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 Green's 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.

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.

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.