X-ray Particle Image Velocimetry Research Articles

Determining 3D Distributions of Pulsatile Blood Flow Using Orthogonal Simultaneous Biplane High-Speed Angiography (SB-HSA) with 1000 fps CdTe Photon Counting Detectors for 3D X-ray Particle Image Velocimetry (3D-XPIV) compared to Results Using Computational Fluid Dynamics (CFD).

3D hemodynamic distributions are useful for the diagnosis and treatment of aneurysms. Detailed blood-flow patterns and derived velocity maps can be obtained using 1000 fps High Speed Angiography (HSA). The novel orthogonal Simultaneous Biplane High-Speed Angiography (SB-HSA) system enables flow information to be quantified in multiple planes, and with additional components of flow at depth, accurate 3D flow distributions are available. Computational Fluid Dynamics (CFD) is the current standard for derivation of volumetric flow distributions, but obtaining solution convergence is computationally expensive and time intensive. More importantly, matching in-vivo boundary conditions is non-trivial. Therefore, an experimentally derived 3D flow distribution method could offer realistic results with less computation time. Using SB-HSA image sequences, 3D X-Ray Particle Image Velocimetry (3D-XPIV) was explored as a new method for assessing 3D flow. 3D-XPIV was demonstrated using an in-vitro setup, where a patient-specific internal carotid artery aneurysm model was attached to a flow loop, and an automated injection of iodinated microspheres was used as a flow tracer. Two 1000 fps photon-counting detectors were placed orthogonally with the aneurysm model in the FOV of both planes. Frame-synchronization of the two detectors made correlation of single-particle velocity components at a given timepoint possible. With frame-rates of 1000 fps, small particle displacements between frames resolved realistic time varying flow, where accurate velocity distributions depended on near-instantaneous velocities. 3D-XPIV velocity distributions were compared to CFD velocity distributions, where the simulation boundary conditions matched the in-vitro setup. Results showed similar velocity distributions between CFD and 3D-XPIV.

Experimentally validated x-ray image simulations of 50 μm x-ray PIV tracer particles

We evaluate Beer–Lambert (BL) ray-tracing and Monte Carlo N-Particle (MCNP) photon tracking simulations for prediction and comparison of x-ray imaging system performance. These simulation tools can aid the methodical design of laboratory-scale x-ray particle image velocimetry (XPIV) experiments and tracer particles by predicting image quality. Particle image signal-to-noise ratio (SNR) is used as the metric of system performance. Simulated and experiment data of hollow, silver-coated, glass sphere tracer particles (AGSF-33) are compared. As predicted by the simulations, the AGSF-33 particles are visible with a SNR greater than unity in 100 ms exposure time images, demonstrating their potential as x-ray PIV or particle tracking velocimetry (XPTV) tracers. The BL approach predicts the image contrast, is computationally inexpensive, and enables the exploration of a vast parameter space for system design. MCNP simulations, on the other hand, predict experiment images slightly more accurately, but are more than an order of magnitude more computationally expensive than BL simulations. For most practical XPIV system design applications, the higher computational expense of MCNP is likely not justified by the modest accuracy improvement compared to BL.

Measurement of the flow behavior index of Newtonian and shear-thinning fluids via analysis of the flow velocity characteristics in a mini-channel

An in-situ measurement technique to determine the rheology of a fluid based on the experimentally measured velocity profile of a flow in a mini-channel is introduced. The velocity profiles of a Newtonian and different shear-thinning fluids along a rectangular channel were measured using shadowgraph particle image velocimetry (PIV). Deionized water and different concentrations of a polyacrylamide solution were used as Newtonian and shear-thinning fluids, respectively and were studied at different Reynolds numbers. The flow indices of the fluids were determined by comparing the experimental velocity profile measurements with developed theory that takes into account the non-Newtonian nature of the fluids rheology. The results indicated that the non-Newtonian behavior of the shear-thinning fluid intensified at lower Reynolds numbers and it behaved more as a Newtonian fluid as the Reynolds number increased. A comparison between the power law index determined from experimental monitoring of the velocity profile at different Reynolds numbers and measurements from a rheometer reflected good agreement. The results from the study validate the new approach of the rheology measurement of Newtonian and non-Newtonian flows through straight, rectangular cross-section channels. The proposed approach can be further utilized using other methods such as X-ray PIV to characterize the rheology of non-transparent fluids and in general, for all non-Newtonian fluids.

