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Biomedical subjects

D R Bacon

Publications and source records attributed to D R Bacon.

At least 19 recordsLinked to original sources
PubMed1992

Optic nerve head microvasculature of the rabbit eye.

Vascular luminal castings of rabbit eyes were microdissected and studied with scanning electron microscopy to elucidate the three-dimensional angioarchitecture of the optic nerve head. Using sequential microdissection, an incomplete arterial circle was identified as terminal branches of two to three short posterior ciliary arteries around the optic nerve head. Several recurrent branches from the arterial circle form a pial arterial network. This pial system supplies the optic nerve head microvasculature and receives numerous venules from them. The only large vessel to enter the optic nerve is a central retinal artery that has few branches within the optic nerve and provides several branches at the surface of the optic disc. Moderately numerous vessels connect the retinal and ciliary vascular layers within the optic nerve head. Few arterioles to the optic nerve head arise from the choroid; however, there are a small number of capillary and numerous venous connections between them. These results indicate that the principal blood supply of the rabbit optic nerve head is derived from the short posterior ciliary arteries by the arterial circle. The retinal arteries contribute to the surface vasculature of the optic nerve head. The pial system also plays a significant role in both supply and drainage of the rabbit optic nerve head.

Animals↗
PubMed1991

Absorption of finite amplitude focused ultrasound.

Predictions of the absorption of focused finite amplitude waves based on weak shock theory have been tested experimentally. The characteristics of this absorption are qualitatively different from those associated with small signal losses. Under appropriate conditions, the absorption of finite amplitude ultrasound is determined largely by source amplitude, field geometry, and the nonlinear properties of the medium and is only weakly dependent upon the small signal absorption coefficient of the material. These effects are seen most dramatically in sharply focused sound fields. To emphasize nonlinear absorption in an experimental test of these predictions, measurements of heating were made in agar which has a very small linear absorption coefficient. Under appropriate conditions, nonlinear losses can make the effective absorption coefficient of this poorly absorbing material somewhat greater than the soft tissues of the body.

Acoustics↗
PubMed1991

The development of academic anesthesiology at the Roswell Park Memorial Institute: James O. Elam, MD, and Elwyn S. Brown, MD.

In the early 1950s, Drs. James Elam and Elwyn Brown were recruited to establish the department of anesthesiology at the Roswell Park Memorial Institute. With substantial financial support from both the New York State coffers and the Institute's director, Dr. George Moore, Elam and Brown were able to create a department of anesthesiology renowned for clinical excellence and basic science research. Their work on carbon dioxide elimination led to a redesigning of the soda lime canisters that is still in clinical use. By popularizing mouth-to-mouth rescue breathing, these two anesthesiologists changed the manner in which emergency aid was given and won international acclaim.

Anesthesia Department, Hospital↗
PubMed1991

Among the first: the career of John Henry Evans, MD.

During his 46-year career, John Henry Evans, MD, significantly guided anesthesia's evolution from a field dominated by lay practitioners toward one in which the preeminent role was played by physicians. Widely recognized as an expert on supplemental oxygen therapy as well as the developer of subcutaneous oxygen as an adjuvant treatment for several chronic diseases, Evans throughout his years of practice held an academic appointment at the University of Buffalo. From that post he tirelessly employed professional political persuasion, combined with a high order of organizational skill, to help create and expand the importance of residency-trained anesthesiologists. As president of the Associated Anesthetists of the United States and Canada, complemented by a quarter-century tour on the International Anesthesia Research Society's Board of Governors, he significantly contributed to the development of anesthesiology into its current form.

Anesthesiology↗
PubMed1990

Ciliary vasoconstriction after topical adrenergic drugs.

