Lei Liu

In vitro study of ultrasound radiation force-driven twinkling sign using PVA-H gel and glass beads tissue-mimicking phantom

The twinkling sign observed in ultrasound coded-excitation imaging (e.g., GE B-Flow) has been reported in previous research as a potential phenomenon to detect micro calcification in soft tissue. However, the mechanism of the twinkling sign has not been clearly understood yet. We conducted an in vitro experiment to clarify the mechanism of the twinkling sign by measuring a soft tissue-mimicking phantom with ultrasonic and optical devices. A soft tissue-mimicking phantom was made of poly(vinyl alcohol) hydro (PVA-H) gel and 200-μm-diameter glass beads. We applied ultrasound to the phantom using medical ultrasound diagnostic equipment to observe the twinkling sign of glass beads. Optical imaging with a laser sheet and a high-speed camera was performed to capture the scatter lights of the glass beads with and without ultrasound radiation. The scatter lights from the glass beads were quantified and analyzed to evaluate their oscillations driven by the ultrasound radiation force. The twinkling sign from the glass beads embedded in the PVA-H gel soft tissue phantom was observed in ultrasound B-Flow color imaging. The intensity and oscillation of the scattered lights from the glass beads showed significant difference between the cases with and without ultrasound radiation. The results showed a close relationship between the occurrence of the twinkling sign and the variations of the scatter lights of glass beads, indicating that ultrasound radiation force-driven micro oscillation causes the twinkling sign of micro calcification in soft tissue.

Development and feasibility study of a two-dimensional ultrasonic-measurement-integrated blood flow analysis system for hemodynamics in carotid arteries

Prevention and early detection of atherosclerosis are critical for protection against subsequent circulatory disease. In this study, an automated two-dimensional ultrasonic-measurement-integrated (2D-UMI) blood flow analysis system for clinical diagnosis was developed, and the feasibility of the system for hemodynamic analysis in a carotid artery was revealed. The system automatically generated a 2D computational domain based on ultrasound color Doppler imaging and performed a UMI simulation of blood flow field to visualize hemodynamics in the domain. In the UMI simulation, compensation of errors was applied by adding feedback signals proportional to the differences between Doppler velocities by measurement and computation while automatically estimating the cross-sectional average inflow velocity. The necessity of adjustment of the feedback gain was examined by analyzing blood flow in five carotid arteries: three healthy, one sclerosed, and one stenosed. The same feedback gain was generally applicable for the 2D-UMI simulation in all carotid arteries, depending on target variables. Thus, the present system was shown to be versatile in the sense that the parameter is patient independent. Moreover, the possibility of a new diagnostic method based on the hemodynamic information obtained by the 2D-UMI simulation, such as a waveform of the cross-sectional average inflow velocity and wall shear stress distributions, was suggested.

Effect of pulse repetition frequency on microcalcification detection in color flow imaging

The twinkling sign is a phenomenon involving rapidly alternating color pixels behind a stationary strongly reflecting medium where an acoustic shadow is expected in the Doppler mode. However, the mechanism of the occurrence of such a sign is not yet clarified. In this study, to clarify the mechanism, we investigated effect of pulse repetition frequency (PRF) on echo signals from a glass bead (diameter 973 µm) in a soft-tissue-mimicking phantom, and observed the appearance pattern of the twinkling sign of the same glass bead. It was observed that the variance of the IQ signal and the appearance frequency of the twinkling sign decreased when PRF increased.

Fundamental study on micro calcification detection using twinkling sign (TS): The effect of stiffness of surrounding tissue on the appearance of TS

The twinkling sign (TS) observed in ultrasound imaging (e.g., color flow mode and pulse Doppler mode) has been reported in previous researches as a potential phenomenon to detect micro calcification in soft tissue. However, the mechanism of the twinkling sign has not been clearly understood yet. The authors investigated the effect of stiffness of surrounding tissue on the appearance of TS using the soft tissue-mimicking phantoms and a medical ultrasound device. The author used Poly (vinyl alcohol) hydro (PVA-H) gel as the material of phantom and developed three phantoms with different PVA concentration; 8 %wt, 10 %wt and 15 %wt those correspond to Youngs modulus (E) as 50 kPa, 100 kPa and 230 kPa, respectively. Micro glass and CaCO3 particles were embedded in the phantoms as pseudo micro calcification. The authors observed TS in each phantom and analyzed the temporal average of TS. The temporal average of TS was largest in the 8 %wt (E = 50kPa) PVA-H gel phantom, and decreased with increasing the phantom stiffness. The result indicated that the micro oscillation of the particles had a close relationship with the occurrence of TS.

