Luca Bassi

ULA-OP: an advanced open platform for ultrasound research

The experimental test of novel ultrasound (US) investigation methods can be made difficult by the lack of flexibility of commercial US machines. In the best options, these only provide beamformed radiofrequency or demodulated echo-signals for acquisition by an external PC. More flexibility is achieved in high-level research platforms, but these are typically characterized by high cost and large size. This paper presents a powerful but portable US system, specifically developed for research purposes. The system design has been based on high-level commercial integrated circuits to obtain the maximum flexibility and wide data access with minimum of electronics. Preliminary applications involving nonstandard imaging transmit/receive strategies and simultaneous B-mode and multigate spectral Doppler mode are discussed.

A reconfigurable and programmable FPGA-based system for nonstandard ultrasound methods

The availability of programmable and reconfigurable ultrasound (US) research platforms may have a considerable impact on the advancement of ultrasound systems technology; indeed, they allow novel transmission strategies or challenging processing methods to be tested and experimentally refined. In this paper, the ULtrasound Advanced Open Platform (ULA-OP), recently developed in our University laboratory, is shown to be a flexible tool that can be easily adapted to a wide range of applications. Five nonstandard working modalities are illustrated. Vector Doppler and quasi-static elastography applications emphasize the real-time potential and versatility of the system. Flow-mediated dilation, pulse compression, and high-frame-rate imaging highlight the flexibility of data access at different points in the reception chain. For each modality, the role played by the onboard programmable devices is discussed. Experimental results are reported, indicating the relative performance of the system for each application.

Real-time vector velocity assessment through multigate doppler and plane waves

Several ultrasound (US) methods have been recently proposed to produce 2-D velocity vector fields with high temporal and spatial resolution. However, the real-time implementation in US scanners is heavily hampered by the high calculation power required. In this work, we report a real-time vector Doppler imaging method which has been integrated in an open research system. The proposed approach exploits the plane waves transmitted from two sub-arrays of a linear probe to estimate the velocity vectors in 512 sample volumes aligned along the probe axis. The method has been tested for accuracy and reproducibility through simulations and in vitro experiments. Simulations over a 0° to 90° angle range of a 0.5 m/s peak parabolic flow have yielded 0.75° bias and 1.1° standard deviation for direction measurement, and 0.6 cm/s bias with 3.1% coefficient of variation for velocity assessment. In vitro tests have supported the simulation results. Preliminary measurements on the carotid artery of a volunteer have highlighted the real-time system capability of imaging complex flow configurations in an intuitive, easy, and quick way, as shown in a sample supplementary movie. These features have allowed reproducible peak velocity measurements to be obtained, as needed for quantitative investigations on patients.

Architecture of an Ultrasound System for Continuous Real-Time High Frame Rate Imaging

High frame rate (HFR) imaging methods based on the transmission of defocused or plane waves rather than focused beams are increasingly popular. However, the production of HFR images poses severe requirements both in the transmission and the reception sections of ultrasound scanners. In particular, major technical difficulties arise if the images must be continuously produced in real-time, i.e., without any acquisition interruption nor loss of data. This paper presents the implementation of the real-time HFR-compounded imaging application in the ULA-OP 256 research platform. The beamformer sustains an average output sample rate of 470 MSPS. This allows continuously producing coherently compounded images, each of 64 lines by 1280 depths (here corresponding to 15.7 mm width and 45 mm depth, respectively), at frame rates up to 5.3 kHz. Imaging tests addressed to evaluate the achievable speed and quality performance were conducted on phantom. Results obtained by real-time compounding frames obtained with different numbers of steering angles between +7.5° and −7.5° are presented.

Multi-line measurements of blood velocity vectors in real-time

Standard Doppler ultrasound investigations are limited to detect the axial blood velocity component, as they cannot directly estimate the flow direction. A typical approach for obtaining velocity vectors consists in combining the Doppler shifts detected by receiving the echoes from two (or more) different directions. Together with plane wave transmission, this strategy can assess the velocity data over a 2D region. Real time performance is achievable, provided that the electronics is capable of beamforming and processing the data acquired from several probe apertures in between consecutive transmissions. Recently, the ULA-OP 256 research scanner was equipped with a beamformer that, exploiting a parallel/serial process strategy, grants the calculation power for beamforming and processing multiple lines. In this work we present a vector Doppler method, based on the transmission of plane waves, which detects the velocity vectors on 8 parallel lines distributed over a 2D region 1 cm wide. The method is implemented on the ULA-OP 256 scanner and it achieves, in real-time, a refresh rate higher than 20 Hz when combined in duplex with a standard B-mode. Experiments on the carotid artery of a volunteer are reported, which show the effectiveness of the real-time implementation in detecting the complex flow patterns present in the carotid.

