Oliver D. Kripfgans

Acoustic droplet vaporization for therapeutic and diagnostic applications

A phase shift droplet emulsion is introduced as an aid to unusual ultrasound (US) applications. The transpulmonary droplet emulsion (90% < 6 microm diameter) is made by mixing saline, bovine albumin and dodecafluoropentane. It has been observed that an acoustic pressure threshold exists, above which the droplets vaporize into bubbles approximately 25 times the original diameter. For frequencies between 1.5 and 8 MHz, the threshold decreases from 4.5 to 0.75 MPa peak rarefactional pressure. This paper presents preliminary results for droplet preparation and their evaporation as a function of applied acoustic pressure and frequency, as well as simulations of the lifetime of these gas bubbles based on gas diffusion. In vivo experiments were simulated by the evaporation of droplets in blood flowing under attenuating material. We propose that this agent might be useful for tissue occlusion in cancer treatment, as well as for phase aberration corrections in acoustic imaging.

Interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements.

In a study involving 10 different sites, independent results of measurements of ultrasonic properties on equivalent tissue‐mimicking samples are reported and compared. The properties measured were propagation speed, attenuation coefficients, and backscatter coefficients. Reasonably good agreement exists for attenuation coefficients, but less satisfactory results were found for propagation speeds. As anticipated, agreement was not impressive in the case of backscatter coefficients. Results for four sites agreed rather well in both absolute values and frequency dependence, and results from other sites were lower by as much as an order of magnitude. The study is valuable for laboratories doing quantitative studies.

Delivery of Chlorambucil Using an Acoustically-Triggered, Perfluoropentane Emulsion

Ultrasound-mediated delivery systems have mainly focused on microbubble contrast agents as carriers of drugs or genetic material. This study uses micron-sized, perfluoropentane (PFP) emulsions as carriers of chlorambucil (CHL), a lipophilic chemotherapeutic. The release of CHL is achieved via acoustic droplet vaporization (ADV), whereby the superheated emulsion is converted into gas bubbles using ultrasound. Emulsions were made using an albumin shell and soybean oil as the CHL carrier. The ratio of the PFP to soybean oil phases in the droplets and the fraction of droplets that vaporize per ultrasound exposure were shown to correlate with droplet diameter. A 60-min incubation with the CHL-loaded emulsion caused a 46.7% cellular growth inhibition, whereas incubation with the CHL-loaded emulsion that was exposed to ultrasound at 6.3 MHz caused an 84.3% growth inhibition. This difference was statistically significant (p < 0.01), signifying that ADV can be used as a method to substantially enhance drug delivery.

Initial Investigation of Acoustic Droplet Vaporization for Occlusion in Canine Kidney

Acoustic droplet vaporization (ADV) shows promise for spatially and temporally targeted tissue occlusion. In this study, substantial tissue occlusion was achieved in operatively exposed and transcutaneous canine kidneys by generating ADV gas bubbles in the renal arteries or segmental arteries. Fifteen canines were anesthetized, among which 10 underwent laparotomy to externalize the left kidney and five were undisturbed for transcutaneous ADV. The microbubbles were generated by phase conversion of perfluoropentane droplets encapsulated in albumin or lipid shells in the blood. A 3.5-MHz single-element therapy transducer was aligned with an imaging array in a water tank with direct access to the renal artery or a segmental artery. In vivo color flow and spectral Doppler imaging were used to identify the target arteries. Tone bursts of 1 kHz pulse repetition frequency with 0.25% duty cycle vaporized the droplets during bolus passage. Both intracardiac (IC) and intravenous (IV) injections repeatedly produced ADV in chosen arteries in externalized kidneys, as seen by B-mode imaging. Concurrent with this in two cases was the detection by pulse-wave Doppler of blood flow reversal, along with a narrowing of the waveform. Localized cortex occlusion was achieved with 87% regional flow reduction in one case using IC injections. Vaporization from IV injections resulted in a substantial echogenicity increase with an average half-life of 8 min per droplet dose. Gas bubbles sufficient to produce some shadowing were generated by transcutaneous vaporization of intrarenal artery or IV-administered droplets, with a tissue path up to 5.5 cm.

