Biomedical subjects

J Pekar

Publications and source records attributed to J Pekar.

At least 19 recordsLinked to original sources

PubMed2001

Hierarchical organization of the human auditory cortex revealed by functional magnetic resonance imaging.

The concept of hierarchical processing--that the sensory world is broken down into basic features later integrated into more complex stimulus preferences--originated from investigations of the visual cortex. Recent studies of the auditory cortex in nonhuman primates revealed a comparable architecture, in which core areas, receiving direct input from the thalamus, in turn, provide input to a surrounding belt. Here functional magnetic resonance imaging (fMRI) shows that the human auditory cortex displays a similar hierarchical organization: pure tones (PTs) activate primarily the core, whereas belt areas prefer complex sounds, such as narrow-band noise bursts.

Adult↗

PubMed2001

Inverse T(2) contrast at 1.5 Tesla between gray matter and white matter in the occipital lobe of normal adult human brain.

T(2) of cortical gray matter is generally assumed to be longer than that of white matter. It is shown here that this is not the case in the occipital lobe, but that this effect is often obscured at lower resolution and concealed in standard T(2)-weighted images. Using a high-resolution (1 x 1.3 x 2 mm(3)) segmented EPI Carr-Purcell-Meiboom-Gill sequence, T(2) relaxation times of the brain were measured at 1.5 T for eight healthy adult volunteers. The average T(2) values of cortical gray and white matter were found to be 88 +/- 2 and 84 +/- 3 msec in the frontal lobe, 84 +/- 2 and 83 +/- 3 msec in the parietal lobe, and 79 +/- 1 and 87 +/- 3 msec in the occipital lobe, respectively. This unexpected occipital T(2) contrast between gray and white matter is attributed to regional differences in iron concentration.

Adult↗

PubMed2001

The development of the Ocutech VES-autofocus telescope and a future binocular version.

Conventional optical low-vision devices are hampered by the physical constraints of magnification-shallow depth of field and narrow field of view, and for telescopes, the need to change focus for different distances. These characteristics make low-vision telescopic aids difficult to use, especially at important midrange distances. The Ocutech VES Autofocus bioptic telescope (VES-AF) funded with National Institutes of Health-Small Business Innovation Research and Ontario (Canada) Ministry of Health grants, eliminates the need to manipulate the device to maintain focus. Clinical experience with the VES-AF has shown that autofocus (AF) enhances the acceptance and utilization of telescopic devices. Difficulty ignoring the fellow eye while sighting through the device has been a complaint of users that has undermined device acceptance. Although occlusion is an option to alleviate the diplopia experienced, the development of a binocular version may further enhance the acceptance, adaptation to, and utilization of telescopic devices and may offer wider fields of view as well.

Adult↗

PubMed2001

Sculpting the developing brain.

The developing brain experiences major construction during fetal life and for at least the first decade of childhood. Many more neurons and synoptic connections are produced than are needed for later function, and the mature brain is what remains after these excess building materials are "sculpted" away. This process is thought to be the basis for the developing brain's plasticity, or the capacity to adapt its behavior and circuitry to stimulation from the external environment. Plastic reorganization of the brain is now being studied in children and adults with new noninvasive tools such as functional brain magnetic resonance imaging. This exploratory tool and other new clinical methods demonstrate how the brain's functional "maps" undergo major reorganization in response to early environmental changes. The neurobiology of brain reorganization during development is also being studied with use of new insights into the molecular mechanisms for activity-dependent neuronal plasticity. Clinical disorders such as lead poisoning, metabolic and epileptic encephalopathies, and psychosocial deprivation may arise from disrupted brain plasticity. Several mental retardation syndromes and cognitive disorders recently recognized as being secondary to genetic disruption of intracellular signaling cascades may also disrupt this process. Understanding how the brain's circuitry is sculpted during development provides an important perspective for thinking about neurodevelopmental disorders.

Brain↗

PubMed1996

Perfusion imaging with compensation for asymmetric magnetization transfer effects.

