Sonication methods and motion compensation for magnetic resonance guided high-intensity focused ultrasound

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk. fi Author Max Köhler Name of the dissertation Sonication methods and motion compensation for magnetic resonance guided high-intensity focused ultrasound Manuscript submitted 9.6. 2009 Manuscript revised 30.10. 2009 Date of the defence 4.12. 2009 Monograph Article dissertation (summary + original articles) Faculty Faculty of Information and Natural Sciences Department Department of Biomedical Engineering and Computational Science Field of research Biomedical Engineering Opponent(s) Professor Kullervo Hynynen, University of Toronto Supervisor Professor Pekka Meriläinen Instructor Gösta Ehnholm, D.Sc. (Tech.) Abstract High-intensity focused ultrasound (HIFU) is an efficient noninvasive therapeutic technique for localized heating of tissues deep within the human body through intact skin. Magnetic resonance imaging (MRI) can provide excellent soft-tissue contrast and can be used for both treatment planning and post-treatment assessment of the induced tissue damage. MRI can also provide temperature sensitive in vivo images via proton resonance frequency shift thermometry. Combined, the use of MRI and HIFU (MR-HIFU) ablation make for a promising therapeutic modality for controlled and noninvasive selective tissue destruction. Sonication strategies, MR thermometry methods, feedback control, and motion compensation for MRHIFU were developed and evaluated in this thesis.High-intensity focused ultrasound (HIFU) is an efficient noninvasive therapeutic technique for localized heating of tissues deep within the human body through intact skin. Magnetic resonance imaging (MRI) can provide excellent soft-tissue contrast and can be used for both treatment planning and post-treatment assessment of the induced tissue damage. MRI can also provide temperature sensitive in vivo images via proton resonance frequency shift thermometry. Combined, the use of MRI and HIFU (MR-HIFU) ablation make for a promising therapeutic modality for controlled and noninvasive selective tissue destruction. Sonication strategies, MR thermometry methods, feedback control, and motion compensation for MRHIFU were developed and evaluated in this thesis. The primary aim of the thesis was to develop a safe and efficient strategy for clinical MR-HIFU ablation. An efficient volumetric method of ablation was achieved by utilizing the phased-array capabilities of the transducer and the inherent heat diffusion of already deposited heat. The induced temperature rise was monitored with rapid multiplane MR thermometry with a volumetric coverage of the heated region. Acquisition and display of temperature images during sonication improved the safety of the therapy. The therapeutic procedure was evaluated in a large animal model and proved to provide a substantial improvement in efficiency as compared to existing methods without compromising safety. The second aim was to improve the reliability of the proposed volumetric sonication strategy. This was achieved with a simple and robust binary feedback algorithm that adjusted the sonication duration of each part of the sonication trajectory based on the temperature rise as obtained by volumetric MR thermometry. The feedback algorithm was evaluated in a large animal model, and was found to reduce the variability in thermal lesion size by approximately 70%. The third aim was to develop a through-plane motion correction method for real-time MR thermometry without disturbing thermometry. This was achieved with a fat-selective navigator. This navigator outperformed the conventional navigator for direct tracking of the kidney under free breathing. The navigator also provided accurate indexing of the look-up-table used to correct the reference phase for MR thermometry of mobile organs. Finally, the combination of through-plane motion correction provided by the fat-selective navigator with existing methods of in-plane motion correction and reference phase correction, allowed for an accurate 3D motion compensation of both MR thermometry and MR-HIFU sonication.