Biomedical subjects

Piero Tortoli

Publications and source records attributed to Piero Tortoli.

12 recordsLinked to original sources

PubMed2006

Noninvasive simultaneous assessment of wall shear rate and wall distension in carotid arteries.

A novel technique has been developed for the noninvasive real-time simultaneous assessment of both blood velocity profile and wall displacements in human arteries. The novel technique is based on the use of two ultrasound beams, one set at optimal angle for wall motion measurements and the other for blood velocity profile measurements. The technique was implemented on a linear array probe divided into two subapertures. A modified commercial ultrasound machine and a custom PC board based on a high-speed digital signal processor was used to process the quadrature demodulated echo signals and display results in realtime. Flow phantom experiments demonstrated the validity of the technique, providing wall shear rate (WSR) estimates within 10% of the theoretical values. The system was also tested in the common carotid arteries of 16 healthy volunteers (age 30 to 53 y). Results of simultaneous diameter distension and WSR measurements were in agreement with published data.

Adult↗

PubMed2006

A programmable real-time system for development and test of new ultrasound investigation methods.

In vitro and/or in vivo experimental tests represent a crucial phase in the development of new ultrasound (US) investigation methods for biomedical applications. Such tests frequently are made difficult by the lack of flexibility of general purpose instruments and commercial US machines typically available in research laboratories. This paper presents a novel, real-time development system specifically designed for US research purposes. Main features of the system are the limited dimensions (it is based on a single electronic board), the capability of transmitting arbitrary waveforms to two probes, of storing the received radio-frequency (RF) echo data in a file and/or of processing them in real-time according to programmable algorithms. As an example of application, results of simultaneous hemodynamic and mechanic investigations in human arteries are reported. However, the high system flexibility and portability make it suitable for a large class of US applications.

Biomedical Research↗

PubMed2006

Multigate transcranial Doppler ultrasound system with real-time embolic signal identification and archival.

An integrated system for acquisition and processing of intracranial and extracranial Doppler signals and automatic embolic signal detection has been developed. The hardware basis of the system is a purpose-built acquisition/processing board that includes a multigate Doppler unit controlled through a computer. The signal-processing engine of the system contains a fast Fourier transform (FFT)-based, spectral-analysis unit and an embolic signal-detection unit using expert system reasoning theory. The system is designed so that up to four receive gates from a single transducer can be used to provide useful reasoning information to the embolic signal-detection unit. Alternatively, two transducers can be used simultaneously, either for bilateral transcranial Doppler (TCD) investigations or for simultaneous intra- and extracranial investigation of different arteries. The structure of the software will allow the future implementation of embolus detection algorithms that use the information from all four channels when a single transducer is used, or of independent embolus detection in two sets of two channels when two transducers are used. The user-friendly system has been tested in-vitro, and it has demonstrated a 93.6% sensitivity for micro-embolic signal (MES) identification. Preliminary in-vivo results also are encouraging.

Adult↗

PubMed2006

Accurate Doppler angle estimation for vector flow measurements.

Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 900 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 900 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 degree), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.

Animals↗

PubMed2005

Clinical validation of common carotid artery wall distension assessment based on multigate Doppler processing.

Mechanical properties of human large arteries result from the interaction between blood pressure, wall distensibility and shear stress. Both the arterial diameter changes through the cardiac cycle (distension) and blood flow velocities can be noninvasively investigated through Doppler ultrasound approaches. Recently, an integrated system processing in real-time all the echo signals produced along an M-line has been developed. This system has been so far demonstrated to be suitable for accurate hemodynamic studies through the detection of blood velocity profiles. This paper reports on the extension of its processing capabilities to the real-time measurement of arterial distension. Tissue motion estimation is based on a modified 2-D autocorrelation algorithm. A novel adaptive approach to track wall position over time using the sum of the high-pass filtered displacement waveform and the low-pass filtered wall position is described. By observing the blood velocity profile, a rapid and accurate positioning of the ultrasound probe and an inherent check on perpendicular observation are provided. First clinical results obtained by measuring the distension of common carotid arteries in a group of 41 volunteers are reported and measurements are validated against those provided by a dedicated wall-track reference system. Average measured distension and diameter were 499 +/- 188 microm and 6.90 +/- 0.66 mm and intraobserver intrasession reproducibility tests showed coefficients of variability of 8.5% and 5.9%, respectively. The agreement between the proposed system and the reference system, expressed as bias +/- 2 SD of the differences, was -34 +/- 141 microm for distension and 0.05 +/- 1.07 mm for diameter.

