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Biomedical subjects

J C Gore

Publications and source records attributed to J C Gore.

At least 109 records · Page 6Linked to original sources
PubMed1996

Radiation therapy dosimetry using magnetic resonance imaging of polymer gels.

Further progress in the development of polymer gel dosimetry using MRI is reported, together with examples of its application to verify treatment plans for stereotactic radiosurgery and high dose rate brachytherapy. The dose distribution image produced in the tissue-equivalent gel by radiation-induced polymerization, and encoded in the spatial distribution of the NMR transverse relaxation rates (R2) of the water protons in the gel, is permanent. Maps of R2 are constructed from magnetic resonance imaging data and serve as a template for dose maps, which can be used to verify complex dose distributions from external sources or brachytherapy applicators. The integrating, three-dimensional, tissue-equivalent characteristics of polymer gels make it possible to obtain dose distributions not readily measured by conventional methods. An improved gel formulation (BANG-2) has a linear dose response that is independent of energy and dose rate for the situations studied to date. There is excellent agreement between the dose distributions predicted using treatment planning calculations and those measured using the gel method, and the clinical practical utility of MRI-based polymer gel dosimetry is thereby demonstrated.

Acrylamide↗
PubMed1996

Medical physics.

Explore the source record for details and available documents.

Biophysics↗
PubMed1996

Quantitative blood flow measurements with cine phase-contrast MR imaging of subjects at rest and after exercise to assess peripheral vascular disease.

OBJECTIVE: The purpose of this study was to determine whether cine phase-contrast MR volume flow measurements can identify patients with peripheral vascular disease. SUBJECTS AND METHODS: We performed MR measurements of volume blood flow in the popliteal artery of subjects at rest and after 5 min of plantar flexion exercise in 10 volunteers (mean age, 28 years old), in five patients suspected of having peripheral vascular disease (mean age, 58 years old), and in five other volunteers of a similar age (mean age, 57 years old). RESULTS: Volume blood flow at rest was similar in volunteers and in patients. Four patients who had abnormal ankle-brachial indexes had lower flow increases after exercise (2.6-fold) compared with the five older normal volunteers (4.8-fold; p < .03, t test). These flow increases correlated well with ankle-brachial indexes: r = .97. The four patients with abnormal ankle-brachial indexes had monophasic resting waveforms, whereas all other subjects had triphasic waveforms. CONCLUSION: MR volume blood flow measurement may aid in evaluating peripheral vascular disease. Studies of larger patient groups will be necessary.

Adult↗
PubMed1995

Barbiturate-reversible reduction of water diffusion coefficient in flurothyl-induced status epilepticus in rats.

Rat brains (n = 17) with flurothyl-induced status epilepticus (SE) have been imaged with a gradient-echo diffusion-weighted imaging sequence at 2.0 T. The apparent water diffusion coefficient (ADC) decreased during seizure discharges. The magnitude of the ADC reduction correlated well with the duration of flurothyl exposure. A 17% reduction in the water ADC compared with preseizure condition was observed in rats with the longest flurothyl exposure time. In 13 rats, pentobarbital was used to arrest the electrographic seizure activity. ADC values began to return to normal a few minutes after the injection. In four rats with no pentobarbital administration, ADC values remained depressed up to 1 h after seizure onset. The results suggest that diffusion-weighted MR imaging may be useful for mapping recent intense seizure activity in human patients with medically intractable epilepsy.

Animals↗
PubMed1995

Theoretical model for water diffusion in tissues.

Water diffusion in a tissue model is studied both analytically and numerically. Tissue is regarded as a periodic array of boxes surrounded by partially permeable membranes (cells), embedded in an extracellular medium. intracellular and extracellular diffusion coefficients may differ. Expressions for the apparent diffusion coefficients (ADC) in isotropic and nonisotropic tissues are derived and compared with Monte Carlo simulations. Calculated ADCs disagree with values obtained from the widely used "fast exchange" formula. Effects of differences between intracellular and extracellular T2 relaxation times on measured values of ADC and T2 are discussed. The general analysis is specifically applied to the changes occurring in ADC following ischemic insults to brain tissue. It is found that although membranes affect ADC significantly, the observed changes in diffusion cannot be due to reduced membrane permeabilities. They may result from the combined effect of changes in cellular volume fraction, extracellular and intracellular diffusion.

