In grating-based x-ray phase sensitive imaging, dark-field contrast refers to the extinction of the interference fringes due to small-angle scattering. For configurations where the sample is placed before the beamsplitter grating, the dark-field contrast has been quantified with theoretical wave propagation models. Yet when the grating is placed before the sample, the dark-field contrast has only been modeled in the geometric optics regime. Here we attempt to quantify the dark-field effect in the grating-before-sample geometry with first-principle wave calculations and understand the associated particle-size selectivity. We obtain an expression for the dark-field effect in terms of the sample material’s complex refractive index, which can be verified experimentally without fitting parameters. A dark-field computed tomography experiment shows that the particle-size selectivity can be used to differentiate materials of identical x-ray absorption.

The intravoxel incoherent motion (IVIM) method was implemented in closed-chest dogs to obtain measurements on microcirculation in the left ventricular wall in vivo. Specifically, it enabled us to measure the mean microflow velocity (400 ؎ 40 m/s) and the vascular volume fraction (VVF) (11.1% ؎ 2.2%), and observe the directional preference of capillary orientation. The apparent diffusion coefficients (ADCs) of water along and perpendicular to myofibers were also measured. With vasodilatation by adenosine infusion, a 25% increase in the VVF and a 7% increase in the mean microflow velocity were observed, while no change in the ADC was detected. Microcirculation in tissue has generated great interest in recent years (1-3) for clinical purposes and in studies of oxygen delivery. Magnetic resonance imaging (MRI) is one of the few noninvasive methods that are being used to investigate this topic. MRI techniques employ either exogenous contrast agents or endogenous contrast mechanisms to delineate blood flow at the capillary level. Gd-DTPAbased studies give estimates of perfusion rates (4), while strictly intravascular agents provide estimates of perfusion rates, vascular volume, and water exchange rates (4,5). Two endogenous contrast methods-remote arterial spinlabeling and local spin-labeling-are used to measure perfusion rates, water distribution between vascular and extravascular compartments, and flow transit times (6 -8).We used an endogenous contrast technique based on the intravoxel incoherent motion (IVIM) phenomenon (9,10) to measure the vascular volume fraction (VVF) and microflow velocity, and to detect directional anisotropy in the capillary morphology in the myocardium. The IVIM technique is based on the assumption that in diffusionweighted images, blood flow in the capillaries and small arterioles/venules also appears as incoherent movement when observed at the scale of an image pixel, and causes a decrease in the image intensity in addition to the effect of Brownian diffusion of the water molecules. Because both flow and diffusion measurements can be obtained with this technique, we also investigated the correlation between the two by increasing blood flow with adenosine infusion.We conducted for the first time IVIM-based microcirculation measurements in the canine heart in vivo. This technique has been used successfully in vascular brain perfusion studies (11-13) and renal perfusion imaging (14,15), but IVIM-based flow detection has not been attempted in the heart-mainly because of the technical difficulties caused by heartbeat motion. Diffusionweighted sequences are critically affected by bulk deformation and rotation, and therefore they require careful registration of the heart position and strain status. Nevertheless, the IVIM method is a useful tool for examining the relationship between vascular anatomy and flow distribution in the myocardium (16), and between microcirculation and diffusion (17).The directional preference of capillary orientation in the myocardium has been observed in many optical ...

We describe an x-ray differential phase contrast imaging method based on two-dimensional transmission gratings that are directly resolved by an x-ray camera. X-ray refraction and diffraction in the sample lead to variations of the positions and amplitudes of the grating fringes on the camera. These effects can be quantified through spatial harmonic analysis. The use of 2D gratings allows differential phase contrast in several directions to be obtained from a single image. When compared to previous grating-based interferometry methods, this approach obviates the need for multiple exposures and separate measurements for different directions, and thereby accelerates imaging speed.