Biomedical subjects

D G Albert

Publications and source records attributed to D G Albert.

10 recordsLinked to original sources

PubMed2001

Acoustic waveform inversion with application to seasonal snow covers.

The amplitude and waveform shape of atmospheric acoustic pulses propagating horizontally over a seasonal snow cover are profoundly changed by the air forced into the snow pores as the pulses move over the surface. This interaction greatly reduces the pulse amplitude and elongates the waveform compared to propagation above other ground surfaces. To investigate variations in snow-cover effects, acoustic pulses were recorded while propagating horizontally over 11 different naturally occurring snow covers during two winters. Two inversion procedures were developed to automatically match the observed waveforms by varying the snow-cover parameters in theoretical calculations. A simple frequency-domain technique to match the dominant frequency of the measured waveform suffered from multiple solutions and poor waveform matching, while a time-domain minimization method gave unique solutions and excellent waveform agreement. Results show that the effective flow resistivity and depth of the snow are the parameters controlling waveform shape, with the pore shape factor ratio of secondary importance. Inversion estimates gave flow resistivities ranging from 11 to 29 kN s m(-4), except for two late-season cases where values of 60 and 140 were determined (compared to 345 for the vegetation-covered site in the summer). Acoustically determined snow depths agreed with the measured values in all but one case, when the depth to a snow layer interface instead of the total snow depth was determined. Except for newly fallen snow, the pore shape factor ratio values clustered near two values that appear to correspond to wet (1.0) or dry (0.8) snow.

Acoustics↗

PubMed2001

Blast noise propagation above a snow cover.

A porous medium model of a snow cover, rather than a viscoelastic treatment, has been used to simulate measured, horizontally traveling acoustic waveform propagation above a dry snow cover 11-20 cm thick. The waveforms were produced by explosions of 1-kg charges at propagation distances of 100 to 1400 m. These waveforms, with a peak frequency around 30 Hz, show pulse broadening effects similar to those previously seen for higher-frequency waves over shorter propagation distances. A rigid-ice-frame porous medium ("rigid-porous") impedance model, which includes the effect of the pores within the snow but ignores any induced motion of the ice particles, is shown to produce much better agreement with the measured waveforms compared with a viscoelastic solid treatment of the snow cover. From the acoustic waveform modeling, the predicted average snow cover depth of 18 cm and effective flow resistivities of 16-31 kPa s m(-2) agree with snow pit observations and with previous acoustic measurements over snow. For propagation in the upwind direction, the pulse broadening caused by the snow cover interaction is lessened, but the overall amplitude decay is greater because of refraction of the blast waves.

Journal Article↗

PubMed1999

Chronic hepatitis C virus patients with breakthroughs during interferon treatment can successfully be retreated with consensus interferon. The Consensus Interferon Study Group.

Patients with chronic hepatitis C who have not had a sustained hepatitis C virus (HCV)-RNA response or serum alanine transaminase (ALT) response to a 6-month course of interferon (IFN) may respond to higher dose retreatment with consensus interferon (CIFN). Some nonresponders to initial IFN treatment have a transient response defined as undetectable HCV RNA or normalization of ALT during treatment, but subsequently have a "breakthrough" while still on treatment. The aim of this study was to determine if nonresponders who had breakthroughs responded differently to CIFN retreatment than nonresponders without breakthroughs using data from a large, multicenter trial. ALT and HCV RNA were monitored frequently during initial IFN therapy (either 9 mcg CIFN or 3 MU IFN-alpha2b 3 times per week). HCV-RNA breakthroughs were observed in 86 of 467 (18%) of all treated patients, and ALT breakthroughs were observed in 90 of 467 (19%) of all treated patients. There was no association between breakthroughs and the presence of either binding or neutralizing anti-IFN antibodies. When the patients who were nonresponders to initial IFN treatment were retreated with CIFN (15 mcg) for 12 months, 27% of those with viral breakthroughs had a sustained viral response compared with 8% in prior nonresponders without breakthroughs (P =.102). Sustained ALT responses were observed in 39% with breakthroughs compared with 10% in those without breakthroughs (P =.014). The data suggest that prior nonresponders with breakthroughs have a greater chance of responding to retreatment than do nonresponders without breakthroughs. However, most breakthrough patients would be missed unless repeated HCV-RNA testing were conducted during therapy.

Alanine Transaminase↗

PubMed1998

Re-treatment of chronic hepatitis C with consensus interferon .

A multicenter, open-label, phase 3 study was conducted in 337 patients with chronic hepatitis C virus (HCV) infection who had either not responded to previous interferon therapy or had relapsed after discontinuation of therapy with either consensus interferon (9 microg) or interferon alpha-2b (3 million U) three times a week for 24 weeks. Patients were randomized to receive a higher dose of consensus interferon (15 microg) administered subcutaneously three times a week for 24 or 48 weeks and then were observed for an additional 24 weeks. Patients who had relapsed after prior interferon therapy were more likely to have a sustained alanine aminotransferase response and HCV RNA response (as measured by reverse transcription-polymerase chain reaction with a sensitivity of < 100 copies/mL) than were patients who had not responded to prior interferon therapy. For relapsers, the sustained HCV RNA response rate was 58% (48 weeks) and 28% (24 weeks). The sustained alanine aminotransferase response for relapsers was 52% (48 weeks) and 39% (24 weeks). The sustained HCV RNA response rate among prior nonresponders was 13% (48 weeks) and 5% (24 weeks), and the sustained alanine aminotransferase response rate for nonresponders was 17% (48 weeks) and 12% (24 weeks). The administration of 15 microg of consensus interferon was well tolerated and was not associated with an increase in the incidence of side effects. These data demonstrate that re-treatment with 15 microg of consensus interferon is safe and effective therapy for patients with chronic hepatitis C who have either not responded to previous interferon therapy or relapsed after discontinuation of interferon therapy.

Adult↗

PubMed1973