Acute Disease↗
PubMed1995

An ROC approach for evaluating functional brain MR imaging and postprocessing protocols.

A method that can be used to evaluate the performance of MRI methods for detecting discrete regional activations using functional MRI is presented. Computer derived receiver-operator-characteristic (ROC) curves have been used to evaluate quantitatively a range of conditions encountered in functional MRI studies. ROC analysis allows multiple acquisition strategies and multiple postprocessing strategies to be quantitatively and objectively compared. The authors first present this analysis technique and then illustrate its use for assessing the relative performances of different functional MRI data acquisition strategies using different gradient echo, echoplanar imaging protocols. In addition, the authors have used the ROC analysis to evaluate and compare several methods for analyzing functional MRI data to extract regions of activation. This approach to assessing the performance of different methods is of general use and can be applied to evaluate other data acquisition protocols and postprocessing methods.

Algorithms↗
PubMed1995

Dynamic echo planar imaging of exercised muscle.

Echo planar imaging was used to record dynamic changes in tissue transverse relaxation rates (delta R2) in the anterior tibialis muscle during dorsi-flexion exercise and recovery. Using a single spin-echo technique to calculate the change in relaxation rate produced by the exercise a time resolution of 4 s was achieved for each measurement of delta R2. For a fixed workload of 70% of maximum voluntary contraction (MVC), the duration of dorsi-flexion exercise was varied and measurements of delta R2 were obtained throughout exercise and for at lease 5 min of recovery. Comparisons were made between the single echo results and those obtained using multiple echo measurements of T2 with much lower time resolution, to verify that the two techniques gave the same results. We found on average that delta R2 decreased by an average of 8.7 s-1 within the tibialis with an average rate of decrease during exercise of delta R2/ delta t(ex) = -0.061 s-2. For the high time resolution studies we consistently observed that there was a continued decrease in the measured value of delta R2 after the exercise, reaching a minimum value about a minute after the exercise ceased. This average rate of undershoot during the postexercise period was given by delta R2/ delta t(us) = -0.035 s-2. This effect has not been noted previously in MR imaging studies and may be attributed to increased flow within the tissue as contracting muscle fibers relax following exercise. The results can be interpreted using simple fast exchange or slow exchange models for tissue water relaxation. For the case of rapid exchange the changes in delta R2 may be indicative of an increase in the net water volume within the muscles, whereas in the case of slow exchange delta R2 is primarily a measure of intracellular volume increases.

Adult↗
PubMed1995

Face-sensitive regions in human extrastriate cortex studied by functional MRI.

1. We have previously identified face-selective areas in the mid-fusiform and inferior temporal gyri in electrophysiological recordings made from chronically implanted subdural electrodes in epilepsy patients. In this study, functional magnetic resonance imaging (fMRI) was used to study the anatomic extent of face-sensitive brain regions and to assess hemispheric laterality. 2. A time series of 128 gradient echo echoplanar images was acquired while subjects continuously viewed an alternating series of 10 unfamiliar faces followed by 10 equiluminant scrambled faces. Each cycle of this alternating sequence lasted 12 s and each experimental run consisted of 14 cycles. The time series of each voxel was transformed into the frequency domain using Fourier analysis. Activated voxels were defined by significant peaks in their power spectra at the frequency of stimulus alternation and by a 180 degrees phase shift that followed changes in stimulus alternation order. 3. Activated voxels to faces were obtained in the fusiform and inferior temporal gyri in 9 of 12 subjects and were approximately coextensive with previously identified face-selective regions. Nine subjects also showed activation in the left or right middle occipital gyri, or in the superior temporal or lateral occipital sulci. Cortical volumes activated in the left and right hemispheres were not significantly different. Activated voxels to scrambled faces were observed in six subjects at locations mainly in the lingual gyri and collateral sulci, medial to the regions activated by faces. 4. Face stimuli activated portions of the midfusiform and inferior temporal gyri, including adjacent cortex within occipitotemporal sulci.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Functional magnetic resonance imaging of sensory and motor cortex: comparison with electrophysiological localization.

