Caleb H. Farny

Nucleating cavitation from laser-illuminated nano-particles

Vapor bubble generation from laser-illuminated gold nano-particles has been investigated as a means of providing nucleation sites for cavitation induced by high-intensity focused ultrasound (HIFU). Pulses from a 532-nm Nd:Yag laser were synchronized with a pulsed 1.1-MHz HIFU source in an acrylamide phantom seeded with 82-nm-diameter gold particles. Emissions from bubble collapses were detected by a 15-MHz focused transducer at a laser pulse energy and HIFU focal pressure of 0.10 mJ and 0.92 MPa, respectively. In comparison, a HIFU peak focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination.

Dynamics and control of cavitation during high-intensity focused ultrasound application

In this paper the results of two studies related to high-intensity focused ultrasound (HIFU) and cavitation are reported. The first study described used polyacrylamide phantoms to gain insight into the behavior of cavitation activity in the focal region of the HIFU transducer. Results indicate that cavitation is the source of a previously observed enhanced heating effect in HIFU. The second study discussed used agar-graphite phantoms to see if changing the duty cycle of the driving could effect some measure of control over the cavitation activity; the results indicate that it can.

Cavitation detection during and following HIFU exposure in vitro

Is the existence of a sustained hyperechogenic region on B‐scan images following HIFU treatment a necessary and sufficient condition for cavitation to have occurred during HIFU exposure? Three means of cavitation monitoring were used synchronously, before, during, and after continuous‐wave HIFU exposure of an agar‐graphite tissue phantom. A 1.1‐MHz HIFU transducer was confocally aligned with a 15‐MHz passive cavitation detection (PCD) transducer and a 5‐MHz scan head. The HIFU pressure amplitude was increased in steps of 0.26 MPa every 5 s. A peak detector recorded the peak PCD signal level and a dynamic signal analyzer monitored broadband noise emissions (5–10 MHz). The sudden onset of a PCD output signal occurred at a peak‐negative focal pressure of 1.25 MPa; no post‐HIFU hyperechogenic region was visible on the B‐scan images. A hyperechogenic region did eventually appear, but only for focal pressures in excess of 1.8 MPa. Inertial cavitation can therefore occur during HIFU exposure in the absence of a ...

Feasibility of ultrasound phase contrast for heating localization

Ultrasound-based methods for temperature monitoring could greatly assist focused ultrasound visualization and treatment planning based on sound speed-induced change in phase as a function of temperature. A method is presented that uses reflex transmission integration, planar projection, and tomographic reconstruction techniques to visualize phase contrast by measuring the sound field before and after heat deposition. Results from experiments and numerical simulations employing a through-transmission setup are presented to demonstrate feasibility of using phase contrast methods for identifying temperature change. A 1.088-MHz focused transducer was used to interrogate a medium with a phase contrast feature, following measurement of the baseline reference field with a hydrophone. A thermal plume in water and a tissue phantom with multiple water columns was used in separate experiments to produce a phase contrast. The reference and phase contrast field scans were numerically backprojected and the phase difference correctly identified the position and orientation of the features. The peak temperature reconstructed from the phase shift was within 0.2 degrees C of the measured temperature in the plume. Simulated results were in good agreement with experimental results. Finally, employment of reflex transmission imaging techniques for adopting a pulse-echo arrangement was simulated, and its future experimental application is discussed.

Observations of cavitation activity and lesion growth in optically clear tissue phantoms

Above a certain acoustic pressure threshold the heating rate of a tissue‐mimicking phantom (as well as in vivo tissue) by high‐intensity focused ultrasound (HIFU) is greatly enhanced. This enhanced heating regime has been shown to correlate well with an increase in cavitation activity; thus, it is believed that the enhanced heating is the result of bubbles formed at the focus of the HIFU source. In this talk we report the results of work carried out to observe the cavitation activity in the focus of a 1.1‐MHz source, using optically clear acrylamide/BSA tissue phantoms. Three different methods were employed to make the measurements: simultaneous passive cavitation detection (PCD) and video imaging, simultaneous PCD and light emission measurements (using a photomultiplier tube), and video imaging with back light. Results complement previous work by other groups which showed that thermal lesion growth progresses towards the HIFU source; however in contrast to those studies, our results indicate that at some...

