Jason L. Raymond

Acoustic characterization of echogenic liposomes: Frequency-dependent attenuation and backscatter

Ultrasound contrast agents (UCAs) are used clinically to aid detection and diagnosis of abnormal blood flow or perfusion. Characterization of UCAs can aid in the optimization of ultrasound parameters for enhanced image contrast. In this study echogenic liposomes (ELIPs) were characterized acoustically by measuring the frequency-dependent attenuation and backscatter coefficients at frequencies between 3 and 30 MHz using a broadband pulse-echo technique. The experimental methods were initially validated by comparing the attenuation and backscatter coefficients measured from 50-μm and 100-μm polystyrene microspheres with theoretical values. The size distribution of the ELIPs was measured and found to be polydisperse, ranging in size from 40 nm to 6 μm in diameter, with the highest number observed at 65 nm. The ELIP attenuation coefficients ranged from 3.7  ±  1.0 to 8.0  ±  3.3 dB/cm between 3 and 25 MHz. The backscatter coefficients were 0.011  ±  0.006 (cm str)(-1) between 6 and 9 MHz and 0.023 ± 0.006 (cm str)(-1) between 13 and 30 MHz. The measured scattering-to-attenuation ratio ranged from 8% to 22% between 6 and 25 MHz. Thus ELIPs can provide enhanced contrast over a broad range of frequencies and the scattering properties are suitable for various ultrasound imaging applications including diagnostic and intravascular ultrasound.

The effect of static pressure on the inertial cavitation threshold

The amplitude of the acoustic pressure required to nucleate a gas or vapor bubble in a fluid, and to have that bubble undergo an inertial collapse, is termed the inertial cavitation threshold. The magnitude of the inertial cavitation threshold is typically limited by mechanisms other than homogeneous nucleation such that the theoretical maximum is never achieved. However, the onset of inertial cavitation can be suppressed by increasing the static pressure of the fluid. The inertial cavitation threshold was measured in ultrapure water at static pressures up to 30 MPa (300 bars) by exciting a radially symmetric standing wave field in a spherical resonator driven at a resonant frequency of 25.5 kHz. The threshold was found to increase linearly with the static pressure; an exponentially decaying temperature dependence was also found. The nature and properties of the nucleating mechanisms were investigated by comparing the measured thresholds to an independent analysis of the particulate content and available models for nucleation.

Pulsed ultrasound enhances the delivery of nitric oxide from bubble liposomes to ex vivo porcine carotid tissue

Ultrasound-mediated drug delivery is a novel technique for enhancing the penetration of drugs into diseased tissue beds noninvasively. By encapsulating drugs into microsized and nanosized liposomes, the therapeutic can be shielded from degradation within the vasculature until delivery to a target site by ultrasound exposure. Traditional in vitro or ex vivo techniques to quantify this delivery profile include optical approaches, cell culture, and electrophysiology. Here, we demonstrate an approach to characterize the degree of nitric oxide (NO) delivery to porcine carotid tissue by direct measurement of ex vivo vascular tone. An ex vivo perfusion model was adapted to assess ultrasound-mediated delivery of NO. This potent vasodilator was coencapsulated with inert octafluoropropane gas to produce acoustically active bubble liposomes. Porcine carotid arteries were excised post mortem and mounted in a physiologic buffer solution. Vascular tone was assessed in real time by coupling the artery to an isometric force transducer. NO-loaded bubble liposomes were infused into the lumen of the artery, which was exposed to 1 MHz pulsed ultrasound at a peak-to-peak acoustic pressure amplitude of 0.34 MPa. Acoustic cavitation emissions were monitored passively. Changes in vascular tone were measured and compared with control and sham NO bubble liposome exposures. Our results demonstrate that ultrasound-triggered NO release from bubble liposomes induces potent vasorelaxation within porcine carotid arteries (maximal relaxation 31%±8%), which was significantly stronger than vasorelaxation due to NO release from bubble liposomes in the absence of ultrasound (maximal relaxation 7%±3%), and comparable with relaxation due to 12 μM sodium nitroprusside infusions (maximal relaxation 32%±3%). This approach is a valuable mechanistic tool for assessing the extent of drug release and delivery to the vasculature caused by ultrasound.

