In vivo blood flow measurement is important for hemorheological analysis of blood flow related to cardiovascular diseases, such as atherosclerosis and cardiogenesis. Noninvasive imaging techniques with high spatial resolutions are required for obtaining detailed information on opaque blood flow in hemodynamic phenomena in the circulatory vascular system. Recently, in vivo hemodynamic experimental studies have been performed using various medical imaging modalities, such as magnetic resonance imaging (MRI), echocardiography, and X-ray angiography. Each technique has technical limitations which prohibit in vivo quantitative analysis on the hemodynamic characteristics of the blood flows. Therefore, in order to understand the blood flows related with vascular disorders, there are strong demands on the development of an advanced analysis tool which can provide detailed quantitative hemodynamic information of blood flows with a high precision. For this, an X-ray particle image velocimetry (PIV) technique was developed by combining the merits of X-ray radiography and PIV velocity field measurement technique. However, it is still required to improve the X-ray PIV technique for direct in vivo measurements of real blood flows in animals.
In this thesis, the X-ray PIV system was largely advanced in terms of three-dimensional (3D) velocity field measurement, increase of temporal resolution and fabrication of tracer particles for X-ray imaging experiments. The advanced X-ray PIV system was applied to measure the flow characteristics of two-phase flows and in vivo blood flows in a rat.
In X-ray imaging techniques, the volumetric features of an object along the pathway of X-ray propagation are compressed on the projected two-dimensional (2D) X-ray image, and the projected cross-correlation function contains the information on compressed velocity vectors. 3D velocity field information (tomographic technique) was reconstructed from 2D projected velocity fields at various angles. The velocity field was reconstructed by adopting the least squares minimum residue method and simultaneous multiplicative algebraic reconstruction technique algorithm. The X-ray tomographic PIV system would be useful for 3D velocity field information of opaque flows.
Although X-ray PIV provides a high spatial resolution (less than 10 μm), significant hemodynamic parameters are difficult to obtain in actual physiological conditions because of the limited temporal resolution of the technique, which in turn is due to the relatively long exposure times (~10 ms) involved in X-ray imaging. The temporal resolution was improved by combining an image intensifier with a high-speed camera to reduce exposure time. 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 at a specific physiological condition.
In applying the X-ray PIV to blood flow measurement, the most pivotal prerequisite is suitable flow tracers which should be detected effectively by the X-ray imaging system. Two kinds of tracer particles were fabricated. The first one is PVA-GA-Iopamidol microparticle in which X-ray contrast agent Iopamidol was encapsulated into the poly(vinyl alcohol) (PVA) microparticles crosslinked by glutaraldehyde (GA). The characteristics of the fabricated particles were determined by optical microscopy, scanning electron microscopy, dynamic light scattering, laser Doppler electrophoresis and nuclear magnetic resonance spectroscopy (1H NMR). The other is Gold-chitosan microparticle in which gold nanoparticles (AuNPs) were incorporated into chitosan microparticles. Gold (Au) is one of the useful materials possessing high X-ray absorption ability and also biocompatibility. We choose chitosan as a AuNP delivery cargo because it can effectively trap the AuNP with high yield. Depending on the molecular weight of the employed chitosan, the physical properties of the Au-chitosan microparticles are controlled. The X-ray absorption ability of both particles was examined by a synchrotron X-ray imaging technique. The particles exhibit excellent X-ray absorption contrast which is fairly applicable in biological systems.
The X-ray PIV system was applied to measuring two-phase characteristics including bubble size, velocity and void fraction. X-ray phase-contrast imaging, particularly Fresnel diffraction, was employed to measure the shape, size, and position of the microbubbles, and the two-frame PTV algorithm was used to measure the displacement of the microbubbles. Void fraction was estimated from X-ray absorption according to the Beer?Lambert law and the definition of void fraction. Microbubble information and void fraction of highly fluctuating flows was accurately determined with high temporal and spatial resolutions.
The X-ray PIV system was also applied to measuring in vivo blood flows in a rat. AuNPs incorporated chitosan microparticles were applied as biocompatible flow tracers. After intravenous injection of the AuNP-chitosan particles into 7- to 9-week-old male rat vein, X-ray images of particle movement inside the cranial vena cava were consecutively captured. Individual AuNP-chitosan particles in the venous blood flow were clearly observed, and the corresponding velocity vectors were successfully extracted. The measured velocity vectors are in good agreement with the theoretical velocity profile suggested by Casson.
Conclusively, the advanced X-ray PIV system developed in this study is a promising technology that can be utilized for in vivo measurements of real blood flow. This study would eventually contribute to the basic understanding on the fluid-m