For mounting the PZT transducer in the test cell the same mounting arrangement was used as that in the planar phased array ultrasound transducer for creating mild hyperthermia described by Aitkenhead and co-workers (Aitkenhead et al., 2008). The PZT transducer and the electrodes were mounted in the lid of the test cell. A copper wire having 1mm diameter and 10mm length was inserted through a hole in the lid of the test cell from the rear of the Perspex lid of the test cell. The tip of the copper wire was made flat using sand paper to allow a solder contact to be made. With the PZT elements held in the place in the lid, a thin wire was soldered between the front face of the PZT element and the tip of the copper wire in the recess. The solder contact on the front of the PZT element was kept small to ensure that it did not protrude above the surface level of the matching layer. This would allow for the matching layer which would later fill the recess and provide electrical insulation between this solder contacts and the medium. However, since the front face of the PZT element was electrically common and would be connected to ground, the electrical isolation provided by the matching layer would not be critical for safety if such an element is used in a clinical context. The high voltage drive was applied to the rear face of the PZT, precluding the possibility of a high voltage coming into contact with the medium.
Once the soldering has been completed at the front of the element, first Blu-Tack and then adhesive tape was applied to form a seal at the rear face of the PZT element with the rear of the Perspex lid. The alignment of the PZT element within the frame was checked repeatedly. The Blu-Tack was required to fill the free space of the back side of the PZT element. The tape and Blu-Tack will prevent the liquid epoxy from seeping through between the PZT elements and the Perspex frame. Once this sealing had been done, the recess on the front surface was filled with epoxy resin until it stood proud of the Perspex surface. Then it was allowed to cure at room temperature. Once cured the epoxy was ground back using wet abrasive paper until the surface was once again level with the Perspex frame. The surface of Perspex within the matching recession was abraded to ensure a reliable bond to form with the epoxy matching layer.
Araldite 2020 was chosen as a suitable epoxy due to its low viscosity in its liquid form and its water resistance nature in its cured state. It was mixed at a ratio of 100 : 35 (resin : hardener) by volume according to the manufacturer’s recommendation. After filling the recess on the front surface, the araldite 2020 needed to allow 72 hours (or 24 hours at elevated temperature) at room temperature to cure properly. Enough time was needed to ensure adequate epoxy conversion (Karayannidou et al., 2006). The low viscosity of
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Araldite 2020 and slow curing allowed any gas bubbles formed during its application to rise to the surface of the epoxy. Grinding the protruding araldite 2020 layer with fine grade abrasive paper (P300A) and water allowed a smooth, level and air bubble free matching layer to be created.
The thickness of the matching layer was defined by the depth of the recess in Perspex lid. Once the epoxy was ground level with the lid surface, the thickness of the frame was measured with a micrometer at several points to ensure that it was an even thickness across the face of the array. The speed of sound in araldite 2020 is 2610 ms-1 (Ma et al., 2007), and hence the wavelength of a 1MHz sound wave is 2.6 mm. To provide a matching layer of thickness λ/4 for optimal matching between the PZT and the medium, a layer of 0.65mm thickness was required. However, for construction of the transducer element a 1mm thick matching layer was used to ensure that the solder connections at the front of PZT lay entirely within the matching layer. It has been shown that the thickness of the matching layer is not critical to ensure adequate coupling into the medium (Wojcik et al., 1996).
The completed Perspex top of the test cell was then ready for a drive signal to be applied to the PZT element. A 1.50m length of 50Ω coaxial cable (type RG178B/U) was used to supply the drive signals for the PZT elements. The cables entered the transducer from the back, connecting to sections of strip board fitted back Perspex top of the test cell. Wires from the PZT transducer element and the coaxial cable were connected at this strip board. The strip board ensured that the solder connections to the PZT were not stressed by the movement of the coaxial cables, making sure that the wiring of the transducer element was robust. The outer conductor of the coaxial cable was grounded to shield the inner conductor which carried the drive signal. The rear of the Perspex lid is shown in the Figure 3-21 where cable connections for the drive pulse are shown.
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Figure 3-21 Showing the rear of the Perspex lid of the test cell. A coaxial cable is connected to a strip board fitted at the rear of the lid. Wires are connected from strip board to the back and front of the transducer to provide drive pulse to the transducer.
Four 8ba brass / stainless steel bolts were fitted to the threaded holes in the top of the test cell so that nuts fitted to the bolts formed the electrodes for electrical impedance measurements. The protrusions of the bolts into the medium were adjusted such that the nut face and end the bolt were flush.