<p>This dissertation investigates the use of intraprocedure acoustic radiation force impulse (ARFI) imaging for visualization of radiofrequency ablation (RFA) lesions during cardiac transcatheter ablation (TCA) procedures. Tens of thousands of TCA procedures are performed annually to treat atrial fibrillation (AF) and other cardiac arrhythmias. Despite the use of sophisticated electroanatomical mapping (EAM) techniques to validate the modification of the electrical substrate, post-procedure arrhythmia recurrence is common due to incomplete lesion delivery and electrical conduction through lesion line discontinuities. The clinical demand for an imaging modality that can visually confirm the presence and completeness of RFA lesion lines motivated this research.</p><p>ARFI imaging is an ultrasound-based technique that transmits radiation force impulses to locally displace tissue and uses the tissue deformation response to generate images of relative tissue stiffness. RF-induced heating causes irreversible tissue necrosis and contractile protein denaturation that increases the stiffness of the ablated region. Preliminary in vitro and in vivo feasibility studies determined RF ablated myocardium appears stiffer in ARFI images.</p><p>This thesis describes results for ARFI imaging of RFA lesions for three research milestones: 1) an in vivo experimental verification model, 2) a clinically translative animal study, and 3) a preliminary clinical feasibility trial in human patients. In all studies, 2-D ARFI images were acquired in normal sinus rhythm and during diastole to maximize the stiffness contrast between the ablated and unablated myocardium and to minimize the bulk cardiac motion during the acquisition time.</p><p>The first in vivo experiment confirmed there was a significant decrease in the measured ARFI-induced displacement at ablation sites during and after focal RFA; the displacements in the lesion border zone and the detected lesion area stabilized over the first several minutes post-ablation. The implications of these results for ARFI imaging methods and the clinical relevance of the findings are discussed.</p><p>The second and third research chapters of this thesis describe the system integration and implementation of a multi-modality intracardiac ARFI imaging-EAM system for intraprocedure lesion evaluation. EAM was used to guide the 2-D ARFI imaging plane to targeted ablation sites in the canine right atrium (RA); the presence of EAM lesions markers and conduction disturbances in the local activation time (LAT) maps were used to find the sensitivity and specificity of predicting the presence of RFA lesion with ARFI imaging. The contrast and contrast-to-noise ratio between RFA lesion and unablated myocardium were calculated for ARFI and conventional ICE images. The opportunities and potential developments for clinical translation are discussed. </p><p>The last research chapter in this thesis describes a feasibility study of intracardiac ARFI imaging of RFA lesions in clinical patients. ARFI images of clinically relevant ablation sites were acquired, and this pilot study determined ARFI-induced displacements in human myocardium decreased at targeted ablation sites after RF-delivery. The challenges and successes of this pilot study are discussed.</p><p>This work provides evidence that intraprocedure ARFI imaging is a promising technology for the visualization of RFA lesions during cardiac TCA procedures. The clinical significance of this research is discussed, as well as challenges and considerations for future iterations of this technology aiming for clinical translation.</p> / Dissertation

Abstract not available. / by Wong Lai-hung, Grace. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 218-252). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.

<p>Ultrasound can be used for a variety of therapeutic purposes. High-intensity focused ultrasound (HIFU) has progressed over the past decade to become a viable therapeutic method and is valuable as a non-invasive alternative to many surgical procedures. Ultrasonic thermal therapies can also be used to release thermally sensitive liposomes encapsulating chemotherapeutic drugs. In the brain, the permeability of the blood-brain barrier to drugs, antibodies, and gene transfer can be increased with a mechanical mechanism using ultrasound and contrast agent.</p><p>The work presented in this dissertation tests the hypothesis that a diagnostic system can be used for combined imaging and therapeutic applications. In order to evaluate the effectiveness of a diagnostic system for use in therapeutic applications, a set of non-destructive tests is developed that can predict the potential for high acoustic output. A rigorous, nondestructive testing regimen for standard, diagnostic transducers to evaluate their potential for therapeutic use is formulated. Based on this work, transducer heating is identified as the largest challenge. The design and evaluation of several custom diagnostic transducers with various modifications to reduce internal heating are described. These transducers are compared with diagnostic controls using image contrast, face heating, hydrophone, and ARFI displacement measurements. From these results, we conclude that the most promising design is a passively and actively cooled, PZT-4 multilayer composite transducer, while the acoustically lossless lens and capactive micro-machined transducers evaluated herein are determined to be ineffective.</p><p>Three therapeutic applications are evaluated for the combined system. Image-guided spot ablations, such as in the treatment of early stage liver cancers, could not be successfully performed; however, the additional acoustic output requirements are determined to be on the order of 2.4 times those that can be currently produced without transducer damage in a clinically relevant amount of time (10-20 seconds per spot). The potential of a diagnostic system for a hyperthermia application is shown by producing temperatures for the duration necessary to release chemotherapeutic agents from thermally-activated liposomes without damage to the transducer. Finally, a mechanically-based therapeutic method for opening the BBB with ultrasonic contrast agent and specialized sonication regimes under ultrasonic B-mode guidance is demonstrated.</p><p>These studies indicate that a diagnostic system is capable of both moderate thermal and mechanical therapeutic applications under co-registered image-guidance.</p> / Dissertation