Adolescent↗

PubMed2005

Real-time software processing and audio reproduction of directional Doppler signals.

In ultrasound Doppler systems, directional signals are typically obtained by processing quadrature demodulated data with dedicated analog or digital circuits. In this paper, a software approach is proposed, that allows fully exploiting the reproduction and recording capabilities of low-cost personal computer sound cards and/or embedded chips. Forward/reverse signals are separated through a wideband Hilbert filter. No limitations are imposed on the input signal sample rate, which is matched to the standard output format of sound cards through a band-limited interpolation filter controlled in a feedback loop. The digital audio streaming is performed in real-time in a Windows-based application. The processed data are in a standard format compatible with real-time recording in waveform or compressed files, as requested in many research applications. Simulations and in vivo tests show a typical cross talk of -50 dB between forward and reverse components, with low latency time (39 ms) and central processing unit load compatible to currently available personal computers.

Equipment Design↗

PubMed2005

Optical observations of acoustical radiation force effects on individual air bubbles.

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 microm to 79 microm was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 microm. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position.

Acoustics↗

PubMed2005

Different effects of microbubble destruction and translation in Doppler measurements.

In flow measurements in which microbubbles are involved, the amplitude and phase of the received echo signal are noticeably influenced by the transmitted ultrasound intensity. Previous studies have shown that, when such intensity is progressively increased, the Doppler spectrum is accordingly distorted, i.e., it is asymmetrically broadened toward the negative frequency side. Such deformation has been attributed to radiation force, which pushes the microbubbles into the sound propagation direction, thus yielding additional phase delays in the received echoes. However, the possible contribution of microbubble destruction to this spectral deformation has not been considered yet. In this paper, this issue is investigated by analyzing the experimental spectra produced by two different types of microbubbles suspended in a moving fluid and insonified in pulsed wave (PW) mode at programmable pulse repetition frequency (PRF) and pressure. Conditions are created in which either the radiation force or the destruction mechanism is expected to be dominant. Effects produced by the two phenomena on the Doppler spectrum are shown to be different. When the PRF is low (2 kHz), so that, according to theoretical simulations, the radiation force effect is negligible, a 26 dB noise floor increase is observed for a 13 dB pressure increment. For a higher PRF (16 kHz), the same pressure increase not only affects the noise floor, but also causes the bubbles to deviate from their original streamlines, yielding a Doppler bandwidth increase by a factor of 5. It is concluded that asymmetrical spectral broadening is mainly due to radiation force, and microbubble destruction mainly results in an increased noise floor without affecting the spectral shape.

Algorithms↗

PubMed2004

Intraluminal ultrasound intensity distribution and backscattered Doppler power.

Ultrasound (US) incident obliquely on a cylindrical vessel is redistributed in space when the propagation path includes walls with acoustic impedance different from that of the surrounding media. We investigated this using low-density polyethylene (PE) as the vessel wall material. Both simulations and experiments were carried out. Direct hydrophone measurements of the acoustic field were made within a half section of the PE tube, and the distribution of backscattered Doppler power along a scan line was obtained using a range-Doppler instrument. Both simulation and hydrophone results demonstrate lateral shadow regions within the lumen. In every one of various Doppler flow experiments conducted, the backscattered Doppler power, compensated for on-axis transducer behaviour, increased with depth. Simulation results for an incident continuous-wave (CW) plane wave show that it tends to be focused by the curvature of the PE tube walls. The wall interactions are, however, angle-dependent and so the behaviour of a focused US beam depends on the beam as well as the walls. This study demonstrates alterations in the spatial distribution of US within a cylindrical vessel as a result of known vessel wall properties. It also provides evidence that local intensity variations within the lumen affect the relative Doppler power backscattered from small sample volumes.