Rabbits underwent the single-dose or long-term therapeutic administration of the adrenergic drugs phenylephrine hydrochloride, timolol maleate, and betaxolol hydrochloride. After a single dose, all three drugs caused substantial, localized constriction in the arterioles that supply the ciliary processes but did not affect the downstream bore of the same vessels. After seven weeks of a daily dose, tolerance reduced the response to betaxolol to insignificant levels and that to phenylephrine substantially, whereas timolol maleate continued to produce identical levels of vasoconstriction to those seen with single-dose administration. In addition to the consequent lowering of perfusion of the ciliary processes and presumptive impact on aqueous humor production, vasoconstriction also reinforces concerns about impaired vascular perfusion of eyes undergoing long-term ocular therapy.

Animals↗
PubMed1990

A multiple-frequency hydrophone calibration technique.

A method is described for comparing the sensitivity of two hydrophones over the frequency range 1-15 MHz. This technique forms the basis for the dissemination of national ultrasonic standards in the U.K. over this frequency range. A reference hydrophone is placed in an ultrasonic field and then the device being calibrated is substituted and the two output voltages are compared. This substitution method utilizes a broadband ultrasonic field produced by nonlinear propagation. Thus it is possible to cover the whole frequency range with a single measurement on each hydrophone. The overall uncertainty in the intercomparison of two hydrophones increases from +/- 4.2% at 1 MHz to +/- 8.2% at 15 MHz (95% confidence level). The method has been compared with discrete-frequency substitution, time-delay spectrometry, and absolute calibrations using the National Physical Laboratory (NPL) Primary Standard Laser Interferometer. Various designs and sizes of hydrophones were compared, and agreement was within the combined random uncertainties for all the comparisons.

Acoustics↗
PubMed1990

Increased heating by diagnostic ultrasound due to nonlinear propagation.

The heating of tissues by the absorption of ultrasound is an important safety consideration in the use of diagnostic ultrasound. This paper shows that models of ultrasonic heating for this situation need to take account of nonlinear propagation. Measurements were made of the temperature rise in a sample of tissue-mimicking gel, caused by the application of 3.6-MHz focused ultrasonic beams for 3 min. The propagation path to the focus was in water, to mimic the situation where the fetus is scanned through the full bladder. The effect of nonlinear propagation was seen by changing the pressure amplitude of the pulse, while altering the pulsing regime to preserve a constant spatial-peak temporal-average intensity of 1 W cm-2. When nonlinear distortion was present, an enhancement in the temperature rise was observed, which correlated with the value of the shock parameter. The enhancement ratio was typically up to a factor of 3, and the maximum temperature rise observed was 2 degrees C. This enhanced heating was seen both at the surface of the tissue-mimicking gel and after propagation through 23 mm of the material. Under conditions of nonlinear propagation, the maximum heating usually occurs in the prefocal region, rather than at the focus.

Body Temperature Regulation↗
PubMed1989

Trabecular repopulation by anterior trabecular meshwork cells after laser trabeculoplasty.

To study further the transient increase in trabecular cell division within the first two days after laser trabeculoplasty in human corneoscleral explant organ cultures, we used a pulse-chase protocol in which immediately after laser treatment 3H-thymidine was added to the culture medium for 48 hours (the pulse period). Fresh medium without radiolabel was then added for variable times (the chase period) before termination of the experiment. Autoradiography was used to follow changes in the regional distribution of the cells that divided during the pulse period and had 3H-thymidine-labeled DNA. Laser-treated explants, evaluated after a pulse with no chase, showed a fourfold increase in cell division (P less than .001) over nontreated controls. Nearly 60% of this cell division was localized to the anterior, nonfiltering region of the trabecular meshwork where it inserts into the cornea beneath Schwalbe's line. Trabecular cell division in other regions of the meshwork was not increased over controls at this time. After seven or 14 days of chase without radiolabel, the regional distribution of radiolabeled cells changed in laser-treated explants but not in controls. By 14 days, only 26% of the labeled cells remained in this anterior insert region, while 60% were found in the region of the burn sites. Macroautoradiography of whole explants corroborated these observations. Our data support the hypothesis that laser trabeculoplasty causes early cell division by a population of cells in the anterior meshwork; these new cells then migrate and repopulate the burn sites over the next few weeks.

