Dodecafluoropentane ultrasonic contrast enhancement in carotid diagnosis: preliminary results

The impact of vaporized nanoemulsions on ultrasound-mediated ablation

Background

The clinical feasibility of using high-intensity focused ultrasound (HIFU) for ablation of solid tumors is limited by the high acoustic pressures and long treatment times required. The presence of microbubbles during sonication can increase the absorption of acoustic energy and accelerate heating. However, formation of microbubbles within the tumor tissue remains a challenge. Phase-shift nanoemulsions (PSNE) have been developed as a means for producing microbubbles within tumors. PSNE are emulsions of submicron-sized, lipid-coated, and liquid perfluorocarbon droplets that can be vaporized into microbubbles using short (<1 ms), high-amplitude (>5 MPa) acoustic pulses. In this study, the impact of vaporized phase-shift nanoemulsions on the time and acoustic power required for HIFU-mediated thermal lesion formation was investigated in vitro.

Methods

PSNE containing dodecafluoropentane were produced with narrow size distributions and mean diameters below 200 nm using a combination of sonication and extrusion. PSNE was dispersed in albumin-containing polyacrylamide gel phantoms for experimental tests. Albumin denatures and becomes opaque at temperatures above 58°C, enabling visual detection of lesions formed from denatured albumin. PSNE were vaporized using a 30-cycle, 3.2-MHz, at an acoustic power of 6.4 W (free-field intensity of 4,586 W/cm²) pulse from a single-element, focused high-power transducer. The vaporization pulse was immediately followed by a 15-s continuous wave, 3.2-MHz signal to induce ultrasound-mediated heating. Control experiments were conducted using an identical procedure without the vaporization pulse. Lesion formation was detected by acquiring video frames during sonication and post-processing the images for analysis. Broadband emissions from inertial cavitation (IC) were passively detected with a focused, 2-MHz transducer. Temperature measurements were acquired using a needle thermocouple.

Results

Bubbles formed at the HIFU focus via PSNE vaporization enhanced HIFU-mediated heating. Broadband emissions detected during HIFU exposure coincided in time with measured accelerated heating, which suggested that IC played an important role in bubble-enhanced heating. In the presence of bubbles, the acoustic power required for the formation of a 9-mm³ lesion was reduced by 72% and the exposure time required for the onset of albumin denaturation was significantly reduced (by 4 s), provided that the PSNE volume fraction in the polyacrylamide gel was at least 0.008%.

Conclusions

The time or acoustic power required for lesion formation in gel phantoms was dramatically reduced by vaporizing PSNE into bubbles. These results suggest that PSNE may improve the efficiency of HIFU-mediated thermal ablation of solid tumors; thus, further investigation is warranted to determine whether bubble-enhanced HIFU may potentially become a viable option for cancer therapy.

Value of a contrast agent in equivocal carotid ultrasound studies: pictorial essay

The aim of the present study was to assess the use of an echo-enhancing agent (Levovist; Schering AG) in equivocal carotid bifurcation ultrasound studies and compare the information obtained with digital subtraction angiography (DSA). Contrast-enhanced carotid ultrasound studies were performed on 30 carotid bifurcations in 28 patients. The standard carotid ultrasound examinations were considered equivocal for two reasons: apparent acute internal carotid artery occlusions (n = 10), and possibly patent but critically stenosed internal carotid arteries with the residual flow lumen being incompletely visualized (n = 20). All patients underwent subsequent carotid digital subtraction angiography. All patients with apparent acute carotid occlusions (n = 10) were correctly characterized on contrast-enhanced ultrasound when compared with DSA. The majority were complete occlusions (n = 8) although in two cases there were critical carotid stenoses requiring surgical endarterectomy. In the 'incompletely visualized lumen' group (n = 20), the majority (n = 16) were correctly characterized on contrast enhanced ultrasound: 13 cases of critically stenotic but patent internal carotid arteries, two cases without a haemodynamically significant stenosis and one case of a carotid occlusion with patent vasa vasorum. One of the critical carotid stenoses was prospectively reported as occluded on the 'gold standard' angiography. In three cases the flow lumen was still incompletely visualized due to calcified plaque despite an echo-enhancing agent; angiography revealed no significant stenosis in all cases. There was one false negative for internal carotid occlusion. This occurred early in the series and could be considered to be a technical error. Importantly, there were no false positives for carotid occlusion. Contrast-enhanced carotid ultrasound significantly improves diagnostic confidence in equivocal carotid ultrasound studies. In appropriate clinical settings this may reduce the need for subsequent carotid angiography.