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Biomedical subjects

J C Gore

Publications and source records attributed to J C Gore.

At least 199 records · Page 11Linked to original sources
PubMed1977

Echo structure in medical ultrasonic pulse-echo scanning.

The factors contributing to the structure (i.e. amplitude and shape) of echoes from major reflecting boundaries in tissues are reviewed. It is shown that the pulsed nature of a scanner's emitted sound field introduces complex variations in echo shape which are not usually taken into account. Moreover, echo structure is strongly modified by the geometry of the interface and not only by the physical composition of the adjacent materials. These effects are shown to be characteristic of the basic reflection process with pulsed fields. The influence of some receiver transfer properties on the displayed echo is also indicated. It is suggested that the analysis of sound fields scattered from small-scale internal structures (echo ensemble analysis) is a preferred method of tissue characterization.

Ultrasonics↗
PubMedDate not supplied

Time course and mechanism of alterations in proton relaxation during liver regeneration in the rat.

We studied the proton T1 and T2, water and lipid content of regenerating rat liver from 1 to 7 days after 70% hepatectomy. Liver from normal and sham-operated animals and splenic tissue from all animals were studied as controls. In vivo proton spectroscopy and imaging of liver was performed in a separate group of control and posthepatectomy rats. The T2 of regenerating liver, but not of sham or normal control liver, was prolonged. Changes in T1, relative to normal tissue, were found in liver and spleen of both operated groups. Lipid content, assessed both by extraction of tissue samples and by in vivo spectroscopy, was increased in regenerating tissue but not in controls. Water content was similarly increased in regenerating liver tissue. Changes in water and lipid content appeared to contribute to the alterations in proton relaxation which we observed.

Animals↗
PubMedDate not supplied

Metabolic state of the rat liver with ethanol: comparison of in vivo 31phosphorus nuclear magnetic resonance spectroscopy with freeze clamp assessment.

In vivo 31phosphorus nuclear magnetic resonance spectroscopy was used to measure the hepatic metabolic state in various groups of rats given ethanol, a control liquid diet or a solid chow diet. The use of selective presaturation pulses applied to the broad phosphorus resonances of immobile phospholipids permitted reliable determination of ATP/ADP ratios by quantitation of the ATP-beta and ATP-gamma peak areas. ATP/ADP ratios were depressed by both techniques in rats chronically ingesting ethanol compared to pair-fed animals consuming the control liquid diet. These differences were observed regardless of whether ethanol feeding was continued up to the time of investigation or whether it was discontinued for 24 hr prior to study. Acute alcohol administration in chow-fed rats, not previously ingesting ethanol, did not lower hepatic ATP/ADP ratios by either methodology. In all cases, liver ATP/ADP ratios assessed by 31phosphorus nuclear magnetic resonance spectroscopy were higher than those measured by high-performance liquid chromatography. However, parallel decreases in hepatic ATP/ADP ratios were observed with chronic ethanol consumption by both 31phosphorus nuclear magnetic resonance spectroscopy and the biochemical method, confirming the utility of in vivo 31phosphorus nuclear magnetic resonance spectroscopy for assessment of the hepatic bioenergetic status. The difference in absolute ATP/ADP ratios by the two methods may to some degree be explained by binding effects of ADP with proteins or mitochondrial membranes, rendering it partially invisible to nuclear magnetic resonance or alternatively, by breakdown of high energy phosphate bonds with freeze clamp extraction.

Adenosine Diphosphate↗
PubMedDate not supplied

Coupled-spin fast spin-echo MR imaging.

Distinguishing between lipid and water-containing tissues is clinically important. Current techniques rely on the chemical shift difference between fat and water resonances or differences in relaxation times of the tissues, or a combination of both. A method is presented for separating the signals of lipid protons from those of water protons by using fast spin-echo magnetic resonance imaging based on the principle that lipid protons behave differently from water protons in multi-echo sequences. Two images are acquired with different echo train lengths and echo spacing but with identical TEs, and then subtracted to exploit differences in the behavior of lipid and water protons in multi-echo sequences. The method is insensitive to B0 inhomogeneities or susceptibility effects and provides separate lipid and water images with a high signal-to-noise ratio. The advantages of the method are demonstrated with phantom studies and clinical examples.

Female↗
PubMedDate not supplied

Validation of volume flow measurements with cine phase-contrast MR imaging for peripheral arterial waveforms.

