In this thesis high intensity focused ultrasound (HIFU) is utilized for myocardial ablation(thermal mode) and for the erosion of atherosclerotic plaque modelling(mechanical mode). These two applications were investigated in-vitro and in-vivo models. MRI was utilized to monitor the effects (thermal or mechanical) created by HIFU. The first application was tested in freshly excised lamb heart tissue, and in rabbit in-vivo. The second application (plaque erosion) was tested in cylindrical chalks and Hydroxyapatite-polyalactide(HA-PLA) composite in-vitro. In the thermal mode the aim was to evaluate a flat rectangular (3x10 mm2) MRI compatible transducer operating at 5.3 MHz. The main task was to explore the feasibility of creating deep lesions at a maximum 15 mm depth in myocardial tissue. The size of thermal necrosis in heart tissue was estimated as a function of power and time using a simulation model. The system was then tested in an excised lamb heart. In this thesis, we were able to create lesions of 15mm deep with acoustic power of 6W for an exposure of approximately one minute. The contrast to noise ratio (CNR) between lesion and heart tissue was evaluated using Fast Spin Echo (FSE). The CNR value was approximately 22 using T1W FSE. Maximum CNR was achieved with repetition time (TR) between 300 and 800 ms. Using T2W FSE, the corresponding CNR was approximately 13 for the 14 in-vivo experiments, The average lesion depth in the in-vivo experiments was 11.93 mm with a standard deviation of 0.62 mm. The size of the lesion in the other two dimensions was close to 3x10 mm2 (size of the transducer element). A feasibility study, was also carried out, in order to investigate the effectiveness of a therapeutic protocol in removing atherosclerotic plaque using pulsed HIFU(mechanical mode-cavitation). ln order to achieve this, the effect of various parameters such as intensity, Pulse- Repetition-Frequency(PRF.), Duty-Factor(DF), presence of bubbles and focal depth were explored. Various in-vitro experiments were carried out, mainly on cylindrical chalks. The efficiency of pulsed-cavitational HIFU was also investigated further, in-vitro during experiments on HA-PLA composite. The study showed that the size of the generated holes on chalks as expected increases with increased parameters being investigated ( i.e. intensity, power factor and duty factor). Duty factor was proven to be a critical parameter. An optimum value of the duty factor is approximately 10%. Lowest values of DF mean lost of the cavitation effect. The effect of HIFU in removing plaque was also verified using MRI images. As mentioned above, the effect of bubbles was also investigated. It is obvious, that with the presence of bubble . cavitation was enhanced achieving better penetration results on the chalks.The size of the holes generated on the chalks by the planar transducer in a plane parallel to the transducer face is slightly less(8.5x3 mm2) than the transducer area (10x3 mm2). Positive results were obtained in carrying out experiments on HA-PLA composites using both, spherical and planar transducers. In conclusion this research provides evidence which suggests that HIFU can be used as an alternative technique for the treatment of heart arrthythmias and for the erosion of atherosclerotic plaque.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:578049 |
| Date | January 2012 |
| Creators | Couppis, Andreas |
| Publisher | City University London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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