Impulse response method for characterization of echogenic liposomes

An optical characterization method is presented based on the use of the impulse response to characterize the damping imparted by the shell of an air-filled ultrasound contrast agent (UCA). The interfacial shell viscosity was estimated based on the unforced decaying response of individual echogenic liposomes (ELIP) exposed to a broadband acoustic impulse excitation. Radius versus time response was measured optically based on recordings acquired using an ultra-high-speed camera. The method provided an efficient approach that enabled statistical measurements on 106 individual ELIP. A decrease in shell viscosity, from 2.1 × 10(-8) to 2.5 × 10(-9) kg/s, was observed with increasing dilatation rate, from 0.5 × 10(6) to 1 × 10(7) s(-1). This nonlinear behavior has been reported in other studies of lipid-shelled UCAs and is consistent with rheological shear-thinning. The measured shell viscosity for the ELIP formulation used in this study [κs = (2.1 ± 1.0) × 10(-8) kg/s] was in quantitative agreement with previously reported values on a population of ELIP and is consistent with other lipid-shelled UCAs. The acoustic response of ELIP therefore is similar to other lipid-shelled UCAs despite loading with air instead of perfluorocarbon gas. The methods described here can provide an accurate estimate of the shell viscosity and damping for individual UCA microbubbles.

Loss of gas from echogenic liposomes exposed to pulsed ultrasound.

The destruction of echogenic liposomes (ELIP) in response to pulsed ultrasound excitations has been studied acoustically previously. However, the mechanism underlying the loss of echogenicity due to cavitation nucleated by ELIP has not been fully clarified. In this study, an ultra-high speed imaging approach was employed to observe the destruction phenomena of single ELIP exposed to ultrasound bursts at a center frequency of 6 MHz. We observed a rapid size reduction during the ultrasound excitation in 139 out of 397 (35%) ultra- high-speed recordings. The shell dilation rate, which is defined as the microbubble wall velocity divided by the instantaneous radius, [Formula: see text] /R, was extracted from the radius versus time response of each ELIP, and was found to be correlated with the deflation. Fragmentation and surface mode vibrations were also observed and are shown to depend on the applied acoustic pressure and initial radius. Results from this study can be utilized to optimize the theranostic application of ELIP, e.g. by tuning the size distribution or the excitation frequency.

Acute Effects of High Intensity Focused Ultrasound on Blood Vessels In Vivo

The objective was to conduct a parameter study to examine how High Intensity Focused Ultrasound (HIFU) acoustic variables such as frequency, power, pulse duration, and duty cycle affect vascular occlusion in intact arteries. We used the New Zealand rabbit ear model and varied the ultrasound parameters using a dose‐escalation approach. In each experiment, the animal’s central auricular artery was exposed to HIFU. Frequency (2.2–4.7 MHz), power (10–87 W applied electrical power), pulse duration (5–20s), and duty cycle (5–100%) were varied to determine the optimal conditions for reducing/stopping arterial blood flow. In each experiment, the HIFU applicator was mechanically positioned above the target vessel and blood flow was monitored before and after HIFU treatment using Ultrasound Doppler and Laser Doppler Velocimetry. Vessel damage was confirmed with post‐treatment histology. Results show that higher frequencies (3.5 MHz and 4.7 MHz) yielded occlusion more often than the lower frequency transducer (2.2 MHz). The results for lower duty‐cycle sonications revealed that occlusion could be achieved at a duty cycle as low as 60%. We conclude that for vessels with minimal adjacent connective tissue, such as in the rabbit ear, HIFU treatments delivered at lower frequencies are less effective than treatments delivered at higher frequencies. Additionally, although the majority of exposures to date have been made using continuous‐wave ultrasound, our results suggest that it is possible to achieve favorable results using lower duty‐cycle (pulsed) exposures.The objective was to conduct a parameter study to examine how High Intensity Focused Ultrasound (HIFU) acoustic variables such as frequency, power, pulse duration, and duty cycle affect vascular occlusion in intact arteries. We used the New Zealand rabbit ear model and varied the ultrasound parameters using a dose‐escalation approach. In each experiment, the animal’s central auricular artery was exposed to HIFU. Frequency (2.2–4.7 MHz), power (10–87 W applied electrical power), pulse duration (5–20s), and duty cycle (5–100%) were varied to determine the optimal conditions for reducing/stopping arterial blood flow. In each experiment, the HIFU applicator was mechanically positioned above the target vessel and blood flow was monitored before and after HIFU treatment using Ultrasound Doppler and Laser Doppler Velocimetry. Vessel damage was confirmed with post‐treatment histology. Results show that higher frequencies (3.5 MHz and 4.7 MHz) yielded occlusion more often than the lower frequency transducer (2.2 M...