The effects of off-resonance radio-frequency irradiation on the intensity of the MR signal from water protons in the cat brain are asymmetric around the chemical shift of the water signal. This asymmetry, which could arise from a shift in the magnetization transfer spectrum approximately 1.5 ppm upfield from the solvent water signal, must be taken into account to compensate for magnetization transfer effects inherent in arterial spin tagging approaches that use a single radio-frequency coil. Two approaches that either correct for, or circumvent, the apparent upfield shift of the magnetization transfer spectrum are presented, and a perfusion image of the cat brain, using flow-induced adiabatic inversion of arterial water protons, is presented. Other problems in obtaining quantitative cerebral blood flow values using the arterial spin tagging approach are discussed.

Animals↗

PubMed1995

Simultaneous measurement of cerebral oxygen consumption and blood flow using 17O and 19F magnetic resonance imaging.

17O and 19F magnetic resonance (MR) imaging were used to determine simultaneously the concentrations of H2 17O and CHF3 in 0.8-cc voxels in the cat brain during inhalation of a gas mixture containing both 17O2 and CHF3. The arterial time course of CHF3 was determined by "on-line" mass spectrometer detection of expired CHF3, and the arterial time course of H2 17O was determined by 17O MR analysis of arterial samples withdrawn during the inhalation period. The brain data and the arterial data for the two tracers were combined to calculate the cerebral oxygen consumption (CMRO2) and the CBF. The protocol was repeated on seven cats, using pentobarbital anesthesia. The average values of CMRO2 and CBF for a 0.8-cc voxel in the parietal cortex were 1.5 +/- 0.5 mmol kg-1 min-1 and 38 +/- 15 ml 100 g-1 min-1, respectively. In individual animals the average uncertainty in CMRO2 and CBF, calculated from Monte Carlo approaches, was +/- 9%.

Animals↗

PubMed1995

Determination of cerebral oxygen consumption and blood flow by magnetic resonance imaging.

Simultaneous 17O and 19F magnetic resonance imaging were used to determine the concentrations of H2(17)O and CHF3 in 0.8 cc voxels in the cat brain during in- and exhalation of a gas mixture containing both 17O2 and CHF3. The arterial time course of H2(17)O was determined by 17O MR analysis of arterial samples withdrawn during the inhalation period and the arterial time concentration of CHF3. The brain data and the arterial data for the two tracers were used to calculate the cerebral oxygen consumption (CMRO2) and the cerebral blood flow (CBF). The average values of CMRO2 and CBF for a 0.8 cc voxel in the parietal cortex were 1.5 +/- 0.5 mmol/kg/min and 38 +/- 15 ml/100g/min, respectively. 17O/19F MR imaging approach has the potential to image CMRO2 and CBF simultaneously in humans and might become a strong diagnostic tool.

Animals↗

PubMed1994

Water diffusion and acute stroke.

The occlusion of the middle cerebral artery was used as an experimental acute stroke model in 30 cats. The diffusion of water was followed by diffusion-sensitized MRI between 1 and 15 h after induction of stroke. It is demonstrated that images representing the trace of the diffusion tensor provide a much more accurate delineation of affected area than images representing the diffusion in one direction only. The reason is that the strong contrast caused by the anisotropy and orientation of myelin fibers is completely removed in the trace of the diffusion tensor. The trace images show a small contrast between white and gray matter. The diffusion coefficient of white matter is decreased in acute stroke to approximately the same extent as gray matter. It is further shown that the average lifetime of water in extra and intracellular space is shorter than 20 ms both for healthy and ischemic tissue indicating that myelin fibers are permeable to water. The anisotropy contrast did not change before or after induction of stroke, nor after sacrifice. Together, these observations are consistent with the view that the changes in water diffusion during acute stroke are directly related to cytotoxic oedema, i.e., to the change in relative volume of intra- and extracellular spaces. Changes in membrane permeability do not appear to contribute significantly to the changes in diffusion.

Acute Disease↗

PubMed1994

19F magnetic resonance imaging of cerebral blood flow with 0.4-cc resolution.

