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Temperature inhomogeneity in hyperthermia treatments often limits the total thermal dose that can be delivered to the tumour region. To reduce such inhomogeneities, a prototype dynamically modifiable square array of saline-filled patches which attenuate microwave energy was developed for superficial treatments that use external microwave applicators. The array was situated inside the coupling water bolus that is often used with external applicators. The prototype has been previously tested clinically with promising results. A more complete theoretical analysis of the performance of this new bolus design and improvements to its design by modelling are presented here.

The analysis was performed by performing five iterative simulations of the SAR pattern produced inside a tissue structure by a waveguide applicator with a water bolus containing the dynamic patch array attached. Between iterations the patch array configuration was modified in an attempt to improve the ability of the bolus to confine heating to an 'L'-shaped tumour region. These simulations were performed using the finite element method. The steady-state temperature profile was then computed using a finite element method based simulation of heat transfer that assumed a given applicator power level and water bolus temperature. Several iterations of these heat transfer simulations were performed with varying applicator power level and water bolus temperature to improve the confinement of heating to the target region.

The analysis showed that the dynamic patch array should be capable of conforming heating to an 'L'-shaped target tumour region while limiting the heating to the surrounding normal tissue to an acceptable level.

Cartilage laser thermoforming (CLT) is a new surgical procedure that allows in situ treatment of deformities in the head and neck with less morbidity than traditional approaches. While some animal and human studies have shown promising results, the clinical feasibility of CLT depends on preservation of chondrocyte viability, which has not been extensively studied. The present paper characterizes cellular damage due to heat in rabbit nasal cartilage. Damage was modelled as a first order rate process for which two experimentally derived coefficients, A = 1.2 × 10⁷⁰ s⁻¹ and E_a = 4.5 × 10⁵ J mole⁻¹, were determined by quantifying the decrease in concentration of healthy chondrocytes in tissue samples as a function of exposure time to constant-temperature water baths. After immersion, chondrocytes were enzymatically isolated from the matrix and stained with a two-component fluorescent dye. The dye binds nuclear DNA differentially depending upon chondrocyte viability. A flow cytometer was used to detect differential cell fluorescence to determine the percentage of live and dead cells in each sample. As a result, a damage kinetic model was obtained that can be used to predict the onset, extent and severity of cellular injury to thermal exposure.

A temperature feedback controller routine using a physical model for temperature evolution was developed for use with focused ultrasound surgery. The algorithm for the controller was a multi-input, single-output linear quadratic regulator (LQR) derived from Pennes' bioheat transfer equation. The controller was tested with simulated temperature data that had the same characteristics as those obtained with magnetic resonance imaging (MRI). The output of the controller was the appropriate power level to be used by the transducer. Tissue parameters estimated prior to the simulated treatments were used to determine the controller parameters. The controller performance was simulated in three dimensions with varying system parameters, and sufficient temperature tracking was achieved. The worst-case overshoot was 7 °C and the steady-state error was 5 °C. The simulated behaviour of the controller suggests satisfactory performance and that the controller may be useful in controlling the power output during MRI-monitored ultrasound surgery.

Four-dimensional (4D) methods strive to achieve highly conformal radiotherapy, particularly for lung and breast tumours, in the presence of respiratory-induced motion of tumours and normal tissues. Four-dimensional radiotherapy accounts for respiratory motion during imaging, planning and radiation delivery, and requires a 4D CT image in which the internal anatomy motion as a function of the respiratory cycle can be quantified. The aims of our research were (a) to develop a method to acquire 4D CT images from a spiral CT scan using an external respiratory signal and (b) to examine the potential utility of 4D CT imaging. A commercially available respiratory motion monitoring system provided an 'external' tracking signal of the patient's breathing. Simultaneous recording of a TTL 'X-Ray ON' signal from the CT scanner indicated the start time of CT image acquisition, thus facilitating time stamping of all subsequent images. An over-sampled spiral CT scan was acquired using a pitch of 0.5 and scanner rotation time of 1.5 s. Each image from such a scan was sorted into an image bin that corresponded with the phase of the respiratory cycle in which the image was acquired. The complete set of such image bins accumulated over a respiratory cycle constitutes a 4D CT dataset. Four-dimensional CT datasets of a mechanical oscillator phantom and a patient undergoing lung radiotherapy were acquired. Motion artefacts were significantly reduced in the images in the 4D CT dataset compared to the three-dimensional (3D) images, for which respiratory motion was not accounted. Accounting for respiratory motion using 4D CT imaging is feasible and yields images with less distortion than 3D images. 4D images also contain respiratory motion information not available in a 3D CT image.