Enhancement of measurement accuracy of X-ray PIV in comparison with the micro-PIV technique.

The X-ray PIV (particle image velocimetry) technique has been used as a non-invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two-dimensional velocity field data from the three-dimensional volumetric information contained in X-ray images is technically unclear. In this study, a new two-dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X-ray PIV and micro-PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro-PIV experiments were conducted for single-plane and summation images, which provide similar positional information of particles as X-ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (VS) and centre-plane images (VC). The velocity differences between VS and VC along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two-dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two-dimensional velocity field information at arbitrary positions could be successfully extracted from X-ray images by using the correction coefficient and several velocity parameters derived from VS.

In vivo measurement of hemodynamic information in stenosed rat blood vessels using X-ray PIV.

Measurements of the hemodynamic information of blood flows, especially wall shear stress (WSS), in animal models with circulatory vascular diseases (CVDs) are important to understand the pathological mechanism of CVDs. In this study, X-ray particle image velocimetry (PIV) with high spatial resolution was applied to obtain velocity field information in stenosed blood vessels with high WSS. 3D clips fabricated with a 3D printer were applied to the abdominal aorta of a rat cadaver to induce artificial stenosis in the real blood vessel of an animal model. The velocity and WSS information of blood flows in the stenosed vessel were obtained and compared at various stenosis severities. In vivo measurement was also conducted by fastening a stenotic clip on a live rat model through surgical intervention to reduce the flow rate to match the limited temporal resolution of the present X-ray PIV system. Further improvement of the temporal resolution of the system might be able to provide in vivo measurements of hemodynamic information from animal disease models under physiological conditions. The present results would be helpful for understanding the relation between hemodynamic characteristics and the pathological mechanism in animal CVD models.

Usage of CO2 microbubbles as flow-tracing contrast media in X-ray dynamic imaging of blood flows.

X-ray imaging techniques have been employed to visualize various biofluid flow phenomena in a non-destructive manner. X-ray particle image velocimetry (PIV) was developed to measure velocity fields of blood flows to obtain hemodynamic information. A time-resolved X-ray PIV technique that is capable of measuring the velocity fields of blood flows under real physiological conditions was recently developed. However, technical limitations still remained in the measurement of blood flows with high image contrast and sufficient biocapability. In this study, CO2 microbubbles as flow-tracing contrast media for X-ray PIV measurements of biofluid flows was developed. Human serum albumin and CO2 gas were mechanically agitated to fabricate CO2 microbubbles. The optimal fabricating conditions of CO2 microbubbles were found by comparing the size and amount of microbubbles fabricated under various operating conditions. The average size and quantity of CO2 microbubbles were measured by using a synchrotron X-ray imaging technique with a high spatial resolution. The quantity and size of the fabricated microbubbles decrease with increasing speed and operation time of the mechanical agitation. The feasibility of CO2 microbubbles as a flow-tracing contrast media was checked for a 40% hematocrit blood flow. Particle images of the blood flow were consecutively captured by the time-resolved X-ray PIV system to obtain velocity field information of the flow. The experimental results were compared with a theoretically amassed velocity profile. Results show that the CO2 microbubbles can be used as effective flow-tracing contrast media in X-ray PIV experiments.

Time-resolved X-ray PIV technique for diagnosing opaque biofluid flow with insufficient X-ray fluxes

X-ray imaging is used to visualize the biofluid flow phenomena in a nondestructive manner. A technique currently used for quantitative visualization is X-ray particle image velocimetry (PIV). Although this technique provides a high spatial resolution (less than 10 µm), significant hemodynamic parameters are difficult to obtain under actual physiological conditions because of the limited temporal resolution of the technique, which in turn is due to the relatively long exposure time (~10 ms) involved in X-ray imaging. This study combines an image intensifier with a high-speed camera to reduce exposure time, thereby improving temporal resolution. The image intensifier amplifies light flux by emitting secondary electrons in the micro-channel plate. The increased incident light flux greatly reduces the exposure time (below 200 µs). The proposed X-ray PIV system was applied to high-speed blood flows in a tube, and the velocity field information was successfully obtained. The time-resolved X-ray PIV system can be employed to investigate blood flows at beamlines with insufficient X-ray fluxes under specific physiological conditions. This method facilitates understanding of the basic hemodynamic characteristics and pathological mechanism of cardiovascular diseases.

