A Numerical Investigation of the Time Reversal Mirror Technique for Trans-skull Brain Cancer Ultrasound Surgery

Introduction: The medical applications of ultrasound on human brain are highly limited by the phase and amplitude aberrations induced by the heterogeneities of the skull. However, it has been shown that time reversing coupled with amplitude compensation can overcome these aberrations. In this work, a model for 2D simulation of the time reversal mirror technique is proposed to study the possibility of targeting any point within the brain without the need for craniotomy and to calculate the acoustic pressure field and the resulting temperature distribution within the skull and brain during a High Intensity Focused Ultrasound (HIFU) transcranial therapy. Materials and Methods: To overcome the sensitivity of the wave pattern to the heterogeneous geometry of the skull, a real MRI derived 2D model is constructed. The model should include the real geometry of brain and skull. The model should also include the couplant medium which has the responsibility of coupling the transducer to the skull for the penetration of ultrasound. The clinical substance used as the couplant is water. The acoustic and thermal parameters are derived from the references. Next, the wave propagation through the skull is computed based on the Helmholtz equation, with a 2D finite element analysis. The acoustic simulation is combined with a 2D thermal diffusion analysis based on Pennes Bioheat equation and the temperature elevation inside the skull and brain is computed. The numerical simulations were performed using the FEMLAB 3.2 software on a PC having 8 GB RAM and a 2.4 MHz dual CPU. Results: It is seen that the ultrasonic waves are exactly focalized at the location where the hydrophone has been previously implanted. There is no penetration into the sinuses and the waves are reflected from their surface because of the high discrepancy between the speed of sound in bone and air.  Under the focal pressure of 2.5 MPa and after 4 seconds of sonication the temperature at the focus reached 51 °C and the temperature of the pre-target bone increased to 56.31 °C. In the post-target region the temperature of the sphenoid bone increased to 47.1 °C while the temperature of the occipital bones reached up to 46 °C. It is also shown that by using a cold water cooling system and cooling down the pre-target bones to 20 °C before sonication, the maximum pre-target bone temperature will not exceed 40 °C and hence the pre-target bone cells will remain intact. Discussion and Conclusion: In this study, it is well demonstrated that by using the time reversal mirror technique it is possible to target any point within the skull without the need for craniotomy. Although at higher acoustic frequencies compared to the lower ones such as 300 kHz the ultrasound undergoes more severe aberrations while passing through media having geometrical heterogeneity and discrepant sound velocities, the simulations performed in this work show that even at such frequencies it is still possible to correct these aberrations using the time reversal mirror technique. The thermal simulations show that by using this method the temperature of the deep seated tumors can be increased to cytotoxic temperature in a few seconds

Recurring Multi-layer Moving Window Approach to Forecast Day-ahead and Week-ahead Load Demand Considering Weather Conditions

The incorporation of weather variables is crucial in developing an effective demand forecasting model because electricity demand is strongly influenced by weather conditions. The dependence of demand on weather conditions may change with time during a day. Therefore, the time stamped weather information is essential. In this paper, a multi-layer moving window approach is proposed to incorporate the significant weather variables, which are selected using Pearson and Spearman correlation techniques. The multi-layer moving window approach allows the layers to adjust their size to accommodate the weather variables based on their significance, which creates more flexibility and adaptability thereby improving the overall performance of the proposed approach. Furthermore, a recursive model is developed to forecast the demand in multi-step ahead. An electricity demand data for the state of New South Wales, Australia are acquired from the Australian Energy Market Operator and the associated results are reported in the paper. The results show that the proposed approach with dynamic incorporation of weather variables is promising for day-ahead and week-ahead load demand forecasting

Bacterial cellulose as a potential vascular graft: Mechanical characterization and constitutive model development.

Bacterial cellulose (BC) is a polysaccharide produced by Acetobacter Xylinum bacteria with interesting properties for arterial grafting and vascular tissue engineering including high-burst pressure, high-water content, high crystallinity, and an ultrafine highly pure fibrous structure similar to that of collagen. Given that compliance mismatch is one of the main factors contributing to the development of intimal hyperplasia in vascular replacement conduits, an in depth investigation of support mechanical properties of BC is required to further supporting its use in cardiovascular-grafting applications. The aim of this study was to mechanically characterize BC and also study its potential to accommodate vascular cells. To achieve these aims, inflation tests and uniaxial tensile tests were carried out on BC samples. In addition, dynamic compliance tests were conducted on BC tubes, and the results were compared to that of arteries, saphenous vein, expanded polytetrafluoroethylene, and Dacron grafts. BC tubes exhibited a compliance response similar to human saphenous vein with a mean compliance value of 4.27 × 10(-2) % per millimeter of mercury over the pressure range of 30-120 mmHg. In addition, bovine smooth muscle cells and endothelial cells were cultured on BC samples, and histology and fluorescent imaging analysis were carried out showing good adherence and biocompatibility. Finally, a method to predict the mechanical behavior of BC grafts in situ was established, whereby a constitutive model for BC was determined and used to model the BC tubes under inflation using finite element analysis

In-situ scanning electron microscopy study of fracture events during back-end-of-line microbeam bending tests

This paper demonstrates the direct observation of crack initiation, crack propagation, and interfacial delamination events during in-situ microbeam bending tests of FIB milled BEOL structures. The elastic modulus and the critical force of fracture of the BEOL beam samples were compared for beams of different length and width.status: publishe

Introducing the EUV CNT pellicle

EUV lithography insertion is anticipated at the 7 nm node and below; however, defects added to the mask during use is a lingering concern. Defectivity in the scanner is non-zero and an EUV pellicle membrane to protect the mask for high volume manufacturing power levels does not yet exist. The EUV photons are strongly absorbed by all materials. Sibased membranes leverage the low absorption coefficient k value (k = 0.0018 at 13.5 nm) for reasonable transmission, but poly Si becomes fragile and wrinkles during the high temperatures associated with exposure. An alternate approach to high transmission is deploying very thin or porous layers so that there are fewer atoms to absorb light. For example, carbon nanomaterials have a reasonably low k value (k = 0.0069), but are strong enough to be fabricated in very thin layers. Graphene, graphite, carbon-nanosheets and carbon nanotubes are all candidate carbon nanomaterials for this application, but we focus here on carbon nanotubes (CNTs). Our first measurements on CNT films of ~60 nm thick were found to have 96.5% transmission at 13.5 nm. Adding CNT layers also enhanced the strength of a thin SiN membrane significantly. In this paper, critical pellicle metrics will be evaluated in more detail: EUV transmission, bulge test for mechanical strength, emissivity measurements for heat management, and exposure testing in a hydrogen environment

Simulation of Balloon-Expandable Coronary Stent Apposition with Plastic Beam Elements

International audienceThe treatment of the coronary artery disease by balloon-expandable stent apposition is a fully endovascular procedure. As a consequence, limited imaging data is available to cardiologists, who could benefit from additional per-operative information. This study aims at providing a relevant prediction tool for stent apposition, in the form of a mechanically precise simulation, fast enough to be compatible with clinical routine. Our method consists in a finite element discretisation of the stent using 1D connected beam elements, with nonlinear plastic behaviour. The artery wall is modelled as a surface mesh interacting with the stent. As a proof of concept, the simulation is compared to micro-CT scans, which were acquired during the apposition of a stent in a silicone coronary phantom. Our results show that the simulation is able to accurately reproduce the stent final geometry, in a computational time greatly lower than for classic 3D finite element codes. Although this first validation step is preliminary, our work is to be extended towards more realistic scenarios, notably with the introduction of a personalised artery model and the corresponding in vivo validation