Acoustic characterisation of ultrasound contrast agents at high frequency
Abstract
This thesis aims to investigate the acoustic properties of ultrasound contrast agents
(UCAs) at high ultrasound frequencies. In recent years, there has been increasing
development in the use of high frequency ultrasound in the fields of preclinical,
intravascular, ophthalmology and superficial tissue imaging. Although research
studying the acoustic response of UCAs at low diagnostic ultrasonic frequencies has
been well documented, quantitative information on the acoustical properties of UCAs
at high ultrasonic frequencies is limited.
In this thesis, acoustical characterisation of three UCAs was performed using a
preclinical ultrasound scanner (Vevo 770, VisualSonics Inc., Canada).
Initially the
acoustical characterisation of five high frequency transducers was measured using a
membrane hydrophone with an active element of 0.2 mm in diameter to quantify the
transmitting frequencies, pressures and spatial beam profiles of each of the
transducers. Using these transducers and development of appropriate software, high
frequency acoustical characterisation (speed and attenuation) of an agar-based tissue
mimicking material (TMM) was performed using a broadband substitution
technique. The results from this study showed that the acoustical attenuation of
TMM varied nonlinearly with frequency and the speed of sound was approximately
constant 1548m·s-1 in the frequency range 12-47MHz.
The acoustical properties of three commercially available lipid encapsulated UCAs
including two clinical UCAs Definity (Lantheus Medical Imaging, USA) and
SonoVue (Bracco, Italy) and one preclinical UCAs MicroMarker (untargeted)
(VisualSonics, Canada) were studied using the software and techniques developed
for TMM characterisation. Attenuation, contrast-to-tissue ratio (CTR) and
subharmonic to fundamental ratio were measured at low acoustic pressures.
The
results showed that large off-resonance and resonant MBs predominantly contributed
to the fundamental response and MBs which resonated at half of the driven
frequency predominantly contributed to subharmonic response. The effect of needle
gauge, temperature and injection rate on the size distribution and acoustic properties
of Definity and SonoVue was measured and was found to have significant impacts.
Acoustic characterisations of both TMM and UCAs in this thesis extend our
understanding from low frequency to high frequency ultrasound and will enable the
further development of ultrasound imaging techniques and UCAs design specifically
for high frequency ultrasound applications.
