The strong link between High Energy Physics and Medicine has always been one that inspires innovation.
Medipix, in essence, is an integrated circuit connected to a sensitive element to form a small particle detector. It is a spin-off of the electronics developed for detectors used at the Large Hadron Collider. Its single photon counting feature enables it to produce X-ray images which are high resolution and noise-free, making it excellent for use in medical imaging and a broad range of applications involving radiation detection
The Medipix family consists of the Medipix 1 chip, the Medipix 2 chip, with an improved resolution, Timepix, a modified version of Medipix2 with the additional functionality of time measurements and Medipix3, which not only counts all of the photons but also determines the energy level of each individual photon detected.
A particle detector can observe each individual proton, electron, neutron or other exotic particle that fall within its measurement range. The intricacies of the LHC operation require the detectors to have certain qualities – they must be fast, noise-free and have high resolutions. CERN researchers saw the potential of transferring this technology outside the High Energy Physics domain, thus the Medipix project was born.
The Medipix chip is a single photon counting chip. In conventional methods several photons are needed to obtain a (black) image, but the Medipix chip requires just one and, therefore, requires less radiation. Another important advantage compared to conventional techniques is that no signal is measured if no photon enters. This means that there is no noise irrespective of the period of exposure.
Both the sensors and the microchips - which together form a hybrid detector - are divided into tiny sensitive elements (pixels), similar to those in a digital camera. These hybrid pixel detectors produce images with high resolution, high contrast and almost no noise. They are so sensitive that they can detect individual X-ray photons.
Figure 3: A transverse section through a mouse’s chest in which the x-ray colour of different radiographic pharmaceuticals is demonstrated.
Figure 4: Spatial resolution of MCP-bases photon detector tubes. The left image was taken with a photo tube which uses the Medipix2 chip. The right image shows the same test object imaged with the Timepix chip in ToT mode and corrected using centre of gravity weighting.
Radiography and computer tomography (CT) use X-ray photons to study the human body. The Medipix chip with its single proton counting technique is an excellent technology to be used in these fields. It has been applied in X-ray CT, in prototype systems for digital mammography, in CT imagers for mammography and for beta and gamma autoradiography of biological samples. Moreover, with the Medipix3 chip, these images will not be black and white anymore – they will have colours to indicate different energy levels of the photons. This colour X-ray imaging technique will produce clearer and more accurate pictures which will help doctors give their patients better diagnoses.
The Medipix2 chip is also being exploited for commercial X-ray materials analysis. Together with an industrial partner, the Medipix2 collaboration has developed a revolutionary detector which can be used for a myriad of purposes such as the characterization of pharmaceuticals, evaluation and synthesis of new materials, and the detection of counterfeit drugs.
Before the decommissioning of a nuclear power plant, the area needs to be meticulously inspected for contamination. Such a job requires equipment with high sensitivity and low background noise – exactly the strong features of the Medipix technology. A compact gamma camera based on the Medipix2 readout chip has been simulated, designed and built specifically for this purpose.
Look into a clear night sky and you will see twinkling stars. Although an inspiration for nursery rhymes, this ‘twinkling’ of stars is actually a nuisance for telescope users – they produce blurry images due to distortions caused by the earth’s atmosphere. But this ‘twinkling’ could be compensated for with the use of a sensitive high-speed camera, and that is where Medipix comes in. The Medipix2 and Timepix chips have been incorporated in photo-tubes with integrated micro-channel plate (MCPs) and applied in astronomical adaptive optics.
The portability and user-friendliness of the Medipix chip and its accompanying USB readout system allows it to be easily used as an educational tool in classrooms. The ability to visually distinguish the signals of alpha, beta and gamma particles and the video style frame rate makes it a compelling demonstration tool for educators teaching about radiation. Students also learn about computer aided data analysis by using the USB readout system that directly records the data on the chip onto a PC. A group of students from the Simon Langton Grammer School (Kent, UK) has developed a space based cosmic ray detector using the Timepix chip which is set to launch on a satellite in year 2012.
The success and maturity of the technology has also led to the chips being used to develop future systems within High Energy Physics (HEP). The Timepix chip in particular, can be used in an array of HEP applications. Evolved from its Medipix ancestor, the Timepix chip is a variation of the Medipix2 development but with extra features. It not only counts the particle arriving in the pixel, but also measures the time of their arrival. And it does all these with very high precisions – a perennial requirement for HEP applications. The Medipix design team is working on using the Timepix as a prototype for the upgrade of the LHCb (one of the LHC experiments) vertex locator. When successfully put into production, it will be yet another milestone for Medipix.
The Medipix2 collaboration was started with the aim of disseminating hybrid pixel detector technology from High Energy Physics to other fields. The collaboration was initially composed of 13 European research institutes. Over the ensuing 10 years the collaboration expanded to reach a peak of 17 member institutes. Although the main scientific focus has been the development of the Medipix2 and Timepix single photon counting pixel detector readout chips, the collaboration members have expanded the range of applications for the technology to many more scientific fields than initially foreseen.
In 2005, the partnership was continued in an altered form to develop the third Medipix chip, a technically advanced version of the second chip. Unlike Medipix 2, Medipix 3 not only counts all of the photons but it also determines the energy level of each individual photon detected. This means that it will be possible, for example, to examine the cogs of a watch from behind its metal casing. This advanced version of the Medipix chip will radically improve the efficiency of X-ray imaging techniques by introducing colour imaging systems. This will lead to better discrimination of materials and tissues and an increased contrast of the images produced. Research is currently being done for example on ways to label cancer cells with pharmaceuticals with specific colours.
For more information about the Medipix project, please visit http://cern.ch/MEDIPIX