Preliminary results relating to the design, fabrication, and characterization of a 3600 (60 x 60) element, fully sampled, 5 MHz two dimensional (2D) array are presented. The viable element yield of the new array was estimated at 98.3%. Single-element pulse-echo experiments indicate that the center frequency is 4.7 MHz - 7.8% below the resonant frequency determined by Finite Element Analysis (FEA) simulation. Pulse-echo signal fractional bandwidth was measured to be 60.3% at the -6 dB level. Ringdown was longer than anticipated in experimental pulse echo voltage waveforms, which we attribute in part to a lack of matching layer and a low-loss backing material. Based on plane-wave pulse-echo experiments in a water-tank, single element signal-to-noise ratio (SNR) was calculated to be 6.0 when using a plane-wave transmit (all elements excited). Experimental angular beam patterns were more directional than predicted with the standard soft-baffle equation, but in good agreement with FEA simulations that take account of finite acoustic crosstalk.
We describe a very low cost handheld ultrasound system that we are currently developing for routine applications such as image guided needle insertion. We provide a system overview and focus discussion on our beamforming strategy, direct sampled I/Q (DSIQ) beamforming. DSIQ beamforming is a low cost approach that relies on phase rotation of in-phase/quadrature (I/Q) data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional baseband demodulation. We describe our efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We also present the results of experiments and simulations that compare the DSIQ beamformer to more conventional approaches, namely time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for routine applications.
Using a scanned laser to generate ultrasound, via the thermoelastic effect, offers an alternative approach for realizing high density, high frequency ultrasound imaging arrays. The approach bypasses the complexity and intricacy required for forming conventional piezoelectric array elements and their associated electrical connections. Thus, it is particularly well suited to 2D arrays. In this paper, the devices considered comprise a carbon black loaded PDMS polymer layer on top of a glass or PDMS substrate. PZFlex Finite Element Analysis (FEA) was used to investigate the impact of a variety of design variables including: laser spot size, substrate material and thermoelastic coupling medium. Predicted single element angular response broadly matched responses obtained by experiment. Specifically, if a low acoustic loss glass substrate is used then measurable sidelobes occur at approximately 40 degrees. However, if the glass substrate is replaced by a PDMS material, then the traveling waves that give rise to sidelobes are no longer supported and a smooth single element angular response is obtained in both experiment and FEA simulation. FEA suggests that there are other modes in addition to the Rayleigh mode observed in the experiment. It is believed that these modes are more quickly damped in the experimental case. Therefore, while FEA provides a very versatile and valuable analysis tool, the accuracy of its predictions are contingent on accurate knowledge of device geometry and relevant material properties.