19F magnetic resonance imaging techniques were used to determine "wash-in" and "wash-out" curves of the inert, diffusible gas CHF3 from 0.4-cc voxels in the cat brain, and mass spectrometer gas detection was used to determine the CHF3 concentration in expired air. These two sets of data were used to calculate cerebral blood flow values in the 0.4-cc voxels, and the blood flow images were registered with high-resolution 1H magnetic resonance images. Data were collected both during the wash-in and wash-out phases of the experiment, but the two sets of data were analyzed separately to obtain independent estimates of the blood flow during the two phases, i.e., Qin and Qout. Repeated determinations of cerebral blood flow images were performed in individual animals, and the entire protocol was repeated on five different animals. The average values of Qin and Qout for a typical 0.4-cc voxel in the parietal cortex were 83 ml 100 g-1 min-1 and 72 ml 100 g-1 min-1, respectively. Monte Carlo calculations utilizing the noise in the 19F NMR signal from this voxel predict an average standard deviation for Qin and Qout of +/- 10%. The average standard deviation for repeated measurements (in the same animal) of Qin and Qout in this voxel was +/- 14%. We conclude that 19F magnetic resonance imaging approaches have the potential to image cerebral blood flow in humans.

Animals↗

PubMed1993

Use of spectacle mounted telescope systems by the visually impaired.

BACKGROUND: Spectacle mounted telescope low vision aids are designed to magnify objects Spectacle telescopes are often rejected by the visually impaired because of their unusual cosmetic appearance which may call attention to their disability. METHODS: Fifty-six subjects were recruited at four independent low vision clinics and divided into two categories: 24 were current wearers of expanded field spectacle mounted telescope systems, and 32 were new wearers. New wearers underwent a randomized cross-over trial, comparing the experimental device (Ocutech VES) to one of two controls (DFV Expanded Field or Walter's Keplarian Close-focus Telescope). RESULTS: Our data show that previous telescope wearers increased their use of telescopes after introduction of the experimental device (Ocutech VES) and there was an equivalent utilization rate for new wearers. CONCLUSIONS: Visual activity patterns are not a sensitive measure of telescope benefit, but frequency of use patterns do change and appear to demonstrate a valid benefit of spectacle mounted telescopes for the visually impaired.

Adolescent↗

PubMed1992

On the precision of diffusion/perfusion imaging by gradient sensitization.

Computer simulation is used to assess the precision and accuracy of diffusion and perfusion parameters derived from a set of gradient-sensitized images. Under ideal experimental conditions, a moderate signal-to-noise level (ca. 40) suffices to estimate diffusion coefficients to within 20% relative precision. However, estimation of a typical cerebral perfusion fraction of 5% to within 20% relative precision requires signal-to-noise levels of ca. 400. Simulations also show that systematic errors in perfusion fraction estimation, as well as underestimation of the uncertainties in perfusion parameters (by chi-squared analysis), will be found at moderate signal-to-noise levels.

Algorithms↗

PubMed1992

In vivo 17O NMR study of rat brain during 17O2 inhalation.

In vivo 17O NMR has been used to monitor the H2(17)O concentration in rat brain during inhalation of 17O2. The results are discussed in terms of oxygen consumption in the brain and recirculation into the brain of H2(17)O produced in other organs.

Animals↗

PubMed1992

Challenges in applying autofocus technology to low vision telescopes.

Autofocus (AF) low vision telescopes offer the potential to increase the acceptance and utilization of such low vision aids (LVA) by the visually impaired. Many patients resist conventional manual focus telescopes for a variety of reasons including appearance, field of view, weight, and utility. The elderly who comprise the significant part of the target population may also resist telescopes due to an avoidance of the technical challenge of its use. Although an AF telescope is technically advanced, it may allow for less manipulation by the wearer and hence enable its more effective application to visual tasks, especially in the near- to mid-range where depths of field narrow and the demands for focusing increase. There are many challenges involved in the application of AF technology to LVA including modification of the focusing range, signal processing for physiologically acceptable performance, and power and weight considerations. A preliminary infrared (IR) AF prototype based upon our recent work with the Ocutech Vision Enhancing System (VES) has been produced. Initial findings are presented which address the requirements of a subsequent version as well as the challenges that will be faced to optimize such a device.

