DEI imaging sharpens x-ray radiography
In recent years, x-ray imaging has grown beyond medical imaging to become a critical part of national security thanks to the use of high-energy x-rays for inspection applications. X-rays can reveal information about the object under test in three ways. Absorption contrast is the best-known method, producing the images commonly seen in medical x-rays and airport security checkpoints. X-rays also refract on the order of a few microradians as they pass through materials of differing densities, creating refraction contrast. Extinction contrast, an additional effect, is produced by scattering of a microradian or less.
A new method called diffraction-enhanced imaging (DEI) leverages all three phenomena to produce greater contrast than standard absorption radiographs for a variety of material densities (see figure 1). In DEI, a pair of Bragg crystals collimates a high-energy x-ray beam from a synchrotron source. The object under test is placed between the monochromator and a third Bragg crystal, called the analyzer. By altering the angle of the analyzer crystal, users can create refraction-contrast images, absorption-contrast images, extinct-contrast images, or a combination of two or more effects. This technique could provide enough contrast to allow radiologists to identify breast cancer, for instance, with much greater confidence, eliminating the need for unnecessary biopsies.
Absorption contrast radiographs are basically shadowgraphs of the object under test. Obtaining absorption radiographs with fine resolution is difficult because conventional anti-scatter grids cannot filter out extinction x-rays that vary from the optical axis by a microradian or less and reduce contrast.
A group at the Illinois Institute of Technology (Chicago, IL) in association with collaborators at the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, NY) and the synchrotron source at Argonne National Laboratory (ANL; Argonne, IL) have developed a standardized experimental setup for DEI imaging techniques. The collimating Bragg crystals are attached to kinetic or moveable mounts. A PC-controlled piezo-driven stage with steps of less than 0.03 µm helps maintain parallelism between the two crystals.
A fan of monochromatic x-rays emerges from the monochromator and passes through an ion chamber that measures the intensity of the x-ray radiation. The object under test traverses this fan-shaped beam. Transmitted, refracted, and diffracted x-rays continue to the analyzer crystal, also on a kinetic mount. This crystal, which is non-dispersive in relation to the second crystal of the monochromator, uses diffraction to direct the x-rays to a Fuji Medical Systems (Stamford, CT) BAS2000 or AC3 silicon image plate reader. The reader traverses the beam in the opposite direction to that of the object under test to reduce image blur.
The analyzer crystal's azimuthal angle changes in 0.1-µm increments, providing an angular resolution of 0.1 µrad for the incident x-rays. The intensity of the x-rays on the detector changes as x-rays outside the optimum incident angle are reflected away from the detector. When the analyzer crystal angle matches that of the monochromator crystals, refracted x-rays are filtered out, increasing the contrast of the absorption image. As the analyzer angle changes in respect to the monochromator, it filters out transmitted x-rays and passes refracted x-rays on to the detector, which is very useful for creating images of structures with high-contrast edges. By isolating absorption and extinction from refraction, DEI provides images with startlingly clear contrast compared to standard absorption radiographs (see figure 2).
"We're currently trying to build a system that does not require a synchrotron source so we can take DEI out into the world," explains system co-developer Dean Chapman, now a professor at University of Saskatchewan (Saskatoon, Canada). "Because we put the crystals in the way to collimate and analyze the beam, a [conventional] x-ray source will leave us shy by a factor of 104 [photons]." In round numbers, it would require 10,000 times as long to produce a DEI image as a normal radiograph. "DEI is optimized for higher x-ray energies. We can generate radiographs if we want because absorption [of x-rays], and therefore the dose to a person, is low at higher energies. DEI is [only] weakly dependent on the absorption contrast mechanism," Dean adds.
Finding an inexpensive source capable of producing enough x-rays is the challenge for a commercial DEI system, according to Ali Khounsary of ANL. "Not everyone can afford a synchrotron," he says, noting that DEI is one of three techniques under development to improve x-ray image contrast, including x-ray interferometry and phase-contrast radiography.