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Biomedical subjects

A L McKenzie

Publications and source records attributed to A L McKenzie.

At least 19 recordsLinked to original sources
PubMed2006

Scatter from radiotherapy beams emerging from primary barriers: an aid to bunker design.

Charts to assist in linear accelerator bunker design have been produced using the Monte Carlo framework, Geant4. These charts assess the amount of forward scatter produced at different angles to the beam axis by concrete and steel barriers irradiated by 6 MV and 10 MV photon beams at normal incidence. These new charts complement existing charts of broad-beam transmission through walls. This is because the existing charts give no indication of the amount of scatter emerging at large angles from the beam axis, for example, towards the maze entrance.

Equipment Design↗
PubMed2004

Assessment of effective dose from concomitant exposures required in verification of the target volume in radiotherapy.

The requirement of the Ionising Radiation (Medical Exposure) Regulation 2000 [IR(ME)R] of justifying all exposures to ionizing radiation includes those from radiotherapy double exposure portal images resulting in exposure to normal tissues outside the treatment volume. Typical effective doses were calculated for a range of common sites using CT data to outline those parts of specific organs subject to concomitant radiation and generate dose-volume histograms. The product of the mean dose and the relative probability of inducing a fatal cancer in specific organs was used to determine a representative total effective dose in mSv per monitor unit for each site. A table of representative effective doses, ranging from 0.32 mSv to 2.56 mSv per monitor unit, was produced, which may be used to monitor cumulative effective doses of individual patients from double exposure portal images, in addition to those received from localization procedures.

Female↗
PubMed2003

Somatosensory representation of the digits and clinical performance in patients with focal hand dystonia.

OBJECTIVE: The purpose of this study was to incorporate magnetoencephalography and clinical testing to describe differences in somatosensory organization and sensorimotor function of the hand in patients with focal hand dystonia, a target-specific disorder of voluntary movement that interferes with fine motor control during the performance of rapid, repetitive, skilled movements. DESIGN: This descriptive study included prospective, quasi-experimental comparisons between groups. RESULTS: Patients with focal hand dystonia demonstrated deficits in physical variables, sensory processing, and motor control when compared with age- and sex-matched controls. They also had altered patterns of firing (amplitude and latency integrated over time) and abnormal somatosensory representations on magnetoencephalography. CONCLUSIONS: These study findings suggest that there are alterations in both somatosensory representation of the digits and clinical performance in patients with focal hand dystonia. Future studies to determine if alterations in the sensorimotor feedback loop contribute to the development of focal hand dystonia are indicated. If so, intervention strategies may need to include specific types of somatosensory retraining as part of the rehabilitation program for patients with focal hand dystonia.

Adult↗
PubMed2000

The width of margins in radiotherapy treatment plans.

Publication of ICRU Reports 50 and 62 has highlighted the need to devise protocols for the process of drawing the planning target volume (PTV) around the clinical target volume (CTV). The margin surrounding the CTV should be wide enough to account for all geometric errors so that no part of the CTV accumulates a dose less than, for instance, 95% of that prescribed. One approach to the problem has been to draw a margin around the CTV delineated at the treatment preparation stage which is sufficiently wide that the mean position of the CTV will be encompassed in a specific percentage of cases, for example 90%. This accounts for the systematic errors. A further margin is then drawn to account for random set-up and organ-motion uncertainties during treatment. The width of this second margin has previously been shown to be 1.64(sigma - sigmap). Here sigma, a vector quantity, is the standard deviation which results from convolving the penumbra spread function of standard deviation sigmap with the Gaussian distributions of the daily positional uncertainties of organ motion and set-up error. However, it is shown in this paper that the calculation should take into account the beam configuration of the treatment plan. In a typical coplanar multibeam plan, usually in the transverse plane, any given edge of the target volume is normally defined by a single beam or two parallel and opposed beams. However, because of the presence of the other beams, the effect of the blurring of the edge-defining beam(s) is reduced, which changes the value of the required margin to beta (sigma - sigmap) where, for example, beta can be as low as 1.04 in the transverse plane of a three-beam plan. The width of the required margins is calculated for up to six beams and presented in a table. It is shown that, while the table was derived using an idealized plan of equally weighted plane beams irradiating a spherical target, it is also valid for non-uniform beam weightings, wedged-beam plans, target volumes of general shape and intensity-modulated radiotherapy (IMRT).

Humans↗
PubMed2000

How should breathing motion be combined with other errors when drawing margins around clinical target volumes?

Geometric uncertainties in radiotherapy treatment may be accommodated by drawing an adequate margin around the clinical target volume (CTV). The width of the margin is commonly based upon the quadrature sum of the standard deviations of the contributory errors. This approach is satisfactory when the probability distributions of the component errors are Gaussian, but breathing-induced motion of the CTV is generally not Gaussian. This paper shows that the blurred dose distribution D(z) arising from a step function beam edge that is moving cyclically along the z-axis is given by D(z) = 2t/tau, where t(z) relates time to position and tau is the cycle duration. Applying this relation to realistic breathing models results in a dose distribution in which the sharp edge of the step function is still evident even after blurring caused by the motion. This suggests that breathing-induced motion should be accounted for separately, with the breathing margin added linearly to the quadrature sum of the other contributing errors.

