High-intensity focused ultrasound (HIFU) is an emerging therapeutic technique that can be used to heat tissue locally and non-invasively through skin. Heating the tissue to a temperature high enough, e.g., to 57°C, induces instantaneous cell death through thermal coagulative necrosis. Alternatively, HIFU can be used for hyperthermia, in which tissue temperature is maintained within a range of 40–45°C for a longer period of time (e.g., 60 min) to enhance the effect of other therapy modalities, such as radiotherapy and chemotherapy. Magnetic resonance imaging (MRI) as guidance of HIFU treatments (MR-HIFU) provides means for spatially targeting the treatment, measuring the temperature evolution in real-time during heating, and evaluating tissue damage after therapy. The primary aim of this Thesis was to develop a hyperthermia solution for an MR-HIFU system to enable safely heat large volumes at clinically relevant depths from the skin with reliable MRI-based temperature mapping. The solution combined mechanical movements of the ultrasound transducer, electronic ultrasound focus steering, and selective use of transducer elements to control the temperature distribution within the entire acoustic beam path. Several MRI temperature mapping slices were used to control and monitor the heating in real-time. Safe hyperthermia heating with good temperature uniformity was achieved in animal experiments in vivo. Furthermore, in vivo animal experiments and patient imaging study showed that the developed hyperthermia solution was feasible for hyperthermia of recurrent rectal cancer. Finally, technical solutions enabled long-duration MRI-based thermometry with accuracy better than 1°C. The Secondary aim of this Thesis was to present new ways to use phased-array transducers in shaping the emitted acoustic field to gain improvement in performance in different MR-HIFU applications. First, multifoci heating was able to reduce the peak pressure experienced by the tissue in hyperthermia heating when compared to single steered-focus heating. Second, the adjustment of transducer-element phases utilizing time-of-flight estimation based on MRI images improved the focus sharpness in heterogeneous phantom simulating conditions of breast tissue. Third, simulated acoustic intensity predicted heating of bones, which enables fast quantitative reduction of bone heating through deactivation of transducer elements in the context of intercostal sonications. The technological solutions presented in this Thesis advance the field of MR-HIFU towards translation into clinical practice.
|Translated title of the contribution||Edistyneet tekniikat magneettiresonanssikuvausohjatussa korkeaintensiteettisessä fokusoidussa ultraääniterapiassa|
|Publication status||Published - 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
- thermal ablation
- magnetic resonance