Audiovisual Aids↗

PubMed1991

Complete separation of intracellular and extracellular information in NMR spectra of perfused cells by diffusion-weighted spectroscopy.

A method is outlined that completely separates intracellular and extracellular information in NMR spectra of perfused cells. The technique uses diffusion weighting to exploit differences in motional properties between intra- and extracellular constituents. This allows monitoring of intracellular metabolism, and of transport of small drugs and nutrients through the cell membrane, under controlled physiological conditions. As a first example, proton spectra of drug-resistant MCF-7 human breast cancer cells are studied, and uptake of phenylalanine is monitored.

Cell Membrane Permeability↗

PubMed1991

Double-quantum surface-coil NMR studies of sodium and potassium in the rat brain.

Double-quantum filtered and conventional single-pulse sodium and potassium NMR spectra were obtained from in situ rat brain at 7.0 T. using a surface coil. In contrast to the ca. 14% decrease observed in single-pulse sodium NMR spectra upon death, increases as large as ca. 800% were observed in double-quantum filtered sodium spectra. Conversely, a ca. 26% increase was observed in single-pulse potassium spectra upon death, while double-quantum filtered potassium spectra decreased below the noise level, for the shortest preparation time used. The decay rate of double-quantum sodium coherence in the in situ rat brain after death was dependent upon the double-quantum preparation time: this behavior results from the nonuniformity of the brain, and may be related to physiological compartmentalization. The potential application to sodium NMR imaging of cerebral functionality is briefly discussed.

Animals↗

PubMed1991

Restricted and anisotropic displacement of water in healthy cat brain and in stroke studied by NMR diffusion imaging.

The occlusion of the middle cerebral artery in cat brain was used as an experimental stroke model to investigate the physical basis of the recently reported lowered diffusion constant of water in acute infarcted brain tissue (Moseley et al., Magn. Reson. Med. 14, 330, 1990). The original findings were confirmed in this study of 12 animals investigated with the diffusion-sensitized stimulated echo sequence. The following additional results were obtained: First, the onset of significant lowering of the diffusion constant in the stroke area varied significantly (up to 2.5 h depending on the animal). Second, the affected area is much more clearly outlined in diffusion-weighted images than in T2-weighted images, even in the period between 3 to 12 h following occlusion. Third, for diffusion times between 50 and 2000 ms. the diffusion constant of water is independent of diffusion time in healthy tissue, as well as in the stroke area. Fourth, the diffusion anisotropy is similar in healthy and in stroke area and remains similar regardless of the diffusion time used.

Animals↗

PubMed1991

In vivo measurement of cerebral oxygen consumption and blood flow using 17O magnetic resonance imaging.

We used 17O NMR imaging techniques to measure the H2(17)O concentration in a 0.8-ml voxel in the cat brain following injection of an arterial bolus of enriched H2(17)O and during inhalation of enriched 17O2. We also measured the H2(17)O concentration in arterial blood during 17O2 inhalation. The data from the first measurement were used to calculate the blood flow in the voxel. The data from all three measurements were combined to calculate the oxygen consumption in the voxel. The values of cerebral blood flow and oxygen consumption calculated with 17O NMR techniques agree reasonably well with values calculated for a similar region of the cat brain using autoradiographic techniques.

Administration, Inhalation↗

PubMed1991

The Ocutech Vision Enhancing System (VES): utilization and preference study.

An NIH-sponsored user and prescriber preference trial was conducted comparing a new low vision telescope system, the Ocutech Vision Enhancing System (VES) to two conventional Keplarian spectacle telescope systems in a controlled cross-over clinical study performed at four independent low vision clinics. Fifty-five visually impaired individuals, 18-81 years of age, with both previous and no telescope experience, were followed for 8-16 weeks. The experimental and control devices were compared on a series of subject-selected visual tasks using a standardized clinical protocol. Subjects were trained in the use of the systems with standardized clinical methods. The data shows a statistically significant preference of the new design by both patient and clinician with the overriding factors relating to appearance, weight, adjustability and the fit of the frame.

Adolescent↗