We report new effective linear attenuation coefficients (μ_eff) for refined gypsum for Co-60 and 18 MV x-rays. 6 MV and 10 MV μ_eff agree with published data. For a 100 cm² field size (FS), tissue defect x = 1.35 cm, target depth d = 1.65 cm beneath the tissue defect x, μ_eff is 0.123 cm⁻¹ (Co-60) and 0.0934 cm⁻¹ (6 MV). For 100 cm², x = 1.35 cm, and d = 5.65 cm beneath x, μ_eff is 0.072 cm⁻¹ (10 MV), and 0.0614 cm⁻¹ (18 MV). For d, μ_eff decreases about 10% from 25 to 400 cm². For a given FS, μ_eff decreases with d by 3%–5% for Co-60, and 3% for 6 MV, 10 MV and 18 MV, but depends on (d − x). For d, when x is large (8 cm), depending on energy and FS, μ_eff is 2%–4% less than when x is small (2 cm). These data were used in a treatment-planning computer to design compensator filters for a step phantom. Compensation was to within 10% in the compensation plane (CP). Above and below CP, computer-calculated ratios of doses with and without filters were 0.75–1.13. Chamber dose ratios with and without filters were 0.75–1.12.

We developed a transport-equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. The deterministic algorithm has been based on the integral transport equation. The algorithm provided us with the capability of computing dose distributions for multiple isotropic point and/or volumetric sources in a homogenous/heterogeneous medium. The algorithm results have been benchmarked against the results from the literature and MCNP results for isotropic point sources and volumetric sources.

Re-entry is an important mechanism of cardiac arrhythmias. During re-entry a wave of electrical activation repeatedly propagates into recovered tissue, rotating around a rod-like filament. Breakdown of a single re-entrant wave into multiple waves is believed to underlie the transition from ventricular tachycardia to ventricular fibrillation. Several mechanisms of breakup have been identified including the effect of anisotropic conduction in the ventricular wall. Cells in the inner and outer layers of the ventricular wall have different action potential durations (APD), and support re-entrant waves with different periods. The aim of this study was to use a computational approach to study twisting and breakdown in a transmural re-entrant wave spanning these regions, and examine the relative role of this effect and anisotropic conduction. We used a simplified model of action potential conduction in the ventricular wall that we modified so that it supported stable re-entry in an anisotropic model with uniform APD. We first examined the effect of regional differences on breakdown in an isotropic model with transmural differences in APD, and found that twisting of the re-entrant filament resulted in buckling and breakdown during the second cycle of re-entry. We found that breakdown was amplified in the anisotropic model, resulting in complex activation in the region of longest APD. This study shows that regional differences in cardiac electrophysiology are a potentially important mechanism for destabilizing re-entry and may act synergistically with other mechanisms to mediate the transition from ventricular tachycardia to ventricular fibrillation.

A novel electromagnetic method of obtaining total body water is proposed, in which the water content is obtained from the dielectric properties as measured by a resonant perturbation technique. A screened room acts as a radio-frequency cavity, in our case resonating at 59 MHz, a frequency at which both real and imaginary parts of the complex permittivity of tissues are correlated to their moisture content. The presence of a human subject in the room leads to both a negative shift in the room's resonant frequency and a reduction in its Q-factor. We simulated the room and the body using the transmission line matrix (TLM) method, a computational electromagnetic code which models the problem in the time domain. Experiment and numerical model showed good agreement for two orientations of the subject. The sensitivity of the technique was investigated by measuring the response before and after the subject drank a small quantity of water, less than 2% of body mass. The resulting change in the resonant frequency was significant, and was also predicted by the numerical model. The proposed technique for studying body composition is simple, non-invasive and employs non-ionizing radio waves at low power.

Mechanical stresses in arterial walls are known to be implicated in the development of atherosclerosis. While shear stress and circumferential stress have received a lot of attention, axial stress has not. Yet, stenoses can be intuitively expected to produce a supplementary axial stress during flow systole in the region immediately proximal to the constriction cone. In this paper, a model for the estimation of this effect is presented, and ten numerical examples are computed. These examples show that the cyclic increase in axial stress can be quite considerable in severe stenoses (typically 120% or more of the normal stress value). This result is in best agreement with the known mechanical or morphological risk factors of stenosis progression and restenosis (hypertension, elevated pulse pressure, degree of stenosis, stenosis geometry, residual stenosis, etc). The supplementary axial stress generated by a stenosis might create the damages in the endothelium and in the elastic membranes which potentiate the action of the other risk factors (hyperlipidaemia, diabetes, etc). It could thus be an important cause of stenosis progression and of restenosis.

Cryogen spray cooling (CSC) is used to protect the epidermis during dermatologic laser surgery. To date, the relative influence of the fundamental spray parameters on surface cooling remains incompletely understood. This study explores the effects of mass flow rate and average droplet velocity on the surface heat flux during CSC. It is shown that the effect of mass flow rate on the surface heat flux is much more important compared to that of droplet velocity. However, for fully atomized sprays with small flow rates, droplet velocity can make a substantial difference in the surface heat flux.

This article is in full in the PDF file below, with a reply by van der Laarse.

This article is in full in the PDF file below (following the letter by MacDougall).