Singular value decomposition: A better way to analyze cavitation-noise data

Detection of inertial and stable cavitation is important for guiding high-intensity focused ultrasound (HIFU). Acoustic transducers can passively detect broadband noise from inertial cavitation and the scattering of HIFU harmonics from stable cavitation bubbles. Conventional approaches to separating these signals typically involve a custom comb-filter applied in the frequency domain followed by an inverse-Fourier transform, which cannot be implemented in real-time. We present an alternative technique based on singular value decomposition (SVD) that efficiently separates the broadband emissions and HIFU harmonics in a single step. Spatio-temporally resolved cavitation detection was achieved using a 128-element, 5-MHz linear-array system operating at 15 frames/s. A 1.1-MHz transducer delivered HIFU to tissue-mimicking phantoms for a duration of 5 s. Beamformed radiofrequency signal corresponding to each scan line and frame were assembled into a matrix and SVD was performed. Eigen vectors that corresponded t...

Inertial cavitation thresholds during standing wave disruption using random phase modulation.

In low‐frequency transcranial ultrasound therapeutic applications, such as sonothrombolysis, standing waves have been identified as a potential source of undesirable collateral damage due to cavitation. Recently, a method for disrupting the standing wave pattern was developed by modulating the phase of the driving signal, producing a beam pattern with significantly lower ripple in pressure along the acoustic axis. Here we present a study conducted to determine the inertial cavitation threshold in tissue‐mimicking phantoms placed in a standing‐wave‐inducing chamber when the 272‐kHz focused therapy transducer was excited separately by a monochromatic or phase‐modulated continuous‐wave signal. A 10‐MHz receiver was used as a passive cavitation detector whose signal was digitally sampled to examine changes in the broadband signal amplitude. Results indicate that the pressure threshold for inertial cavitation is higher during random phase modulation, suggesting that such an approach would be suitable for trans...

Controlled nucleation of microcavitation with laser‐illuminated nano‐particles

The safe utilization of controlled cavitation in HIFU therapy requires the presence of nucleation sites for bubble formation. Effective cavitation nuclei do not exist in most tissues; nucleation thresholds pressures in excess of 4–5 MPa have been reported. We investigate the efficacy of transient vapor cavity generation from laser‐illuminated gold nano‐particles as a means for nucleating cavitation with high‐intensity focused ultrasound. An acrylamide tissue phantom seeded with 82‐nm diameter gold particle was exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was precisely synchronized with a pulsed 1.1 MHz HIFU pressure field. Acoustic emissions from inertial cavitation were detected by a 15 MHz focused transducer at a laser energy of 0.10 mJ/pulse and a HIFU peak‐negative focal pressure as low as 0.92 MPa. In comparison, a peak‐negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination; nano‐particles were present in both cases. Since the particles are durable, one can re‐activate them as needed, essentially yielding cavitation nuclei on demand. [Work supported by the Dept. of the Army (award No. DAMD17‐02‐2‐0014) and the Center for Subsurface Sensing and Imaging Systems (NSF ERC Award No. EEC‐9986821).]

A technique for monitoring and controlling cavitation activity during high intensity focused ultrasound application

It has been reported that cavitation can lead to enhanced heating in the focal region of a HIFU source. In order to exploit this heating for in vivo use, it is essential that the cavitation only occur in the focal region. Thus, the onset and evolution of inertial cavitation activity must be monitored and controlled during HIFU therapy. One candidate sensor is a confocally‐aligned passive cavitation detector; however this would add complexity to a clinical HIFU applicator. Instead we propose that the HIFU source itself can serve as a monitoring device. The combination of broadband acoustic emissions (inertial cavitation) and backscatter (stable cavities) emanating from the HIFU focus manifests itself as fluctuations of the otherwise constant driving voltage amplitude, providing a convenient means for sensing cavitation activity. We will present results of experiments assessing the feasibility of using the variance in the amplitude of the HIFU drive voltage as a feedback control signal. Success is determine...

Controlling a high intensity focused ultrasound induced cavitation field via duty cycle

Cavitation has been implicated in the lack of control over the shape of thermal lesions generated by high‐intensity focused ultrasound (HIFU). A coincident effect the decline in the acoustic emissions from cavitation at the focus suggests that the HIFU energy is shielded from the focal region, possibly by prefocal bubble activity. Most clinical techniques employ continuous‐wave (CW) ultrasound, which can exacerbate the problem depending on the acoustic intensities employed. This talk presents a series of experiments investigating techniques to control HIFU energy delivered to, and cavitation activity within, a tissue phantom. A passive cavitation detector (PCD) is employed as a sensor of cavitation activity. For 1.1‐MHz CW ultrasound at focal pressures above 3 MPa, bubble shielding was inferred from a steady decline in the PCD signal over time. By lowering the duty cycle the PCD output remained constant over time. Finally, driving the HIFU source initially with a CW signal and then switching to a pulsed s...