In this thesis high intensity focused ultrasound (HIFU) is utilized for cancer treatment (thermal mode) and treatment of ischaemic stroke (mechanical mode). These two applications were investigated in vitro and in vivo models. MRI was utilized to monitor the lesions created by HIFU either in thermal or cavitation mode in freshly excised lamb brain tissue in vitro, and in rabbit brain in vivo. Additionally, MRI was used to monitor lesions deep in tissue for both in vitro and in vivo exposures. All three MRI sequences used (T1-W FSE, T2-W FSE and FLAIR) were able to detect lesions. Both thermal and bubbly lesions were best monitored using T1-W FSE with excellent contrast, proving the potential of HIFU to treat reliably tumours in the brain. A HIFU system was also used to assist thrombolysis in cooperation with a thrombolytic drug such as recombinant tissue plasminogen activator (rt-PA) in vitro and in vivo. It was shown that higher intensity results to higher volume of dissolved clot, but there is a limit of the intensity to be used in order to avoid heating of the clot and the surrounding tissue. The goal in this study was to achieve temperature elevation not exceeding 1ºC (called safe temperature). It was found that the larger the beam area the larger the dissolved clot volume. Also, the lower the frequency, the larger the volume of the dissolved clot. The results reported herein point to the use of frequency around 0.5 MHz and pulsing to optimize thrombolysis and skull penetration and at the same time avoiding unwanted heating. Finally, an Acrylonitrile Butadiene Styrene (ABS) phantom skull model was developed in order to evaluate the propagation of ultrasound using a single element transducer. The skull model was appropriately designed so that it has the same attenuation as a human skull. It was demonstrated that using a frequency of 0.5 MHz versus 1 MHz, ultrasound propagation through the phantom skull was higher. Therefore, higher frequency has poor skull penetration and a small beam size at the focus, while low frequencies have better skull penetration but with the risk of reaching the unpredictable effect of cavitation. The developed system has proven to successfully create large lesions in the brain and at the same time, these lesions are successfully monitored with excellent contrast using MRI (T1-W FSE) enabling the accurate determination of the margins of these lesions. The results reported in this study point to the use of frequency around 0.5 MHz and pulsing to optimize thrombolysis and skull penetration and at the same time avoiding unwanted heating. For treating tumours located deep in the brain and for dissolving thrombus causing an acute ischaemic stroke, further extensive clinical studies will be needed before this technology is applied to humans.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:556229 |
| Date | January 2012 |
| Creators | Hadjisavvas, Venediktos |
| Publisher | City University London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://openaccess.city.ac.uk/1399/ |
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