Biomedical subjects

G Larry Bretthorst

Publications and source records attributed to G Larry Bretthorst.

4 recordsLinked to original sources

PubMed2003

Equilibrium water exchange between the intra- and extracellular spaces of mammalian brain.

This report describes the measurement of water preexchange lifetimes and intra/extracellular content in intact, functioning mammalian brain. Intra- and extracellular water magnetic resonance (MR) signals from rat brain in vivo were quantitatively resolved in the longitudinal relaxation domain following administration of an MR relaxation agent into the extracellular space. The estimated intracellular water content fraction was 81% +/- 8%, and the intra- to extracellular exchange rate constant was 1.81 +/- 0.89 s(-1) (mean +/- SD, N = 9), corresponding to an intracellular water preexchange lifetime of approximately 550 ms. These results provide a temporal framework for anticipating the water exchange regime (fast, intermediate, or slow) underlying a variety of compartment-sensitive measurements. The method also supplies a means by which to evaluate membrane water permeability and intra/extracellular water content serially in intact tissue. The data are obtained in an imaging mode that permits detection of regional variations in these parameters.

Animals↗

PubMed2003

Sources of plasma glucose by automated Bayesian analysis of 2H NMR spectra.

Sources of blood glucose can be determined after oral ingestion of (2)H(2)O followed by isolation of plasma glucose and measurement of the relative (2)H enrichments in select positions within the glucose molecule. Typically, (2)H enrichments are obtained by mass spectrometry but (2)H NMR offers an alternative. Here it is demonstrated that the entire analysis may be automated by Bayesian analysis of a (2)H free induction decay signal of monoacetone glucose to obtain a direct readout of the relative contributions of glycogenolysis, glycerol, and phosphoenol pyruvate to plasma glucose production. Furthermore, Markov Chain Monte Carlo (MCMC) simulations of the posterior probability density provide uncertainties in all metabolic parameters from a single patient, thereby allowing comparisons in glucose metabolism from one individual to another. The combined MCMC Bayesian methodology is operationally simple and requires little intervention from the operator.

Autoanalysis↗

PubMed2003

Statistical model for diffusion attenuated MR signal.

A general statistical model that can describe a rather large number of experimental results related to the structure of the diffusion-attenuated MR signal in biological systems is introduced. The theoretical framework relies on a phenomenological model that introduces a distribution function for tissue apparent diffusion coefficients (ADC). It is shown that at least two parameters--the position of distribution maxima (ADC) and the distribution width (sigma)--are needed to describe the MR signal in most regions of a human brain. A substantial distribution width, on the order of 36% of the ADC, was found for practically all brain regions examined. This method of modeling the MR diffusion measurement allows determination of an intrinsic tissue-specific ADC for a given diffusion time independent of the strength of diffusion sensitizing gradients. The model accounts for the previously found biexponential behavior of the diffusion-attenuated MR signal in CNS.

Brain↗

PubMed2002

Quantitative in vivo assessment of lung microstructure at the alveolar level with hyperpolarized 3He diffusion MRI.

The study of lung emphysema dates back to the beginning of the 17th century. Nevertheless, a number of important questions remain unanswered because a quantitative localized characterization of emphysema requires knowledge of lung structure at the alveolar level in the intact living lung. This information is not available from traditional imaging modalities and pulmonary function tests. Herein, we report the first in vivo measurements of lung geometrical parameters at the alveolar level obtained with 3He diffusion MRI in healthy human subjects and patients with severe emphysema. We also provide the first experimental data demonstrating that 3He gas diffusivity in the acinus of human lung is highly anisotropic. A theory of anisotropic diffusion is presented. Our results clearly demonstrate substantial differences between healthy and emphysematous lung at the acinar level and may provide new insights into emphysema progression. The technique offers promise as a clinical tool for early diagnosis of emphysema.

Diffusion↗