A flow phantom was used to study MR volume flow measurements for monophasic and triphasic waveforms over the flow range expected in peripheral arteries at rest and with exercise (2-24 mL/sec, n = 50). The improvement in accuracy with phase-correction image processing to eliminate errors caused by eddy currents was measured. Volume flow estimates with Doppler sonography were also measured. MR volume flow measurements correlated with volume collection with r = .996 and mean error = 4.6%. Phase-correction processing decreased mean error from 12.6% to 4.6% (P < .001, paired t-test). Doppler sonography had a higher mean error of 10.3% (P < .001, unpaired t-test). Cine phase-contrast MR imaging provides accurate estimates of volume blood flow for waveforms and flow ranges expected in peripheral arteries.

Adult↗
PubMedDate not supplied

Diffusion-weighted imaging in tissues: theoretical models.

Typical diffusion measurements use Stejskal-Tanner pulsed gradient spin echo sequences to provide information about the average diffusion and displacement profiles of particles in a sample. To derive structural information, a measured displacement profile has to be related by means of a model to the physical and geometrical properties of the tissue, such as diffusion coefficients and shapes of semi-permeable membranes of compartments in the system. The behavior of the NMR signal and the measured ADC are greatly affected by the cellular architecture of a tissue, mainly because cellular membranes are relatively impermeable to water. For long diffusion times, and small signal attenuations, ADC is relatively insensitive to how it is measured. In general, however, ADC values are not readily interpreted unless the measuring conditions are specified in detail. For given measuring conditions, ADC depends on intra- and extracellular diffusion coefficients, membrane permeabilities, cell sizes and the cellular volume fraction. If intra- and extracellular T2 relaxation rates are different enough, ADC may also depend on the relaxation properties of the system and the echo time. An improved understanding of the precise influence of these factors has been obtained by detailed consideration of theoretical and computer models that can be related to experimental data in simple systems. Further refinements of such models should advance our understanding of water diffusion in tissues.

Body Water↗
PubMedDate not supplied

Diffusion-weighted NMR imaging changes caused by electrical activation of the brain.

The apparent diffusion coefficient of brain water was decreased by frontal cortical electroshock, usually but not always associated with brief epileptic afterdischarge detectable at the parietal cortex. Previous studies have shown that status epilepticus causes similar larger decreases, which are largely reversible by the termination of seizure discharge with pentobarbital. Cerebral blood flow is elevated in these conditions, and biochemical energy failure does not occur. The brain water diffusion coefficient also decreases in spreading depression, without depletion of energy stores. All of these findings may be due in part to the reduction of brain extracellular space caused by cell swelling, which occurs to some degree in all three conditions. However, major biological differences between brain activation and brain ischemia and new evidence for increased cytosolic viscosity in the latter both suggest that other mechanisms deserve further investigation. Use-dependent motility of dendritic spines and other phenomena that may allow direct detection of neural activity by diffusion-weighted NMR imaging are of special interest.

Animals↗
PubMedDate not supplied

Regional differences in rat brain displayed by fast MRI with superparamagnetic contrast agents.

Gradient echo images of coronal sections through the brains of normal rats have been recorded at 2.0 T both before and at various times after intravenous bolus administrations of a superparamagnetic iron oxide suspension. The decrease in NMR signal due to the transit of the contrast agent has been quantified as a function of time. The image immediately following the injection can be subtracted from that pre-contrast to reveal regional differences related to blood flow.

Animals↗
PubMedDate not supplied

Beagle puppy model of perinatal asphyxia: alterations in cerebral blood flow and metabolism.

Perinatal asphyxia remains a major cause of neurodevelopmental handicap. The neuropathologic and clinical sequelae of perinatal asphyxia are likely attributable to alterations in cerebral blood flow (CBF) to the developing brain with uncoupling of CBF and metabolism. The newborn beagle puppy model was used to study the control of CBF in physiologic and pathologic conditions in the developing brain. Pups, 2-10 days of age, were randomized to asphyxial insult (i.e., ventilator lines clamped for 5 min) or no insult. In the first series, pups underwent radioactive microsphere determinations of CBF immediately prior to insult (t = 0), at the end of insult (t = 5), and after 60 min of observation (t = 65). In the second series, utilizing a 2.0 tesla superconducting magnet, pups underwent continuous serial in vivo 31P nuclear magnetic resonance spectral analysis of cerebral hemispheric metabolic state. Animals exposed to insult had significant alterations in PO2, PCO2, pH, and mean arterial pressure at the end of insult compared to control pups (all pups: p less than 0.001). Serial CBF data demonstrated decreases in flow in cortical gray and white matter regions during insult and return to baseline 1 hour later; brainstem structures were hyperemic during insult. Analysis of phosphorylated metabolites inorganic phosphorus (Pi), phosphocreatine (PCr), and ATP demonstrated that during insult both the PCr/Pi and ATP/Pi ratios were depressed but by 20-30 min following insult, these ratios of cerebral phosphorylated metabolites had almost returned to baseline values. These data demonstrate that even after a severe asphyxial episode which results in electrocerebral silence, CBF and metabolism may return to normal.

