Pulse wave velocity measurement along the ulnar artery in the wrist region using a high frequency ultrasonic array.

A pulse wave velocity (PWV) measurement method performed above a small blood vessel using an ultrasonic probe is studied and reported in this paper. These experimentations are carried out using a high-frequency probe (14-22 MHz), allowing a high level of resolution compatible with the vessel dimensions, combined with an open research ultrasound scanner. High frame-rate (HFR) imaging (10 000 frames per second) is used for a precise PWV estimation. The measurements are performed in-vivo on a healthy volunteer. The probe is placed above the ulnar artery on the wrist in order to make longitudinal scans. In addition to conventional duplex ultrasound evaluation, the measurement of the PWV using this method at this location could strengthen the detection and diagnosis of cardiovascular diseases (CVDs), in particular for arm artery diseases (AADs). Moreover, these experimentations are also carried out within the scope of a demonstration for a potential miniaturized and wearable device (i.e., a probe with fewer elements, typically less than 32, and its associated electronics). The study has shown results coherent with expected PWV and also promising complementary results such as intima-media thickness (IMT) with spatiotemporal resolution on the order of 6.2 µm and 0.1 ms.

Ultrafast compound Doppler imaging: providing full blood flow characterization.

Doppler-based flow analysis methods require acquisition of ultrasound data at high spatio-temporal sampling rates. These rates represent a major technical challenge for ultrasound systems because a compromise between spatial and temporal resolution must be made in conventional approaches. Consequently, ultrasound scanners can either provide full quantitative Doppler information on a limited sample volume (spectral Doppler), or averaged Doppler velocity and/or power estimation on a large region of interest (Doppler flow imaging). In this work, we investigate a different strategy for acquiring Doppler information that can overcome the limitations of the existing Doppler modes by significantly reducing the required acquisition time. This technique is called ultrafast compound Doppler imaging and is based on the following concept: instead of successively insonifying the medium with focused beams, several tilted plane waves are sent into the medium and the backscattered signals are coherently summed to produce high-resolution ultrasound images. We demonstrate that this strategy allows reduction of the acquisition time by a factor of up to of 16 while keeping the same Doppler performance. Depending on the application, different directions to increase performance of Doppler analysis are proposed and the improvement is quantified: the ultrafast compound Doppler method allows faster acquisition frame rates for high-velocity flow imaging, or very high sensitivity for low-flow applications. Full quantitative Doppler flow analysis can be performed on a large region of interest, leading to much more information and improved functionality for the physician. By leveraging the recent emergence of ultrafast parallel beamforming systems, this paper demonstrates that breakthrough performances in flow analysis can be reached using this concept of ultrafast compound Doppler.

High-frequency ultrasound backscattering by blood: analytical and semianalytical models of the erythrocyte cross section.

This paper proposes analytical and semianalytical models of the ultrasonic backscattering cross section (BCS) of various geometrical shapes mimicking a red blood cell (RBC) for frequencies varying from 0 to 90 MHz. By assuming the first-order Born approximation and by modeling the shape of a RBC by a realistic biconcave volume, different scattering behaviors were identified for increasing frequencies. For frequencies below 18 MHz, a RBC can be considered a Rayleigh scatterer. For frequencies less than 39 MHz, the general concept of acoustic inertia tensor is introduced to describe the variation of the BCS with the frequency and the incidence direction. For frequencies below 90 MHz, ultrasound backscattering by a RBC is equivalent to backscattering by a cylinder of height 2 microm and diameter 7.8 microm. These results lay the basis of ultrasonic characterization of RBC aggregation by proposing a method that distinguishes the contribution of the individual RBC acoustical characteristics from collective effects, on the global blood backscattering coefficient. A new method of data reduction that models the frequency dependence of the ultrasonic BCS of micron-sized weak scatterers is also proposed. Applications of this method are in tissue characterization as well as in hematology.

Three-dimensional spatial and temporal temperature imaging in gel phantoms using backscattered ultrasound.

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed suboptimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging recently was proposed as a means to measure noninvasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional (2-D) nature of traditional ultrasound imaging, and the complexity of the temperature dependence of sound speed for biological tissues. In this paper, we present methodology, results, and validation of a 3-D spatial and temporal ultrasound temperature estimation technique in an alginate-based gel phantom to track the evolution of heat deposition over a treatment volume. The technique was experimentally validated for temperature rises up to approximately 10 degrees C by comparison with measurements from thermocouples that were embedded in the gel. Good agreement (rms difference = 0.12 degrees C, maximum difference = 0.24 degrees C) was observed between the noninvasive ultrasound temperature estimates and thermocouple measurements. Based on the results obtained for the temperature range studied in this paper, the technique demonstrates potential for applicability in image guidance of thermal therapy for determining the location of the therapeutic focal spot and assessing the extent of the heated region at subablative intensities.

Analytical modeling of the instantaneous maximal transvalvular pressure gradient in aortic stenosis.