Evaluation of tyre products as ground improving geomaterials

Applications of scrap tyre-derived recycled products (such as tyre chips and tyre shreds) in geotechnical engineering projects have been increasing largely because of their potential economic and environmental benefit. This paper first addresses and evaluates two novel Japanese experiences of tyre-derived recycled products in ground improvement applications. It then discusses and evaluates the deformation mechanism of such materials through non-destructive testing techniques. Direct shear testing procedures are first described, in which sand as well as layers of sand and tyre chips were examined to compare the deformation behaviour. The shear behaviour of the materials could be investigated through X-ray computed tomography (CT) during the testing. The particle image velocimetry (PIV) technique was then used to visualise the deformation properties. In addition, triaxial shear testing of tyre chips as well as tyre chip–sand mixtures is described, in which the compressibility and the deformation of the tyre chip particles were examined by X-ray CT scanning and PIV. Based on these test results, the mechanical properties and the internal structure during the deformation of tyre chips were finally evaluated for their applications as ground improving geomaterials. Model test results indicate that the presence of tyre chips (either as cushion or as reinforcing material) could substantially reduce the earthquake-induced permanent displacement of structures. Non-destructive test results indicate that the deformation characteristics of mixtures of tyre chips and sand are different from those of pure sand. Less shear strain is generated in tyre chips and tyre chips mixed with sand than that in pure sand, and the potential of onset of localisation diminishes with decreasing sand fraction. Also, it was found that tyre chips–sand mixtures do not undergo liquefaction if a suitable mixing percentage (sand fraction) is used. Therefore, tyre chips can be successfully used as a reinforcing material to help prevent the liquefaction of sandy soils.

Note: Development of a compact x-ray particle image velocimetry for measuring opaque flows. II. Three-dimensional velocity field reconstruction

An x-ray particle image velocimetry (PIV) system using a cone-beam type x-ray was developed. The field of view and the spatial resolution are 36 × 24.05 mm(2) and 20 μm, respectively. The three-dimensional velocity field was reconstructed by adopting the least squares minimum residue and simultaneous multiplicative algebraic reconstruction techniques. According to a simulation study with synthetic images, the reconstructions were acceptable with 7 projections and 50 iterations. The reconstructed and supplied flow rates differed by only about 6.49% in experimental verification. The x-ray tomographic PIV system would be useful for 3D velocity field information of opaque flows.

Flow tracing microparticle sensors designed for enhanced X-ray contrast

In applying the X-ray particle image velocimetry (PIV) technique to biofluid flows, the most pivotal prerequisite is suitable flow tracing sensors which should be detected effectively by the X-ray imaging system. In this study, to design those flow tracing sensors, X-ray contrast agent Iopamidol was encapsulated into the poly(vinyl alcohol) (PVA) microparticles crosslinked by glutaraldehyde (GA). The characteristics of the fabricated particle sensors were determined by optical microscopy, scanning electron microscopy, dynamic light scattering, laser Doppler electrophoresis and nuclear magnetic resonance spectroscopy (1H NMR). The amount of Iopamidol in the microparticles was measured using the energy dispersive X-ray spectroscopy (EDS) and 1H NMR. The physical properties of the PVA microparticles are effectively controlled in terms of the average particle size, degree of crosslinking, degree of swelling and encapsulation efficiency of Iopamidol. By changing the amount of crosslinker, the degree of crosslinking and the efficiency of the Iopamidol encapsulation reached to the optimal. To some extent, the zeta-potential of the PVA microparticles is increased in less ionic media where the particles can effectively repel each other prohibiting aggregation. The X-ray absorption ability of the designed tracing sensors was examined by a synchrotron X-ray imaging technique. The X-ray absorption coefficients of the particle sensors were expressed by an exponential law assuming the spherical shape of the microparticles. The X-ray contrast agent, Iopamidol, was successfully encapsulated into the bio-compatible and bio-degradable PVA. With the controlled physical properties of the flow tracing sensors designed in this study, the particle sensors exhibit excellent X-ray absorption contrast fairly applicable in biological systems.

Development of a compact x-ray particle image velocimetry for measuring opaque flows

A compact x-ray particle image velocimetry (PIV) system employing a medical x-ray tube as a light source was developed to measure quantitative velocity field information of opaque flows. The x-ray PIV system consists of a medical x-ray tube, an x-ray charge coupled device camera, a programmable shutter for a pulse-type x ray, and a synchronization device. Through performance tests, the feasibility of the developed x-ray PIV system as a flow measuring device was verified. To check the feasibility of the developed system, we tested a tube flow at two different mean velocities of 1 and 2 mm/s. The x-ray absorption of tracer particles must be quite different from that of working fluid to have a good contrast in x-ray images. All experiments were performed under atmospheric pressure condition. This system is unique and useful for investigating various opaque flows or flows inside opaque conduits.

