Necesidad y uso de información en el marco de los estudios de usuarios

In order to compare and evaluate the actual result, a 5.9MHz, 32 × 32 elements (4.8mm × 4.8mm) transducer array is modeled using Field II. To simulate the transducer, a 1D linear array is used to transmit and focus in the azimuth direction. Kerf, element width, and element height are set to 0.03mm, 0.12mm, and 4.8mm, respectively, to mimic the actual RC-CMUT layout. Sub-elements are configured to 3 in the x-direction (i.e. azimuth) and 20 in the y-direction (i.e.

elevation) in order to mimic the actual array configuration. The same configuration is used to model receive aperture, except the 1D array is rotated orthogonally. A single cycle sinusoidal pulse with a center frequency of 5.9MHz excites the transducers. Phased-array focusing is done to focus 20mm away from the aperture on the center axis. The one-way radiation pattern was generated by finding the maximum value of the demodulated signal detected by an ideal point receiver scanned at a plane that is parallel to the aperture at a distance of 20mm. The point spread function was generated by scanning an ideal point target located at [x, y, z] = [0, 0, 20]

mm. The maximum value of the demodulated received beamformed signal was plotted as a function of the lateral distance in the azimuth and elevation direction 20 mm away from the aperture. The demodulated signals are normalized and presented with an 80 dB dynamic range.

A B-mode image is generated for wire targets parallel to the aperture. The wires run in the azimuth direction. The 90° sector plane in Figure 5-5 is taken in the elevation dimension perpendicular to the aperture. The beam is focused at 20mm (z). The three wires on the right are positioned at coordinates (-0.5mm, 15mm), (0mm, 20mm), and (0.5mm, 25mm) in the y-z plane while the left most wire is positioned close to (-1.5mm, 27.5mm). As expected, the left most wire, positioned near the edge of the transmitted beam’s focal height shows more defocusing than the wires closer to the center.

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Figure 5-5: Simulated B-Mode of wire targets.

The actual wire targets under the same configuration as simulated are imaged and shown in Figure 5-6. Acquired RF data were averaged over 32 times to remove noise and enhance image.

Figure 5-6: Measured and reconstructed B-mode image of actual wire targets. Unfocused wire is seen on the bottom left.

The set-up of the pinheads is shown in Figure 5-7. Three 1/16 inch (1.59mm) pinheads are laid out ~2mm apart in both x- and y-direction and 4mm apart in depth. The bottom pin is

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tilted to better direct sound to the aperture. Figure 5-8 illustrates nine sequential B-modes planes acquired over the volume that confines the pinheads. Each B-mode shows the reconstructed data from a single Tx event acquired with the PXI imager. Here, 0° is defined as the orthogonal plane that intersects the aperture at the center. Since the reflected signals are weaker when further away from the focal spot (20mm), software equivalent of time-gain-compensation (TGC) is applied.

The pinheads are connected to a sponge that absorbed sound to reduce reflection. The sponge is supported by a stage while being submerged in a vegetable oil-filled tank.

Figure 5-7: Side view of the oil-tank set-up. The top view of the 3 pins is also shown.

The total time for each Tx/Rx event, including the time it takes for data to transfer from the FPGA to Host PC, is 5ms. It takes 205ms to complete a single ± 20º volume scan with no averaging. An additional 20ms is required for the image reconstruction. Figure 5-9 is a 3D rendered image created from the data set shown in Figure 5-8. The rendered image is processed by the 3D Ultrasound Visualization (UltraVis) platform developed by Vision and Image Processing group at the University of Waterloo.

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Figure 5-8: Reconstructed image of the pin heads at 8 different azimuth angles.

-8° -6° -4°

-2° 0° 2°

4° 6° 8°

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Figure 5-9: 3D Rendered image of the volume created by the 3D Ultrasound Visualization (UltraVis) platform developed by Vision and Image Processing group at UW.

Discussion

5.3.1 Focusing Volume

With the RC method, the cost of achieving a fast scanning rate and minimal connection count is that focusing is lost in one direction during both transmit and receive event. For example, when a wavefront is generated during transmit, phased-array focusing is only applied to the azimuth direction while natural focusing occurs in the elevation direction. As a result, the transmitted pulse weakens immediately outside the natural focal zone, preventing sound from focusing directed above or below the focal height. This phenomenon was verified from the simulation results where the the -12dB beam height at 20mm away is ~5mm, which is approximately the height of the aperture. Thus, it can be concluded that a flat RC array design greatly limits the field of view. Increasing the aperture size would be the most straight forward approach to increase the scan volume and the resolution. As calculated, by increasing the array size from 32x32 to 64x64 elements, the lateral resolution improves from 1.3mm to 0.7mm. Although the aperture size cannot be compromised for IVUS/ICE applications, there is more room for play in applications such as breast or prostate cancer screening. Therefore, using a curved array would be a promising solution to address the limitation.

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