Objective: The assessment of blood-flow volume (BFV) is clinically relevant for the diagnosis and monitoring of cardiovascular dysfunctions and the prevention of subsequent secondary diseases. Non-invasive BFV measurement based on ultrasound methods are appealing for lower cost, real-time operation, and equipment portability. Recently, complex ultrasound research scanners with 1024 channels controlling the elements of a 2-D matrix array probe, have been demonstrated suitable for off-line accurate BFV estimates. In this work, a streamlined approach, using a 256-channel research scanner paired with a 256-element 2-D sparse spiral array, is proposed and validated. Methods: This setup allows for simultaneous scanning of the vessel's longitudinal and transverse sections through an interleaved transmission sequence. In real-time, the longitudinal scan is used to determine the flow direction, while the transverse scan captures both the dynamic cross-sectional area and the local velocities by high frame rate color flow mapping. Results: Flow phantom experiments under steady and pulsatile flow conditions were conducted to assess the performance by comparing the measurements with the outputs of a reference flow sensor. The proposed method provided accurate and precise BFV values for both flow conditions, with mean percentage error and standard deviation always lower than 9.4% and 2.8%, respectively. Furthermore, preliminary in vivo experiments have produced results consistent with those reported in the literature. Conclusion: The proposed method based on the use of a sparse array has permitted accurate and precise phantom BFV measurements and has been shown suitable for real-time arterial BFV measurements.

Wall shear rate (WSR), a key marker of vascular health, is useful for cardiovascular risk assessment. Traditionally, its non-invasive evaluation via ultrasound relies on longitudinal imaging of the artery, a method that can be restrictive for comprehensive hemodynamic monitoring. Here a bi-plane ultrasound method using a 2D sparse array for fast, cross-sectional WSR estimation is presented. The technique provides 12 simultaneous, angularly distributed WSR estimates per frame, overcoming limitations of conventional methods and avoiding the hardware complexity of full 3D imaging. Phantom experiments were conducted at different depths with good accuracy (bias<16%) and repeatability (<21%).

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.

Volumetric ultrasound imaging of blood flow with microbubbles enables a more complete visualization of the microvasculature. Sparse arrays are ideal candidates to perform volumetric imaging at reduced manufacturing complexity and cable count. However, due to the small number of transducer elements, sparse arrays often come with high clutter levels, especially when wide beams are transmitted to increase the frame rate. In this study, we demonstrate with a prototype sparse array probe and a diverging wave transmission strategy, that a uniform transmission field can be achieved. With the implementation of a spatial coherence beamformer, the background clutter signal can be effectively suppressed, leading to a signal to background ratio improvement of 25 dB. With this approach, we demonstrate the volumetric visualization of single microbubbles in a tissue-mimicking phantom as well as vasculature mapping in a live chicken embryo chorioallantoic membrane.

Ultrasound vector Doppler techniques for three-dimensional (3-D) blood velocity measurements are currently limited by low temporal resolution and high computational cost. In this paper, an efficient 3-D high-frame-rate vector Doppler method, which estimates the displacements in the frequency domain, is proposed. The novel method extends to 3-D an approach so far proposed for two-dimensional (2-D) velocity measurements by approximating the (x, y, z) displacement of a small volume through the displacements estimated for the 2-D regions parallel to the y and x directions, respectively. The new method was tested by simulation and experiments for a 3.7 MHz, 256-element, 2-D piezoelectric sparse spiral array. Simulations were also performed for an equivalent 7 MHz Capacitive Micromachined Ultrasonic Transducer spiral array. The results indicate performance (bias ± standard deviation: 6.5 ± 8.0) comparable to the performance obtained by using a linear array for 2-D velocity measurements. These results are particularly encouraging when considering that sparse arrays were used, which involve a lower signal-to-noise ratio and worse beam characteristics with respect to full 2-D arrays.

Current 3-D Vector Flow Imaging (VFI) techniques are typically limited by either low temporal resolution and/or high computational cost. In this work, we illustrate a 3-D high frame rate VFI method, which efficiently splits the 3-D (x, y, z) displacement estimation in two separate 2-D VFI estimations operated in the frequency domain. The echo-data received after the transmission of plane waves are beamformed and high-pass filtered before being VFI processed. The new method was experimentally tested by using the ULA-OP 256 research system connected to a 3 MHz, 256-element, 2-D spiral array. Phantom experiments were conducted in steady laminar flow conditions for different probe-to-flow angles. For the tested conditions, the mean relative bias was 11.5% with a standard deviation of 10.9%. The 2-step approach applied to the 2-D VFI technique makes the new 3-D VFI method accurate and computationally efficient.