Functional magnetic resonance (MR) imaging was performed using a 1.5-tesla MR system to localize sensorimotor cortex. Six neurologically normal subjects were studied by means of axial gradient-echo images with a motor task and one or more sensory tasks: 1) electrical stimulation of the median nerve; 2) continuous brushing over the thenar region; and 3) pulsed flow of compressed air over the palm and digits. An increased MR signal was observed in or near the central sulcus, consistent with the location of primary sensory and motor cortex. Four patients were studied using echo planar imaging sequences and motor and sensory tasks. Three patients had focal refractory seizures secondary to a lesion impinging on sensorimotor cortex. Activation seen on functional MR imaging was coextensive with the location of the sensorimotor area determined by evoked potentials and electrical stimulation. Functional MR imaging provides a useful noninvasive method of localization and functional assessment of sensorimotor cortex.

Adult↗
PubMed1995

Changes in magnetic resonance transverse relaxation times of two muscles following standardized exercise.

Magnetic resonance (MR) imaging studies of exercising leg muscles were performed to compare the changes in MR transverse relaxation times (T2) that result from exercise of the anterior tibialis (AT) and extensor digitorum/hallicus longus (E) in the anterior compartment of the lower leg with those T2 changes in the medial and lateral gastrocnemius (G) in the posterior compartment. Spin-echo MR images were obtained at 1.5 Tesla before and during the first 14 min of recovery from dynamic exercise. In order to normalize the exercise, workloads for each subject were set at 25% of the measured maximum voluntary contraction (MVC) of the anterior and posterior compartments. In separate exercise sessions, a nonmagnetic, pneumatic exercise apparatus was employed for either dorsiflexion or plantarflexion against a fixed constant resistance for two different exercise durations (1 min 45 s or 5 min). Transaxial MR images (TR = 1000 ms, TE = 30, 60, 90, 120 ms, 128 x 256 matrix, 1.5 cm slice) were used to calculate T2 values. Although subjects performed approximately 7-fold more work (P < or = 0.001, dorsiflexion vs plantarflexion) during plantarflexion than during dorsiflexion at both exercise duration's, the exercise induced T2, while being greater than those at rest (P < or = 0.001), were not significantly different in the different compartments. We conclude that, when exercised at the same workload (25% of MVC), these two muscles produce T2 changes that are not significantly different from each other.

Adult↗
PubMed1994

Brain morphology in normal and dyslexic children: the influence of sex and age.

Morphometric magnetic resonance imaging techniques were used to compare the convolutional surface area of the planum temporale, temporal lobe volume and superior surface area, and an estimate of overall brain volume in a homogeneous sample of 17 dyslexic children (7 girls) and 14 nonimpaired children (7 girls). Substantial sex differences were apparent for all measured regions, with all the measurements in boys being significantly larger. Age, even within the narrow range employed here (7.5-9.7 years), was positively correlated with the size of each brain region. While initial analyses suggested smaller left hemisphere structures in dyslexics compared to control subjects, subsequent analyses controlling for age and overall brain size revealed no significant differences between dyslexics and nonimpaired children on a variety of measures, in particular surface area and symmetry of the planum temporale. We suggest that differences in subject characteristics (i.e., sex, age, handedness, and definition of dyslexia) as well as procedural variations in the methods used to acquire images and to define and measure anatomical regions of interest such as the planum temporale all may play an important role in explaining apparent discrepant results in the neuroimaging literature on dyslexia.

Age Factors↗
PubMed1994

Intravascular susceptibility contrast mechanisms in tissues.