Embedded System for Real-Time Digital Processing of Medical Ultrasound Doppler Signals

Ultrasound (US) Doppler systems are routinely used for the diagnosis of cardiovascular diseases. Depending on the application, either single tone bursts or more complex waveforms are periodically transmitted throughout a piezoelectric transducer towards the region of interest. Extraction of Doppler information from echoes backscattered from moving blood cells typically involves coherent demodulation and matched filtering of the received signal, followed by a suitable processing module. In this paper, we present an embedded Doppler US system which has been designed as open research platform, programmable according to a variety of strategies in both transmission and reception. By suitably sharing the processing tasks between a state-of-the-art FGPA and a DSP, the system can be used in several medical US applications. As reference examples, the detection of microemboli in cerebral circulation and the measurement of wall _distension_ in carotid arteries are finally presented.

Multi transmit beams for fast cardiac imaging towards clinical routine

Multiple-line transmit (MLT) techniques, involving the simultaneous transmission of multiple focused beams into different directions, increase the frame rate without significantly compromising the resolution or contrast. The higher frame rate can facilitate the assessment of regional myocardial deformation or of left ventricular dyssynchrony. However, application in clinical routine is still hindered by the lack of real-time implementations. This paper reports on the first real-time implementation of MLT-based imaging and on a pulse-width modulator for three-level pulsers, so as to transmit complex waveforms like those needed by MLT. Experiments with the ULA-OP 256 research scanner connected to a 128-element phased array probe show that real-time imaging is feasible at up to 212 Hz without significant reduction in image quality or field-of-view, pushing fast MLT cardiac imaging one-step forward to clinical routine.

An integrated system for the evaluation of flow Mediated Dilation

An integrated system capable of estimating either the stimulus (wall shear rate change) and the effect (diameter change) in Flow Mediated Dilation (FMD) ultrasound investigations is presented. The FMD integrated system consists of a modified ULA-OP research platform and a post-processing software. The ULA-OP provides a real-time visual feedback on the positioning of the ultrasound probe on the artery of interest. Both B-mode images and blood velocity profiles can be checked during the entire exam so that the operator continuously controls the morphology and the hemodynamics of the region of interest. In addition, ULA-OP is used to acquire the raw I/Q demodulated data. These are post-processed through a Matlab® platform to estimate the arterial diameter and the wall shear rate changes. Experimental examples are reported.

Compact hardware for real-time multi-line beamforming

Parallel beamforming is increasingly used, since it can deliver imaging frame rates (IFRs) higher than those achievable with standard approaches. In plane wave imaging, for example, IFRs of several kHz can be obtained. In a direct real-time implementation, each line is simultaneously processed by a corresponding hardware beamformer (BF), so that N-line parallel bemforming needs N hardware BFs. This makes the electronics complex and cumbersome, and only powerful GPUs or multiple FPGAs can achieve the required performance. In this work, a simple and compact architecture for real-time multi-line beamforming is proposed. A single BF implemented in FPGA efficiently works at the maximum frequency permitted by the device, so that several lines from the same channel-data are serially processed. The multi-line BF has been implemented in the ULA-OP research scanner and tested on B-Mode and Color-Mode applications.

Real-time vector velocity profile measurement based on plane wave transmission

Standard Doppler ultrasound investigations are limited to detect the axial blood velocity component, as they cannot directly estimate the flow direction. A typical approach for obtaining a 2D velocity vector consists in combining the echoes received from two PW lines investigating the region of interest from different angles. The estimate is usually limited to the sample volume (SV) where the focused lines intersect. To get a complete picture of flow distribution, at least the entire velocity profile across a vessel must be reconstructed. In this work, we propose exploiting the plane waves originated from two different sub-arrays of a linear probe, to estimate the vector velocities in 512 SVs aligned to cover the diameter of large vessels. The method was tested on a laminar flow in a 8 mm diameter pipe. The probe was placed longitudinally over the pipe and the angle, α, between the probe surface and the pipe axis, was changed in the range 0°~20°. In particular, at α = 12°, the flow was perpendicular to one of the plane waves. The direction measurements featured a gain accuracy of 0.89 and a standard deviation (SD) <; 1.8°, while the velocity magnitude featured an average 2.8% overestimation with a coefficient of variation less than 1%.