|Title:||Characterization of biogenic nanobubble for molecular ultrasound imaging and therapy|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xiv, 110 pages : illustrations (some color)|
|Abstract:||Microbubble is a widely used ultrasound contrast agent with a spherical shell encapsulating inertial gas. However, due to the micrometer size, microbubble is limited within blood circulation, so it is mainly used for mapping blood vessels and endothelium cells therapy. Reducing size may make it possible to image extravascular region providing the particle can leak out of vessel wall. However, in traditional artificial synthesis method, the bubble in nano-meter size (nanobubble) is unstable and hard to be surface modified. The poor stability results in short circulation time and make it hard accumulate in focused region. Recently, biogenetic nanobubble called nano gas vesicle at the size of ~250nm is reported. It is much more stable than synthesized bubble, and become possible to travel across the endothelial layer to reach cells beyond. To further understand its behaviour in acoustic field and biological environment, this research investigated the acoustic property of the nanobubble, and understand its potential in contrast enhanced ultrasound imaging. In addition, the surface modification ability and the biological effect on cell attachment and endocytosis were also studied. Nanobubble was isolated from Anabaena flos-aquae by lysing and centrifuge method. The basic properties of nanobubble were characterized such as concentration, size distribution, microstructure, zeta-potential and stability. The morphologycharacterization result is consistent with previous research. Biogenic nanobubble is demonstrated to be extremely stable in vitro.|
To study the acoustic property of nanobubble, attenuation coefficient was measured using pulse echo method by comparing results with and without nanobubble present. Attenuation coefficient is the indicator of resonance frequency, damping and Q-value. The experiments were repeated by three customized high frequency transducers with centre frequencies of 22 MHz, 40 MHz and 48 MHz covering a broad frequency band from 7 MHz to 103 MHz. Resonance frequency is found to be 88 MHz. Harmonic Property was measured by two transducers, one for transmit and the other for receiver. The cencer frequency of receiver transducer is twice the transmit one. Different incident ultrasound pressure gradient was tested with transmit ultrasound wave at half of the resonance frequency of nanobubble. Significant second harmonic frequency was observed. The nanobubble stability under sonication is demonstrated to be able to last for over 1 hr. The biogenic nanobubble has both strengthened stability and non-linear property indicating its potential for accumulate in extravascular region and perform molecular imaging. Biological effect of nanobubble at cellular level was characterized by incubation with HeLa tumor cells. For nanobubble floating effect, two different cell distribution was tested, one attached to the culture dish and one disperse in the culture medium. In addition, the ability of nanobubble for cell attachment and endocytosis was studied. The fluorescence probe PpIX was bond to nanobubble surface and confocal microscope was used to characterize the intracellular distribution of nanobubble. Experiments have demonstrated that biogenic nanobubble can be internalized by tumor cells. Enhanced stability ensures nanobubble enough circulation time to accumulate in extravascular site; non-linear property can improve the contrast in ultrasound imaging; easily surface modified property indicates its ability to be a smart molecular or cellular probe with antibody ligand; endocytosis ability make it possible to deliver drug or gene into cytoplasm. Compared with conventional nanobubble, these four enhanced properties demonstrate the great potential of biogenic nanobubble for molecular imaging and therapy.
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