# MRI Compatible Planar Material Acoustic Lenses
## 3 Experimental Set-Up  
It is required to validate the results obtained from the theoretical models with other solutions such as numerical models and experimental measurements. In this sense, obtaining experimental results is fundamental to validate the numerical models. A complex measurement and acquisition system is needed to perform the experiments given the technical difficulties to control the underwater devices. The Center for Physics Technologies: Acoustics, Materials and Astrophysics of the Universitat Politecnica de Valencia has a robotized and automated system for high precision ultrasound measurements. The robot is built based on the size of the immersion tank where the tests and experiments are carried out, which contains distilled and degassed water, with dimensions of 0.5 m wide by 0.5 m high by 1 m long. These dimensions suppose that the immersion tank must contain around 200 L of distilled water to be functional, and allow both transducers and devices to be completely submerged, and avoid reflections due to the impedance changes produced by the change medium.  
The measurement system is composed by a fixed emitter and a hydrophone coupled to the robotic system. This system obtains reliable and precise results that allow for the evaluation of the acoustic phenomena involved in these types of lenses. A plane immersion piston transducer built by Imasonic with 250 kHz of central working frequency and an active diameter of 32 mm has been used as the emitter. Also, a Precision Acoustics hydrophone, model 1.0 mm Needle Hydrophone is used as the receiver. This hydrophone is capable of measuring high frequencies, even if they have a very weak signal level. The sensitivity of the hydrophone is 850 nV/Pa (\(-\)241.4 dB 1V/\(\upmu\)Pa) with a tolerance of \(\pm\)3 dB. The frequency response is flat \(\pm\)2 dB between 3 and 12 MHz and \(\pm\)4 dB between 200 kHz and 15 MHz. The bandwidth ranges from 5 kHz to 15 MHz. Figure 3 shows an experimental set-up in a measurement. Two types of different configurations are used to generate and amplify the signals. The first one is to use an external function generator connected to a high power amplifier. The second configuration is to use a pulse generator (5077 PR of Panametrics) with integrated amplifier. This generator and amplifier allows generating pulses with frequencies between 100 kHz and 20 MHz, a pulse repetition frequency (PRF) from 100 Hz to 5 kHz and a pulse amplitude between 100 and 400 V.  
All the results shown below are obtained for a working frequency of 250 kHz. For the experimental comparison, three lenses have been implemented, two made of PLA and one of brass. Every lens considered in this work was designed with 11 Fresnel zones and an outer radius of 88.8 mm. The thickness of the brass lens was 1 mm. For manufacturing reasons, the rest of the lenses had a total thickness of 5 mm. Figure 4 shows both PLA and brass lenses. Both PLA lenses are identical, the only difference being an inner air chamber to achieve a higher impedance contrast. As described in the previous section, it is not possible to differentiate them by the naked eye.  
Figure 3: Experimental set-up.