Justin Reed

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.

The effects of microgravity on nanoparticle size distributions generated by the ultrasonic reduction of an aqueous gold-chloride solution

A solution of gold chloride was reduced using ultrasound irradiation to prepare metallic gold nanoparticles under conditions of microgravity and normal gravity at sea level. Particle size distributions were measured using TEM analysis. A mean particle diameter of 10 nm was obtained in microgravity while a mean diameter of 80 nm was obtained in the laboratory. Absorbance measurements on the reacted solution found an enhanced reduction rate in the reduction of gold chloride in microgravity compared to that in the laboratory.

Nonlinear Mechanisms of Lesion Formation by High Intensity Focused Ultrasound

Nonlinear mechanisms of lesion formation by high intensity focused ultrasound (HIFU) were investigated experimentally and numerically in a transparent polyacrylamide gel phantom. Numerical predictions were made with a finite‐amplitude acoustic propagation model. A 2‐MHz transducer of 42‐mm diameter and 44.5‐mm radius of curvature was operated above the cavitation threshold of the gel phantom at various peak acoustic powers and duty cycles. Acoustic waveforms were recorded in the gel by a fiber optic hydrophone. Bubble activity was detected actively by B‐mode diagnostic imaging, passively by a remote focused hydrophone, and optically by CCD and high‐speed cameras. Elevated static pressure was applied to suppress bubble activity and increase the boiling temperature, thus isolating the pure effect of acoustic nonlinearity. In overpressure experiment performed at 32 W acoustic power, both cavitation and nonlinear ultrasound propagation accelerated lesion inception and growth, but acoustic nonlinearity, which ...

Separating nonlinear propagation and cavitation effects in HIFU

High intensity focused ultrasound (HIFU) can destroy tumors or stop internal bleeding. The primary physical mechanism in HIFU is the conversion of acoustic energy to heat, which as HIFU amplitude increases is enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. The goal of this work is to study and separate the effects of nonlinear propagation and cavitation during HIFU heating of tissue. Transparent polyacrylamide gel was used as a tissue-mimicking phantom to visualize HIFU lesion growth. Lesion size was also measured in excised turkey breast. Lesions were produced by the same time-averaged intensity, but with different peak acoustic pressure amplitudes compensated by different duty cycles. In order to separate cavitation and nonlinear wave effects, experiments were performed under static pressure (10.34MPa) greater than the peak negative pressure amplitude of the sound waves (8.96MPa). Suppression of cavitation by overpressure was measured by reduced acoustic scattering and transmission loss in the treatment region. We found that, with the same time-averaged intensity, a shorter, higher amplitude wave created a larger lesion than a longer, lower amplitude wave with or without overpressure.

The relative effects of cavitation and nonlinear ultrasound propagation on HIFU lesion dynamics in a tissue phantom

The relative importance of the effects of acoustic nonlinearity and cavitation in HIFU lesion production is studied experimentally and theoretically in a polyacrylamide gel. A 2‐MHz transducer of 40‐mm diameter and 45‐mm focal length was operated at different regimes of power, and in cw or duty‐cycle regimes with equal mean intensity. Elevated static pressure was applied to suppress bubbles, increase boiling temperature, and thus to isolate the effect of acoustic nonlinearity in the enhancement of lesion production. Experimental data were compared with the results of simulations performed using a KZK acoustic model combined with the bioheat equation and thermal dose formulation. Boiling and the typical tadpole‐shaped lesion shifting towards the transducer were observed under standard atmospheric pressure. No boiling was detected and a symmetric thermal lesion formed in the case of overpressure. A delay in lesion inception time was registered with overpressure, which was hypothesized to be due to suppresse...

Dual frequency high intensity focused ultrasound to accelerate hemostasis

In acoustic hemostasis, cavitation appears to help emulsify and heat tissue to form a paste which seals lacerations. Our objective was to mix two frequencies to enhance cavitation, increase heat deposition, and accelerate hemostasis. A focused transducer (curvature 5 cm) with a 4.3‐MHz center element (diameter 2 cm) and a 125‐kHz outer annular element (inner and outer diameters, 3 and 5 cm) was engineered. This dual frequency transducer was used to create lesions in a transparent gel phantom and to induce hemostasis in five rabbits. Hemostasis time was determined with both frequencies (4.3 MHz at 620 W/cm2, 125 kHz at 4 W/cm2) and with the high frequency only (4.3 MHz at 620 and 780 W/cm2). In gels, dual frequency HIFU created larger, broader lesions than 4.3‐MHz HIFU alone. Bubbles created by just low frequency could be seen and, with sufficient time, created small local lesions. In rabbits, hemostasis times (∼1 min for 25‐ml/min bleeds) were decreased 20% with dual frequency compared to the 4.3‐MHz wave...

The characterization of the lesion growth in time

Thermal heating effects of high intensity focused ultrasound (HIFU) on the dynamics of lesion formation were characterized automatically to assess the role of vapor bubbles in distorting the shape. Tissue mimicking phantom was used in experiments by a 4.2 MHz curve‐linear transducer with 44 mm diameter and 44 mm radius of curvature. A variety of HIFU intensities were produced by different amplitudes. Images were acquired by a CCD camera and HDI‐1000 ultrasound imager, recorded to VHS, and digitized to measure lesion size and shape. Each image was subtracted with noise reduction in order to detect the HIFU on time and to segment the boundaries of the lesions performed by Matlab programming. Area, length, width, and ratio of lesion area proximal to center line over area distal to center line were calculated along HIFU exposure time. Slight increase in HIFU intensity, means hyperecho forms earlier, and lesion shape change. The data supported the hypothesis that lesion dramatically distorts well after hyperec...

Separating thermal coagulation and cavitation effects in HIFU attenuation measurements

HIFU can be used to destroy tumors. The conversion of acoustic energy into heat causes protein coagulation (Lesion) in tissue. Attenuation measurements have been proposed to monitor the progression of thermal therapy. The goal of this work is to study and separate the effects of cavitation and thermal coagulation in attenuation measurements. A HIFU transducer was used to treat Bovine liver. A receiving transducer mounted across from the transmitting HIFU transducer measured attenuation during the treatment. A pressure chamber provided static pressure greater than the pressure amplitude of the HIFU wave, which suppressed cavitation. rf data from a commercial ultrasound scanner was also obtained. A large increase in attenuation was observed with cavitation present, while a subtle increase in attenuation was observed with cavitation suppressed. Attenuation estimated from the RF data showed an increase in attenuation downstream of the location of the lesion with cavitation present, while a subtle increase in ...