Scientists have often sought to reduce harmful ionizing radiation, the high-energy particles that penetrate tissue to reveal internal organs and bone structures that can damage DNA, from single x-ray records or computed tomography (CT) scans. Although this ionizing radiation can be reduced, it usually comes at the expense of the image resolution and type of detector.
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Recognizing this, a European health consortium is developing a set of low-radiation, low-cost, flat-panel x-ray detectors that use novel photonics technology to make diagnosis safer for patients as well as hospital and dental staff, generating some of the highest resolution images ever seen in rapid moving body functions, such as malicious growths or the beating heart of a baby.
As a result of the two types of x-ray technology that exist today, diagnosis can only take the form of a high-resolution image, seen with direct converters that are used in mammography, or a low radiation dose, seen with indirect converters that are used in radiography or fluoroscopy.
Direct converters are based on semiconductors that directly convert x-rays into electrical signals (for example, amorphous selenium), while indirect converters make use of scintillators (e.g., caesium iodide) that emit light upon x-ray irradiation, and convert it into electrical signals by a photodetector. One is fine for micro-anomalies in breast tissue while the other for larger structures like bones or larger blood vessels, but never has there been a hybrid of the two.
Combining these advantages, the Direct Conversion Hybrid-Organic X-ray Detectors on Metal Oxide Backplane (DiCoMo) project is developing a new digital x-ray detector capable of producing high-resolution images that, in the envisioned product, rival a 16 Mpixel photograph.
Current indirect detectors, which offer a lower dose of radiation, generate light in all directions as soon as an x-ray photon is absorbed in it, hitting a large number of pixels on the photodetector array and creating limited spatial resolution. "The result is like a blurred photograph, or 'frosted' bathroom glass, where light bounces off the surface at different angles," explains project coordinator Dr. Sandro Tedde of Siemens Healthineers (Erlangen, Germany). "In indirect converters, there is always a compromise between resolution and sensitivity."
DiCoMo combines the advantages of both technologies by fusing radical innovations in the frontplane, the part of the device converting x-rays into electrical signals, and the backplane, which stores and drives the signals from the pixels to the readout circuitry and digital image reconstruction.
|A 5 µm view (top) of scintillating micro-particles (white) embedded in an organic semiconducting bulk (dark) and active pixel metal-oxide backplane (bottom).|
It works by getting an indirect converter, with low radiation output, to behave in the way we would expect to see a direct converter perform, a technique DiCoMo calls the quasi-direct effect. This allows the images to have the high resolution typical for direct converters by using scintillators and the great sensitivity they enable.
Contrary to the indirect approach, where the scintillator is stacked on top of the photodetector array, the DiCoMo technique embeds the scintillator into the photodetector itself, meaning the absorption of light occurs right where it has been generated and no optical crosstalk between pixels occurs. This produces high spatial resolution typically seen in direct converters, a result never before achieved.
DiCoMo’s second innovation is in the technology of the backplane, which is developed by TNO and IMEC, where the active pixels made from metal-oxide thin-film transistors amplify the charge within the backplane pixel itself, creating a stronger signal in situ.
Active pixels are state of the art in CMOS imagers, but have not been used so far in imagers based on transistor technologies made of amorphous semiconductors, like those used in medical x-ray detectors. A customized readout chip (ROIC) developed by project partner ICSense (Leuven, Belgium) is key for enabling the readout of the active pixel array and achieving the envisioned high frame rates.
"Similar to CMOS camera technology, where millions of tiny photosensors and transistors create an electrical current when exposed to light, our active pixel technology gives us a 'louder' signal at a much higher frame rate than ever before," Tedde says. "The fastest flat-panel x-ray detectors with state-of-the-art technology deliver around 60 frames/s, whereas DiCoMo aims to double this by capturing at 130 frames/s, permitting a physician to examine vital organs not only in high resolution, but also moving images in slow motion."
DiCoMo came up with the solution of using a hybrid x-ray absorption and conversion layer by embedding scintillating micro-particles into an organic semiconductor photodetecting bulk. This prevents the light from being widely spread over several pixels since the light to electrical signal conversion occurs in the same pixel where the x-ray photon has been absorbed preserving the spatial information.
The project aims to have the new x-ray detectors ready within five years.
For more information, please visit www.dicomo-project.eu.