Humans↗
PubMed1989

Prediction of in situ exposure to ultrasound: an improved method.

The acoustic output of medical ultrasonic equipment is usually measured in water but to determine the safety (or performance) of a machine it is necessary to know the in situ acoustic pressure levels inside a patient. At present, when estimates of these levels are made, a linear propagation model is used although in practice the propagation is nonlinear. This paper shows that such a model can lead to large errors (80% or more in pressure) and in particular that in situ predictions of the peak-negative acoustic pressure are too low. To describe the field in water and in tissue it is necessary to take account of diffraction, attenuation, and dispersion as well as nonlinear propagation. It is difficult to use acoustic output levels in water to predict in situ values because the amplitude in water approaches a limit, an effect known as saturation. Nevertheless, a method of making such predictions is presented in this paper and is validated by comparison with experiment. The method is relatively time-consuming to implement and has not yet been applied to pulsed fields, so there is a need for more complete and simpler methods.

Environmental Exposure↗
PubMed1989

Comparison of two theoretical models for predicting non-linear propagation in medical ultrasound fields.

Non-linear propagation models are required to predict the fields from medical ultrasonic equipment, particularly diagnostic devices and lithotripters. This need arises because of the requirement to know the safety and effectiveness of these instruments. Several theoretical models have been developed to take account of non-linear propagation as well as diffraction, focusing and attenuation, but little work has been done to validate them. This paper compares two theoretical models with each other and with measurements in the field of a 3.5 MHz focused transducer. One model uses an approximation based on modelling the beam profile with a Gaussian function, whereas the other utilises a full three-dimensional finite difference method, using a uniform transducer excitation function. Comparisons are made in the time and frequency domain at the focus for four different source levels and in general the results agree to within about 10%. However, an important conclusion is that the finite amplitude field of a real transducer may differ significantly from that of an ideal piston source, particularly for the weakly focused beams used in diagnostic ultrasound.

Acoustics↗
PubMed1988

Controlled vascular corrosion casting of the rabbit eye.

We have refined the technique of vascular corrosion casting with methacrylate to permit the reproduction of physiological states of vascular tone and to produce sturdy castings of ocular microvasculature. The method entails careful maintenance of homeostasis up to the moment of plastic perfusion, avoidance of vascular rinsing or fixation with the attendant anoxia, reduction of the viscosity of the casting resin without impairing the properties of the resultant polymer, addition of a cross-linking agent to increase the strength of the plastic, and injection at physiological temperature and pressure. This casting regimen reproduces the normal anatomical conditions of blood vessels and can be used to demonstrate altered conditions of vascular tone. In all instances, the second, untouched eye serves as a control for unilateral manipulations. Special problems of replicating the ocular vasculature are related to the intraocular pressure, which opposes the vascular perfusion pressure and constitutes an impediment to perfusion.

Animals↗
PubMed1987

Pressure waveforms generated by a Dornier extra-corporeal shock-wave lithotripter.

Pressure waveforms in the acoustic field generated by a Dornier (HM3) shock-wave lithotripter have been measured using a bilaminar shielded PVDF membrane hydrophone in water. Using these waveforms, values of the peak-positive (p+) and peak-negative pressure (p-) at various positions in the field have been estimated. At the focus, p+ is 38.6 MPa (standard deviation = 9.0 MPa) and p- is 10.1 MPa (standard deviation = 1.0 MPa) at 20 kV discharge potential and an electrode separation in the range 1.3 to 2.4 mm. The peak-positive pressure is found to fall to 50% (-6 dB level) at about 60 mm either side of the focus on the major axis of the reflector and on a 10 mm radius circle around the focus in the focal plane. A shot-to-shot variation of +/- 25% in p+ is attributed to the inherent variability of the electrical discharge which may result in changes in the exact position and strength of the acoustic field. The results reported are considered to be more accurate than those of previous measurements due to the relatively flat frequency response of this type of hydrophone.

Acoustics↗