R. Mori

Microbubble characterization through acoustically induced deflation

Ultrasound contrast agents (UCA) populations are typically polydisperse and contain microbubbles with radii over a given range. Although the behavior of microbubbles of certain sizes might be masked by the behavior of others, the acoustic characterization of UCA is typically made on full populations. In this paper, we have combined acoustic and optical methods to investigate the response of isolated lipid-shelled microbubbles to low-pressure (49 and 62 kPa peak negative pressure) ultrasound tone bursts. These bursts induced slow deflation of the microbubbles. The experimental setup included a microscope connected to a fast camera acquiring one frame per pulse transmitted by a single-element transducer. The behavior of each bubble was measured at multiple frequencies, by cyclically changing the transmission frequency over the range of 2 to 4 MHz during subsequent pulse repetition intervals. The bubble echoes were captured by a second transducer and coherently recorded. More than 50 individual microbubbles were observed. Microbubbles with radii larger than 3 ?m did not experience any size reduction. Smaller bubbles slowly deflated, generally until the radius reached a value around 1.4 ?m, independent of the initial microbubble size. The detected pressure amplitude backscattered at 2.5 cm distance was very low, decreasing from about 5 Pa down to 1 Pa at 2 MHz as the bubbles deflated. The resonant radius was evaluated from the echo amplitude normalized with respect to the geometrical cross section. At 2-MHz excitation, deflating microbubbles showed highest normalized echo when the radius was 2.2 ?m while at higher excitation frequencies, the resonant radius was lower. The relative phase shift of the echo during the deflation process was also measured. It was found to exceed ?/2 in all cases. A heuristic procedure based on the analysis of multiple bubbles of a same population was used to estimate the undamped natural frequency. It was found that a microbubble of 1.7 ?m has an undamped natural frequency of 2 MHz. The difference between this size and the resonant radius is discussed as indicative of significant damping.

Charge collection measurements on slim-edge microstrip detectors

We have generated slim edges on manufactured silicon strip detectors by cleaving the non-active edge material and passivating the very smooth edge with a thin coat of silicon oxide. We report a comparison of I-V measurements and charge collection and noise measurements on two identical sensors, one with and one without slim edge treatment. The current voltage measurements of the entire sensor and individual strips indicate that the large current increase due to the treatment is confined to the guard ring, while the strips show essentially no increase in leakage currents. The noise on all strips, including the one adjacent to the slim edge, is not changed by the cut. The signal from a beta source before and after cutting is the same within 4%.

IMRT field profiling by high-quality polycrystalline CVD diamond

A dosimeter made with state of art polycrystalline Chemical Vapour Deposited (pCVD) diamond for applications in clinical radiotherapy has been manufactured, based on a large area (2.5 × 2.5 cm2) film equipped with a matrix of contacts. A dose profile under an Intensity Modulated Radiotherapy (IMRT) beam for prostate cancer has been monitored using a set of in-line pads along a section of the field map. Results compare well with data obtained with a commercial bidimensional silicon dosimeter, confirming that our pCVD diamond device is suitable for mapping the conformed dose in IMRT treatments.

The use of microbubbles in Doppler ultrasound studies

Ultrasound contrast agents (UCAs) are widely used in Doppler studies, either for simple echo enhancement purposes, or to increase the low signal-to-clutter ratio typical of microcirculation investigations. Common to all Doppler techniques, which are briefly reviewed in this paper, is the basic assumption that possible phase and amplitude changes in received echoes are only associated with UCA microbubble movements due to the drag force of blood. Actually, when UCAs are insonified, phenomena such as rupture, displacement due to radiation force, and acoustically driven deflation might influence the results of Doppler investigations. In this paper, we investigate the possible Doppler effects of such phenomena by means of a numerical simulation model and a special acousto-optical set-up which allows analysis of the behavior of individual microbubbles over relatively long time intervals. It is thus found that all phenomena produce evident Doppler effects in vitro, but that bubble displacement and deflation in particular, are not expected to significantly interfere with clinical measurements in standard conditions.

Correspondence - Nonlinear oscillations of deflating bubbles

Phospholipid-coated ultrasound contrast agents may deflate or even collapse because of stress resulting from ultrasound-induced oscillations. In this work, we investigate the behavior of isolated contrast agent microbubbles during prolonged ultrasound excitation. Isolated microbubbles placed in a thin capillary tube were excited with hundreds of ultrasound pulses at a low mechanical index, and their oscillations were recorded using the Brandaris-128 ultra-high-speed camera. Results show that microbubbles undergo an irreversible, non-destructive deflation process. Such deflation seems to occur in discrete steps rather than as a continuous process; furthermore, the dynamics of the bubble change during deflation: radial oscillations, both symmetric and asymmetric around the resting radius of the bubble, occur at various stages of the deflation process. Strongly asymmetric oscillations, such as compression-only and expansion-only behavior, were also observed: notably, expansion-only behavior is associated with a rapid size reduction, whereas compression-only behavior mostly occurs without a noticeable change of the bubble radius. We hypothesize that bubble deflation results from at least two distinct phenomena, namely diffusive gas loss and lipid material shedding from the encapsulating shell.

