Daniele Righi

An FFT-based flow profiler for high-resolution in vivo investigations.

Pulsed Doppler spectral analysis is a well-established diagnostic technique in the assessment of arterial diseases. Because of hardware limitations, its use has been so far restricted to the analysis of a single sample volume located along the ultrasound beam axis. In this paper, we discuss the operation of a newly developed multigate instrument capable of performing, in real time, 64-point fast Fourier transforms of Doppler signals sampled from 64 different range cells. The new instrument is capable of accurately detecting the actual blood flow behavior in major human vessels. Significant examples of velocity profiles obtained in real time from carotid arteries in healthy subjects are reproduced here for the first time. Multigate extension of spectral analysis is demonstrated to be a suitable means for detailed in vivo investigation of blood flow dynamics.

Comparison of carotid artery blood velocity measurements by vector and standard Doppler approaches.

Although severely affected by the angle dependency, carotid artery peak systolic velocity measurements are widely used for assessment of stenosis. In this study, blood peak systolic velocities in the common and internal carotid arteries of both healthy volunteers and patients with internal carotid artery stenosis were measured by two vector Doppler (VD) methods and compared with measurements obtained with the conventional spectral Doppler approach. Although the two VD techniques were completely different (using the transmission of focused beams and plane waves, respectively), the measurement results indicate that these techniques are nearly equivalent. The peak systolic velocities measured in 22 healthy common carotid arteries by the two VD techniques were very close (according to Bland-Altman analysis, the average difference was 3.2%, with limits of agreement of ± 8.6%). Application of Bland-Altman analysis to comparison of either VD technique with the spectral Doppler method provided a 21%-25% average difference with ± 13%-15% limits of agreement. Analysis of the results obtained from 15 internal carotid arteries led to similar conclusions, indicating significant overestimation of peak systolic velocity with the spectral Doppler method. Inter- and intra-operator repeatability measurements performed in a group of 8 healthy volunteers provided equivalent results for all of the methods (coefficients of variability in the range 2.7%-6.9%), even though the sonographers were not familiar with the VD methods. The results of this study suggest that the introduction of vector Doppler methods in commercial machines may finally be considered mature and capable of overcoming the angle-dependent overestimation typical of the standard spectral Doppler approach.

On the interaction between ultrasound and contrast agents during Doppler investigations

Knowledge of interaction mechanisms between ultrasound (US) and contrast agents (CA) suspended in blood is important for a correct interpretation of clinical investigation results. Experiments performed in different laboratories have shown that, as a consequence of primary radiation force, CA tend to move away from the US transducer. Accordingly, Doppler spectra produced by particles suspended in moving water turn out to be significantly altered from what is theoretically expected. The purpose of this paper is twofold. First, an original model describing the bubble dynamics as the outcome of the balance between US radiation force and fluid drag force is validated for the case in which bubbles are suspended in blood. The high fluid viscosity is shown to prevent significant bubble deviations from the unperturbed fluid streamlines so that, in large vessels, a residual spectral distortion may exist only at the highest intensity levels permitted by current regulations. Finally, the relative importance and differences between the effect of primary radiation force and streaming mechanisms that, in principle, could lead to similar effects, are discussed.

A simplified approach for real-time detection of arterial wall velocity and distension

Arterial stiffness is known to increase with age and with many vascular diseases, but its noninvasive assessment in patients still represents a difficult task. The measurement of diameter change during the cardiac cycle (distension) has been proposed as a means to estimate arterial compliance and stiffness. Therefore, we have developed a simple PC-based device and algorithm for noninvasive quantification of vessel wall motion and diameter change in humans. This goal is achieved in real-time by processing the base-band signals from a commercial ultrasound Doppler system. Real-time operation is of crucial importance, because it allows a rapid achievement of optimal measurement conditions. The system was evaluated in a laboratory using a string phantom and was tested on the carotid arteries of 10 volunteers. Wall velocities from 0.05 to 600 mm/s and displacements lower than 2 /spl mu/m were detected with phantoms. The measured carotid diameter change in the volunteers ranged from 7.5 to 11.8% (mean=9.8%) and agrees closely with values reported in the literature. The difference between values taken one hour apart ranged from 0.2 to 0.5%. We conclude that the new system provides rapid, accurate, and repeatable measurements of vessel distension in humans.

Quantitative analysis of Doppler spectrum modifications yielded by contrast agents insonified at high pressure

In this report, the authors consider the modifications yielded in the Doppler spectrum when acoustic fields of increasing intensities are applied to encapsulated gas bubbles. Their in vitro experimental results show that the spectrum bandwidth is nearly proportional to the incident acoustic pressure, when its amplitude is maintained below about 200 kPa. At higher pressure levels, it even may happen that, in a steady, unidirectional flow (which should generate only positive Doppler frequencies), the Doppler spectrum is enlarged up to the point that negative Doppler shifts also are produced. Possible explanations in terms of either radiation force or streaming are discussed for this asymmetrical bandwidth enlargement.

