Mairead Butler

The Speed of Sound and Attenuation of an IEC Agar-Based Tissue-Mimicking Material for High Frequency Ultrasound Applications

This study characterized the acoustic properties of an International Electromechanical Commission (IEC) agar-based tissue mimicking material (TMM) at ultrasound frequencies in the range 10–47 MHz. A broadband reflection substitution technique was employed using two independent systems at 21°C ± 1°C. Using a commercially available preclinical ultrasound scanner and a scanning acoustic macroscope, the measured speeds of sound were 1547.4 ± 1.4 m∙s−1 and 1548.0 ± 6.1 m∙s−1, respectively, and were approximately constant over the frequency range. The measured attenuation (dB∙cm−1) was found to vary with frequency f (MHz) as 0.40f + 0.0076f2. Using this polynomial equation and extrapolating to lower frequencies give values comparable to those published at lower frequencies and can estimate the attenuation of this TMM in the frequency range up to 47 MHz. This characterisation enhances understanding in the use of this TMM as a tissue equivalent material for high frequency ultrasound applications.

The "quasi-stable" lipid shelled microbubble in response to consecutive ultrasound pulses

Controlled microbubble stability upon exposure to consecutive ultrasound exposures is important for increased sensitivity in contrast enhanced ultrasound diagnostics and manipulation for localised drug release. An ultra high-speed camera operating at 13 × 106 frames per second is used to show that a physical instability in the encapsulating lipid shell can be promoted by ultrasound, causing loss of shell material that depends on the characteristics of the microbubble motion. This leads to well characterized disruption, and microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures.

Single Microbubble Response Using Pulse Sequences: Initial Results

The study of acoustic scattering by single microbubbles has the potential to offer improved signal processing techniques. A microacoustic system that employs a hydrodynamically-focused flow was used to detect radiofrequency (RF) backscatter from single microbubbles. RF data were collected using a commercial scanner. Results are presented for two agents, namely Definity (Lantheus Medical Imaging, N. Billerica, MA, USA) and biSphere (Point Biomedical Corp, San Carlos, CA, USA). The agents were insonified with amplitude-modulated pulses, and it was observed in both agents that a subpopulation of microbubbles did not produce a measurable echo from the first-half amplitude pulse, but did produce a response from the full amplitude pulse and from a subsequent half amplitude pulse. The number of microbubbles in this subpopulation was seen to increase with increasing transmit amplitude. These results do not bear out the simple theory of microbubble-pulse sequence interaction and invite a reassessment of signal processing approaches.

In vitro acoustic characterization of three phospholipid ultrasound contrast agents from 12 to 43 MHz.

The acoustic properties of two clinical (Definity, Lantheus Medical Imaging, North Billerica, MA, USA; SonoVue, Bracco S.P.A., Milan, Italy) and one pre-clinical (MicroMarker, untargeted, Bracco, Geneva, Switzerland; VisualSonics, Toronto, ON, Canada) ultrasound contrast agent were characterized using a broadband substitution technique over the ultrasound frequency range 12–43 MHz at 20 ± 1°C. At the same number concentration, the acoustic attenuation and contrast-to-tissue ratio of the three native ultrasound contrast agents are comparable at frequencies below 30 MHz, though their size distributions and encapsulated gases and shells differ. At frequencies above 30 MHz, native MicroMarker has higher attenuation values and contrast-to-tissue ratios than native Definity and SonoVue. Decantation was found to be an effective method to alter the size distribution and concentration of native clinical microbubble populations, enabling further contrast enhancement for specific pre-clinical applications.

P1F-6 Development of a Novel Experimental Set-Up to Allow Investigation of the Ultrasonic Backscatter from Microbubble Contrast Agents Attached to Surfaces

Developing applications of ultrasound contrast agents include targeting to areas such as inflamed plaque, drug delivery and gene therapy. In order to develop these techniques to their full potential the interaction of individual attached microbubbles with ultrasound and the mechanisms of transfer of material from microbubble to targeted areas needs to be fully understood. Aim: To develop an experimental set-up and technique suitable for determining the ultrasonic backscatter from individual, attached microbubbles and for the study of the transfer of material from single bubbles. A tank allowing acoustic and optical imaging was constructed with dimensions of: optical imaging section 34 times 16 times 4 cm, acoustic imaging section 30 times 16 times 30 cm. A microscope slide incorporated into the base of the optical imaging section maximised the quality of the microscope images. The imaging modalities were separate in the tank to allow precise characterisation of the acoustic field and to minimise reflections from surfaces other than the membrane. A holder comprising two Perspex rings held a 12 mum polyester membrane. A sliding device allowed movement of the membrane holder between the ultrasound field and the microscope. A membrane hydrophone with active element of 0.5 mm was used to determine the acoustic field at the surface of the membrane and within the tank. Copper (Cu) spheres of diameter 40-80 mum, attached to the membrane using poly-L-lysine (PLL), were used to assess the experimental set-up and to aid with alignment of the microscope optics with the acoustic field. A Sonos5500 scanner with S3 transducer of frequency range 1.26-3.75 MHz was used. The transducer was placed at an angle to the membrane to minimise the received echo, the area of interest on the membrane was 7.5 cm from the transducer. A sequence of 6 pulses was used. Previously the minimum detectable pressure with the ultrasound settings was determined to be 0.1 Pa. Cu spheres were used to assess the suitability of the set-up for studying microbubbles. Commercially available Definity microbubbles, mean diameter 1.1-3.3 mum, have been attached using PLL and imaged with 2 MHz ultrasound. The acoustic field was characterised for 1.26-3.75 MHz, for acoustic pressures of 300 and 550 kPa. Using the sliding device, Cu spheres attached to the membrane were found to reposition over the microscope objective within 20 mum of the original position which will allow optical imaging of contrast agents before and after insonation. The tank and set up has been shown to allow detection of the ultrasonic backscatter from individual particles attached to a membrane where the ultrasonic field at the location of the particle can be well calibrated. Definity attached to a membrane can be imaged and future experiments will investigate ultrasonic backscatter from attached single microbubbles for frequencies up to 11.1 MHz