Inertial cavitation threshold dependence on static pressures

A bubble will be nucleated in a liquid if the tension (or negative pressure) overcomes the forces that oppose the bubbles growth (static pressure and surface tension). The magnitude of the acoustic pressure required to nucleate a bubble capable of producing macroscopically measurable phenomena (e.g. light or shock waves) is the termed the inertial cavitation threshold. The hydrostatic pressure dependence of the inertial cavitation threshold in water was measured using standing waves in a spherical resonator. The resonator was driven at a radially symmetric mode with resonant frequency of 25.6 kHz at 1 atmosphere. The preliminary experimental measurements, reported here, showed the inertial cavitation threshold to be linear (r2= 0.98) over the range of static pressures measured, 10-250 bar. This is consistent with data reported for threshold reported at modest static pressures.

Characterization of a large volume spherical resonator for studies of acoustically induced cavitation in liquids

This paper describes a spherical acoustical resonator system used to study acoustic cavitation phenomena in liquids as part of an effort to scale up the energy density of collapse of transient cavitation. The resonator is formed by a stainless steel spherical shell 24.1 cm in diameter (OD) and either 1.27 cm or 1.90 cm thick designed for generating transient cavitation at high static pressures. An external transducer attached to the surface of the resonator was used to excite an acoustic standing wave in the liquid in order to generate a pressure maximum near the center of the liquid. We will present the results of our characterization of this device, including hydrophone measurements of the acoustic pressure generated in the liquid and vibration analysis on the surface of the resonator carried out using laser Doppler vibrometry. The resonance frequency spectrum and modal structure are compared to numerical predictions using theory developed by (Mehl, JASA 78(2), 782–788 (1985)) [Work supported by SMDC Contract No. W9113M‐07‐C‐0178.]

Reciprocity calibration of hydrophones in the Megahertz frequency range [biomedical ultrasound applications]

In biomedical ultrasound, experimental work may be performed at various temperatures whereas hydrophone calibration data are typically specified at only one temperature. It is necessary to know the temperature dependence, if any, of hydrophone sensitivity. Comparison of the results of a three-transducer reciprocity calibration of an Imotec type 80-0.5-4.0 passive PVDF probe at 3/spl deg/C, 15/spl deg/C and 37/spl deg/C to the calibration at 22/spl deg/C show that within the limits of precision of this experiment, there is no difference in hydrophone sensitivity at these four temperatures.

Suppression of an acoustic mode by an elastic mode of a liquid-filled spherical shell resonator.

The purpose of this paper is to report on the suppression of an approximately radial (radially symmetric) acoustic mode by an elastic mode of a water-filled, spherical shell resonator. The resonator, which has a 1-in. wall thickness and a 9.5-in. outer diameter, was externally driven by a small transducer bolted to the external wall. Experiments showed that for the range of drive frequencies (19.7-20.6 kHz) and sound speeds in water (1520-1570 m/s) considered in this paper, a nonradial (radially nonsymmetric) mode was also excited, in addition to the radial mode. Furthermore, as the sound speed in the liquid was changed, the resonance frequency of the nonradial mode crossed with that of the radial one and the amplitude of the latter was greatly reduced near the crossing point. The crossing of the eigenfrequency curves of these two modes was also predicted theoretically. Further calculations demonstrated that while the radial mode is an acoustic one associated with the interior fluid, the nonradial mode is an elastic one associated with the shell. Thus, the suppression of the radial acoustic mode is apparently caused by the overlapping with the nonradial elastic mode near the crossing point.