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Biomedical subjects

J McLellan

Publications and source records attributed to J McLellan.

At least 19 recordsLinked to original sources
PubMed2000

Astigmatism and the development of myopia in children.

While it is now established that astigmatism is more prevalent in infants and young children than in the adult population, little is known about the functional significance of this astigmatism, especially its role, if any, in emmetropization and the development of myopia. Manifest refractions (mean of 16 per subject) were obtained from 245 subjects starting in the first year, with 6-23 years of regular follow-up. Results showed that infantile astigmatism is associated with increased astigmatism and myopia during the school years. Two possible mechanisms underlying this association are discussed: (1) infantile astigmatism disrupts focusing mechanisms; and (2) ocular growth induces astigmatism and myopia.

Adolescent↗
PubMed1999

Role of genetic factors in the etiology of juvenile-onset myopia based on a longitudinal study of refractive error.

In an attempt to determine the role of genetic factors in the development of myopia, we examined the relationship of infantile refractive error and parental history to juvenile-onset myopia and analyzed 43 pedigrees affected by juvenile-onset myopia. Refraction data collected at regular intervals from a sample of juvenile subjects participating in a 24-year longitudinal study of refractive error were used. Results showed that children with two myopic parents were 6.42 times as likely to become myopic as children with one or no myopic parents. Furthermore, children who had refractions in the lower half of the distribution at 6 to 12 months of age were 4.33 times as likely to develop myopia as children who had refractions in the upper half of the distribution at 6 to 12 months of age. The pedigree analysis indicated that 63% of individuals considered at risk for developing juvenile-onset myopia actually became myopic, with an equal number of affected males and females. These results suggest that juvenile-onset myopia of moderate amounts may be inherited as a complex trait involving both genetic and environmental factors.

Adolescent↗
PubMed1994

Inclusion of energy straggling in a numerical method for electron dose calculation.

Energy straggling along electron trajectories has been incorporated into a numerical algorithm for electron beam dose calculations. Landau's theory is used to predict, at any point in the absorber, the broadening of the primary electron energy spectrum due to energy loss straggling. Numerical calculations have been performed for electron beams with energies of 10-30 MeV incident upon water in order to determine the variation of dose with depth and variation of energy spectra with pathlength. These calculations are compared with the results of Monte Carlo simulations performed with the EGS4 code. The inclusion of energy loss straggling in the numerical calculations leads to predictions of energy spectra and dose deposition that are in good agreement with modified Monte Carlo simulations in which bremsstrahlung is ignored and the energy given to knock-on electrons is deposited at the site of their creation. Less satisfactory agreement was achieved when these calculations were compared to full Monte Carlo simulations that included the bremsstrahlung events and transported the knock-on electrons. It is concluded that bremsstrahlung energy loss must also be included into this algorithm, if an acceptable dose computation accuracy is to be achieved for clinical applications.

Computer Simulation↗
PubMed1994

A method for the calculation of electron energy-straggling spectra.

To calculate electron beam dose distributions accurately, numerical methods of electron transport calculations must account for the statistical variation (or "straggling") in electron energy loss. This paper shows that the various energy straggling theories that are applicable to short path lengths all derive from a single statistical model, known as the compound Poisson process. This model in turn relies on three assumptions: (1) the number of energy-loss events in a given path length is Poisson distributed; (2) events are mutually independent; and (3) each event has the same probability distribution for energy loss (i.e., the same energy-loss cross section). Applying the principles of the compound Poisson process and using fast Fourier transforms, a new method for calculating energy-loss spectra is developed. The spectra calculated using this method for 10, 20, and 30 MeV electrons incident on graphite and aluminum absorbers agreed with Monte Carlo simulations (EGS4) within 1% in the spectral peak. Also, stopping powers derived from the calculated spectra agreed within 1.2%, with stopping powers tabulated by the International Commission on Radiation Units and Measurements. Several numerical transport methods "propagate" the electron distribution (in position, direction, and energy) over small discrete increments of path length. Thus the propagation of our calculated spectra over multiple path length increments is investigated. For a low atomic number absorber (graphite in this case), calculated spectra agreed with EGS4 Monte Carlo simulations over the full electron range, provided the path length increments were sufficiently small (less than 0.5 g/cm2). It is concluded from these results that numerical methods of electron transport should restrict the size of path length increments to less than 0.5 g/cm2 if energy straggling is to be modeled accurately.

Electron Transport↗
PubMed1993

Deep venous thrombosis caused by femoral exostosis.

OBJECTIVE: To present the first case of deep venous thrombosis caused by femoral exostosis reported in Australia. CLINICAL FEATURES: An 11-year-old prepubertal Caucasian girl had a two-year history of a posterior femoral exostosis. She then presented with a deep venous thrombosis 24 hours after riding a horse for the first time. The deep venous thrombosis was diagnosed by Doppler ultrasound, which showed an intimate relationship between the femoral exostosis and the femoral vein. Coagulation abnormalities were excluded. INTERVENTION AND OUTCOME: The patient was given anticoagulation therapy with heparin intravenously and warfarin orally. The popliteal vein recanalised within two days. The exostosis was excised 10 weeks after initial presentation, with warfarin being continued for four weeks postoperatively. Two months after excision the patient was symptom free. CONCLUSIONS: It is possible for venous compression by an exostosis to result in thrombosis in a patient with no underlying coagulation abnormality. Palpation and plain radiography of the region will demonstrate the exostosis; ultrasound is the next investigation of choice.