Probing microbubble targeting with atomic force microscopy

Microbubble science is expanding beyond ultrasound imaging applications to biological targeting and drug/gene delivery. The characteristics of molecular targeting should be tested by a measurement system that can assess targeting efficacy and strength. Atomic force microscopy (AFM) is capable of piconewton force resolution, and is reported to measure the strength of single hydrogen bonds. An in-house targeted microbubble modified using the biotin-avidin chemistry and the CD31 antibody was used to probe cultures of Sk-Hep1 hepatic endothelial cells. We report that the targeted microbubbles provide a single distribution of adhesion forces with a median of 93pN. This interaction is assigned to the CD31 antibody-antigen unbinding event. Information on the distances between the interaction forces was obtained and could be important for future microbubble fabrication. In conclusion, the capability of single microbubbles to target cell lines was shown to be feasible with AFM.

The acoustic response from individual attached and unattached rigid shelled microbubbles

A comparison of the acoustic response of attached and unattached individual microbubbles is presented. biSphere™ was insonated by 1.6 MHz ultrasound, peak negative acoustic pressures of 550 and 800 kPa, and the unprocessed radio frequency echo data collected. Comparison of the mean backscatter pressure from unattached and attached biSphere™ at these pressures showed no significant difference. No backscatter was detected for attached microbubbles insonated at 300 kPa. We surmised that at 550 and 800 kPa the rigid-shelled microbubble cracked on insonation, resulting in backscatter from an air microbubble in both cases. The dissolution rate of the gas was slower for attached biSphere™.

The acoustic signature of decaying resonant phospholipid microbubbles

Sub-capillary sized microbubbles offer improved techniques for diagnosis and therapy of vascular related disease using ultrasound. Their physical interaction with ultrasound remains an active research field that aims to optimize techniques. The aim of this study is to investigate whether controlled microbubble disruption upon exposure to consecutive ultrasound exposures can be achieved. Single lipid-shelled microbubble scattered echoes have been measured in response to two consecutive imaging pulses, using a calibrated micro-acoustic system. The nonlinear evolution of microbubble echoes provides an exact signature above and below primary and secondary resonance, which has been identified using theoretical results based on the Mooney-Rivlin strain softening shell model. Decaying microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures. The characteristics of the microbubble motion causes varying amounts of shell material to be lost during microbubble decay. Incident ultrasound field parameters can thus accurately manipulate the regulated shedding of shell material, which has applications for both imaging applications and localized drug delivery strategies.

Cardiac imaging with high frame rate contrast enhanced ultrasound: In-vivo demonstration

This work presents the first in-vivo High-frame rate Contrast Enhanced Ultrasound (HFR CEUS) for cardiac application. The in-vivo acquisition has been made on a sheep. A coherent compounding of diverging waves combined with Pulse Inversion (PI) transmission allow a frame rate of 250 frame per seconds which is 8 times faster than standard CEUS acquisition in cardiac application. The proposed method improves the image contrast compared to the CEUS and allows a better tracking of fast movement of the heart.

Influence of temperature, needle gauge and injection rate on the size distribution, concentration and acoustic responses of ultrasound contrast agents at high frequency

This paper investigated the influence of needle gauge (19G and 27G), injection rate (0.85ml·min(-1), 3ml·min(-1)) and temperature (room temperature (RT) and body temperature (BT)) on the mean diameter, concentration, acoustic attenuation, contrast to tissue ratio (CTR) and normalised subharmonic intensity (NSI) of three ultrasound contrast agents (UCAs): Definity, SonoVue and MicroMarker (untargeted). A broadband substitution technique was used to acquire the acoustic properties over the frequency range 17-31MHz with a preclinical ultrasound scanner Vevo770 (Visualsonics, Canada). Significant differences (P<0.001-P<0.05) between typical in vitro setting (19G needle, 3ml·min(-1) at RT) and typical in vivo setting (27G needle, 0.85ml·min(-1) at BT) were found for SonoVue and MicroMarker. Moreover we found that the mean volume-based diameter and concentration of both SonoVue and Definity reduced significantly when changing from typical in vitro to in vivo experimental set-ups, while those for MicroMarker did not significantly change. From our limited measurements of Definity, we found no significant change in attenuation, CTR and NSI with needle gauge. For SonoVue, all the measured acoustic properties (attenuation, CTR and NSI) reduced significantly when changing from typical in vitro to in vivo experimental conditions, while for MicroMarker, only the NSI reduced, with attenuation and CTR increasing significantly. These differences suggest that changes in physical compression and temperature are likely to alter the shell structure of the UCAs resulting in measureable and significant changes in the physical and high frequency acoustical properties of the contrast agents under typical in vitro and preclinical in vivo experimental conditions.