Abstract:
The influence of highly regular, anisotropic, microstructured materials on high frequency
ultrasonic wave propagation was investigated in this work. Microstructure, often only treated as
a source of scattering, significantly influences high frequency ultrasonic waves, resulting in
unexpected guided wave modes. Tissues, such as skin or muscle, are treated as homogeneous
by current medical ultrasound systems, but actually consist of highly anisotropic micron-sized
fibres. As these systems increase towards 100 MHz, these fibres will significantly influence
propagating waves leading to guided wave modes. The effect of these modes on image quality
must be considered. However, before studies can be undertaken on fibrous tissues, wave
propagation in more ideal structures must be first understood.
After the construction of a suitable high frequency ultrasound experimental system, finite
element modelling and experimental characterisation of high frequency (20-200 MHz)
ultrasonic waves in ideal, collinear, nanostructured alumina was carried out. These results
revealed interesting waveguiding phenomena, and also identified the potential and significant
advantages of using a microstructured material as an alternative acoustic matching layer in
ultrasonic transducer design. Tailorable acoustic impedances were achieved from 4-17 MRayl,
covering the impedance range of 7-12 MRayl most commonly required by transducer matching
layers. Attenuation coefficients as low as 3.5 dBmm-1 were measured at 100 MHz, which is
excellent when compared with 500 dBmm-1 that was measured for a state of the art loaded
epoxy matching layer at the same frequency. Reception of ultrasound without the restriction of
critical angles was also achieved, and no dispersion was observed in these structures (unlike
current matching layers) until at least 200 MHz.
In addition, to make a significant step forward towards high frequency tissue characterisation,
novel microstructured poly(vinyl alcohol) tissue-mimicking phantoms were also developed.
These phantoms possessed acoustic and microstructural properties representative of fibrous
tissues, much more realistic than currently used homogeneous phantoms. The attenuation
coefficient measured along the direction of PVA alignment in an example phantom was 8
dBmm-1 at 30 MHz, in excellent agreement with healthy human myocardium. This method will
allow the fabrication of more realistic and repeatable phantoms for future high frequency tissue
characterisation studies.
