Dean Chapman

Diffraction enhanced x-ray imaging

Diffraction enhanced imaging is a new x-ray radiographic imaging modality using monochromatic x-rays from a synchrotron which produces images of thick absorbing objects that are almost completely free of scatter. They show dramatically improved contrast over standard imaging applied to the same phantom. The contrast is based not only on attenuation but also the refraction and diffraction properties of the sample. This imaging method may improve image quality for medical applications, industrial radiography for non-destructive testing and x-ray computed tomography.

Multiple-image radiography

Conventional radiography produces a single image of an object by measuring the attenuation of an x-ray beam passing through it. When imaging weakly absorbing tissues, x-ray attenuation may be a suboptimal signature of disease-related information. In this paper we describe a new phase-sensitive imaging method, called multiple-image radiography (MIR), which is an improvement on a prior technique called diffraction-enhanced imaging (DEI). This paper elaborates on our initial presentation of the idea in Wernick et al (2002 Proc. Int. Symp. Biomed. Imaging pp 129-32). MIR simultaneously produces several images from a set of measurements made with a single x-ray beam. Specifically, MIR yields three images depicting separately the effects of refraction, ultra-small-angle scatter and attenuation by the object. All three images have good contrast, in part because they are virtually immune from degradation due to scatter at higher angles. MIR also yields a very comprehensive object description, consisting of the angular intensity spectrum of a transmitted x-ray beam at every image pixel, within a narrow angular range. Our experiments are based on data acquired using a synchrotron light source; however, in preparation for more practical implementations using conventional x-ray sources, we develop and evaluate algorithms designed for Poisson noise, which is characteristic of photon-limited imaging. The results suggest that MIR is capable of operating at low photon count levels, therefore the method shows promise for use with conventional x-ray sources. The results also show that, in addition to producing new types of object descriptions, MIR produces substantially more accurate images than its predecessor, DEI. MIR results are shown in the form of planar images of a phantom and a biological specimen. A preliminary demonstration of the use of MIR for computed tomography is also presented.

Implementation of diffraction-enhanced imaging experiments: at the NSLS and APS

Di!raction-enhanced imaging is a recently developed X-ray imaging technique that has demonstrated enhanced contrast for dense, highly absorbing materials of interest in materials science and medicine. The implementation of this technique in experiments at the National Synchrotron Light Source and at the Advanced Photon Source is described in detail. ( 2000 Elsevier Science B.V. All rights reserved.

Extraction of extinction, refraction and absorption properties in diffraction enhanced imaging

Diffraction enhanced imaging is a radiographic technique that derives contrast from an objects x-ray absorption, refraction gradient and small angle scatter properties (extinction). In prior work, images obtained using two analyser settings were combined to obtain refraction angle and apparent absorption images. A more general method of determining independently the refraction, absorption and extinction of the object is presented. This approach has been used to model the transmission, refraction and scatter distribution of the sample and to visualize these three physical phenomena separately.

A physical model of multiple-image radiography

We recently proposed a phase-sensitive x-ray imaging method called multiple-image radiography (MIR), which is an improvement on the diffraction-enhanced imaging technique. MIR simultaneously produces three images, depicting separately the effects of absorption, refraction and ultra-small-angle scattering of x-rays, and all three MIR images are virtually immune to degradation caused by scattering at higher angles. Although good results have been obtained using MIR, no quantitative model of the imaging process has yet been developed. In this paper, we present a theoretical prediction of the MIR image values in terms of fundamental physical properties of the object being imaged. We use radiative transport theory to model the beam propagation, and we model the object as a stratified medium containing discrete scattering particles. An important finding of our analysis is that the image values in all three MIR images are line integrals of various object parameters, which is an essential property for computed tomography to be achieved with conventional reconstruction methods. Our analysis also shows that MIR truly separates the effects of absorption, refraction and ultra-small-angle scattering for the case considered. We validate our analytical model using real and simulated imaging data.