Delivery of water-soluble drugs using acoustically triggered perfluorocarbon double emulsions.

ABSTRACTPurposeUltrasound can be used to release a therapeutic payload encapsulated within a perfluorocarbon (PFC) emulsion via acoustic droplet vaporization (ADV), a process whereby the PFC phase is vaporized and the agent is released. ADV-generated microbubbles have been previously used to selectively occlude blood vessels in vivo. The coupling of ADV-generated drug delivery and occlusion has therapeutically synergistic potentials.MethodsMicron-sized, water-in-PFC-in-water (W1/PFC/W2) emulsions were prepared in a two-step process using perfluoropentane (PFP) or perfluorohexane (PFH) as the PFC phase. Fluorescein or thrombin was contained in the W1 phase.ResultsDouble emulsions containing fluorescein in the W1 phase displayed a 5.7±1.4-fold and 8.2±1.3-fold increase in fluorescein mass flux, as measured using a Franz diffusion cell, after ADV for the PFP and PFH emulsions, respectively. Thrombin was stably retained in four out of five double emulsions. For three out of five formulations tested, the clotting time of whole blood decreased, in a statistically significant manner (p < 0.01), when incubated with thrombin-loaded emulsions exposed to ultrasound compared to emulsions not exposed to ultrasound.ConclusionsADV can be used to spatially and temporally control the delivery of water-soluble compounds formulated in PFC double emulsions. Thrombin release could extend the duration of ADV-generated, microbubble occlusions.

Acoustic droplet vaporization threshold: effects of pulse duration and contrast agent

The use of superheated liquid perfluorocarbon droplets encased in albumin shells has been proposed as a minimally invasive alternative to current treatment of cancer by means of occlusion therapy. In response to an applied acoustic field, these droplets, which are small enough to pass through capillaries, vaporize into large gas bubbles that subsequently lodge in the vasculature. This technique, known as acoustic droplet vaporization (ADV) has been shown to successfully reduce blood flow in vivo, but for in situ conditions where attenuation is present, lower acoustic frequency and ADV threshold may be desirable. Thus, two methods to lower the ADV threshold at a lower 1.44 MHz were explored. The first part of this study investigated the role of pulse duration on ADV. The second part investigated the role of inertial cavitation (IC) external to a droplet by lowering the IC threshold in the host liquid with the presence of Definityreg contrast agent (CA). The threshold was found to be 5.5-5.9 MPa for short microsecond pulses and decreased for millisecond pulses (3.8-4.6 MPa). When CAs were present and long millisecond pulses were used, the ADV threshold decreased to values as low as 0.41 MPa

Acoustic Droplet Vaporization for Enhancement of Thermal Ablation by High Intensity Focused Ultrasound

RATIONALE AND OBJECTIVES Acoustic droplet vaporization (ADV) shows promise for spatial control and acceleration of thermal lesion production. The investigators hypothesized that microbubbles generated by ADV could enhance high-intensity focused ultrasound (HIFU) thermal ablation by controlling and increasing local energy absorption. MATERIALS AND METHODS Thermal lesions were produced in tissue-mimicking phantoms using focused ultrasound (1.44 MHz) with a focal intensity of 4000 W · cm(-2) in degassed water at 37°C. The average lesion volume was measured by visible change in optical opacity and by T2-weighted magnetic resonance imaging. In addition, in vivo HIFU lesions were generated in a canine liver before and after an intravenous injection of droplets with a similar acoustic setup. RESULTS Thermal lesions were sevenfold larger in phantoms containing droplets (3 × 10(5) droplets/mL) compared to phantoms without droplets. The mean lesion volume with a 2-second HIFU exposure in droplet-containing phantoms was comparable to that made by a 5-second exposure in phantoms without droplets. In the in vivo study, the average lesion volumes without and with droplets were 0.017 ± 0.006 cm(3) (n = 4; 5-second exposure) and 0.265 ± 0.005 cm(3) (n = 3; 5-second exposure), respectively, a factor of 15 difference. The shape of ADV bubbles imaged with B-mode ultrasound was very similar to the actual lesion shape as measured optically and by magnetic resonance imaging. CONCLUSION ADV bubbles may facilitate clinical HIFU ablation by reducing treatment time or requisite in situ total acoustic power and provide ultrasonic imaging feedback of the thermal therapy.