In presence of aortic stenosis, a jet is produced downstream of the aortic valve annulus during systole. The vena contracta corresponds to the location where the cross-sectional area of the flow jet is minimal. The maximal transvalvular pressure gradient (TPG(max)) is the difference between the static pressure in the left ventricle and that in the vena contracta. TPG(max) is highly time-dependent over systole and is known to depend upon the transvalvular flow rate, the effective orifice area (EOA) of the aortic valve and the cross-sectional area of the left ventricular outflow tract. However, it is still unclear how these parameters modify the TPG(max) waveform. We thus derived an explicit analytical model to describe the instantaneous TPG(max) across the aortic valve during systole. This theoretical model was validated with in vivo experiments obtained in 19 pigs with supravalvular aortic stenosis. Instantaneous TPG(max) was measured by catheter and its waveform was compared with the one determined from the derived equation. Our results showed a very good concordance between the measured and predicted instantaneous TPG(max). Total relative error and mean absolute error were on average 9.4+/-4.9% and 2.1+/-1.1 mmHg, respectively. The analytical model proposed and validated in this study provides new insight into the behaviour of the TPG(max) and thus of the aortic pressure at the level of vena contracta. Because the static pressure at the coronary inlet is similar to that at the vena contracta, the proposed equation will permit to further examine the impact of aortic stenosis on coronary blood flow.

Characterization of digital waveforms using thermodynamic analogs: applications to detection of materials defects.

We describe characterization of digital signals using analogs of thermodynamic quantities: the topological entropy, Shannon entropy, thermodynamic energy, partition function, specific heat at constant volume, and an idealized version of Shannon entropy in the limit of digitizing with infinite dynamic range and sampling rate. We show that analysis based on these quantities is capable of detecting differences between digital signals that are undetectable by conventional methods of characterization based on peak-to-peak amplitude or signal energy. We report the results of applying thermodynamic quantities to a problem from nondestructive materials evaluation: detection of foreign objects (FO) embedded near the surface of thin graphite/epoxy laminates using backscattered waveforms obtained by C-scanning the laminate. The characterization problem was to distinguish waveforms acquired from the region containing the FO from those acquired outside. In all cases the thermodynamic analogs exhibit significant increases (up to 20-fold) in contrast and for certain types of FO materials permit detection when energy or amplitude methods fail altogether.

Effect of red cell clustering and anisotropy on ultrasound blood backscatter: a Monte Carlo study.

When flowing at a low shear rate, blood appears hyperechogenic on ultrasound B-scans. The formation of red blood cell (RBC) aggregates that also alters blood viscosity is the microscopic mechanism explaining this acoustical phenomenon. In this study, Monte Carlo simulations were performed to predict how RBC clustering increases ultrasound scattering by blood. A bidimensional Gibbs-Markov random point process parameterized by the adhesion energy epsilon and an anisotropy index nu was used to describe RBC positions for a hematocrit H = 40%. The frequency dependence of the backscattering coefficient chi(f) was computed using Born approximation. The backscattering coefficient chi0 at 5 MHz and the spectral slopes n(x) and n(y) (chi alpha f(nx) or f(ny)) measured, respectively, when the insonification is parallel and perpendicular with the RBC cluster axis were then extracted. Under isotropic conditions, chi0 increased up to 7 dB with epsilon and n(x) = n(y) decreased from 4.2 to 3.4. Under anisotropic conditions, the backscattering was stronger perpendicularly to aggregate axis, resulting in n(x) < n(y). The anisotropy in scattering appeared more pronounced when epsilon or nu increased. These two dimensional results generally predict that low-frequency blood backscatter is related to cluster dimension, and higher-frequency properties are affected by finer morphological features as anisotropy. This numerically establishes that ultrasound backscatter spectroscopy on a large frequency range is pertinent to characterize in situ hemorheology.

Non-Gaussian statistics and temporal variations of the ultrasound signal backscattered by blood at frequencies between 10 and 58 MHz.

Very little is known about the blood backscattering behavior and signal statistics following flow stoppage at frequencies higher than 10 MHz. Measurements of the radio frequency (rf) signals backscattered by normal human blood (hematocrit = 40%, temperature = 37 degrees C) were performed in a tube flow model at mean frequencies varying between 10 and 58 MHz. The range of increase of the backscattered power during red blood cell (RBC) rouleau formation was close to 15 dB at 10 and 36 MHz, and dropped, for the same blood samples, below 8 dB at 58 MHz. Increasing the frequency from 10 to 58 MHz raised the slope of the power changes at the beginning of the kinetics of aggregation, and could emphasize the non-Gaussian behavior of the rf signals interpreted in terms of the K and Nakagami statistical models. At 36 and 58 MHz, significant increases of the kurtosis coefficient, and significant reductions of the Nakagami parameter were noted during the first 30 s of flow stoppage. In conclusion, increasing the transducer frequency reduced the magnitude of the backscattered power changes attributed to the phenomenon of RBC aggregation, but improved the detection of rapid growth in aggregate sizes and non-Gaussian statistical behavior.

Simulation of ultrasound backscattering by red cell aggregates: effect of shear rate and anisotropy.

Tissue characterization using ultrasound (US) scattering allows extraction of relevant cellular biophysical information noninvasively. Characterization of the level of red blood cell (RBC) aggregation is one of the proposed application. In the current paper, it is hypothesized that the microstructure of the RBCs is a main determinant of the US backscattered power. A simulation model was developed to study the effect of various RBC configurations on the backscattered power. It is an iterative dynamical model that considers the effect of the adhesive and repulsive forces between RBCs, and the effect of the flow. The method is shown to be efficient to model polydispersity in size, shape, and orientation of the aggregates due to the flow, and to relate these variations to the US backscattering properties. Three levels of aggregability at shear rates varying between 0.05 and 10 s(-1) were modeled at 40% hematocrit. The simulated backscattered power increased with a decrease in the shear rate or an increase in the RBC aggregability. Angular dependence of the backscattered power was observed. It is the first attempt to model the US power backscattered by RBC aggregates polydisperse in size and shape due to the shearing of the flow.