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 ...

A moiré pattern is created by superimposing two black-and-white or gray-scale patterns of regular geometry, such as two sets of evenly spaced lines. We observed an analogous effect between two transparent phase masks in a light beam which occurs at a distance. This phase moiré effect and the classic moiré effect are shown to be the two ends of a continuous spectrum. The phase moiré effect allows the detection of sub-resolution intensity or phase patterns with a transparent screen. When applied to x-ray imaging, it enables a polychromatic far-field interferometer (PFI) without absorption gratings. X-ray interferometry can non-invasively detect refractive index variations inside an object1–10. Current bench-top interferometers operate in the near field with limitations in sensitivity and x-ray dose efficiency2, 5, 7–10. The universal moiré effect helps overcome these limitations and obviates the need to make hard x-ray absorption gratings of sub-micron periods.

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.

Coherent X-ray scattering is related to the electron density distribution by a Fourier transform, and therefore a window into the microscopic structures of biological samples. Current techniques of scattering rely on small-angle measurements from highly collimated X-ray beams produced from synchrotron light sources. Imaging of the distribution of scattering provides a new contrast mechanism which is different from absorption radiography, but is a lengthy process of raster or flue scans of the beam over the object. Here, we describe an imaging technique in the spatial frequency domain capable of acquiring both the scattering and absorption distributions in a single exposure. We present first results obtained with conventional X-ray equipment. This method interposes a grid between the X-ray source and the imaged object, so that the grid-modulated image contains a primary image and a grid harmonic image. The ratio between the harmonic and primary images is shown to be a pure scattering image. It is the auto-correlation of the electron density distribution at a specific distance. We tested a number of samples at 60–200 nm autocorrelation distance, and found the scattering images to be distinct from the absorption images and reveal new features. This technique is simple to implement, and should help broaden the imaging applications of X-ray scattering.

Purpose: The purpose of this study is to develop a single-shot version of the grating-based phase contrast x-ray imaging method and demonstrate its capability of in vivo animal imaging. Here, the authors describe the principle and experimental results. They show the source of artifacts in the phase contrast signal and optimal designs that minimize them. They also discuss its current limitations and ways to overcome them. Methods: A single lead grid was inserted midway between an x-ray tube and an x-ray camera in the planar radiography setting. The grid acted as a transmission grating and cast periodic dark fringes on the camera. The camera had sufficient spatial resolution to resolve the fringes. Refraction and diffraction in the imaged object manifested as position shifts and amplitude attenuation of the fringes, respectively. In order to quantify these changes precisely without imposing a fixed geometric relationship between the camera pixel array and the fringes, a spatial harmonic method in the Fourier domain was developed. The level of the differential phase ͑refraction͒ contrast as a function of hardware specifications and device geometry was derived and used to guide the optimal placement of the grid and object. Both ex vivo and in vivo images of rodent extremities were collected to demonstrate the capability of the method. The exposure time using a 50 W tube was 28 s. Results: Differential phase contrast images of glass beads acquired at various grid and object positions confirmed theoretical predictions of how phase contrast and extraneous artifacts vary with the device geometry. In anesthetized rats, a single exposure yielded artifact-free images of absorption, differential phase contrast, and diffraction. Differential phase contrast was strongest at bonesoft tissue interfaces, while diffraction was strongest in bone. Conclusions:The spatial harmonic method allowed us to obtain absorption, differential phase contrast, and diffraction images, all from a single raw image and is feasible in live animals. Because the sensitivity of the method scales with the density of the gratings, custom microfabricated gratings should be superior to off-the-shelf lead grids.

The ordered alignment of the mineralized collagen fibrils in compact bone was reflected in the anisotropic scattering signal in this bone. In trabecular bone, the porosity of the mineralized matrix accounted for the granular pattern seen on the scattering and PC images.

http://radiology.rsnajnls.org/cgi/content/full/246/1/229/DC1.