"Color" Could Expand Radiography's Usefulness

Researchers in the U.K. report that they are on their way to creating an X-ray scanner that gives a chemical analysis of its target.

Bob Cernik, Ph.D. University of Manchester

Chirstopher Hall, D. Phil., S.Sc University of Edinburgh

Bob Cernik, Ph.D., a professor of synchrotron radiation and materials science at the University of Manchester, U.K., has worked with colleagues Kern Hauw Khor, Ph.D., and Conny Hansson, B.Sc., to develop a prototype "color" X-ray scanner that uses additional wavelengths of light to detect the object's chemical structure. The technique is called tomographic energy dispersive diffraction imaging (TEDDI).

X-ray beams produce wavelengths along the whole electromagnetic spectrum of frequencies but most detectors are unable to read differences between the wavelengths. "In conventional black and white X-ray scanning, that extra information is thrown away because you don't have an energy-sensitive detector that's able to distinguish those different colors," said Dr. Cernik.

Using advanced detectors and collimators created at the University of Cambridge, Rutherford Appleton Laboratory and Daresbury Laboratory, the Manchester team has built a 16 x 16 pixel array of collimating tubes and a pixelated, energy-sensitive detector with a tungsten filter. The TEDDI method then records the X-ray diffraction pattern as wavelengths and collects fluorescence information from the scattered spectra—information that all goes into identifying the scanned material.

"When the X-ray beam with all those different wavelengths hits the object, you can look at the scattered light, which contains characteristic fingerprints of the material that gave rise to the scattering," said Dr. Cernik. "Not only can you see the outline of a bone or something suspicious in a suitcase, you can actually tell, from the scattering pattern on the color X-rays, what exactly gave rise to the scattering. If you're looking at normal tissue or cancer tissue, they have different types of scattering patterns—they each have a very characteristic fingerprint."

The idea of bringing color to radiology is not new, as scientists have long acknowledged that the human eye is capable of distinguishing many more shades of color than shades of grey.

Building Detectors a Major Challenge

Dr. Cernik has worked intermittently on the color X-ray scanner for about five years, beginning with just a single beam, one detector and one collimator. The scanning process "took forever," lasting 16 to 20 hours, said Dr. Cernik. "We had to find a way to bring this into a useable timescale," he said. "The only way was to have thousands of detectors, which were bulky, expensive and had to be liquid-nitrogen cooled. Building the detectors was one of the major challenges."

Bob Cernik, Ph.D., and colleagues at the University of Manchester use this experimental setup as they work to develop a "color" X-ray scanner that uses additional wavelengths of light to detect the object's chemical structure. The technique is called tomographic energy dispersive diffraction imaging (TEDDI). Photo courtesy of Bob Cernik, Ph.D.

The solution involved using a silicon surface that counts the photons, but building such parallel energy-sensitive, pixelated detectors meant shrinking conventional electronics down to one pixel of an integrated circuit.

At the same time, Dr. Cernik attempted to build a multiple collimator array. Again, the trick was the size of the collimator holes. "They have to be 6,000 times deeper than wide, so it was a major engineering challenge," said Dr. Cernik. After two and a half years of trial and error, using laser drilling techniques pioneered at Cambridge, the Manchester team had their 16 x 16 collimator and detector system.

The team demonstrated the prototype's effectiveness by imaging aluminum samples and testing welds for the aerospace industry. The team has also shown that diffraction patterns can be obtained from polymer materials and has worked with biological samples such as deer antlers.

"The problem with all of those things is that silicon is quite a light atom—it doesn't have that many electrons and is therefore not very good at capturing high-energy X-rays," said Dr. Cernik. Since only high-energy X-rays come through a thick object, prototype scanning has been restricted to light atom samples and very thin sections. "This might be the case with biopsy samples," said Dr. Cernik.

"We have to replace the silicon and we're working with a number of materials," Dr. Cernik continued. "The favorite of the moment is cadmium zinc telluride and we're testing some of these detectors with the prototype." The next generation device will also have an 80 x 80 detector. "I anticipate that two years from now we will have the first high-energy prototype that can scan large samples," said Dr. Cernik.

Christopher Hall, D.Phil., D.Sc., a professor of materials and director of research at the School of Engineering and Electronics at the University of Edinburgh, previously conducted research with the single-collimator TEDDI process. "What Dr. Cernik has done is demonstrate a method for making this more practical," said Dr. Hall. "It is a major breakthrough and extremely clever what they've done, because it depends on simultaneous developments in instrumentation and hardware—it's a long, long way from where we were 10 years ago. It's a real tour de force as far as instrumentation development is concerned."

Clinical Uses Envisioned by Some

With Dr. Cernik and colleagues building larger and more powerful and practical color X-ray scanners, questions arise as to how they might be used, particularly in medicine.

Dr. Cernik said he is hopeful that color X-rays will eventually be used to not only depict human anatomy but also suggest diagnoses. "This device could tell you the exact tissue type at each point in the image," he said. "Certain cancer types, or the difference between normal or diseased bone, could be recognized. The scanner could be trained to look for certain types of pathological tissue."

Dr. Hall was a little more skeptical about the medical potential of TEDDI. "What we're getting is information that comes from crystalline X-ray diffraction," he said. "In my view, this technique is beautifully suited to investigating complex heterogeneous crystalline materials—engineering components, welds, archeological materials, ceramics, things of that kind." Medical X-rays, however, are used mainly to look at non-crystalline soft tissue, he said. The lack of crystalline scattering, he said, would reduce the additional information.

Crystalline scattering is absent in soft organic tissue, said Dr. Cernik, adding, "I'm not quite so pessimistic about identifying soft tissue types." Other researchers have described calcifications and tissue differences using energy dispersive diffraction, he said. "I would say that it is more difficult than examining crystalline systems, but not impossible. We will need detectors with a higher efficiency than we currently have, if we are going to get the signal-to-noise to acceptable levels."

In the meanwhile, color X-rays may be used to scan engineering welds and analyze geological samples. They may also have a role in archeology, which yields objects that can't be dissected due to their value. "To be able to put an object like that into an instrument like this and get new information about its detailed chemical or mineralogical composition is very interesting," said Dr. Hall.