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Biomedical subjects

L Borland

Publications and source records attributed to L Borland.

6 recordsLinked to original sources
PubMed2001

Escape time in anomalous diffusive media.

We investigate the escape behavior of systems governed by the one-dimensional nonlinear diffusion equation theta(t)rho=theta(x)[theta(x)Urho]+Dtheta(x)2rho(nu), where the potential of the drift, U(x), presents a double well and D,nu are real parameters. For systems close to the steady state, we obtain an analytical expression of the mean first-passage time, yielding a generalization of Arrhenius law. Analytical predictions are in very good agreement with numerical experiments performed through integration of the associated Ito-Langevin equation. For nu not equal to 1, important anomalies are detected in comparison to the standard Brownian case. These results are compared to those obtained numerically for initial conditions far from the steady state.

Journal Article↗
PubMed1997

Consequences of using a simplified kinetic model for dynamic PET data.

UNLABELLED: We compared a physiological model of 82Rb kinetics in the myocardium with two reduced-order models to determine their usefulness in assessing physiological parameters from dynamic PET data. METHODS: A three-compartment model of 82Rb in the myocardium was used to simulate kinetic PET ROI data. Simulations were generated for eight different blood-flow rates reflecting the physiological range of interest. Two reduced-order models commonly used with myocardial PET studies were fit to the simulated data, and parameters of the reduced-order models were compared with the physiological parameters. Then all three models were fit to the simulated data with noise added. Monte Carlo simulations were used to evaluate and compare the diagnostic utility of the reduced-order models. A description length criterion was used to assess goodness of fit for each model. Finally, fits to simulated data were compared with fits to actual dynamic PET data. RESULTS: Fits of the reduced-order models to the three-compartment model noise-free simulated data produced model misspecification artifacts, such as flow parameter bias and systematic variation with flow in estimates of nonflow parameters. Monte Carlo simulations showed some of the parameter estimates for the two-compartment model to be highly variable at PET noise levels. Fits to actual PET data showed similar variability. One-compartment model estimates of the flow parameter at high and low flow were separated by several s.d.s for both the simulated and the real data. With the two-compartment model, the separation was about one s.d., making it difficult to differentiate a high and a low flow in a single experiment. Fixing nonflow parameters reduced flow parameter variability in the two-compartment model and did not significantly affect variability in the one-compartment model. Goodness of fit indicated that, at realistic noise levels, both reduced-order models fit the simulated data at least as well as the three-compartment model that generated the data. CONCLUSION: The one-compartment reduced-order model of 82Rb dynamic PET data can be used effectively to compare myocardial blood-flow rates at rest and stress levels. The two-compartment model can differentiate flow only if a priori values are used for nonflow parameters.

Animals↗
PubMed1992

Liver transplantation for arteriohepatic dysplasia (Alagille's syndrome).

Thirteen out of 268 children (less than 18 years old) underwent hepatic transplantation (OLT) for end-stage liver disease (ESLD) associated with arteriohepatic dysplasia (AHD). Seven children are alive and well with normal liver function. Six children died, four within 11 days of the operation and the other two at 4 and 10 months after the OLT. Vascular complications with associated septicemia were responsible for the deaths of three children. Two died of heart failure and circulatory collapse, secondary to pulmonary hypertension and congenital heart disease. The remaining patient died of overwhelming sepsis not associated with technical complications. Seven patients had a portoenterostomy or portocholecystostomy early in life; five of these died after the OLT. Severe cardiovascular abnormalities in some of our patients suggest that complete hemodynamic monitoring with invasive studies should be performed in all patients with AHD, especially in cases of documented hypertrophy of the right ventricle. The improved quality of life in our surviving patients confirms the validity of OLT as a treatment of choice in cases of ESLD due to AHD.

Adolescent↗
PubMed1988

Packing of the 30 nm chromatin fiber in the human metaphase chromosome.

The human genetic material is packed hierarchically within the metaphase chromosome: the DNA molecule together with histone proteins form 11 nm diameter nucleosomes, which are then ordered into the 30 nm thick chromatin fiber. Little is known about the packing of this fiber within the chromosome. We have developed a tracking algorithm with which we followed its path within a three-dimensional reconstruction of a human chromosome computed from a series of electron micrographic projections. Fiber segments were seen to form loops of 100-350 nm diameter. Our observations indicate that these loops--which themselves show no preferred orientation--are organised into regions of roughly 200 nm axial extent.

Chromatin↗
PubMed1987

Three-dimensional reconstruction of a human metaphase chromosome from electron micrographs.

A complete human metaphase chromosome has been reconstructed from a series of electron microscopical projections obtained by tilting the specimen stage at 3 degree intervals from -60 to +60 degrees. The reconstructed structure is about 3.0 microns long, 1.6 micron wide, and 0.8 micron thick. The mass distribution was fairly homogeneous within the chromatids and neither a hollow nor a dense core was observed. The distribution and course of fibers observed are most consistent with a looping model of chromosome structure.

Algorithms↗