Yunhui Zhu investigates new ways of enhancing X-ray technology by combining lesser-used properties of propagation.

If the doctor orders a CT scan of your broken wrist, you know what to expect: An X-ray machine conjuring ghostly images of joints and bones. Or you know X-rays from the airport: a security official scrolling through pictures of transparent luggage and rejecting your toiletry kit because you exceeded the liquid limit.

ECE Assistant Professor Yunhui Zhuworks with X-rays, but she is looking beyond bones and bags, to the future of imaging. She is combining different physical properties of X-rays to enhance their capabilities, granting researchers a sharper, richer look within.

There wouldn't be much to see of a lime using traditional X-ray imaging. But a combination of phase and attenuation provides a stronger signal.

Doctors and TSA agents make use of a specific characteristic of X-rays: their attenuation. As X-rays travel through the body, they are absorbed in different amounts by different tissues. Bones and other hard materials easily absorb this type of radiation, and the images are based on the varying intensity of the X-rays.

"People love X-rays because they have a strong penetrating ability and they let them see what's happening inside," said Zhu. "But traditional X-ray technology is extremely limited. It produces very fuzzy images, and there are many materials that hardly absorb X-rays at all."

Most notably, Zhu said, there are liquid explosive materials that can't be detected by airport security equipment, and state-of-the-art breast cancer scans often can't pick up potentially cancerous lumps in breast tissue.

Instead, Zhu is investigating little-used properties of the radiation, particularly their phase, which takes advantage of how the light waves bend through tissue, and small-angle scattering, which exploits how the X-rays bounce off a sample and scatter.

Combined with attenuation, these propagations of light bring X-ray images into much sharper focus. Zhu's test images have already increased the contrast by a thousand times.

Using only X-ray attenuation, the image is basically noise (left), but Zhu's techniques result in more information per photon. After tracking the phase signals, the layers of the artery come into stark relief (right). X-ray images created in collaboration with the 3-D optics lab at the Massachusetts Institute of Technology.

In the field of X-ray imaging, the source of the light is key. Conventional applications in airports and hospitals use ordinary X-ray tubes. They have a relatively large volume and, according to Zhu, they yield less accuracy.

Barring the use of a synchrotron, which would be too large and expensive for her uses, Zhu has been advancing her techniques by shrinking, focusing, and cooling her light source, which allows for phase and scatter imaging with higher resolution. She's also been experimenting with arrays of light sources, modulating how the light is shining in terms of spatial patterns and pulses.

Arrays also work well for 3-D X-ray reconstruction, which is another of Zhu's goals. A 3-D reconstruction is based on stacked projection images from different angles and a 3-D reconstruction algorithm. An array set up around a sample (or a patient) makes for a faster, more accurate image.

While the applications of X-ray phase imaging research are wide-ranging, Zhu is currently focusing on two projects.

The first seeks to mitigate the discomfort and inaccuracy associated with breast cancer screenings. "Breasts are soft tissue, and to get a good image, they are mashed and distorted in painful ways," said Zhu. "By using phase imaging, the patient wouldn't have to be put through such a difficult procedure, and the images will be more accurate."

She is also interested in applying the new techniques to monitor and improve the printing process for 3-D printed materials, which are good candidates for this type of imaging. "If I can track the stress distribution inside the 3-D configuration in any partsmetal, carbon, and nylonthat will be a tremendous thing for materials and mechanical engineering," said Zhu.