Bubble evolution in acoustic droplet vaporization at physiological temperature via ultra-high speed imaging

Acoustic droplet vaporization in a rigid tube at body temperature was investigated experimentally using an ultra-high speed camera. This study was motivated by gas embolotherapy, a developmental cancer treatment in which gas microbubbles that are selectively formed by acoustically vaporizing liquid droplets in vivo are used to occlude tumor blood flow. The evolution of microbubbles formed by acoustic droplet vaporization was analyzed and a four-stage empirical curve was fit to the growth. Viscous resistance from the tube was shown to dampen oscillations of the microbubbles even though the bubble diameter was smaller than the tube diameter. The results suggest that, for some parameter values, vaporization may still be occurring when the bubble expansion starts and indicate the importance of this in modeling the growth of bubbles formed by acoustic droplet vaporization.

Cavitation nucleation agents for nonthermal ultrasound therapy

The use of a nucleation-promoting agent can greatly enhance therapeutically useful nonthermal bioeffects. A blank agent (saline), Optison ultrasound contrast agent, a stabilized perfluoropentane droplet suspension (SDS), and retained air space were compared as nucleation agents in whole blood. Fresh canine whole blood with added agent was exposed in 1.3-ml disposable pipette bulbs to lithotripter shock waves (2-Hz rate; +24.4, -5.2 MPa peak pressure amplitudes). Cavitation activity was assessed by measuring hemolysis. The droplet suspension performed nearly as well as retained air when added at a concentration sufficient to provide a roughly equal volume of gas after vaporization. Optison also yielded nucleation, but a concentration of 10%-20% was needed for large enhancement of hemolysis comparable to 5% SDS. Exposure at room temperature, which was less than the 29 degrees C boiling point of perfluoropentane, eliminated the enhancement of the hemolysis effect relative to the blank. Application of 100-kPa excess pressure during exposure reduced but did not eliminate the nucleation ability of Optison, SDS, or retained air. However, this small pressure (relative to the peak positive pressure of the shock waves) eliminated the hemolysis induced with the blank agent. The stabilized perfluoropentane droplet suspension appears to be a good nucleation agent for nonthermal ultrasound therapy applications.

Measurement of Volumetric Flow

Objective. The purpose of this study was to evaluate a 3‐dimensional (3D) sonographic method for the measurement of volumetric flow under conditions of known flow rates and Doppler angles. Methods. A GE/Kretz Voluson 730 system (GE Healthcare, Milwaukee, WI) and RAB2‐5 probe were used to acquire 3D Doppler measurements in a custom flow phantom. Blood‐mimicking fluid circulated by a computer‐controlled pump provided a range of flow velocities (2–15 mL/s). A 6‐axis positioning system maneuvered the ultrasound probe through a range of angles (40°–70° and 110°–140°) with respect to the tube (orthogonal to the tube being 90°). Volume data sets were obtained spanning 29° lateral and 20° elevational angles encompassing the flow tube in a scanning time of less than 10 seconds. Power Doppler data were used to correct for partial volume effects. Results. Using a single angle (110°) with respect to the flow tube, measured and actual volume flow rates were within the 95% confidence interval over the full range of flow rates. At flow rates of 5 and 10 mL/s, the measured volume flow rates were all within ±15% of actual values for the range of angles tested and also stayed within the 95% confidence interval. Conclusions. Direct comparisons of volume flow rates estimated with 3D sonography and known flow rates showed that the method has good accuracy. Subsequent comparisons under pulsatile and in vivo conditions will be needed to verify this performance for clinical applications.