Radiation imaging detectors for robotic X-ray imaging system
ADVACAM s.r.o. is a Czech high-tech company that develops radiation imaging detectors based on the Medipix technology. It is a technology that is under long term development lead by CERN. The collaboration involves institutions from many countries including ESA and NASA. The technology is licensed to ADVACAM for commercialisation. ADVACAM builds using this technology radiation and X-ray imaging devices. The largest market is science and the industrial markets are growing as well.
The imaging detectors utilise Timepix chips. (Timepix is a modified version of the Medipix chip). The Timepix readout chip is an array of 256x256 pixels. Each pixel is 55x55 µm2 in size and contains the full signal processing electronics and counter. Each electronics pixel is connected to an appropriate pixel in a sensor that is attached to the readout. The sensors are of various materials (Si, CdTe, CZT or GaAs) types and sizes (100 to 2000 µm). All are direct conversion sensors, it means that the radiation generates directly electric charge that is processed in the readout chip pixel electronics. This detection approach offers several major advantages compared to common X-ray imaging detectors that are based on CCD or CMOS readouts. The major advantages are: • Elimination of electronics noise by use of internal energy discrimination threshold. • Photon counting. • High contrast. • High sensitivity to low and high energy X-ray photons. • Energy sensitivity (measurement of X-ray wavelength). Therefore, these devices offers excellent contrast especially for light materials such as composites used in aerospace industry. However, the limitation that currently limits broader application of these detectors outside of scientific fields is their size. A single Timepix chip has size of 14x14 mm2. ADVACAM has a patented technology to arrange these chips to larger arrays. The largest device combines 10x10 of these chips leading to 140x140 mm2. However, the price reaches level of about €500k making it too expensive for most of applications. More accessible are detector version utilising arrays of 1x5 or 2x5 chips. The price is here comparable with regular X-ray imaging devices.
Innovations & Advantages
The most outstanding difference to common X-ray imaging detectors is the ability of Medipix/Timepix chips measuring X-ray wavelength. This is a major step forward in X-ray imaging because the knowledge of X-ray spectrum gives a tool to identify also different materials in the sample based on their “trace”. It can be in a way described as the difference between taking a black and white and colour photo of a stained-glass window in a church. Both images reveal the structure, but only the colour photo gives a full information about used glass types as some of the glass plates may have the same grey level in the black and white photo. The situation in X-ray imaging is similar. Measuring the wavelength allows using for example so called k-edge imaging technique that was otherwise possible only with synchrotron accelerators that emit monochromatic tuneable X-ray spectrum. The new detector technology makes it possible to use regular (polychromatic) X-ray tubes, make the whole system compact and orders of magnitude cheaper. An example of this technique is the application in art inspection that we being developed by another associated start-up InsightART . Some of the results could be found in a BBC World shot available at https://youtu.be/1xUD0BUzgtQ. Nevertheless, as described in the previous section, the limit is size of the sensor that that so to be overcome by different scanning approaches and in the case of robotics the compact size even offers new options, hence new markets.
The direct conversion of X-rays into electric charge and possibility to suppress electronic noise allows achieving an excellent contrast in light materials such as plastics, composites, but also biological tissue. That opens up new applications of X-rays where the common X-ray imaging systems fails or do not provide sufficient quality of images (common X-ray imaging systems work well for heavy, highly attenuating objects, such as steel parts, but fail in light materials). Therefore, the new imaging detectors push the boundary of X-ray imaging applications to a new level.