Effect of complex bolus-tissue load configurations on SAR distributions from dual concentric conductor applicators. Specific absorption rate

Power deposition [specific absorption rate (SAR)] distributions from a two-element array configuration of 4-cm-square 915-MHz dual concentric conductor (DCC) microwave antennas were characterized theoretically for several clinically realistic complex bolus-tissue load models using the finite difference time domain (FDTD) numerical method. The purpose of this effort was to determine the perturbing effects on SAR of three often unavoidable heterogeneities in the bolus-tissue load. The three cases studied in this work consist of bone (two ribs spaced 1 cm apart) embedded 5-mm or 1-cm deep in muscle or layered fat-muscle tissue, small air bubbles trapped between the coupling bolus and tissue surface, and variable thickness water bolus layer due to sharply contoured anatomy. Results of the FDTD simulations demonstrate rather small effects on SAR distribution for both rib-sized bones > or = 5-mm deep in muscle and small air pockets < or = 1-mm thick. Larger air bubbles > 1-cm diameter by 3-mm depth showed a distinct concentration of SAR near the lateral sides of the air bubbles, and a blocking effect under the bubbles when located directly under the center of a DCC aperture where there is a higher normal E-field component. Variation from 2.5- to 7.5-mm bolus thickness under the two aperture array produced only minor perturbation of the uniformity and penetration of SAR, along with minor reduction in SAR under the thicker bolus which should be accommodated sufficiently by changes in applied power to the array elements.     

Heteromolecular condensation theory applied to particle growth

The growth of aqueous sulfuric acid droplets originating from a heteromolecular nucleation process is calculated for particle sizes up to 1000 Å. Two cases are discussed: (1) there exists a given initial concentration of a condensable trace gas, and (2) there exists a constant production rate of a condensable trace gas. The particle growth and the final particle size are calculated as functions of relative humidity from 10 to 99 per cent. Our previous studies on heteromolecular nucleation (Kiang et al., 1973) as applied to gas to particle conversion are extended to the particle growth process and found to agree roughly with experiments by Cox (1973).     

Use of percolation clusters in nucleation theory

The capillarity approximation of classical nucleation theory assumes that the concept of surface area and surface tension can be used even for very small droplets (clusters); this surface energy enters the average number of clusters of a given size and thus the nucleation rate. Numerical evidence from these average cluster numbers in a simple model (percolation theory) suggests that this concept of a well-defined surface contribution can be extrapolated down to very small clusters and even single molecules, provided one uses an effective surface tension. We also estimate numerically the pre-exponential factor in the cluster size distribution and find it to vary as (cluster size)^−θ with 1 θ 2 in two dimensions.     

Analysis of pin characteristics for a variable geometry mold

The sheet metal industry has considered the use of a variable geometry mold for years. Its application to the vacuum forming of plastic sheets, however, is rather new. One significant problem is the dimpling effect resulting from the discrete nature of the mold surface. The purpose of this research is to determine the most appropriate pin type and configuration for a viable variable geometry mold, considering the dimpling effect and other quality issues. Through analyzing the consequences of several pin types and configurations, we conclude that a flat top, square pin in a closed-pack configuration is the most appropriate option.     

An audit of diagnostic reference levels in interventional cardiology and radiology: are there differences between academic and non-academic centres?

A wide variation in patient exposure has been observed in interventional radiology and cardiology. The purpose of this study was to investigate the patient dose from fluoroscopy-guided procedures performed in non-academic centres when compared with academic centres. Four procedures (coronary angiography, percutaneous coronary intervention, angiography of the lower limbs and percutaneous transluminal angioplasty of the lower limbs) were evaluated. Data on the dose-area product, fluoroscopy time and number of images for 1000 procedures were obtained from 23 non-academic centres and compared with data from 5 academic centres. No differences were found for cardiology procedures performed in non-academic centres versus academic ones. However, significantly lower doses were delivered to patients for procedures of the lower limbs when they were performed in non-academic centres. This may be due to more complex procedures performed in the academic centres. Comparison between the centres showed a great variation in the patient dose for these lower limb procedures.     

Compaction and size segregation in a liquid-saturated grain packing due to pulsation effect during air injection

Injecting air into two-dimensional vertical liquid-saturated assemblies of grains causes a rearrangement of the grains. The interaction of the air flow injected at the bottom with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Furthermore, the grain packing is compacted due to a rearrangement process caused by the pulsating movement of air channels. The compaction process is found to obey a slow exponential growth law. Additionally, the pulsation introduces size segregation in the packing. This is visualized by a set of tracing experiments showing that the coarser grains tend to accumulate at the downstream end of the preferential air pathway. However, the pulsating strength decreases sharply as the grain size increases. Therefore no segregation was observed in the coarse packing. A stabilization process of the preferential air channel can be described by a lower bound of critical channel size assuming the validity of Hagen–Poiseuille air flow inside the channel. Nevertheless, the channel size could not exceed an upper size which is determined by the capillary instability, assuming a quasi-static equilibrium of the dilating process of the air channel.     

Radiation patterns of dual concentric conductor microstrip antennas for superficial hyperthermia

The finite difference time domain (FDTD) method has been used to calculate electromagnetic radiation patterns from 915-MHz dual concentric conductor (DCC) microwave antennas that are constructed from thin and flexible printed circuit board (PCB) materials. Radiated field distributions are calculated in homogeneous lossy muscle tissue loads located under variable thickness coupling bolus layers. This effort extends the results of previous investigations to consider more realistic applicator configurations with smaller 2-cm-square apertures and different coupling bolus materials and thicknesses, as well as various spacings of multiple-element arrays. Results are given for practical applicator designs with microstrip feedlines etched on the backside of the PCB antenna array instead of previously tested bulky coaxial-cable feedline connections to each radiating aperture. The results demonstrate that for an optimum coupling bolus thickness of 2.5-5 mm, the thin, flexible, and lightweight DCC antennas produce effective heating to the periphery of each aperture to a depth of approximately 1 cm, and may be combined into arrays for uniform heating of large area superficial tissue regions with the 50% power deposition contour conforming closely to the outer perimeter of the array.