Acid-Base Equilibrium↗
PubMedDate not supplied

The relationship of problems in biomedical MRI to the study of porous media.

The NMR methods that are used to characterize inanimate porous media measure relaxation times and related phenomena and material transport, fluid displacement and flow. Biological tissues are comprised of multiple small, fluid-filled compartments, such as cells, that restrict the movement of the bulk solvent water and whose constituents influence water proton relaxation times via numerous interactions with macromolecular surfaces. Several of the methods and concepts that have been developed in one field of application are also of great value in the other, and it may be expected that technical developments that have been spurred by biomedical applications of MR imaging will be used in the continuing study of porous media. Some recent specific studies from our laboratory include the development of multiple quantum coherence methods for studies of ordered water in anisotropic macromolecular assemblies, studies of the degree of restriction of water diffusion in cellular systems, multiple selective inversion imaging to depict the ratios of proton pool sizes and rates of magnetization transfer between proton populations, and diffusion tensor imaging to depict tissue anisotropies. These illustrate how approaches to obtain structural information from biological media are also relevant to porous media. For example, the recent development of oscillating gradient spin echo techniques (OGSE), an approach that extends our ability to resolve apparent diffusion changes over different time scales in tissues, has also been used to compute surface to volume measurements in assemblies of pores. Each of the new methods can be adapted to provide spatially resolved quantitative measurements of properties of interest, and these can be efficiently acquired with good accuracy using fast imaging methods such as echo planar imaging. The community of NMR scientists focused on applications to porous media should remain in close communication with those who use MRI to study problems in biomedicine, to their mutual benefits.

Animals↗
PubMedDate not supplied

Activation of human prefrontal cortex during spatial and nonspatial working memory tasks measured by functional MRI.

Separate working memory domains for spatial location, and for objects, faces, and patterns, have been identified in the prefrontal cortex (PFC) of nonhuman primates. We have used functional magnetic resonance imaging to examine whether spatial and nonspatial visual working memory processes are similarly dissociable in human PFC. Subjects performed tasks which required them to remember either the location or shape of successive visual stimuli. We found that the mnemonic component of the working memory tasks affected the hemispheric pattern of PFC activation. The spatial (LOCATION) working memory task preferentially activated the middle frontal gyrus (MFG) in the right hemisphere, while the nonspatial (SHAPE) working memory task activated the MFG in both hemispheres. Furthermore, the area of activation in the left hemisphere extended into the inferior frontal gyrus for nonspatial SHAPE task. A perceptual target (DOT) detection task also activated the MFG bilaterally, but at a level approximately half that of the working memory tasks. The activation in the MFG occurred within 3-6 s of task onset and declined following task offset. Time-course analysis revealed a different pattern for cingulate gyrus, in which activation occurred upon task completion. Cingulate gyrus activation was greatest following the SHAPE task and was greater in the left hemisphere. The present results support the prominent role of the PFC and, specifically, the MFG in working memory, and indicate that the mnemonic content of the task affects the relative weighting of hemispheric activation.

Adult↗
PubMedDate not supplied

Medical nuclear magnetic resonance imaging: I. Physical principles.

For over 30 years, nuclear magnetic resonance (NMR) spectroscopy has been used for chemical analysis and for studying molecular behavior, but NMR imaging is a recent addition to the methods available to radiologists for investigating the interior of the body. It uses radiofrequency radiation in the presence of a magnetic field to produce anatomical cross sections. The images may be simple maps of the concentration of the hydrogen nucleus, or they may depend on tissue relaxation times, which describe how rapidly hydrogen nuclei exchange energy with their surroundings. In this article, the basic concepts and physical principles of conventional NMR spectroscopy are introduced. In subsequent articles, the various approaches to producing NMR images will be outlined, and the types of information obtainable from NMR scanners will be discussed.