Blood Vessels↗

PubMed2003

Interaction between secondary velocities, flow pulsation and vessel morphology in the common carotid artery.

The common carotid artery (CCA), one of the vessels more frequently investigated by ultrasound (US), is often modeled as a straight tube in quasi-laminar flow regimens. Experimental investigations based on a prototype multigate system show that blood velocity profiles are parabolic during diastole and early systole, and flat during the systolic peak. However, during late systole/beginning of diastole, they have an "M" shape, where the velocity near the walls is higher than in the vessel center. Moreover, the profile shape changes when the sound beam direction is moved over a given cross-section; thus, suggesting a nonaxisymmetrical velocity distribution, which contradicts the straight tube assumption. The purpose of this paper was twofold. First, the actual velocity distribution in "normal" CCAs was reconstructed. The analysis of several velocity profiles confirms that the velocity distribution is markedly asymmetrical, especially during the deceleration phase following the systolic peak. Second, a tentative explanation for such behavior is given by correlating it with the growth of secondary flows caused by the slight vessel curvature and viscous effects. This explanation is supported by the comparison between in vitro results and numerical solution of the Navier-Stokes equations in laminar pulsed-flow regimens.

Adult↗

PubMed2003

Noninvasive in vivo measurements of hematocrit.

OBJECTIVE: To develop a clinically applicable method for noninvasive acoustic determination of hematocrit values in vivo. METHODS: The value of hematocrit was determined initially in vitro from the pulseecho measurements of acoustic attenuation. The testing was carried out in a laboratory setup with an ultrasonic transducer operating at 20 MHz and with the use of human blood samples at 37 degrees C. The attenuation coefficient measurements in blood in vivo were implemented by multigated, 20-MHz pulsed Doppler insonation. The Doppler signal was recorded in the brachial and radial arteries. Both in vitro and in vivo hematocrit data were compared with those obtained by the centrifuge method. RESULTS: The attenuation coefficient in vitro was determined from the measurements of 168 samples with hematocrit values varying between 23.9% and 51.6%. The attenuation from 20-MHz data was equal to 3.66 + 0.089 hematocrit (decibels per centimeter). The uncertainty of in vivo measurements in the brachial artery was determined to be within +/- 5% hematocrit. However, the measurements in the radial artery resulted in a clinically unacceptable uncertainty of +/- 20% hematocrit. CONCLUSIONS: The method proposed appears to be promising for in vivo determination of hematocrit, because 5% hematocrit error is adequate for monitoring changes in patients in shock or during dialysis. It was found that the multigate system largely simplified placement of an ultrasonic probing beam in the center of the blood vessel. Current work focuses on enhancing the method's applicability to arbitrarily selected vessels and to reducing the hematocrit measurement error to much less than 5% hematocrit.

Acoustics↗

PubMed2002

Toward a better quantitative measurement of aortic flow.

Ultrasound investigation of aortic blood flow (ABF) still represents a technically challenging task, because of the complex geometry of such a deep artery. In this paper, we present a unique experimental set-up capable of providing detailed information about blood dynamics in the aorta. The set-up is based on an esophageal probe (EP) connected to a multigate Doppler-processing system. The EP, developed for the noninvasive hemodynamic monitoring of ABF in patients under general anesthesia or in the intensive care area, must be inserted at a thoracic depth where the esophagus and the aorta are nearby and parallel. Doppler processing of pulsed wave echoes in the multigate system provides the distribution of all Doppler frequencies detected along the probe beam axis (spectral profile) in real time. The results of this investigation confirm that flow in the aorta is extremely complex, especially at the level of the aortic arch or in nonphysiologic circumstances. In general, the velocity profiles tend to be flat only during the systolic acceleration, but not during the full cardiac cycle. In most cases, they are asymmetrical, including both positive and negative components. In particular, it is shown that an appropriate positioning of the ultrasound transducer and/or the correct integration of different velocities is mandatory to make reliable ABF measurements.

Aorta↗