New Medical Imaging Technique First to Use Low-Dose X-Rays to Reveal Soft Tissue
Shades of Grey
DEI makes use of the special beams of x-rays available at synchrotron sources such as the NSLS. In contrast to x-rays from conventional sources, synchrotron x-ray beams are thousands of times more intense, as well as being highly collimated, or extremely concentrated into a narrow beam. In addition, synchrotron light can be tuned to be “monochromatic,” or essentially of one wavelength or color.
To make a conventional radiograph, x-rays are beamed at, say, a hand that is placed between the x-ray source and a piece of x-ray film or a digital recorder. The density of the structures being pictured and, hence, their x-ray absorption determine what the radiograph looks like.
When the negative is developed, bone and other calcified structures appear clear, or white, and metallic objects, such as Anna-Bertha Röntgen’s ring, are seen as bright white. Soft tissue, because it absorbs fewer x-rays than does bone but has small differences in density, is seen as shades of grey.
Instead of making use of absorption, DEI, as its name says, relies upon diffraction, which is the variation in the intensities of light after it scatters off structures of different densities and organization. Because DEI relies upon diffraction instead of absorption to produce contrast among various structures, the x-ray dose is lower, while the image quality is higher. “Because absorption is necessary to produce contrast in the image, the radiation dose received from traditional x-rays comes from the x-rays that are absorbed by the body,” explains Zhong. “But in DEI, we do not need to use absorption as a contrast mechanism because we are only following the x-rays that pass through the tissue. Therefore, we can use higher-energy x-rays, which pass through with little absorption so the dose is lower.”
To make a diffraction enhanced image, x-rays from the synchrotron are first tuned to one wavelength before being beamed at an anatomical structure, such as a hand or foot. As the monochromatic beam passes through, the tissue within the appendage scatters the x-rays at different angles and causes the x-rays to refract, or change directions. The subtle scattering and refraction are detected by what is called an analyzer crystal, which diffracts, or changes the intensity, of the x-rays by different amounts according to their scattering angles.
The diffracted beam is passed on to a radiographic plate or digital recorder, which documents the differences in intensity to show the interior details.