Objective: The aim of this study was to assess the feasibility and imaging options of contrast-enhanced volumetric ultrasound kidney vasculature imaging in a porcine model using a prototype sparse spiral array. Methods: Transcutaneous freehand in vivo imaging of two healthy porcine kidneys was performed according to three protocols with different microbubble concentrations and transmission sequences. Combining high-frame-rate transmission sequences with our previously described spatial coherence beamformer, we determined the ability to produce detailed volumetric images of the vasculature. We also determined power, color and spectral Doppler, as well as super-resolved microvasculature in a volume. The results were compared against a clinical 2-D ultrasound machine. Results: Three-dimensional visualization of the kidney vasculature structure and blood flow was possible with our method. Good structural agreement was found between the visualized vasculature structure and the 2-D reference. Microvasculature patterns in the kidney cortex were visible with super-resolution processing. Blood flow velocity estimations were within a physiological range and pattern, also in agreement with the 2-D reference results. Conclusion: Volumetric imaging of the kidney vasculature was possible using a prototype sparse spiral array. Reliable structural and temporal information could be extracted from these imaging results.

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Volumetric echocardiography can potentially give a more complete picture of cardiac dynamics than its two-dimensional (2D) counterpart. Current clinical volumetric imaging probes have relatively low frame rates, and often require ECG gating to stitch together an entire volume. This makes measuring fast dynamics of the heart as well as imaging patients with irregular heartbeats difficult. We have previously designed and manufactured 2D sparse arrays with elements seeded in a density-tapered spiral pattern for cardiac imaging. Using these prototypes, we demonstrate in this paper the first high-frame-rate volumetric closed-chest porcine cardiac as well as open-chest myocardial blood flow results. These preliminary results suggest the potential of performing high-frame-rate volumetric cardiac imaging using the sparse spiral arrays.@en

Two-dimensional (2-D) arrays offer volumetric imaging capabilities without the need for probe translation or rotation. A sparse array with elements seeded in a tapering spiral pattern enables one-to-one connection to an ultrasound machine, thus allowing flexible transmission and reception strategies. To test the concept of sparse spiral array imaging, we have designed, realized, and characterized two prototype probes designed at 2.5-MHz low-frequency (LF) and 5-MHz high-frequency (HF) center frequencies. Both probes share the same electronic design, based on piezoelectric ceramics and rapid prototyping with printed circuit board substrates to wire the elements to external connectors. Different center frequencies were achieved by adjusting the piezoelectric layer thickness. The LF and HF prototype probes had 88% and 95% of working elements, producing peak pressures of 21 and 96 kPa/V when focused at 5 and 3 cm, respectively. The one-way -3-dB bandwidths were 26% and 32%. These results, together with experimental tests on tissue-mimicking phantoms, show that the probes are viable for volumetric imaging.

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Transducer arrays for 3D imaging are characterized by elements distributed over a 2D surface. The dimensions of each element are typically one half-wavelength in both x- and y-directions. Such small elements inherently have a high electrical impedance. When the elements are connected to a probe cable, the high cable capacitance decimates the delivered voltage and results in a poor Signal to Noise Ratio (SNR). This may have dramatic effects, especially in sparse arrays, where a small number of elements contributes to the beamformed signal. In this paper we demonstrate that the use of in-probe preamplifiers in a sparse PZT probe is valuable to significantly increase the SNR.

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Volumetric medical ultrasound imaging currently provides either high resolution or high frame rates. We propose a sparse array probe with only 256 elements that are density tapered. All elements are directly wired to an ultrasound engine, thus being flexible in transmission and reception. Here we present the design, realization, and characterization of a probe prototype for carotid 3D high frame rate scanning (5 MHz). The design is based on rapid prototyping with printed circuit board substrates to wire the elements to external connectors. The results show that 95% of the elements was acoustically active, producing a peak pressure of 400 kPa when focused at 24 mm with a 20 V amplitude, 3-cycle tone burst. The fractional one-way −6dB bandwidth (FBW) was 32 %, which is narrower than the designed one (50% FBW), due to an unexplained ringing effect. These results show that the probe is viable for volumetric imaging tests, albeit that it can be further optimized.@en

3D ultrasound imaging is costly and complicated mostly due to the need of controlling 2D probes with high numbers of elements (≥1024). Sparse arrays are a convenient way to reduce system complexity maintaining reasonable performance. A 256-element density tapered spiral array has been recently designed and realized in a piezoelectric probe prototype. The probe has been connected to the ULA-OP 256 open scanner, which can be programmed to permit volumetric scanning in real-time. This paper reports on the first experimental measurements of transmit ultrasound fields produced by such prototype system, including multi-line transmit fields that, in combination with parallel receive beamforming, may permit real-time volumetric imaging. An average −21.3dB side-lobe level (SLL) was measured on the transmit fields., while volume rates of 35 volumes per second can be achieved.@en