Deformation of Stenotic Blood Vessel Model Made From Poly (Vinyl Alcohol) Hydrogel by Hydrostatic Pressure

Vascular plaque deformation reduces blood flow, increases arterial embolism risk, and may lead to ischemic stroke. Plaque stiffness varies widely and is an important factor influencing both plaque and parent artery deformation. These geometric changes affect local hemodynamics, which impact plaque initiation influencing disease progression. However, most previous studies used non-elastic stenotic vessel models. For more realistic analysis, we constructed a stenosis model comprising an elastic poly (vinyl alcohol) hydrogel (PVA-H) parent artery and plaque of variable stiffness. Our previous study using this flexible model demonstrated substantial effects of hydrostatic pressure. Here ultrasonography was conducted under changing hydrostatic pressure to measure geometric changes at the narrowest cross section. PVA-H specimens were constructed with the stiffness of a hard lipid core, smooth muscle, and plaque, as estimated by tensile tests using 5, 12, and 15 wt% PVA, respectively. The change in cross-sectional aspect ratio (height/face length) at the narrowest site is largest (∼1.3) for the 5 wt% PVA-H plaque and smallest (∼1.2) for the 12 wt% PVA-H plaque. Stenotic artery deformation depends on both artery and plaque elasticity. Hydrostatic pressure has a substantial effect on both vessel and plaque geometries, which markedly alter blood flow.Copyright

Microscopic observation of glass bead movement in soft tissue-mimicking phantom under ultrasound PW mode scanning

Previous studies have demonstrated that stones and calcification in soft tissue show special enhancement in response to color flow (CF) or pulse Doppler (PW) mode ultrasound scan. This phenomenon is known as the “twinkling sign (TS)”. The authors conducted an in vitro experiment to investigate the mechanism of TS occurrence by observing a glass bead in a transparent PVA-H soft tissue-mimicking phantom. The TS in PW mode showed a low-power and slow-velocity spectrum. At the same time, analysis of images by high-speed camera showed that the glass bead in the phantom oscillated following the pulse repetition frequency (PRF) of the PW mode ultrasound scan. The harmonic oscillations were confirmed, as well. The ultrasound radiation force-driven micro-oscillation possibly affects the ultrasound propagation around the scatterer and triggers random signals in the received echo signals. The results indicate that TS is a phenomenon based on complicated acoustic–mechanical interaction of multiple mechanisms. Further investigation is required for gaining a full understanding of the mechanism of TS occurrence and its clinical application.

Evaluation of Compliance of Poly (vinyl alcohol) Hydrogel for Development of Arterial Biomodeling

An in vitro blood vessel biomodeling with realistic mechanical properties and geometrical structures will be helpful for the training of intervention, preoperative simulation, and realizing the same physical conditions of blood flow. Poly (vinyl alcohol) hydrogel (PVA-H) with low surface friction, good transparency, and similar elasticity to blood vessel has been developed and accepted as a good material for biomodeling. However, the compliance of PVA-H vessel model has not been measured. In this study, we changed the elasticity and measured the compliance of a box-typed PVA-H biomodeling, to represent the similar compliance to a real artery. PVA powders were dissolved in a mixture of water and organic solvent so that PVA solutions of 12, 18 wt% were prepared. The model was subjected to hydrostatic pressure by adding distilled water and the inner diameter was measured using an ultrasound system. The compliance of each model was calculated from the strain of inner diameter and the inner pressure. The inner diameter of model increases linearly as the inner pressure increases. The compliance of PVA-H model is constant according to pressure change, while that of a real blood vessel changes nonlinearly. This difference may depend on the isotropic characteristics of PVA-H, whereas a real blood vessel is anisotropic. These results indicate that adding anisotropy, such as orientation of PVA, to model would be necessary to represent the compliance of a real artery.

Fundamental Numerical Experiment for Validation of Ultrasonic-Measurement-Integrated Simulation Using Straight Tube Model

In this study, as a fundamental study of the experimental validation of UMI simulation, we performed numerical experiment of UMI simulation with an axisymmetric straight tube model. We confirmed the validity of UMI simulation by reproducing the analytic solution, and it was found that the ultrasonic beam entry angle influences on the spatial and temporal convergence property of the UMI simulation.Copyright