Conformal interstitial ultrasound thermal therapy

MRI-guided interstitial thermal therapy is a minimally-invasive treatment for localized tumours. Heating applicators are inserted within or adjacent to a tumour and energy is delivered to elevate the temperature across the entire volume above the threshold for thermal coagulation (∼60°C). MR imaging is used to guide device placement, monitor energy delivery and assess thermal damage. A common characteristic of existing interstitial heating technology is limited control over spatial power deposition, resulting in the inability to generate asymmetric thermal damage patterns. The ability to generate thermal damage patterns that conform to the actual tumour volume could make the treatment of tumours close to critical structures possible, and spare the maximum amount of normal tissue in sensitive organs.; The goal of this thesis was to investigate the potential to generate conformal heating patterns from an interstitial heating applicator. Ultrasound technology was selected because of the capability to control spatial power deposition simply with multiple parameters. Linear arrays and multi-layer transducers were evaluated for their heating characteristics through numerical calculations. Linear arrays produced thermal lesions with uniform radial temperature distributions for small target volumes (∼1 cc). Transducers with high impedance ceramic layers were capable of ultrasound transmission at multiple discrete frequencies. This design enabled control over the depth of heating due to the frequency dependence of ultrasound absorption.; The performance of prototype heating applicators was evaluated in ex-vivo heating experiments. The ability to direct heating to defined angular sectors in tissue through device rotation was demonstrated, and the MR compatibility of the heating applicators was verified through measurement of the temperature distributions with MR thermometry. The combination of multi-frequency planar transducers and device rotation was demonstrated to make the generation of arbitrary thermal damage patterns possible. It was also predicted that the extent of thermal damage was less sensitive to the effects of blood perfusion as compared to conventional interstitial heating technology.; It is concluded that interstitial ultrasound heating applicators can be used in conjunction with MRI-guidance to perform conformal thermal therapy.