GaAs Enables Novel X-ray Imaging

A camera originally developed to capture X-rays from objects in space will now be used in the early detection and removal of cancerous tissue, especially in cases of breast cancer. A key aspect of the technology that enables such diverse X-ray applications is epitaxial gallium arsenide (GaAs).

The imager uses epitaxial GaAs instead of the silicon used in charge-coupled devices (CCDs) and CMOS imagers. The much higher effective atomic number of GaAs results in a far greater detection efficiency. The material was developed at the European Space Agency (ESA), under the leadership of Marcos Bavdaz, to meet the demanding requirements of such X-ray sensors. The prototype sensor has undergone extensive and successful testing at the HASYLAB (Hamburger Synchrotronstrahlungslabor, Hamburg, Germany) radiation facility, which is part of the Hamburg DESY (Deutsche Elektronen-Synchrotron) synchrotron ring.

ESA designed the X-ray camera to image celestial objects that emit large quantities of X-radiation but only a little visible light. The researchers are especially interested in imaging the 1-100 keV wavelength band or even up to 200 keV for astrophysics, but for planetary applications the main interest is in imaging the 0.5-10 keV region.

The Science Payloads Technology Division of the Research and Science Support Department developed the X-ray camera at ESA's Science, Technology and Engineering Research Centre (ESTEC, Noordwijk, Netherlands). The ESTEC researchers claim to have produced the first microchip, similar to that found in common video cameras, that is capable of detecting hard X-radiation instead of visible light.

The technology is also applicable to detecting cancerous tissue because relatively hard X-rays are the only type that readily pass through the human body. "For the lymphatic system, a small amount of a radioactive tracer that emits X-rays is injected into or near the breast cancer," said Alan Owens of the ESTEC group. "The trace focuses on those parts of the system that are cancerous. With a small camera it is therefore possible to image this cancerous tissue during surgery."

The most frequently used isotope is metastable technetium-99 (^99mTc), which emits X-rays of 140 keV photon energy. But work is also continuing with other isotopes emitting X-rays in the range of 27-364 keV. The researchers hope that the new X-ray camera will enable information on the state of the lymphatic system to be evaluated in the operating theater with cancerous areas being immediately pin-pointed.

The size of the prototype chip made for the X-ray planetary fluorescence spectrometer for the ESA BepiColombo mission to Mercury is 10 × 10 mm. This is much too small to replace conventional X-ray film in medical applications. Much developmental work will be needed to produce a 100 × 100 mm device for mammography or a 300 × 400 mm device for a typical chest X-ray, Owens said. However, 30 × 30 mm devices for dentistry and 10 × 10 mm devices for intra-operative radio-guided surgical probes could be produced now.

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