Alessandro Dallai

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.

An Automatic Angle Tracking Procedure for Feasible Vector Doppler Blood Velocity Measurements

Two-dimensional angle-independent blood velocity estimates typically combine the Doppler frequencies independently measured by two ultrasound beams with known interbeam angle. A different dual-beam approach was recently introduced in which one (reference) beam is used to identify the flow direction, and the second (measuring) beam directly estimates the true flow velocity at known beam-flow angle. In this paper, we present a procedure to automatically steer the two beams along optimal orientations so that the velocity magnitude can be measured. The operator only takes care of locating the Doppler sample volume in the region of interest and, through the extraction of appropriate parameters from the Doppler spectrum, the reference beam is automatically steered toward right orientation to the flow. The velocity magnitude is thus estimated by the measuring beam, which is automatically oriented with respect to the (known) flow direction at a suitable Doppler angle. The implementation of the new angle tracking method in the ULtrasound Advanced Open Platform (ULA-OP), connected to a linear array transducer, is reported. A series of experiments shows that the proposed method rapidly locks the flow direction and measures the velocity magnitude with low variability for a large range of initial probe orientations. In vitro tests conducted in both steady and pulsatile flow conditions produced coefficients of variability (CV) below 2.3% and 8.3%, respectively. The peak systolic velocities have also been measured in the common carotid arteries of 13 volunteers, with mean CV of 7%.

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.

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.

Voice quality monitoring: A portable device prototype

This paper addresses the important issue of voice monitoring throughout the day under a clinical perspective. This problem is of great concern, for rehabilitation and from the assistive technology point of view. A prototype for a new portable device is proposed, implementing basic voice quality indexes (fundamental frequency F0, jitter, relative average perturbation RAP, noise) by means of robust high-resolution techniques. The device is contact-less, as the transducer is a small microphone included in the device. A feedback for patients outside the clinic is provided, given by a led/audio unit that advices the patient for any abnormal vocal emission, to help patients with carryover of therapy goals outside the clinical environment. The device will collect audio recordings to be submitted to a PC for further analysis, to be performed off-line. Such device for self-diagnosis and vocal rehabilitation could give a valid support, both to clinicians and patients.

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.

Coded Spectral Doppler Imaging: From Simulation to Real-Time Processing

Transmission of coded pulses and matched receive filtering can improve the ultrasound imaging penetration depth while preserving the axial resolution. This paper shows that the pulse compression technique may be integrated in a low-cost scanner to be profitably used also in spectral Doppler investigations. By operating on beamformed, demodulated, and down-sampled data in the frequency domain, a single digital signal processor is proved sufficient to perform both pulse compression and multigate spectral Doppler algorithms in real time. Simulations, phantom, and in vivo experiments demonstrate that the transmission of (2.5 or 5 μs long) linear frequency-modulated chirps with bandwidths over the range 1.6-5.4 MHz, rather than of corresponding sine-burst pulses, provides signal-to-noise ratio (SNR) improvements very close to theory. Even in the presence of selective tissue attenuation, SNR gains up to 11 and 13.3 dB have been obtained for the short and the longer chirp, respectively. This may be important in clinical Doppler applications where the needed penetration depth is not achieved with sufficient SNR unless very long bursts are transmitted.

Simultaneous measurement of wall shear stress and arterial distension in FMD studies

In Flow-mediated dilation (FMD) studies, blood flow in the brachial artery is restricted for about 5 minutes by a cuff. When the restriction is removed, the subsequent increase in wall shear rate (WSR) stimulates the release of nitric oxide, a vasodilator, from the endothelial cells into the smooth muscle. Impaired FMD responses, which are considered independent predictors of possible cardiovascular events, should be reliably detected. While several efforts have been so far dedicated to measure the diameter change, the source of this change, i.e., the WSR increase, has not been measured so far. In this paper, first results of the simultaneous measurement of WSR and diameter performed during FMD studies in the brachial arteries of 15 volunteers are reported. All measurements were obtained through the ULtrasound Advanced Open Platform (ULA-OP).

Implementation of color-flow plane-wave imaging in real-time

The Transmission of Plane Waves (PWT) has recently promoted the development of “ultrafast” Doppler applications. The inherent high computation cost has typically enabled retrospective implementations, i.e. the acquisition of raw data followed by fast off-line beamforming and Doppler processing. In this paper, the combination of PWT with real-time beamforming and Color Flow Mapping (CFM) is experimentally evaluated in comparison with a conventional focussed transmission (FT). The ULA-OP 256 research scanner was configured to implement CFM together with either PWT or FT, and tested on a pulsatile flow phantom. The total computational cost was measured under different conditions allowing frame rates up to 800 Hz for a CFM region of 256×64 points and ensemble length = 8.