R. Matera

An improved Doppler model for obtaining accurate maximum blood velocities

Maximum blood velocity estimates are frequently required in diagnostic applications, including carotid stenosis evaluation, arteriovenous fistula inspection, and maternal-fetal examinations. However, the currently used methods for ultrasound measurements are inaccurate and often rely on applying heuristic thresholds to a Doppler power spectrum. A new method that uses a mathematical model to predict the correct threshold that should be used for maximum velocity measurements has recently been introduced. Although it is a valuable and deterministic tool, this method is limited to parabolic flows insonated by uniform pressure fields. In this work, a more generalized technique that overcomes such limitations is presented. The new approach, which uses an extended Doppler spectrum model, has been implemented in an experimental set-up based on a linear array probe that transmits defocused steered waves. The improved model has been validated by Field II simulations and phantom experiments on tubes with diameters between 2mm and 8mm. Using the spectral threshold suggested by the new model significantly higher accuracy estimates of the peak velocity can be achieved than are now clinically attained, including for narrow beams and non-parabolic velocity profiles. In particular, an accuracy of +1.2±2.5 cm/s has been obtained in phantom measurements for velocities ranging from 20 to 80 cm/s. This result represents an improvement that can significantly affect the way maximum blood velocity is investigated today.

Accurate blood peak velocity estimation using spectral models and vector doppler

Ultrasound blood peak velocity estimates are routinely used for diagnostics, such as the grading of a stenosis. The peak velocity is typically assessed from the Doppler spectrum by locating the highest frequency detectable from noise. The selected frequency is then converted to velocity by the Doppler equation. This procedure contains several potential sources of error: the frequency selection is noise dependent and sensitive to the spectral broadening, which, in turn, is affected by the Doppler angle uncertainty. The result is, often, an inaccurate estimate. In this work we propose a new method that removes the aforementioned errors. The frequency is selected by exploiting a mathematical model of the Doppler spectrum that has recently been introduced. When a very large sample volume is used, which includes all the vessel section, the model is capable of predicting the exact threshold to be used without the need of broadening compensation. The angle ambiguity is solved by applying the threshold to the Doppler spectra measured from two different directions, according to the vector Doppler technique. The proposed approach has here been validated through Field II simulations, phantom experiments, and tests on volunteers by using defocused waves to insonify a large region from a linear array probe. A mean error lower than 1% and a mean coefficient of variability lower than 5% were measured in a variety of experimental conditions.

ULA-OP 256: A portable high-performance research scanner

In the past few years, open scanners have rapidly advanced to offer a variety of solutions to ultrasound researchers. Each system presents specific performance in terms of number of channels, flexibility, processing power and raw data storage capability. This paper describes the 256-channel ULtrasound Advanced Open Platform (ULA-OP 256), designed to provide high performance in a small size. The high-speed interconnection of multiple front-end boards allows the direct and complete control of all transmit (TX) and receive (RX) waveforms. Up to 80 GB of RX data can be stored in the on-board DDR3 memory, while high real-time computational power (equivalent to 2500 GFLOP) is guaranteed by the available digital signal processors. The system implements a massive parallel multi-line beamformer in order to speed up the acquisition in high frame rate imaging techniques. This will enable imaging rates up to 4000 fps, depending on the image frame size.

Validation of a novel vector method for blood peak velocity detection in an anthropomorphic phantom

The peak blood velocity is a parameter of high medical interest, which is used, for example, in the determination of carotid stenosis grade. The standard approach, which typically exploits the maximum frequency detectable in the Doppler spectrum, is prone to two main sources of errors: the ambiguity of the Doppler angle and the spectral broadening. A novel method, based on a mathematical model, was recently introduced and shown to be unaffected by the spectral broadening. The method directly measures the maximum velocity component in a large sample volume that includes all the vessel section. Furthermore, its combination with a vector Doppler approach allows automatically correcting for the angle. This technique produced good results when verified in straight tubes, but tests in a more realistic configuration are necessary for an accurate validation. In this work, the proposed technique is compared against 2 already validated methods by investigating the common and internal branches of an anthropomorphic phantom, which mimics a carotid bifurcation with a 50% stenosis on the internal artery.

Amplitude and phase estimator for real-time biomedical spectral Doppler applications

In a typical echo-Doppler investigation the moving blood is periodically insonated by the transmitting bursts of ultrasound energy. The echoes, shifted in frequency according to the Doppler effect, are received, coherently demodulated and processed through a spectral estimator. The detected frequency shift can be exploited for blood velocity assessment. The spectral analysis is typically performed by the conventional Fast Fourier Transform (FFT), but, recently, the application of the Amplitude and Phase EStimator (APES) was proved to produce a good quality sonogram based on a reduced number of transmissions. Unfortunately, the much higher calculation effort needed by APES hampers its use in real-time applications. In this work, a fixed point DSP implementation of APES is presented. A spectral estimate - based on 32 transmissions - occurs in less than 120μs. Results obtained on echo-Doppler investigations on a volunteer are presented.

