Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology

We have developed an efficient order-N real-space Kubo approach for the calculation of the phonon conductivity which outperforms state-of-the-art alternative implementations based on the Green's function formalism. The method treats efficiently the time-dependent propagation of phonon wave packets in real space, and this dynamics is related to the calculation of the thermal conductance. Without loss of generality, we validate the accuracy of the method by comparing the calculated phonon mean free paths in disordered carbon nanotubes (isotope impurities) with other approaches, and further illustrate its upscalability by exploring the thermal conductance features in large width edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach of more conventional techniques. We show that edge-disorder is the most important scattering mechanism for phonons in graphene nanoribbons with realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication

Contrast-Enhanced Ultrasound Imaging with Chirps: Signal Processing and Pulse Compression

Contrast-enhanced ultrasound imaging creates one of the worst case scenarios for pulse compression due to depth and frequency dependent attenuation, high level of harmonic generation, phase variations due to resonance behavior of microbubbles, and increased broadband noise by microbubble destruction. This study investigates the feasibility of pulse compression with a matched filter in the existence of microbubbles with resonant behavior. Simulations and experimental measurements showed that the scattered pressure from a microbubble population excited by a chirp waveform preserves its chirp rate even for harmonic frequencies. Although, pulse compression by a matched filter was possible due to the conservation of the chirp rate, an increase on sidelobe levels were observed at fundamental and second harmonic frequencies. Therefore, using chirp excitation and a matched filter pair will increase the contrast-to-tissue ratio with a trade-off of decreased image quality

Wideband Excitation of Microbubbles to Maximize the Sonoporation Efficiency and Contrast in Ultrasound Imaging

The importance of the excitation bandwidth is well known in diagnostic ultrasound imaging. However, the effect of excitation bandwidth in therapeutic applications of microbubbles has been mostly overlooked. A majority of contrast agent production techniques generate polydisperse microbubble populations, so a wide range of resonance frequencies exist. Therefore, wideband excitation is necessary to fully utilize microbubble resonance behavior and maximize the reradiated energy from a microbubble population, both for imaging and therapy. Oscillations of sixty SonoVue microbubbles in proximity of a rigid boundary were captured on a high speed camera at 3 Mfps, excited with a peak negative pressure of 50 kPa at 1 MHz. Measurements were analyzed according to their peak radiated pressure, radial oscillations, root mean squared pressure, and shear stress generated by microbubbles. Results showed that long duration and wideband excitation at low intensity levels was preferable for sonoporation, where microbubbles can be driven in a stable oscillation state without experiencing inertial cavitation or destruction

Elevation resolution enhancement in 3D photoacoustic imaging using FDMAS beamforming

Photoacoustic imaging is a non-invasive and non-ionizing imaging technique that combines the spectral selectivity of laser excitation with the high resolution of ultrasound imaging. It is possible to identity the vascular structure of the cancerous tissue using this imaging modality. However, elevation and lateral resolution of photoacoustic imaging is usually poor for imaging target. In this study, three dimension filter delay multiply and sum beamforming technique (FDMAS(3D)) is used to improve the resolution and enhance the signal to noise ratio (SNR) of the 3D photoacoustic image that is created by using linear array transducer. This beamforming technique showed improvement in the elevation by 36% when its compared with three dimension delay and sum beamforming technique (DAS(3D)). In addition, it enhanced the SNR by 13 dB compared with DAS (3D)

Gallium Nitride Based High-Power Switched HIFU Pulser with Real-Time Current/Voltage Monitoring

High-Intensity Focussed Ultrasound (HIFU) techniques make use of ultrasound transducers capable of delivering high powers to be delivered at high frequencies. Real-time monitoring of power delivered can avoid damage to the transducer and injury to patients due to overexposure. This paper demonstrates the real-time current and voltage monitoring capabilities of a new Gallium-Nitride (GaN) based switched mode transmit pulser developed for the University of Leeds High-Intensity Focussed Ultrasound Array Research Platform (HIFUARP) system, which uses a novel approach of using an Analog Front End (AFE) floating on the transmitter output to provide high bandwidth current measurement

Spatial Resolution and Contrast Enhancement in Photoacoustic Imaging with Filter Delay Multiply and Sum Beamforming Technique

Photoacoustic imaging is used to differentiate between tissue types based on light absorption. Different structures, such as vascular density of capillaries in human tissue, can be analysed and provide diagnostic information to detect early stage breast cancer. Delay and sum (DAS) beamforming is the traditional method to reconstruct photoacoustic images. However, for structures located deep in the tissue (>10 mm), signal to noise (SNR) of the photoacoustic signal drops significantly. This study proposes using filter delay multiply and sum (FDMAS) beamforming technique to increase the SNR and enhance the image quality. Experimental results showed that FDMAS beamformer improved the SNR by 6.9 dB and the lateral resolution by 48% compared to the DAS beamformer. Moreover, the effect of aperture size on the proposed method is presented as the sub-group FDMAS, which further increased the improvement in image quality

Improved shear wave-front reconstruction method by aligning imaging beam angles with shear-wave polarization: Applied for shear compounding application

In shear compounding, shear waves are generated at various angles and individual elasticity maps are averaged to reduce noise and improve accuracy. The steered shear waves tilt the tissue motion direction therefore conventional plane wave tracking is not capable of capturing true shear wave amplitude and direction. The proposed method aligns the tracking beams with the shear wave angles, enables beam-axis in the direction of tissue motion to estimate true shear wave motion vector. In this experimental work, shear waves are produced at five different angles and motion is captured using proposed and conventional method. All the experiments are conducted using inclusion-based elasticity phantom. In the results, the displacement maps show that proposed method accurately captured the steered push-beam wave-fronts while conventional method produced push-beam direction artefacts. In the final compounded elasticity maps, the proposed method slightly improved background-to-inclusion elasticity ratio, CNR by 2 dB, and produced inclusion boundary shape sharper than the conventional tracking