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Top1. Introduction
High intensity focused ultrasound (HIFU) has the potential to induce thermal changes in tissue and therefore it is used extensively for medical applications (Stewart et al 2006). Nowadays HIFU is utilized to selectively heat biological tissues for oncological applications with minimal invasiveness by using magnetic resonance imaging (MRI) to provide, to the operator performing the procedure, images of a region within the subject being heated.
HIFU was explored in almost every tissue that is accessible by ultrasound. The following literature represents some examples of some applications explored: eye ablation for the treatment of Glaukoma (Lizzi, 1978), prostate ablation using ultrasound guidance (Chapelon, 1992), liver ablation for the intention to treat cancer (Haar, 1989), brain ablation for the intention to treat cancer (Lele, 1962; Vykhodtseva, 1994) and kidney ablation for oncological applications (Linke, 1973; Hynynen, 1995; Bihrle, 1994).
In commercial systems (for example, (Chapelon, 1992; Bihrle, 1994; Hynynen, 2001) HIFU is either guided by ultrasound or MRI. Ultrasonic imaging is the simplest and most inexpensive method to guide HIFU, however MRI offers superior contrast than ultrasound, having the only disadvantage that is more expensive.
In order to treat large areas by creating tissue necrosis the HIFU transducer which has focal beam of few mms should be moved to create overlapping lesions. The positioning device is responsible for this motion and should operate inside the MRI scanner. The use of the positioning device inside MRI scanner may interfere with the RF field and as a result the quality of the images captured could be harmed. Therefore all the components must be non-magnetic materials. Generally the positioning device has to be small to fit the gantry MRI scanners. An MRI compatible robotic system refers to a system that is not hazardous in any way and neither generates any interference to the MRI scanner nor affected by the strong electromagnetic field of the MRI scanner (Gassert, 2008).