Applications of high-intensity focused ultrasound in medicine: Spotlight on neurological applications

High-intensity focused ultrasound (HIFU) has the potential to become a modality of treatment for a wide range of clinical conditions. HIFU enables non-invasive, selective ablation of tissues including tumors and punctured vessels. Another promising area of research within the field of therapeutic ultrasound is the application of HIFU to treat neurological disorders by selectively targeting the brain, spinal cord, or nerves. This paper provides an overview of the current applications of focused ultrasound in medicine with an emphasis on its use in the fields of neurology and neurosurgery.     

Calculation of optical matrix elements in Gallium Arsenide

Results are presented for optical matrix elements calculated numerically for a 2 × 2 lattice of delta-function impurity potentials in Gallium Arsenide. The calculations show clearly the change in the character of the state with increasing binding energy and particularly the increase in importance of the higher minima over the absolute conduction band minimum at г. Comparison with some experimental work is made from which good qualitative agreement is clear.     

A reduced-order, single-bubble cavitation model with applications to therapeutic ultrasound

Cavitation often occurs in therapeutic applications of medical ultrasound such as shock-wave lithotripsy (SWL) and high-intensity focused ultrasound (HIFU). Because cavitation bubbles can affect an intended treatment, it is important to understand the dynamics of bubbles in this context. The relevant context includes very high acoustic pressures and frequencies as well as elevated temperatures. Relative to much of the prior research on cavitation and bubble dynamics, such conditions are unique. To address the relevant physics, a reduced-order model of a single, spherical bubble is proposed that incorporates phase change at the liquid-gas interface as well as heat and mass transport in both phases. Based on the energy lost during the inertial collapse and rebound of a millimeter-sized bubble, experimental observations were used to tune and test model predictions. In addition, benchmarks from the published literature were used to assess various aspects of model performance. Benchmark comparisons demonstrate that the model captures the basic physics of phase change and diffusive transport, while it is quantitatively sensitive to specific model assumptions and implementation details. Given its performance and numerical stability, the model can be used to explore bubble behaviors across a broad parameter space relevant to therapeutic ultrasound.     

High efficiency, high power 808nm laser array and stacked arrays optimized for elevated temperature operation

Operation of 808-nm laser diode pumping at elevated temperature is crucial to many applications. Reliable operation at high power is limited by high thermal load and low catastrophic optical mirror damage (COMD) threshold at elevated temperature range. We demonstrated high efficiency and high power operation at elevated temperature with high COMD power. These results were achieved through device design optimization such as growth conditions, doping profile, and materials composition of the quantum-well and other layers. Electrical-to-optical efficiency as high as 62% was obtained through lowered threshold current, lowered series resistance and increased slope efficiency. The performance of single broad-area laser diodes scales to that of high power single bars on water-cooled copper micro-channel heatsinks or eonductively-cooled CS heatsinks. No reduction in bar performance or significant spectral broadening is seen when these micro-channel coolers are assembled into 6-bar and 18-bar CW stacks for the h     

A new validation method for X-ray mammogram registration algorithms using a projection model of breast X-ray compression

Establishing spatial correspondence between features visible in X-ray mammograms obtained at different times has great potential to aid assessment and quantitation of change in the breast indicative of malignancy. The literature contains numerous nonrigid registration algorithms developed for this purpose, but existing approaches are flawed by the assumption of inappropriate 2-D transformation models and quantitative estimation of registration accuracy is limited. In this paper, we describe a novel validation method which simulates plausible mammographic compressions of the breast using a magnetic resonance imaging (MRI) derived finite element model. By projecting the resulting known 3-D displacements into 2-D and generating pseudo-mammograms from these same compressed magnetic resonance (MR) volumes, we can generate convincing images with known 2-D displacements with which to validate a registration algorithm. We illustrate this approach by computing the accuracy for two conventional nonrigid 2-D registration algorithms applied to mammographic test images generated from three patient MR datasets. We show that the accuracy of these algorithms is close to the best achievable using a 2-D one-to-one correspondence model but that new algorithms incorporating more representative transformation models are required to achieve sufficiently accurate registrations for this application.     

Millimeter wave rheometry: theory and experiment

A novel millimeter wave (MMW) rheometry is developed to determine the viscosity of fluids based on an unsteady film flow on an inclined plane. The method measures fringes due to MMW interference between the front and back surfaces of a fluid flowing across the field of view of a ceramic wave guide coupled to a MMW receiver operating at 137 GHz. With knowledge of the dielectric constant, the interference fringe spacing is used to calculate the thickness of the fluid layer. This thickness is then transformed into the viscosity by means of a simple hydrodynamic theory. Our results show that the MMW rheometry can practically distinguish between the 30, 100, and 200 Pa·s silicone oils. The geometry of the method allows for potential industrial applications such as measuring viscosity of the flowing slag down the walls of coal gasifiers. The MMW rheometry with simple modifications can be easily extended to measure important non-Newtonian fluid characteristics such as yield stress.     

Nanoscale imaging of Li and B in nuclear waste glass,a comparison of ToF‐SIMS,NanoSIMS, and APT

It has been very difficult to use popular elemental imaging techniques to image Li and B distribution in glass samples with nanoscale resolution. In this study, time‐of‐flight secondary ion mass spectrometry, nanoscale secondary ion mass spectrometry, and atom probe tomography (APT) were used to image the distribution of Li and B in two representative glass samples, and their performance was comprehensively compared. APT can provide three‐dimensional Li and B imaging with very high spatial resolution (≤2 nm). In addition, absolute quantification of Li and B is possible, although there remains room for improving accuracy. However, the major drawbacks of APT include poor sample compatibility and limited field of view (normally ≤100 × 100 × 500 nm³). Comparatively, time‐of‐flight secondary ion mass spectrometry and nanoscale secondary ion mass spectrometry are sample‐friendly with flexible field of view (up to 500 × 500 µm² and image stitching is feasible); however, lateral resolution is limited to only about 100 nm. Therefore, secondary ion mass spectrometry and APT can be regarded as complementary techniques for nanoscale imaging of Li and B in glass and other novel materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of nonlinear propagation, cavitation, and boiling in lesion formation by high intensity focused ultrasound in a gel phantom

The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.     

Nucleation and stabilization of microbubbles in liquids

An important aspect of the processes of cavitation and boiling is the concept of a nucleus that acts as a preferential site for the inception of these events. It is commonly thought that except for rare instances or specially controlled experiments, all cavitation and boiling sites originate at the location of such a nucleus. In order to study these important phenomena, then, it is imperative that as much as possible be known about nucleation in cavitation and boiling. It is generally accepted that free air bubbles normally do not act as nucleation sites because they are inherently unstable to dissolution due to surface tension. Thus, the study of nucleation is necessarily associated with mechanisms for stabilizing microbubbles or pockets of gas within the liquid. In this paper, various stabilization models that have been proposed are reviewed as well as the experimental evidence that supports the specific models. One particular model, the crevice model, is examined in some detail, and its predictions are used to explain several different measurements of boiling and cavitation inception. Finally, some evidence that has recently become available concerning the damaging aspects of high intensity ultrasound is examined. Many aspects of this evidence point to the existence of cavitation as the damage mechanism. Also given in this paper are preliminary explanations of these effects due to the growth of microbubbles or cavitation nuclei by rectified diffusion.