Influences of bubble motion to second-harmonic inversion imaging

In ultrasound contrast harmonic imaging, the generated harmonics during wave propagation in tissues lead to a limited contrast-to-tissue ratio (CTR). Second-harmonic inversion (SHI), based on two 90° phase-shifted transmissions with the same frequencies and amplitudes, can effectively reduce the tissue-harmonic signals and improve CTR. However, the bubbles motion strongly influences the SHI effectiveness. Therefore, in-vitro experiments on circulating UCA were conducted to quantify the influence of moving bubbles to SHI technique, by regulating the pulse repetition frequency (PRF), in order to guarantee and to optimize the SHI technique. Experimental results show that when bubbles move in axial direction, the second-harmonic amplitudes exhibits an oscillating evolution with increasing bubbles motions. The maximum improvement reaches 12 dB when compared to SHI without bubbles motion. The influence of lateral bubbles motion is much less than the influence of axial bubbles motion.

Zero bias thermally stimulated currents in synthetic diamond

Zero bias thermally stimulated currents (ZBTSCs) have been observed in single crystal high pressure high temperature (HPHT) and polycrystalline chemical vapor deposited (pCVD) diamond films. The ZBTSC technique is characterized by an increased sensitivity with respect to a standard TSC analysis. Due to the absence of the thermally activated background current, new TSC peaks have been observed in both HPHT and pCVD diamond films, related to shallow activation energies usually obscured by the emission of the dominant impurities. The ZBTSC peaks are explained in terms of defect discharge in the nonequilibrium potential distribution created by a nonuniform traps filling at the metal-diamond junctions. The electric field due to the charged defects has been estimated in a quasizero bias TSC experiment by applying an external bias.

Thermally Stimulated Currents in Nanocrystalline Titania

A thorough study on the distribution of defect-related active energy levels has been performed on nanocrystalline TiO2. Films have been deposited on thick-alumina printed circuit boards equipped with electrical contacts, heater and temperature sensors, to carry out a detailed thermally stimulated currents analysis on a wide temperature range (5–630 K), in view to evidence contributions from shallow to deep energy levels within the gap. Data have been processed by numerically modelling electrical transport. The model considers both free and hopping contribution to conduction, a density of states characterized by an exponential tail of localized states below the conduction band and the convolution of standard Thermally Stimulated Currents (TSC) emissions with gaussian distributions to take into account the variability in energy due to local perturbations in the highly disordered network. Results show that in the low temperature range, up to 200 K, hopping within the exponential band tail represents the main contribution to electrical conduction. Above room temperature, electrical conduction is dominated by free carriers contribution and by emissions from deep energy levels, with a defect density ranging within 1014–1018 cm−3, associated with physio- and chemi-sorbed water vapour, OH groups and to oxygen vacancies.

Implementation of arbitrary contrast imaging strategies on an US Open Platform

Comparing the performance of different contrast imaging techniques can be difficult and somewhat confusing, since the tests were typically made using different in vitro setups, contrast agents, ultrasound transducers and systems. In this study, we report on the implementation of arbitrary contrast-pulse-sequences (CPS) in the ULA-OP open ultrasound system. Combined with a home-made phantom, a convenient and consistent evaluation of the main parameters characterizing various techniques is obtained. Several experiments were performed exciting BR14 microbubbles at a dilution between 1/200 and 1/800, with an average MI = 0.1, through 10 different excitation schemes including power modulation, chirp reversal and their combination. Chirp Amplitude Modulation (CAM) and Chirp Reversal Amplitude Modulation (CRAM) showed the best performance with an average CTR of 34 dB.

Ultrafast framing optical and acoustical recording of nonlinearly oscillating microbubbles

Microbubble nonlinear behavior improves detection efficacy of ultrasound contrast agents (UCAs), but not all microbubble sizes present in the dispersed UCA population will contribute to the detection signal. This paper experimentally studies the relation between size and nonlinear response of isolated lipid-coated microbubbles, held in a transparent tube. The echo from microbubbles is acoustically recorded with a sensitive custom electronic system, and simultaneously the radial excursion is optically recorded with an ultrafast framing camera. Ultrasound-induced deflation is used to sample the bubble at multiple sizes. At sizes smaller than the resonant size the bubbles show compression-only behavior accompanied by strong second harmonic scattering. Subharmonics are observed for bubble sizes larger than resonant.