Anticoagulants↗
PubMed1992

A numerical method for electron transport calculations.

A numerical algorithm for calculating the penetration of electrons in dense media is presented. The numerical algorithm is intended for future application to radiotherapy dose calculations. The method is generic in the sense that it may be used with different theoretical models describing the angular scattering of electrons with depth. It is also general enough that it may be applied to electron dose calculations in heterogeneous as well as homogeneous media. The assumptions used in the algorithm are examined and equations describing the evolution of the distribution of electrons with depth are presented. Calculations have been performed for 10 MeV broad beams and pencil beams incident on water. It is shown that the Fermi-Eyges analytical solutions are recovered if the angular scattering process is assumed to be a Gaussian Markov process and the cumulative angle of electron travel remains small. In the case where the small angle approximation is not imposed, the numerical method qualitatively reproduces, at large depths, the wide angle scattering 'tails' seen in Monte Carlo generated profiles.

Algorithms↗
PubMed1987

Fatal head injuries in Cape Town.

Between 1 July 1983 and 30 June 1985 there were 563 cases of fatal head injuries in people over the age of 15 years recorded at the Salt River Police Mortuary, Cape Town. The demographic character of these cases was reviewed according to: (i) age, race and sex; (ii) date, day and time of injury; (iii) cause of death; and (iv) positive blood alcohol test. Assaults and transport-related accidents in association with a positive blood alcohol test in coloured males aged 40-60 years were a major characteristic.

Accidental Falls↗
PubMed1975

Film and xeroradiographic images in mammography. A comparison of tungsten and molybdenum anode materials.

Methods of comparing mammograms produced by radiation derived from tungsten and molydenum anodes are described. In film mammography better images were obtained using molybdenum radiation. In xeromammography images of equal quality were produced by either technique. However, the dose required for good xeromammography using a molybdenum source was significantly higher than for tungsten.

Female↗
PubMedDate not supplied

Fulminant hepatic failure with intractable ascites due to an echovirus 11 infection successfully managed with a peritoneo-venous (LeVeen) shunt.

A case is described of an infant who at 5 days of age developed an echovirus 11 infection with hepatic failure and intractable ascites, refractory to medical treatment. The ascites was successfully controlled with a peritoneo-venous (Le Veen) shunt inserted at 6 weeks of age. The shunt was subsequently removed when the infant was 4 months of age with no recurrence of ascites. At long-term follow-up he is growing and developing normally. This case shows that an adult Le Veen shunt can be successfully modified for use in the newborn with worthwhile long-term survival.

Ascites↗
PubMedDate not supplied

Heat production in microwave-irradiated thermocouples.

It has been known for some time that the irradiation of metallic thermometers by microwaves during clinical hyperthermia can lead to artifactual readings. We describe here a series of measurements in which this effect has been quantitatively studied. In particular, the data yield values for the conversion coefficient describing the rate of heat production per unit length of a thermocouple array per watt applied power which can be compared with the rate of heat production in the same volume of tissue. The degree of artifact in the temperature recording depends on the thermal resistance of the protective materials surrounding the array, and this thermal resistance has also been determined. It has been shown that measures taken to reduce the temperature artifact do not compromise the response time of the probe.

Humans↗
PubMedDate not supplied

A restricted angular scattering model for electron penetration in dense media.

A restricted angular scattering model for electron penetration in dense media is presented. In the model, the Fermi-Eyges transport equation is modified through the addition of an extra term which may be interpreted as representing an apparent force opposing the scattering of electrons into wider angles. The introduction of this extra term allows the modeling of the measured saturation in the mean square angular spread of electrons with depth. The restricted scattering model retains the Gaussian features of the Fermi-Eyges model and, therefore, may be readily incorporated into existing dose computation algorithms. Good agreement is obtained with measured angular electron distribution data for a point monodirectional beam over a wide range of incident electron energies (5-20 MeV) and scattering media (atomic numbers of 6 to 82). Also, a comparison of the restricted scattering model predictions with measurements of the lateral pencil beam spread shows an improvement over the predictions of Fermi-Eyges model close to the end of the electron range. Broad beam profiles were generated using both the Fermi-Eyges and restricted scattering models. A comparison of predicted and measured beam profiles shows that the restricted scattering model is a significant improvement over the Fermi-Eyges model for the prediction of beam penumbra shape in homogeneous media.

Aluminum↗
PubMedDate not supplied

Minimizing the self-heating artefacts due to the microwave irradiation of thermocouples.

The self-heating of metallic thermocouples in therapeutic microwave fields has long been recognized as a source of temperature artefacts in clinical hyperthermia dosimetry. We examine several techniques by which the probe and tissue heating artefacts resulting from self-heating may be quantitatively assessed, and discuss these in the context of their applicability to clinical hyperthermia.

Hyperthermia, Induced↗