The factors affecting the rate of loss of transverse magnetization in gradient echo and spin-echo pulse sequences have been quantified using computer modeling for media containing arrays of susceptibility variations. The results are particularly relevant for describing the signal losses that occur in tissues containing capillaries of altered intrinsic susceptibility from the administration of exogenous contrast agents or arising from changes in blood oxygenation. The precise magnitudes and relationship of gradient echo and spin-echo decay rates depend on geometrical factors such as the sizes and spacings of the inhomogeneities, the rate of water diffusion, field strength, and echo times. The conventional separation of contributions to transverse decay rates arising from so-called static field effects and diffusion is shown to be inappropriate for many situations of practical interest because diffusion introduces a motional averaging of the static field even in gradient echo sequences. The result of diffusion in some regimes is to reduce the decay rate from field inhomogeneities in gradient echo sequences, so that T2* is longer in media such as tissue where water diffuses reasonably rapidly, than would be the case for stationary nuclei. The effects of different types of contrast agent and the implications for functional imaging based on the effects of deoxyhemoglobin in brain tissue are considered.

Capillaries↗
PubMed1994

BOLD MRI monitoring of changes in cerebral perfusion induced by acetazolamide and hypercarbia in the rat.

To evaluate MRI methods for estimating cerebrovascular reserve, we computed changes in the R2* and R2 transverse relaxation rate and apparent diffusion coefficient (ADC) at 2.0 Tesla in five rats after administration of 30 mg of acetazolamide and in four rats during inhalation of 20% carbon dioxide gas. Significant decreases in R2*, corresponding to increases in gradient echo MRI signals, occurred in both the acetazolamide (average change -8.3%, P = 0.005) and the carbon dioxide (-2.7%, P = 0.009) treated animals. The computed values for R2 and ADC were unchanged. The magnitude of the gradient echo MRI changes observed should permit anatomic mapping of blood flow reactivity patterns in normal human subjects and in patients at risk for cerebrovascular disease.

Acetazolamide↗
PubMed1994

Functional NMR imaging using fast spin echo at 1.5 T.

Functional NMR imaging of the brains response to a simple visual task has been performed using a fast spin echo (FSE) imaging sequence at 1.5 T. The FSE method refocuses dephasing effects induced by large-scale susceptibility variations, and permits imaging in regions where macroscopic field gradients produce artifacts in gradient echo sequences. At 1.5 T, gradient echo (GRE) sequences are sensitive to the effects of brain activation, but relatively large effects may arise from large vessels and veins, and these may dominate the effects produced by smaller capillaries. Spin echo (SE) sequences with short echo times are relatively immune to large vessel effects and emphasize the susceptibility induced losses from small capillaries, but the imaging time for these sequences is prohibitive for most functional brain studies. We demonstrate that multislice functional brain imaging may be performed in reasonable imaging times at 1.5 T using an FSE imaging sequence. The FSE sequence with short echo spacing but long effective TE is sensitive to susceptibility induced effects at the capillary level. It is not sensitive to larger scale inhomogeneities such as those found in veins and can be used in regions near tissue/air boundaries. Results are shown comparing conventional GRE and FSE images in activation of the visual cortex and these are supported by theoretical calculations and phantom experiments.

Body Water↗
PubMed1994

Development and evaluation of tracking algorithms for cardiac wall motion analysis using phase velocity MR imaging.

Phase velocity magnetic resonance imaging (MRI) has shown considerable potential for tracking distinct regions of the myocardium throughout the cardiac cycle. Phase contrast MR imaging produces multiple images, each phase encoded for velocity in a different direction, in which individual pixels depict the local motion of the tissue. In this work we present in detail three algorithms for tracking motion based on these images. Both simulated and phantom data are used to examine some of the problems encountered in practice in tracking points based on velocity maps. Solutions to these problems are offered when possible. The impact of noise and low order phase errors in the data on each of the three tracking approaches is examined. In addition, problems due to tissue expansion and contraction, to 2D versus 3D tracking, and to round off errors from motion which is small relative to pixel size or slice thickness, are considered. An example using data obtained in vivo is included to demonstrate the efficacy of the best of the three tracking algorithms in measuring left ventricular circumferential shortening preinfarct and postinfarct in a canine model.

Algorithms↗