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Biomedical subjects

L J Hayes

Publications and source records attributed to L J Hayes.

32 records · Page 2Linked to original sources
PubMed1990

The major outer-membrane proteins of Chlamydia trachomatis serovars A and B: intra-serovar amino acid changes do not alter specificities of serovar- and C subspecies-reactive antibody-binding domains.

The major outer-membrane protein (MOMP) of Chlamydia trachomatis is a promising candidate antigen for chlamydial vaccine development. We have sequenced the MOMP genes for a serovar A and a serovar B isolate and have compared these new sequences with those already reported. Intra-serovar changes in the inferred amino acid sequences of the surface-exposed variable segments known to be responsible for binding of neutralizing antibody were observed. Nevertheless, epitope mapping with solid-phase peptides showed that these intra-serovar changes did not affect the binding of serovar- and subspecies-specific, potentially protective antibodies. Variable segment 1 of C. trachomatis serovar A contained two adjacent antibody-binding sites, one of which was C-subspecies specific while the other was serovar A specific. Therefore the subspecies binding site for C-complex organisms is in variable segment 1, whilst that for B-complex organisms is in variable segment 4. This work shows that MOMP sequences are relatively stable within the serovar categorization for isolates taken decades apart from different continents. Within a given serovar, however, limited interchange of functionally related amino acids may occur without impairing the binding of serovar-specific antibody.

Amino Acid Sequence↗
PubMed1989

A theoretical study of the effect of optical properties in laser ablation of tissue.

The role of optical properties in the distribution of laser light and the resulting thermodynamic processes in biological tissue is studied from a theoretical perspective. Light distribution is modeled by a discrete ordinate method and heat transfer and ablation is modeled by an immobilized finite element method. The effect of parametric variation of absorption, scattering anisotropicity on the dynamics of the ablation process is examined. The manifestation of higher than the ablation threshold temperature in the subsurface tissue is observed and discussed. Results indicate significant differences in the ablation behavior which may have important clinical implications.

Hot Temperature↗
PubMed1989

Stimulus equivalence and rule following.

The present study examined the occurrence of a novel behavior pattern with respect to a novel configuration of stimuli enabled by the participation of those stimuli in equivalence classes. In Experiment 1, functional substitutabilities were established via equivalence between two independent sets of musical stimuli. Aspects of stimuli from the two sets were then compounded to produce novel stimulus configurations. Behavioral components enabled by each separate class combined to produce novel musical performances and accurate descriptions of them. In Experiment 2, the impact of experimenter-provided names for equivalence classes on the musical performances was investigated in naive subjects by establishing similar classes without experimenter-provided names. The results indicated few differences in the playing performances under these conditions. These experiments demonstrated a possible method for the analysis of rule following.

Journal Article↗
PubMed1988

Prediction of local cooling rates and cell survival during the freezing of a cylindrical specimen.

A finite element numerical model was implemented to simulate the freezing process of an aqueous salt solution in a cylindrical container. Local cooling rates within the container were computed for several defined cooling protocols applied at the boundary. Characteristic cell survival signatures were used to predict the associated local survival rates throughout the system. These calculations show that there are two definite time domains during a typical freezing process: (1) while the surface temperature is changing and (2) after the surface temperature reaches a constant storage value. The calculations also show significant spatial variations in the local cooling rates within the container and considerable local deviation from the volumetric average survival for various simulated freezing protocols.

Animals↗
PubMed1988

Laser ablation of discs of agar gel.

Discs of agar gel mixed with ink were used to study ablation effects with an argon laser as a light source. Varying amounts of ink were added resulting in a variation of the attenuation coefficient between 0.45 and 6.3 mm-1. For laser beam irradiation horizontally incident on a vertical sample, the average velocity of ablation was found to be approximately constant for thicknesses up to 1.7 mm. When the laser beam was directed vertically on a sample held horizontally, the vaporized debris present in the beam attenuated the incident laser energy to such a degree that the average ablation velocity decreased by a factor of approximately five. Horizontal beam experiments for various attenuation coefficients showed that an attenuation coefficient of about 1.7 mm-1 is optimal for fast penetration of discs thicker than 4 mm. Thus, based upon the optical properties of a given tissue, there may exist an optimum laser wavelength to maximise ablation velocity.

Agar↗
PubMed1988

Segmented arrangement of Borrelia duttonii DNA and location of variant surface antigen genes.

The DNA of an isolate of Borrelia duttonii, an agent of relapsing fever is present as seven major species ranging in size from 10 kb to greater than 150 kb. Additionally, this isolate contains low copy number species, both smaller and larger than these seven major elements. No one of these individual DNA species obviously corresponds to the bacterial chromosome, unlike the situation in Borrelia hermsii, another relapsing fever Borrelia. Thus it appears that B. duttonii has a unique segmented arrangement of its genetic material. Cloned DNA fragments containing coding sequences specific for variant surface antigens of B. duttonii hybridize to a closely migrating, high copy number subset of these genetic elements.

Animals↗
PubMed1985

Effect of laser radiation on tissue during laser angioplasty.