Humans↗
PubMed1998

A simple method for matching electron beams in radiotherapy.

Many solutions have been proposed to solve the problem of 'hot' and 'cold' spots in the junction between abutting electron fields. Although some of these methods have proved satisfactory, the designs of the modifications to the applicators are generally applicator dependent and involve measuring data for individual fields. An idea which was originally proposed as a solution to a different problem is resurrected here because it happens to solve the beam-matching problem and because it is very simple to apply. The idea is to cover the end of the electron applicator completely with a slab of plastic, called a 'spoiler'. Formulae are proposed and tested for estimating the penumbra broadening and the extent of over- or underdosing at field junctions. The technique is applicator independent, and no extra beam-data measurements need be made for computer treatment planning of matching fields. The price paid for such simplicity is that consideration needs to be given to leading-off the other borders of the treatment fields because of the increased penumbra widths.

Data Interpretation, Statistical↗
PubMed1997

What is the explanation for the changes to cobalt-60 tissue-air ratios in BJR Supplement 25?

Values of tissue-air ratio (TAR) in the recent British Journal of Radiology (BJR) Supplement 25 have been increased by nearly 2% over the values which have been accepted for the past 30 years. The need for this was shown by analysis of previous data using scaling laws, together with Monte Carlo calculations and careful re-measurement. However, it was not clear why previous determinations of TAR were in error: it was not, as some workers argued, because scattered radiation had been included in the absorbed dose in the miniphantom, because TAR data in BJR Supplement 17 had been derived from peak scatter factor (PSF), which is not based on the miniphantom concept. The purpose of this paper is to find the real explanation of why the PSF and, therefore, TAR were underestimated for so long. Two definitions of PSF are considered: one based on kerma and one based on dose. This paper relates PSF of either definition to measurements of air kerma by including in the derivation the scatter in the plug which replaces the chamber when it has been removed from the surface of the water phantom. The kerma-based PSF is found to be 2% higher than the simple ratio of chamber readings in phantom and in air. The value of the dose-based definition agrees with that of the kerma-based definition to within 0.2%. It is the scatter in the replacement plug in the surface of the water phantom which was effectively ignored by previous workers, and which explains the underestimates of around 2% in PSF and TAR. The value of the dose-based PSF differs slightly from that of the kerma-based PSF because of the different distributions of primary and scatter photon fluence.

Air↗
PubMed1996

How can the co-ordinate transformation method of beam matching be extended to include separately labelled collimators?

The problem of matching radiation beams was tackled by Siddon in 1980 using co-ordinate transformations. Since then, the need to distinguish between individual collimators in prescriptions of treatment set-up, brought about by the widespread use of 3-D treatment planning systems and asymmetric fields, as well as a reversal of the rotation sense in the turntable co-ordinate system proposed by the International Electrotechnical Commission, have made it necessary to revisit this particular problem. This paper builds upon Siddon's general equations for the particular case of matching beams, and derives expressions for calculating treatment-unit settings which may be used in a computer program without the need to perform matrix manipulation. The expression treat the individual collimator jaws separately.

Humans↗
PubMed1995

Intrathecal administration of endothelin-1 in the rat: impact on spinal cord blood flow and the blood-spinal cord barrier.

The purpose of this study was to examine the integrity of the blood-spinal cord barrier after intrathecal administration of endothelin-1 (ET-1) in the rat spinal cord. A laser Doppler probe was positioned in the exposed subarachnoid space at the T8 vertebral level. In the first experiment, horseradish peroxidase (HRP), a marker of barrier integrity, was given intravenously prior to the intrathecal application of ET-1. Blood flow was then recorded for 3 h, after which the anesthetized animals were euthanized. In the second experiment, animals exposed to endothelin were recovered after confirming a 50% reduction in blood flow. HRP was given 10 min prior to euthanasia at 24 h post infusion. The intensity of staining for HRP was quantified by optical density in fixed sections of spinal cord. There was a significant sustained reduction in spinal cord blood flow and significant barrier breakdown to HRP at both 3 and 24 h after administration of the peptide. Based upon these results we conclude that intrathecal infusion of ET-1 reduces spinal cord blood flow and results in prolonged breakdown of the blood-spinal cord barrier.

Animals↗
PubMed1995

Twenty-five years of MTPR--a theoretical and experimental analysis.

The system of modified tissue phantom ratio (called tissue standard factor, or TSF, in this paper) was introduced to facilitate the calculation of monitor units in isocentric configurations. Twenty-five years later, TSFS have been measured at 6 MV on different linear accelerators, and are in agreement with the original values, highlighting the property of machine independence. The utility of TSF is shown by the simplicity of the equation for calculating dose per monitor unit at any depth, field size and distance from source. New equations are given for relating isocentric systems to fixed-SSD systems, and the table of TSFS is extended to include a symbolic zero-area column, which is shown to be the primary-dose component of TSF in electronic equilibrium.