Diffraction enhanced imaging contrast mechanisms in breast cancer specimens

We have investigated the contrast mechanisms of the refraction angle, and the apparent absorption images obtained from the diffraction enhanced imaging technique (DEI) and have correlated them with the absorption contrast of conventional radiography. The contrast of both the DEI refraction angle image and the radiograph have the same dependence on density differences of the tissues in the visualization of cancer; in radiography these differences directly relate to the contrast while in the DEI refraction angle image it is the density difference and thickness gradient that gives the refraction angle. We show that the density difference of fibrils in breast cancer as measured by absorption images correlate well with the density difference derived from refraction angle images of DEI. In addition we find that the DEI apparent absorption image and the image obtained with the DEI system at the top of the reflectivity curve have much greater contrast than that of the normal radiograph (x8 to 33-fold higher). This is due to the rejection of small angle scattering (extinction) from the fibrils enhancing the contrast.

Spatially resolved measurement of high doses in microbeam radiation therapy using samarium doped fluorophosphate glasses

The measurement of spatially resolved high doses in microbeam radiation therapy has always been a challenging task, where a combination of high dose response and high spatial resolution (microns) is required for synchrotron radiation peaked around 50 keV. The x-ray induced Sm3+ → Sm2+ valence conversion in Sm3+ doped fluorophosphates glasses has been tested for use in x-ray dosimetry for microbeam radiation therapy. The conversion efficiency depends almost linearly on the dose of irradiation up to ∼5 Gy and saturates at doses exceeding ∼80 Gy. The conversion shows strong correlation with x-ray induced absorbance of the glass which is related to the formation of phosphorus-oxygen hole centers. When irradiated through a microslit collimator, a good spatial resolution and high “peak-to-valley” contrast have been observed by means of confocal photoluminescence microscopy.

The design and application of an in-laboratory diffraction-enhanced x-ray imaging instrument

We describe the design and application of a new in-laboratory diffraction-enhanced x-ray imaging (DEXI) instrument that uses a nonsynchrotron, conventional x-ray source to image the internal structure of an object. In the work presented here, a human cadaveric thumb is used as a test-sample to demonstrate the imaging capability of our instrument. A 22 keV monochromatic x-ray beam is prepared using a mismatched, two-crystal monochromator; a silicon analyzer crystal is placed in a parallel crystal geometry with the monochromator allowing both diffraction-enhanced imaging and multiple-imaging radiography to be performed. The DEXI instrument was found to have an experimentally determined spatial resolution of 160+/-7 mum in the horizontal direction and 153+/-7 mum in the vertical direction. As applied to biomedical imaging, the DEXI instrument can detect soft tissues, such as tendons and other connective tissues, that are normally difficult or impossible to image via conventional x-ray techniques.

Medical applications of diffraction enhanced imaging.

We have developed a new X-ray imaging technique, diffraction enhanced imaging (DEI), which can be used to independently visualize the refraction and absorption of an object. The images are almost completely scatter-free, allowing enhanced contrast of objects that develop small angle scattering. The combination of these properties has resulted in images of mammography phantoms and tissues that have dramatically improved contrast over standard imaging techniques. This technique potentially is applicable to mammography and other fields of medical X-ray imaging and to radiology in general, as well as possible use in nondestructive testing and X-ray computed tomography. Images of various tissues and materials are presented to demonstrate the wide applicability of this technique to medical and biological imaging.

Monochromatic energy-subtraction radiography using a rotating anode source and a bent Laue monochromator

A system for area-beam energy-subtraction monochromatic radiography was developed and tested. It utilizes a bent Laue crystal monochromator developed at the National Synchrotron Light Source (NSLS), and a compact rotating anode X-ray source developed at the Science Research Laboratory (SRL). The K(alpha) characteristic lines (both K(alpha 1) and K(alpha 2) of the cerium and barium targets were diffracted by the monochromator and used for the above- and below-K-edge imaging, respectively, of phantoms with iodine contrast agents. Digital subtraction of the images produced an iodine image.