# MRI Compatible Planar Material Acoustic Lenses  
[PERSON]  
1 Centro de Tecnologias Fisicas, Universitat Politecnica de Valencia, Cami de Vera s/n, 46022 Valencia, Spain; [EMAIL_ADDRESS] (D.T.-S.); sercasib@upvnet.upves (S.C.-I.); [EMAIL_ADDRESS] (A.U.) Hospital Francesc de Borja, 46702 Gandia, Valencia, Spain; [EMAIL_ADDRESS]  
1 Centro de Tecnologia Fisicas, Universitat Politecnica de Valencia, Cami de Vera s/n, 46022 Valencia, Spain; [EMAIL_ADDRESS] (D.T.-S.); sercasib@upvnet.upves (S.C.-I.); [EMAIL_ADDRESS] (A.U.) Hospital Francesc de Borja, 46702 Gandia, Valencia, Spain; [EMAIL_ADDRESS]  
1 Centro de Tecnologia Fisicas, Universitat Politecnica de Valencia, Cami de Vera s/n, 46022 Valencia, Spain; [EMAIL_ADDRESS] (D.T.-S.); sercasib@upvnet.upves (S.  
medium. There are studies that have implemented Soret FZP (SZP) by ultrasounds based on these type of lenses [10].  
A material that has a high impedance with respect to water and that allows for the creation of opaque zones to achieve a Soret type lens is brass. However, this type of material has limitations, especially when used in fields such as bioengineering. The use of acoustic lenses in medicine for high intensity focused ultrasounds (HIFU) treatment is one of the current lines of research. magnetic resonance imaging (MRI) is the technique that is most used for guiding HIFU treatment [11].  
MRI is a technique used for soft tissue structure imaging in a non-invasive way. The image is obtained by aligning and relaxing the magnetic moments of the atoms of the introduced elements in the MRI. Tissues are exposed to a strong external time-independent magnetic field. Thus, metallic elements cannot be introduced in the resonance zone due to their interference in the image and because they could damage MRI-systems. To avoid interaction with the electromagnetic field, non-metallic materials should be used in the construction of lenses. One of these materials is polylactic acid (PLA) [12]. The MRI environment requires materials such as PLA for medical instruments and patient supports. Recently, PLA has been used for this purpose and its reliability has been shown [13]. The HIFU transducer is embedded within a specially designed table that fits into the MRI device. This integrated system, has a degassed water bath where the transducer is located. The patient lies over this system on [14]. Although the transducer and the lens must be immersed in this water bath, degradation of the PLA will occur over long-term immersion. PLA degrades in water after a period ranging from months to a year [15]. Therefore it must be taken into account that, in MRI systems, the lens and the transducer are not permanently submerged. After 20 to 25 min, the system is extracted from the water bath, and for this reason, the time of degradation due to being immersed in water can be prolonged considerably.  
In this work, three lenses with three types of compatible materials with MRI environments are compared. In this sense, acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA) and polylactic acid (PLA) materials are used. Furthermore, a SZP built in brass is compared. Although, this material is not MRI compatible, it is the nearest to the ideal SZP that can be implemented in real projects. In the comparison, a not compatible with MRI lens built in brass and an ideal Soret lens will be added. Results are obtained and compared both numerically and experimentally. Numerical results have been obtained using the commercial software COMSOL Multiphysics 4.3a by COMSOL Inc. (Sweden) [16]. In this work, it has been verified that the ratio of the transmission capacity that is related to the ratio of impedances of the medium and the lens, directly influences the focusing capacity.