The thermal properties of adipose and ceramic atherosclerotic plaque deposits and normal arterial vessel wall were measured in the temperature range of 25-95 degrees C. In general, the data indicate that fatty plaques exhibit the lowest thermal conductivity and thermal diffusivity of the three types, whereas calcified plaques seem to have the highest values. By using a video scanning thermograph, temperature rise was recorded in normal vessel wall and plaque during ablation of tissue. Theoretical analysis suggested that realistic modeling of laser angioplasty should account for scattering of light, water content, and ablation. This paper is a preliminary report of these results.

Arteriosclerosis↗
PubMed1985

Steady-state analysis of self-heated thermistors using finite elements.

The purpose of this work is to validate, using numerical, finite element methods, the thermal assumptions made in the analytical analysis of a coupled thermistor probe-tissue model upon which a thermal conductivity measurement scheme has been based. Analytic, closed form temperature profiles generated by the self-heated thermistors can be found if three simplifying assumptions are made: the thermistor is spherical; heat is generated in all regions of the bead; and heat is generated uniformly in the bead. This analytic solution is used to derive a linear relationship between tissue thermal conductivity and the ratio of thermistor temperature rise over electrical power required to maintain that temperature rise. This derived, linear relationship is used to determine thermal conductivity from the observed experimental data. However, in reality, the thermistor bead is a prolate spheroid surrounded by a passive shell, and the heating pattern in the bead is highly nonuniform. In the physical system, the exact relationship between the tissue thermal conductivity and parameters measured by the thermistor is not known. The finite element method was used to calculate the steady-state temperature profiles generated by thermistor beads with realistic geometry and heating patterns. The results of the finite element analysis show that the empirical, linear relationship remains valid when all three simplified assumptions are significantly relaxed.

Biomechanical Phenomena↗
PubMed1985

Influence of hypothermia and circulatory arrest on cerebral temperature distributions.

A finite element model of the bioheat transfer equation has been developed to simulate the temperature distribution in the head of a subhuman primate. Simulations were made of the induction of deep hypothermia and of subsequent hypothermic circulatory arrest (HCA). Simulations of the circulatory arrest phase were performed with different values of surface heat transfer coefficient and tissue metabolic heat generation. Numerical results were compared with experimental data for the same procedure. The simulations indicate the brain cools rapidly to a near isothermal condition in response to an infusion of cold arterial blood. However, extracerebral structures cool much more slowly. The bulk of heat gain by the brain during HCA is due to heat transfer from these warmer extra-cerebral tissues. These results suggest extended cooling by cardiopulmonary bypass (CPB) combined with surface cooling pads should reduce or even prevent the rise of brain temperatures during HCA.

Animals↗
PubMed1983

A finite element model of burn injury in blood-perfused skin.

The burn process resulting from the application of a hot, cylindrical source to the skin surface was modeled using the finite element technique. A rotationally symmetric 125-element mesh was defined within the tissue beneath and outlying to an applied heating disk. The disk temperature and duration of contact were varied, respectively, between 50 and 100 degrees C for up to 30 s. Natural convection with ambient air was assumed for areas of skin surface not in direct contact with the disk. The simulated thermal history was used in a damage integral model to calculate the extent and severity of injury in the radial and axial dimensions.

Animals↗
PubMed1983

A finite element model for phase change heat transfer in a composite tissue with blood perfusion.

This paper describes a finite element model of the thermal response of a human to extreme subfreezing environments and the transient temperature fields that are produced at specific locations within the body. This analysis includes the effects of latent heat of fusion from the liquid to the solid phase and of blood perfusion in selected tissues during the freezing process. A highly simplified physiological model is assumed. This type of analysis can be used as a predictive tool in many applications such as cryosurgery and cryopreservation of organs. The finite element model described herein has many advantages over standard numerical techniques such as finite difference methods for analyzing heat transfer problems. This method can treat very general geometries with many different tissue types, each having unique thermal properties, and unique blood perfusion and metabolic rates. This model can incorporate either convective thermal resistance at the body surface or a prescribed temperature history as a boundary condition, either of which may vary as a function of time or temperature. This analysis extends previous work by incorporating the effects of latent heat and by including a blood perfusion rate that varies with temperature in individual tissues.

Blood Physiological Phenomena↗
PubMedDate not supplied

Prediction of transient temperature fields and cumulative tissue destruction for radio frequency heating of a tumor.

A therapeutic hyperthermia protocol using a radio frequency (rf) electrode placed adjacent to a bronchial wall tumor has been modeled using the finite element technique. Variable physical properties and variable blood perfusion have been assigned to the tumor and to the surrounding normal lung tissue. The Laplace equation was solved on a curvilinear grid for a single rf source electrode to determine the steady-state electric field, which in turn governs the energy deposition function. The heat generation in the tumor and in the lung tissue is then calculated from the energy deposition profile, and the bioheat equation is solved on the same finite element mesh to determine the transient temperature history. The temperatures are displayed as isothermal contours at designated times during the protocol and as temperature histories at selected points. In addition, an Arrhenius-type injury model has been implemented to predict thermally induced damage, from which equal total amounts of energy are deposited into the tissue using a constant power density for an appropriate time or using a cyclic heating pattern. The cyclic heating pattern consisted of a series of equal duration time periods during which the rf current source is alternately turned on and off (50% duty cycle). This study illustrates how a finite element model could be used to evaluate alternative protocols for heating a tumor of a specific geometry and to evaluate thermally induced damage to surrounding normal tissue.

Humans↗