Arteriovenous Anastomoses' Function and Raynaud's Phenomenon

By intrabrachial artery injections of a bolus of human albumin microspheres labeled with 99 mTc in patients with primary or secondary Raynauds phenome non (RP) and in a group of healthy volunteers, the authors developed a method of detecting the patency rate of arteriovenous anastomoses (AVA) in the hand, after local heat and cold stimulation, by quantifying the radioactivity of the lungs expressed as a percentage of an intravenously injected radionuclide dose. With strain gauge plethysmography, simultaneous changes in the digital total (DTF) flow were also measured. After exposure of fingers to cold, 25 of 26 subjects had a clear reduction in both DTF and the AVA patency rate (APR) in comparison with the corresponding heat values. The RP patients, in particular, showed a statistically significant reduction in DTF (P < 0.001) and in APR (P < 0.001). These results appear to be consistent with the onset of critically reduced patency of the AVA of the hand during the ischemic phase of RP.

Our Experience on Light Reflection Rheography (LRR): A New Non-Invasive Method for Lower Limbs Venous Examination

By means of a light reflection rheograph (LRR 100), the authors studied 353 lower limbs: 128 with primary varices, 41 with a post-phlebitic syndrome and 81 controls. The parameters that were selected were: VRT (venous refill time) and ΔLR (as expression of the venous emptying rate). In order to assess the accuracy of LRR measurements, simultaneous recordings of both LRR and venous pressure changes in 97 limbs (58 with primary varicose disease, 21 with post-phlebitic syndrome and 18 normals) were compared with each other. The correlation rate for VRT in the two methods revealed very high agreement, (r = 0.96). A fairly good correlation also occurred between changes in venous pressure and ΔLR (r = 0.74). The reproducibility rates ranged between ± 15.9% s.d. (VRT) and ± 7.7% s.d. (ΔLR), using 100 as the average value obtained from each subject. The authors conclude that LRR appears to offer high specificity, sensitivity and reproducibility rates for non-invasive assessment of valvular incompetence. It appears also potentially reliable for the evaluation of calf musculo-venous pump efficiency.

Multigate Quality Doppler Profiles Technology in Vascular, Obstetrics and Cardiology Applications

Multigate Quality Doppler Profiles (QDP) is a Fast Fourier Transform (FFT) based Doppler technology recently integrated within a commercial ultrasound scanner. QDP represents a novel analysis tool, which is able to simultaneously detect and show the velocity components, present in the blood flow of multiple vessels, without any frame rate reduction. QDP can be used in conjunction with the DIR algorithm to enhance the visibility of the flow direction information. The present work describes the use of QDP in Cardiology (Pediatric and Adult), Vascular (cervical venous and arterial system) and Obstetrics (uterine arteries). In vivo and in vitro tests are reported, showing the QDP behavior with respect to insonation angle changes and its advantages with respect to classic Doppler technologies. The QDP is demonstrated to be a precise positioning tool for the Pulsed Wave Doppler (PW) sample volume (SV) and a qualitative direct real time indicator of blood flow variations. It is also shown ideal for the study of turbulent flow, for minor reflux detection and for easier sampling of the blood flow signal within the examined vessel (or vessels) in case of movements.

Transverse Doppler in-vivo spectrograms

Abstract Objective: It is known that a focused ultrasound beam transversely oriented to the direction of flow gives a Doppler spectrum, which despite being centred on zero frequency, has a non-zero bandwidth proportional to the velocity of the insonified flow. This is the main concept of the so-called ‘Transverse Doppler’ theory. The aim of this study is to show a collection of Doppler waveforms obtained in the as yet unusual transverse beam-to-flow orientation and to explain how to gain additional flow information from them. Methods: The results shown have been collected between 1993 and 1996 in three different laboratories located in Florence, Milan and Philadelphia. Commercial pulsed wave (PW) and Duplex scanners have been used and coupled with a custom spectrum analysis unit to provide clearer spectrograms. Results: Comparisons between transverse and conventional spectrograms obtained from arteries of healthy subjects are reported. We also show how the transverse sonograms can be distinguished from waveforms having similar appearance, but caused by erroneous instrumentation settings. One case of turbulent flow produced by a tight stenosis in a common femoral artery (FMA) is finally analyzed. Conclusion: Transverse Doppler spectrograms generated by any focused transducer, if properly interpreted, can give flow information complementary to that contained in conventional sonograms. Using this additional piece of information, the field of application of Doppler flow measurements is extended from the range of acute angles to all angles.

Blood velocity measurement in healthy and diseased carotid arteries by vector Doppler techniques

Vector Doppler (VD) methods are increasingly attractive for the capability of providing angle-independent blood velocity measurements. This may be particularly important in carotid artery peak systolic velocity (PSV) measurements, which are widely used for stenosis assessments. Here, two recently introduced VD techniques have been compared to the classic spectral Doppler approach through 37 PSV measurements in common carotid arteries (CCAs) and internal carotid arteries (ICAs) of healthy volunteers and of patients with different degrees of stenosis. The PSVs measured by the two VD techniques were very close to each other for both CCAs and ICAs (0.83 slope coefficient and 7 cm/s intercept value, according to the linear regression analysis). Comparing any of the two VD techniques to the classic method provided a (0.42÷0.46) regression coefficient with a (14÷20) cm/s intercept value. The high correlation obtained with the two VD techniques supports their suitability to perform accurate PSV measurements in carotid arteries.