Evaluation of a Magnetic Resonance Imaging guided focused ultrasound system for prostate ablation

Abstract

Prostate cancer is one of the most prevalent health threats for men worldwide. Although there are successful treatments for prostate cancer, a percentage of cases of men diagnosed with prostate cancer is led to death. Since there is an increasing trend in men being diagnosed with prostate cancer, the development of new treatments with higher successful rates and reduced side effects leading to a better quality of life of men is of utmost importance. In this dissertation, high-intensity focused ultrasound (HIFU) is recommended for the treatment of prostate cancer. The technology was combined with medical robotics and the treatment procedure was guided using Magnetic Resonance Imaging (MRI) which is known to be superior than ultrasound guidance. A 4-degrees of freedom (DOF) endorectal robotic system was 3D-designed and developed to treat prostate cancer at early stage. Prior to the evaluation of the robotic system, agar-based tissue-mimicking materials were developed and characterized so as to mimic human tissue. The effect of agar, silicon dioxide, and evaporated milk concentration on the acoustic and thermal properties of tissue-mimicking materials was investigated. A new tissue-mimicking material containing agar and wood powder was proposed for Magnetic Resonance-guided Focused Ultrasound (MRgFUS) applications. The acoustic, thermal, and MR properties of the agar/wood powder tissue-mimicking material were estimated. Wood powder was found to enhance the absorption and attenuation coefficient of agar-based phantoms. The new developed phantom matched adequately the acoustic and thermal properties of human tissues and was evaluated for MRgFUS thermal protocols. A new method to estimate ultrasonic absorption coefficient of phantoms and tissues is described. The absorption coefficient of the developed phantoms was measured using this new, accurate, and fast technique. The change in absorption coefficient according to the variation of agar, silicon dioxide, and evaporated milk was investigated. The agar was found to best control the absorption coefficient while the increase of agar, silicon dioxide (up to 4 %) and evaporated milk concentration resulted in an increase of the absorption coefficient of agar-based tissue-mimicking materials. The initial evaluation procedure involved the selection and evaluation of the focused transducer. A 3.2 MHz transducer was selected after being evaluated through simulations and experiments in agar-based phantoms and excised tissues. The near-field heating of the transducer was also investigated and a 40 s time delay resulted in safe temperatures in the near-field region. The MR compatibility of the 4-DOF robotic system was tested for MR sequences that are used to obtain MR thermometry data during a treatment. There was an insignificant drop to the signal-to-noise ratio of the MR thermometry images and therefore the 4-DOF robotic device was found to be MR compatible and can be used in MRgFUS prostate cancer therapies. The accuracy of the robotic device of the two computer-controlled axes (X and Θ) was estimated. Αt small linear movements (1 mm) of the X-axis, the maximum error was calculated at 73 μm while at small angular movements (1o) of the Θ-axis, the maximum error was 0.09o. The robotic device is capable of performing multiple sonications with high accuracy. Finally, the robotic system was evaluated in phantoms and excised tissues in a laboratory setting and MRI environment. Discrete and overlapping lesions were produced with variable lesion diameter and length, demonstrating the efficacy of the system to produce reproducible and controllable lesions. The evaluation was completed with the in vivo efficiency of the system, utilizing a rabbit thigh model.