Modelling Millimetre Wave Propagation and Absorption in a High Resolution Skin Model: The Effect of Sweat Glands

The aim of this work was to investigate the potential effect of sweat gland ducts (SGD) on specific absorption rate (SAR) and temperature distributions during mm-wave irradiation. High resolution electromagnetic and bio-heat transfer models of human skin with SGD were developed using a commercially available simulation software package (SEMCAD X™). The skin model consisted of a 30 μm stratum corneum, 350 μm epidermis and papillary dermis (EPD) and 1000 μm dermis. Five SGD of 60 μm radius and 300 μm height were embedded linearly with 370 μm separation. A WR-10 waveguide positioned 20 μm from the skin surface and delivering 94 GHz electromagnetic radiation was included in the model. Saline conductivity was assigned inside SGD. SAR and temperatures were computed with and without SGD. Despite their small scale, SAR was significantly higher within SGD than in the EPD without SGD. Without SGD, SAR and temperature maxima were in the dermis near EPD. With SGD, SAR maximum was inside SGD while temperature maximum moved to the EPD/stratum-corneum junction. Since the EPD participates actively in perception, the effect of SGD should be taken into account in nociceptive studies involving mm-waves. This research represents a significant step towards higher spatial resolution numerical modelling of the skin and shows that microstructures can play a significant role in mm-wave absorption and induced temperature distributions

The indentation response of GaAs-AlAs heterostructures

Low-load indentation has been used to investigate the deformation behaviour of a submicron layer of AlAs on a GaAs substrate. High-resolution scanning electron microscopy of cross-sections through the deformed regions under indentations into this structure reveals that the softer AlAs layer is not penetrated by the indenter and that unusual lateral cracking originates during the unloading phase. Data of intendation load against depth demonstrates the effect of the layer through the increase in composite hardness and elastic modulus with increasing load

Photodynamic therapy for glioblastoma: A preliminary approach for practical application of light propagation models

International audiencePurpose Photodynamic therapy (PDT) is a promising treatment modality to be added in the management of glioblastoma multiforme (GBM). Light distribution modeling is required for planning and optimizing PDT. Several models have been developed to predict the light propagation inside biological tissues. In the present study, two analytical methods of light propagation emitted from a cylindrical fiber source were evaluated: a discrete and a continuous method. Methods The two analytical approaches were compared according to their fluence rate results. Several cylindrical diffuse lengths were evaluated, and the relative deviation in the fluence rates was estimated. Moreover, a sensitivity analysis was conducted to compute the variance of each analytical model. Results The discrete method provided fluence rate estimations closer to the Monte-Carlo simulations than the continuous method. The sensitivity study results did not reveal significant differences between the variance of the two analytical models. Conclusions Although the discrete model provides relevant light distribution, the heterogeneity of GBM tissues was not considered. With the improvement in parallel computing that drastically decreased the computing time, replacing the analytical model by a Monte-Carlo GPU-accelerated code appeared relevant to the GBM case. Nonetheless, the analytical modeling may still function in the optimization algorithms, which might be used in the Photodynamic treatment planning system

Doppler signals observed during high temperature thermal ablation are the result of boiling

Purpose—To elucidate the causation mechanism of Spectral Doppler ultrasound signals (DUS) observed during high temperature thermal ablation and evaluate their potential for imageguidance. Methods—Sixteen ex vivo ablations were performed in fresh turkey breast muscle, eight with radiofrequency ablation (RFA) devices, and eight with a conductive interstitial thermal therapy (CITT) device. Temperature changes in the ablation zone were measured with thermocouples located at 1 to 10mm away from the ablation probes. Concomitantly, DUS were recorded using a standard diagnostic ultrasound scanner. Retrospectively, sustained observations of DUS were correlated with measured temperatures. Sustained DUS was arbitrarily defined as the Doppler signals lasting more than 10 s as observed in the diagnostic ultrasound videos captured from the scanner. Results—For RFA experiments, minimum average temperature (T1±SD) at which sustained DUS were observed was 97.2±7.3°C, while the maximum average temperature (T2±SD) at which DUS were not seen was 74.3±9.1°C. For CITT ablation, T1 and T2 were 95.7±5.9°C and 91.6±7.2°C, respectively. It was also observed, especially during CITT ablation, that temperatures remained relatively constant during Doppler activity. Conclusions—The value of T1 was near the standard boiling point of water (99.61°C) while T2 was below it. Together, T1 and T2 support the conclusion that DUS during high temperature thermal ablation are the result of boiling (phase change). This conclusion is also supported by the nearly constant temperature histories maintained at locations from which DUS emanated