Stuart Foster

Nanoparticle-loaded perfluorocarbon droplets for imaging and therapy

Nanoscale perfluorocarbon droplets that are in the liquid phase at physiological temperatures, but which can be converted to gas using ultrasound, offer potential as a contrast agent for the detection and therapy of solid tumours. Nanoparticles such as quantum dots can also be encapsulated within PFC droplets, enabling multi-modal imaging and controlled nanoparticle release. In this work, experiments were conducted to investigate the impact of nanoparticle incorporation on droplet conversion at low and high ultrasound frequencies. It was found that incorporation of quantum dots lowered the inertial cavitation threshold at 1 MHz by 20%. In contrast, quantum dot nanoparticles did not significantly alter the conversion threshold of perfluorohexane or perfluoropentane droplets at 18 MHz. It was also shown that perfluoropentane droplets could be converted to gas and imaged at high frequency in hepatomas in mice, using brief high pressure bursts to achieve the phase conversion. Finally, optically fluorescent quantum dots incorporated within droplets were used to demonstate the feasibility of assessing biodistribution in rabbits using fluorescence histology.

A novel method for the measurement of proximal fibrous cap puncture force in chronic total occlusions: the effect of increasing age.

AIMS The composition of CTO lesions changes significantly as they age. These changes may adversely affect PCI outcome. We present a robust ex vivo technique to measure the force required to puncture the proximal cap of CTOs, and to determine whether the puncture force differs according occlusion age to examine the effects of compositional changes over time on lesion stiffness. METHODS AND RESULTS Occlusions were created in 44 rabbit femoral arteries by thrombin injection. Between two and 15 weeks following induction, vessels were harvested and tested in a custom setup to determine the force required to puncture the proximal cap. The puncture force mean values at 2, 6, 12, and 15 weeks were 0.61N, 0.78N, 1.21N and 1.52N, respectively. The puncture force required in occlusions of ≤6 weeks was significantly lower those≥12 weeks (0.72±0.10N versus 1.45±0.13N, p<0.01). Using a cutoff point of 1N, 86% of lesions≤6 weeks of age required<1N compared to 30% of those≥12 weeks. CONCLUSIONS We have shown an objective and reproducible testing system for measuring CTO puncture force. Puncture force is correlated with occlusion age. This technique would be useful to evaluate therapies that alter CTO composition/compliance, as well as guidewire testing.

High frequency ultrasonics using PZT sol gel composites

Abstract Piezoelectric arrays suitable for high frequency ultrasound have been fabricated using PZT sol gel composite coatings in the thickness range of 5–100μm on aluminum and platinized alumina substrates. The coatings have been quantitatively characterized using impedance measurements to 100 MHz. The analysis provides a direct measure of the piezoelectric coupling coefficient of kt < 0.34. 16 element linear array structures suitable for ultrasound biomicroscopy at 30–40 MHz with elements having dimensions ∼40μm thick, 20–25μm wide and 15μm spacing have been patterned using excimer based laser micromachining. The dielectric and pulse echo responses have been measured.

Convertible perfluorocarbon droplets for cancer detection and therapy

To enhance the detection and treatment of solid tumours, submicron droplets of liquid perfluorocarbon (PFC) have been developed which can accumulate in tumours by extravasation, and which can subsequently convert to highly echogenic gas bubbles using bursts of high-pressure ultrasound. One aim of droplet formulation is to balance the requirements of physical stability with acoustic convertibility. In this study, droplets ranging in size from 200nm to 2μm, composed of liquid perfluoropentane (PFP) or pefluorohexane (PFH) encapsulated by a fluorosurfactant were synthesized by extrusion and sonication. Droplet stability was assessed using fluorescence microscopy of incorporated quantum dots following incubation with macrophage cells over 4 hours. The impact on stability of depositing 1–3 layers of polyelectrolytes on the droplet surface was also assessed. The rate of coalescence was significantly reduced for droplets coated with 3 polyelectrolyte layers. At 10 MHz, the conversion threshold MI decreased from greater than 4 for 200nm PFP to less than 2 for 2μm PFP droplets, and did not depend strongly on the addition of polyelectrolytes. Resulting bubbles from vaporization persisted for at least 1s and were detectable using clinical nonlinear contrast detection pulse sequences, however, the conversion of droplets within cells resulted in cell death.

Adaptive processing on transmit and receive for high frequency composite transducers

High frequency ultrasound imaging systems are used in many fields (eye, skin, preclinical animal imaging). To date, making high frequency composite transducers for such systems is a challenge due to the small sizes of the composite elements. A width-to-height ratio of less than 0.5 is generally required to avoid interferences from lateral resonances in the composites. This is difficult to reach in making thin composites for high frequency applications. In this study, we investigate the use of adaptive driving pulses on transmit and adaptive filters on receive for composite transducers, to cancel out the effects of lateral resonances and generate shorter pulses. The required adaptive driving pulse and adaptive filter can be determined from simulations and experiments. FEM simulations showed that bandwidth and level of secondary pulses, can be significantly improved using adaptive pulses to drive composite transducers with square, triangular and hexagonal PZT pillars. Two composite transducers, with hexagonal PZT pillars and an aspect ratio of 0.72 and 0.48, respectively, were tested. The −6dB bandwidth measured in the far field increased from 71% to 78% for the first transducer and 40% to 115% for the second. The −20dB pulse width decreased by 48% and 67%, respectively. And the level of secondary pulses decreased from −10dB to −23dB and from −6.5dB to −28dB, respectively. Pulse-echo experiments using a 15 µm wire positioned in the far field of the first transducer in water, showed a decrease of the pulse width, from 0.49 µs with a one-cycle driving pulse to 0.21 µs with an adapted driving pulse and to 0.18 µs after adaptive filtering on receive. Pulse echo experiments and phantom images with the second transducer also showed great improvements using this adaptive processing technique.