Contrast enhancement of speckle patterns from blood in synchrotron X-ray imaging

Hemodynamics has been a very important factor in understanding and diagnosing various vascular diseases. Recently, the X-ray particle image velocimetry (X-ray PIV) method using speckle patterns of blood has been introduced as a new quantitative visualization method for blood flows without any seeding tracer or contrast agents. In this study, the peculiar optical characteristics of blood on the synchrotron X-ray imaging method, which were not presented in previous studies, were investigated in depth and systematically. The experimental conditions required for X-ray PIV application were found to be the distance between the sample and the scintillator (∼40 cm), the thickening of the blood sample (>0.3 mm), and hematocrit (20.0–80.0%). In addition, we verified that the X-ray PIV method is reliable as an advanced flow velocimetry by comparing the flow rate evaluated from the X-ray PIV result and the input flow rate supplied from a syringe pump with an error of less than 1%. Through this study, based on the understanding of contrast enhancement mechanisms of speckle patterns from blood, we could establish a trustworthy flow visualization method that can be used effectively in hemodynamic studies.

ENGINEERING IMAGING: USING PARTICLE IMAGE VELOCIMETRY TO SEE PHYSIOLOGY IN A NEW LIGHT

1. Despite the array of sophisticated imaging techniques available for biological applications, none of the standard biomedical techniques adequately provides the capability to measure motion and flow. Those techniques currently in use are particularly lacking in spatial and temporal resolution. 2. Herein, we introduce the technique of particle image velocimetry. This technique is a well-established tool in engineering research and industry. Particle image velocimetry is continuing to develop and has an increasing number of variants. 3. Three case studies are presented: (i) the use of microparticle image velocimetry to study flow generated by high-frequency oscillatory ventilation in a human airway model; (ii) the use of stereoparticle image velocimetry to study stirred cell and tissue culture devices; and (iii) a three-dimensional X-ray particle image velocimetry technique used to measure flow in an in vitro vascular flow model. 4. The case studies highlight the vast potential of applying the engineering technique of particle image velocimetry and its many variants to current research problems in physiology.

Three-dimensional synchrotron x-ray particle image velocimetry

There is great potential to vastly improve biological flow measurement by using a combination of synchrotron imaging and the latest experimental flow measurement techniques. In the current paper, the three-dimensional velocity field within a cylindrical tube is measured using a combination of phase-contrast x-ray imaging and particle image velocimetry (PIV). We greatly refine the techniques previously used to undertake velocity measurements with a synchrotron light source, substantially enhancing accuracy. Furthermore a PIV correlation peak analysis is developed to allow three-dimensional measurement of the velocity field.

X-ray PIV 기법의 개발과 혈액 유동에의 적용연구

An x-ray PIV (Particle Image Velocimetry) technique was developed to measure quantitative information on flows inside opaque conduits and on opaque-fluid flows. At first, the developed x-ray PIV technique was applied to flow in an opaque Teflon tube. To acquire x-ray images suitable for PIV velocity field measurements, refraction-based edge enhancement mechanism was employed using detectable tracer particles. The optimal distance between with the sample and detector was experimentally determined. The resulting amassed velocity field data were in reasonable agreement with the theoretical prediction. The x-ray PIV technique was also applied to blood flow in a microchannel. The flow pattern of blood was visualized by enhancing the diffraction/interference-based characteristics of blood cells on synchrotron x-rays without any contrast agent or tracer particles. That is, the flow-pattern image of blood was achieved by optimizing the sample (blood) to detector distance and the sample thickness. Quantitative velocity field information was obtained by applying PIV algorithm to the enhanced x-ray flow images. The measured velocity field data show a typical flow structure of flow in a macro-scale channel.

X-ray particle image velocimetry for measuring quantitative flow information inside opaque objects

An x-ray particle image velocimetry (PIV) technique was developed to measure quantitative information on flows inside opaque objects. To acquire x-ray images suitable for PIV velocity field measurements, refraction-based edge enhancement was employed using detectable tracer particles with the object and detector separated by an experimentally determined optimal distance. The x-ray PIV method was applied to a flow in an opaque Teflon tube. The resulting amassed velocity field data were in reasonable agreement with theoretical predictions.