Theron J. Hamilton

Low density contrast agents for x-ray phase contrast imaging: the use of ambient air for x-ray angiography of excised murine liver tissue

We report a new preparative method for providing contrast through reduction in electron density that is uniquely suited for propagation-based differential x-ray phase contrast imaging. The method, which results in an air or fluid filled vasculature, makes possible visualization of the smallest microvessels, roughly down to 15 microm, in an excised murine liver, while preserving the tissue for subsequent histological workup. We show the utility of spatial frequency filtering for increasing the visibility of minute features characteristic of phase contrast imaging, and the capability of tomographic reconstruction to reveal microvessel structure and three-dimensional visualization of the sample. The effect of water evaporation from livers during x-ray imaging on the visibility of blood vessels is delineated. The deformed vascular tree in a cancerous murine liver is imaged.

The photoacoustic effect generated by an incompressible sphere

An incompressible sphere with a vanishing thermal expansivity suspended in a fluid can generate a photoacoustic effect when the heat deposited in the sphere by a light beam diffuses into the surrounding liquid causing it to expand and launch a sound wave. The properties of the photoacoustic effect for the sphere are found using a Greens function solution to the wave equation for pressure with Neumann boundary conditions. The results of the calculation show that the acoustic wave for fast heat liberation is an outgoing compressive pulse followed by a reflected pulse whose time profile is modified as a result of frequency dependent reflection from the sphere. For slow heat release by the sphere, the photoacoustic effect is shown to be proportional to the first time derivative of the heat flux at the particle-fluid interface.

Acoustic radiation pressure: A “phase contrast” agent for x-ray phase contrast imaging

We show that the radiation pressure exerted by a beam of ultrasound can be used for contrast enhancement in high-resolution x-ray imaging of tissue and soft materials. Interfacial features of objects are highlighted as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of x-ray phase contrast imaging to density variations. The potential of the method is demonstrated by imaging microscopic tumor phantoms embedded into tissue with a thickness typically presented in mammography. The detection limit of micrometer size masses exceeds the resolution of currently available mammography imaging systems. The directionality of the acoustic radiation force and its localization in space permits the imaging of ultrasound-selected tissue volumes. The results presented here suggest that the method may permit the detection of tumors in soft tissue in their early stage of development.

X-ray phase contrast imaging: Transmission functions separable in cylindrical coordinates

A Fresnel-Kirchhoff integral can be used to calculate x-ray phase contrast images when the transmission function is known. Here expressions for image intensity are derived for objects with axial symmetry for an x-ray source with non-vanishing dimensions. An expression for the image intensity is given for an x-ray source whose intensity distribution is described by a Gaussian function, from which an expression for the limiting case of a point source of radiation is found. The expressions for image intensity are evaluated for cases where the magnification is substantially greater than one, as would be employed in biological imaging. Experiments using a microfocus x-ray tube and charge coupled device camera are reported to determine the capability of the method for imaging small spherical objects, such as gold particles, which might find application as contrast agents in biomedical imaging.

X-ray elastography: Modification of x-ray phase contrast images using ultrasonic radiation pressure

The high resolution characteristic of in-line x-ray phase contrast imaging can be used in conjunction with directed ultrasound to detect small displacements in soft tissue generated by differential acoustic radiation pressure. The imaging method is based on subtraction of two x-ray images, the first image taken with, and the second taken without the presence of ultrasound. The subtraction enhances phase contrast features and, to a large extent, removes absorption contrast so that differential movement of tissues with different acoustic impedances or relative ultrasonic absorption is highlighted in the image. Interfacial features of objects with differing densities are delineated in the image as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of x-ray phase contrast imaging to density variations. Experiments with ex vivo murine tumors and human tumor phantoms point out a diagnostic capability of the method for identifying tumors.

Tissue imaging utilizing the ultrasonic vibration potential

The ultrasonic vibraton potential refers to the production of a voltage that varies in time when ultrasound passes through a colloidal or ionic solution. The vibration potential can be used as an imaging method for soft tissue by recording its phase, time of arrival, and magnitude relative to the launching of a burst of ultrasound. A theory of the effect can be found from Maxwells equations. Experimental results demonstrating the imaging method are shown for bodies with simple geometries.

Ultrasonically modulated x-ray phase contrast and vibration potential imaging methods

We show that the radiation pressure exerted by a beam of ultrasound can be used for contrast enhancement in high resolution x-ray imaging of tissue. Interfacial features of objects are highlighted as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of x-ray phase contrast imaging to density variations. The potential of the method is demonstrated by imaging various tumor phantoms and tumors from mice. The directionality of the acoustic radiation force and its localization in space permits the imaging of ultrasound-selected tissue volumes. In a related effort we report progress on development of an imaging technique using and electrokinetic effect known as the ultrasonic vibration potential. The ultrasonic vibration potential refers to the voltage generated when ultrasound traverses a colloidal or ionic fluid. The theory of imaging based on the vibration potential is reviewed, and an expression given that describes the signal from an arbitrary object. The experimental apparatus consists of a pair of parallel plates connected to the irradiated body, a low noise preamplifier, a radio frequency lock-in amplifier, translation stages for the ultrasonic transducer that generates the ultrasound, and a computer for data storage and image formation. Experiments are reported where bursts of ultrasound are directed onto colloidal silica objects placed within inert bodies.

Photothermal modulation of x-ray phase contrast images

The in-line x-ray phase-contrast imaging method relies on changes in index of refraction within a body to produce image contrast. In soft tissue, index of refraction variations arise from density changes so that phase contrast imaging provides a map of density gradients within a body. An intense, short pulse laser beam that is differentially absorbed by an object within a body will produce a thermal wave with an associated density change that propagates outwardly from the interface between the object and the body. Experiments are described where a pulsed Nd:YLF laser is synchronized to an image intensifier to record the effects of the energy deposited by a pulsed laser.

Ultrasonically modulated X-ray phase contrast imaging

We show that the radiation pressure exerted by a beam of ultrasound can be used for contrast enhancement in high resolution X-ray imaging of tissue. Interfacial features of objects are highlighted as a result of both the displacement introduced by the ultrasound and the inherent sensitivity of X-ray phase contrast imaging to density variations. The potential of the method is demonstrated by imaging various tumor phantoms and tumors from mice. The directionality of the acoustic radiation force and its localization in space permits the imaging of ultrasound-selected tissue volumes. The results suggest that the method may permit the detection of tumors in soft tissue in their early stages of development