Magnetic Resonance Spectroscopy↗
PubMedDate not supplied

Studies of tissue NMR relaxation enhancement by manganese. Dose and time dependences.

Manganese is a powerful paramagnetic material and potential NMR contrast agent. It drastically affects the NMR properties of solutions and tissues and is less toxic than most other transition elements. It also possesses some unusual and advantageous features; it alters T1 and T2 to different degrees, and it can bind to macromolecules to become even more effective at reducing proton relaxation times. The dose dependence of tissue relaxation rate increases has been measured in mice, and proton relaxation enhancement ratios that describe binding effects have been evaluated. These ratios imply that a tenfold reduction in manganese dose is achievable when the ion binds to intracellular components, and it is demonstrated that such binding effects can be a major factor in the efficacy of contrast enhancement. The effect of manganese on the ratio T1/T2 is dose dependent so that lower doses may be more useful for some imaging techniques. The postmortem time course of relaxation times in organs containing manganese varies between organs and with manganese content, and demonstrates that the relationship between tissue relaxation enhancement and metal content is not a simple correlation with concentration since large variations in T1 and T2 can occur even when metal and water content are fixed.

Animals↗
PubMedDate not supplied

Signal-to-noise and contrast in fast spin echo (FSE) and inversion recovery FSE imaging.

Fast spin echo (FSE) imaging has recently experienced a renewed enthusiasm in the clinical setting for its ability to provide high contrast T2-weighted images in short imaging times. This article evaluates the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) properties of the FSE sequence, inversion recovery (IR) FSE sequence, and conventional SE imaging. The results indicate that FSE imaging displays similar contrast properties to SE imaging, but that the SNR and CNR are improved secondary to the longer TRs and longer effective TEs that may be used. The SNR per unit time of the FSE sequence, and hence its efficiency, is at least a factor of 8 better than the SE sequence when 16 echoes are acquired for each excitation. The addition of a slice selective inversion pulse in IR-FSE allows rapid generation of IR images with image contrast similar to that of conventional IR sequences. When used with a multicoil array for abdominal, pelvic, and spine imaging, the IR-FSE sequence produces images that are virtually free of motion artifact from the subcutaneous fat immediately adjacent to the coils. Both FSE and IR-FSE, when compared with SE imaging, provide superior image contrast and SNR in reduced imaging time.

Brain↗
PubMedDate not supplied

Improvements in selective nuclear magnetic resonance excitation using nonlinear gradients.

The potential role of nonlinearities in the magnetic field gradient in magnetic resonance imaging for producing sharper boundaries for the excited spin slice region is investigated. A theoretical analysis based on solving the Bloch equations numerically is presented. Simple criteria for measuring the performance of selective excitation are defined and then used to evaluate the results for selective 90 degrees and 180 degrees pulses.

Humans↗
PubMedDate not supplied

Toward an automated analysis system for nuclear magnetic resonance imaging. II. Initial segmentation algorithm.

Image segmentation algorithms based on hierarchical clustering have been developed for analysis of T1 and T2 nuclear magnetic resonance images. Application of these algorithms to simultaneous T1-T2 images of healthy volunteers extracted fundamental tissue types in the brain. These algorithms also were used both to identify the extent of the region of involvement of a subject with a history of a grade 3 astrocytoma of the right frontal lobe of the brain, and to characterize the tissue within the region of involvement. These results suggest that a simple segmentation algorithm can produce reasonable clustering of tissue types within the brain.

Biometry↗
PubMedDate not supplied

Toward an automated analysis system for nuclear magnetic resonance imaging. I. Efficient pulse sequences for simultaneous T1-T2 imaging.

Simultaneous T1 and T2 images have been obtained using multiple-echo self-normalizing sequences. Differential T1 and T2 discrimination for two of these sequences, double saturation recovery and inversion-saturation recovery, have been explored in detail. The inversion-saturation recovery sequence exhibits T1 discrimination comparable to, and in many cases better than, that of optimal T1 weighted conventional pulse sequences, but is poor for T2 imaging. The double saturation recovery sequence yields comparable tissue discrimination for T1 and T2 imaging, but is poor compared to optimal T1 weighted and T2 weighted conventional pulse sequences. Simultaneous T1 and T2 images represent quantitative maps of intrinsic tissue properties, and hence form a natural basis for an automated image analysis and tissue classification system.

Brain↗