Real-time staggered PRF for vector Doppler blood velocity assessment

Vector Doppler techniques are ready to substitute the standard Doppler methods currently employed in clinical blood velocity investigations. However, like in classic Pulse-Wave modes, the maximum detectable velocity is related to the Pulse Repetition Frequency (PRF), which is limited by the maximum depth and/or by hardware constraints. Unfortunately, the blood velocity, e.g. in stenotic vessels, can easily peak above 1.5 m/s, which often results in aliased and unreliable measurements. Staggered PRF is a technique that, by correlating the Doppler shifts detected at different PRFs, can recover the correct velocity even when it is beyond the Nyquist limit. In this work, we applied staggered PRF to Multi Line Vector Doppler (MLVD), a 2D imaging method that, exploiting plane waves, detects velocity vectors on a 2D region composed of 8 lines placed perpendicular to a linear array probe. Real-time MLVD with staggered PRF was implemented in the ULA-OP 256 research scanner. Tests in a flow-rig show the MLVD capable of recovering the correct peak velocity up to 2.3 times the Nyquist limit.

Real-Time in-Line Industrial Fluids Characterization Using Multiple Pulse Repetition Frequency

The characterization of fluids flowing in industrial pipes is of paramount importance to optimize the production process and guarantee the final product quality in most industries. Rheological parameters of the fluid can be efficiently calculated starting from the Pressure Drop (PD) along a tract of the pipe, and the velocity profile that the flow develops along the pipe diameter, which can be assessed through Ultrasounds Pulsed Wave Doppler (PWD). Unfortunately, in PWD the maximum detectable velocity is restricted by the aliasing limit related to the Pulse Repetition Frequency (PRF). The use of PRF sequences at different rate can recover de-aliased velocities by combining the aliased data. In this work, we extend the capabilities of an embedded PWD ultrasound system used to characterize industrial fluids by implementing, in real-time, the multi-PRF method.

High Dynamics Adaptive Demodulator for Ultrasound Applications: FPGA Implementation

The modern acquisition and processing systems digitalize the input signal directly at radio-frequency (RF) by using Analog-to-Digital (AD) converters, that works up to several hundreds of MHz, and then process the signal numerically in powerful flexible digital devices like the Field Programmable Gate Arrays (FPGAs). A digital coherent demodulation is the first processing step in several applications. Echo-Doppler ultrasound systems belong to this category. An industrial fluid flowing in a pipe can be investigated by transmitting ultrasound bursts at some MHz in the pipe. Fluid particles produce echoes with a frequency shift (typically some kHz) related to the particle velocity through the Doppler effect. The echoes are sampled at several tens of MHz, and coherent demodulated for detecting the Doppler shift. The demodulator processes the high intensity echoes from the steel pipes wall together with the weak signal from the fluid, and thus it should feature very high dynamics. In this paper, it is presented an adaptive numerical demodulator integrated in the FPGA of an ultrasound system for flow profile detection. Its performance is tested with synthetic samples and a signal acquired from a pipe. Finally, the flow profile detected in a pipe, calculated by a processing chain that includes the proposed demodulator, is reported.

Validation of a Doppler method for peak blood velocity detection in a CFD-simulated carotid model

The peak velocity of the blood flowing in carotid is a parameter of high impact in the diagnosis of cardiovascular diseases. Current measurement methods are based on heuristic thresholds applied to the Doppler spectrum. Unfortunately, they often produce inaccurate assessments due to their sensitivity to noise and to the spectral broadening effect. Recently, a new vector technique has been proposed that solves these shortcomings thanks to a threshold calculated through an accurate mathematical model of the Doppler spectrum. However, the evaluation of its accuracy in real-life conditions is hampered by the difficulty of obtaining a reliable gold standard. Computational fluid dynamics (CFD) simulations, based on real carotid geometries, can help in filling this gap. In this work the proposed peak measurement method is tested on a realistic CFD model of the carotid bifurcation with an eccentric plaque in the internal branch. Common (CCA) and internal (ICA) arteries were separately investigated for a whole cardiac cycle, and the measurements from the method under test were compared to the CFD velocity reference. The obtained errors are +4.3% and +5.2% for the whole heart cycle in CCA and ICA, respectively, and +4.6% and +3.1% for the systolic peak.

An improved method of determining peak blood velocity

The peak blood velocity is used in important diagnostic applications, e.g. for determining the stenosis degree. The peak velocity is typically assessed by detecting the highest frequency in the Doppler spectrum. The selected frequency is then converted to velocity by the Doppler equation. This procedure contains multiple potential sources of error: the peak frequency selection is sensitive to noise and affected by spectral broadening, and the frequency to velocity conversion is altered by the Doppler angle uncertainty. The result is an inaccurate estimate. In this work we propose a new method that removes the aforementioned errors. By exploiting a mathematical model of the Doppler spectrum the exact frequency to be converted to velocity, with no need of broadening compensation, is determined. The angle ambiguity is solved by calculating the Doppler spectra backscattered from two different receive apertures. The proposed methods uses, in transmission and receive, defocused steered waves that produce a wide sample volume. This includes the whole vessel section making the probe positioning quick and easy. The method, validated through Field II simulations and phantom experiments, featured a mean error lower than 1%.