Abstract 4235: Functional assessment of mouse tumor microenvironment following sunitinib treatment using an integrated micro-ultrasound and photoacoustic system

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL The development of novel cancer therapies could benefit significantly from the introduction of new functional imaging methods that allow non-invasive longitudinal assessment of tumor response to therapy. We have developed and tested a new instrument in which photoacoustic (PA) imaging is combined with co-registered micro-ultrasound (µUS). The new system allows tumor oxygenation and hemoglobin data derived from PA imaging to be combined with blood volume and perfusion data derived from contrast µUS and be observed during the course of therapy. Using this system, we assessed changes in the tumor microenvironment following treatment with an anti-angiogenic drug, sunitinib (Pfizer, USA). Human metastatic breast cancer cells (231/LM2-4) were surgically implanted in the mammary fat pads of 4 control and 7 treated female nude SCID mice and were allowed to grow for 10 days prior to initiation of experimental treatment, which consisted of either 4 consecutive daily gavage doses of 120mg/kg sunitinib, or control vehicle. Imaging was performed, using the VevoLAZR (VisualSonics, Canada) integrated µUS/PA system, prior to and following treatment. Tumor volume was quantified with 3D ultrasound imaging using a 40MHz frequency probe. Indices of relative blood volume and perfusion were quantified with non-linear contrast imaging using a 21MHz probe during a 50uL (2x109/mL) intravenous bolus injection of microbubbles (MicroMarker, VisualSonics). Blood oxygen saturation, relative tissue oxygen saturation, and hemoglobin concentration were measured with photoacoustic imaging using an integrated photoacoustic probe with 21MHz ultrasound frequency and tuneable 680-970nm laser optics. Following treatment, we observed significant (p<.05) suppression in tumor growth (-35%) decrease in blood volume (-91%), perfusion (-86%), relative tissue oxygen saturation (-60%), and hemoglobin concentration (-40%) in the sunitinib- relative to control-treated mice. When comparing pre- and post-treatment within the control group, there were increases in tumor volume (+120%), however, interestingly, there were also decreases in perfusion (-52%), blood volume (-22%) and relative tissue oxygen saturation (-31%). When comparing contrast µUS and PA data, there was a strong, moderate, and weaker correlations between relative tissue oxygen saturation and perfusion (R2 = 0.722), relative tissue oxygenation and blood volume (R2 = 0.576), and blood volume and hemoglobin concentration (R2 = 0.294) respectively. This study demonstrates the ability of an integrated PA and µUS imaging system to provide quantitative functional assessment of a preclinical breast cancer model following treatment with sunitinib. The degree to which quantitative correlates such as these are indicative of useful therapeutic response and of prognostic value remain to be investigated. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4235. doi:1538-7445.AM2012-4235

The influence of attachment on the nonlinear behaviour of lipid encapsulated microbubbles at high frequencies

Molecular imaging at higher frequencies (f >;10 MHz) has applications in a preclinical context for a range of disease processes and clinically for the detection of vulnerable atherosclerotic plaques and superficial tumours. The subharmonic response from individual microbubbles in either a bound or unbound state was investigated at both 11 and 25 MHz transmit frequencies. At 11 MHz, unbound bubbles (n=53) were found to have strong subharmonic activity for sizes between 2.4-2.6 μm, while bound (n=50) were most active from 2.6-3.0 μm. Destruction rates were found to differ, with very few unbound bubbles undergoing destruction. At 25 MHz, bound bubble (n=57) activity was found to peak at 1.9 μm as compared to 2.1 μm for the unbound case (n=53), with a 20% increase in amplitude. Comparison with simulations indicates that both unbound and bound bubbles undergo “compression-only” behaviour at 11 MHz, and “expansion-dominated” behaviour at 25 MHz. Subharmonic emissions elicited from 0 radian transmit pulses were found to be p/2 radians out of phase with those elicited from a p radian transmit pulse, suggesting inefficient subharmonic preservation from pulse inversion schemes.

High repetition rate bismuth borate optical parametric oscillator pumped by a cavity-dumped Nd:YLF laser

We present a bismuth borate optical parametric oscillator pumped by cavity-dumped Nd:YLF laser delivering low jitter 9.5ns millijoule pulses at 1kHz tunable from 720 to 900nm that is ideally suited for photoacoustic medical imaging.

Functional imaging of the rat brain with micro-ultrasound

Linear array based micro-ultrasound provides 40–150um resolution over a significant depth of field at frames rates as high as 1000fps. Current imaging modalities for investigating in vivo brain function are challenged to provide this combination of imaging parameters. The present experiment was carried out to investigate the potential of micro-ultrasound in neuroimaging of rodents in vivo.