Humans↗
PubMed1995

Immunolocalization of endothelin in the traumatized spinal cord: relationship to blood-spinal cord barrier breakdown.

The purpose of this study was to determine the relationship between endothelin-1 (ET-1), a prominent vasoactive agent, and the breakdown of the blood-spinal cord barrier along the axis of the cord after a moderate spinal cord injury. In the first study rats (n = 10) were euthanized 24 h after spinal cord injury and compared to sham (n = 5) and unoperated (n = 10) controls. Endothelin and immunoglobulins (IgG) were immunolocalized in adjacent sections of spinal cord using semiquantitative immunocytochemical techniques. In the second study animals were pretreated with the endothelin antagonist Bosentan (n = 6) or vehicle (n = 6) prior to spinal cord injury. Animals were euthanized at 24 h postinjury. Ten minutes prior to euthanasia animals were given horseradish peroxidase (HRP) intravenously. After perfusion fixation sections of cord were prepared for quantitative HRP histochemistry. After spinal cord injury there was enhanced staining for endothelin along the axis of the cord that correlated with the anatomical pattern of barrier breakdown to IgG. In those animals that were pretreated with Bosentan, there was a significant reduction in barrier breakdown along the axis of the injured cord as compared to those animals that received vehicle only. Taken together, this data implicate involvement of endothelin in the axial pattern of barrier breakdown after spinal cord injury.

Animals↗
PubMed1994

Pulsed microamperage stimulation: a controlled study of healing of surgically induced wounds in Yucatan pigs.

BACKGROUND AND PURPOSE: Microamperage direct current and microamperage electromagnetic stimulation are used to accelerate healing in bone. Although many clinicians are using microamperage stimulation to relieve pain and facilitate wound healing, there is significant question regarding whether this low-intensity direct current significantly accelerates soft tissue wound repair. The purpose of this study was to determine whether low-voltage pulsed microamperage current (100 microA, 60 V, 0.1 Hz) enhances the healing of superficial, full-thickness, or incisional wounds created to simulate acute abrasions, ulcers, and lacerations. SUBJECTS: Ten adult Yucatan mini pigs served as the subjects for this study. METHODS: Variables associated with healing were measured in 60 matched pairs of surgically induced partial-thickness, full-thickness, and incisional wounds after either sham or anodal (positive-polarity) stimulation with pulsed microamperage current (100 microA) was administered for 1 hour per day for 5 days. Sterile, disposable electrodes (2 x 4 cm) were placed over each wound, which was kept wrapped and protected throughout the study. RESULTS: At 7 days postinjury, all wounds were healing well with no signs of infection. There were no differences in tensile strength, collagen density, maturity, or deposition (hydroxyproline), wound size, or visual appearance between the sham treatment and treatment lesions. No changes in local subcutaneous oxygen or temperature were found in the swine during or after microamperage stimulation. CONCLUSION AND DISCUSSION: This study did not provide any evidence to support the use of microamperage stimulation to accelerate wound healing. No negative effects, however, were found. Further research is needed to determine whether there is a critical interaction between the size of the electrode relative to the wound, the density of the current, the duration of the treatment, the polarity of the treatment electrode, and the acuity or chronicity of wounding and the effectiveness of microcurrent stimulation for wound healing.

Animals↗
PubMed1992

Low-dose ultrasound effects on wound healing: a controlled study with Yucatan pigs.

This study reports on the effect of low-dose ultrasound in accelerating wound healing in matched pairs of surgically induced incisions and full-thickness and partial-thickness lesions in 11 Yucatan mini-pigs after 7 days of healing and 5 days of sonation. Tensile strength, collagen deposition (hydroxyproline), reduction in wound size (full-thickness lesion), and mast-cell degranulation were significantly greater in the sonated lesions than in the sham-treated controls (p = 0.01). Sonation enhanced strength by 24% and collagen deposition by 29%, when compared with controls. There were no significant differences in the quality of healing as measured by an ordinal scale. The results suggest that within the first week of healing, low-dose ultrasound facilitates wound healing. More research is needed to confirm the most effective dose, frequency, and treatment duration and intervention time for maximum healing.

Animals↗
PubMed1991

Intercomparison of whole-body counters using a multinuclide calibration phantom.

Whole-body counters in the UK have been compared using a multinuclide anthropomorphic phantom. A standard Bush phantom was modified by inserting channels into the long axis of each section. Radionuclide sources sealed in a urea-formaldehyde polymer were then inserted into the channels to simulate distributions of radioactivity in a human. The phantom was taken to 10 whole-body counters in the UK and estimates of 134Cs, 137Cs and 40K were obtained both separately and as mixtures. Results showed close agreement between the median estimates and the known activities. The technique also allowed diagnosis of particular problems in calibration for several of the counters.

Calibration↗