often with injected contrast agents), contrast with iZQC images comes from variations in the susceptibility over a distance dictated by gradient strength. Phantom and in vivo (rat brain) data confirm that iZQC images give contrast enhancement. This contrast might be useful in the detection of small tumors, in that susceptibility correlates with oxygen concentration and in functional MRI.

Animals↗