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Large X-ray Detectors for Coloured X-ray Imaging

Final Report Summary - LACX (Large X-ray Detectors for Coloured X-ray Imaging)

The digital revolution in the medical application of X- and gamma-rays for diagnosis opens quite new opportunities. State of the art radiography images provide high contrast and high spatial resolutions, however, information on the energy of the absorbed X-ray photons is mostly not yet available although such information would help doctors to improve their diagnosis and could lead to a reduction of radiation doses for patients. The use of real coloured X-ray imaging (CXI) is limited to experimental laboratory studies such as the dual-energy contrast-enhanced digital subtraction mammography published by Lewin. The reason of this limitation of CXI is the very high requirements for X-ray detectors and read-out electronics which can only be met by semiconductor detectors. They provide high efficiency and high spatial resolution because semiconductors convert radiation directly into charge carriers.

Although real progress has been made over the past years in the development of microelectronics and in material engineering resulting in novel devices, these improvements were not sufficient for the industrial production of semiconductor detectors for large area X-ray imaging at reasonable cost. Conventional semiconductors like silicon and germanium cannot be used for applications with high energy photons, like mammography, nuclear medicine or computer tomography. The absorption coefficient is too low (Si) or the material has to be cooled (Ge). Above approximately 30 keV the efficiency of silicon decreases very rapidly, whereas CdTe or CZT still have an efficiency over 90 % at 100 keV, the later with just 1 mm thickness and detectors with over 5 mm thickness can be built these days. (Cd,Zn)Te sensors can operate at ambient temperatures and the material is chemically stable up to rather high temperatures.

Therefore, although the material properties of CdTe/CZT are promising the best performance for the proposed application, major technological challenges must be overcome to produce large area detector systems. However, despite the clear potential of CdTe/CZT technology for large area sensors that would lead to dose reduction for the patients and high contrast of the radiation imaging, no such system was presently available on the market when this project was launched.

The LACX project goal was to fulfill this need by the development of large X-ray detectors based on CdTe/CZT crystals combined with photon counting readout electronics. One of the main issues limiting the realisation of such large detectors was the high cost and the lack of availability of detector grade material. Indeed, the growth of large single crystals with the specific properties required for this type of application is complex and although ingots can be grown with a diameter as large as 140 mm, the average useful size of homogeneous single crystals is often reduced to areas smaller than 40 x 40 mm². This is sufficient for some applications but not for our present objective. The solution to overcome these limitations was the deposition of CdTe/CZT thick films with the required detector performance.

The following technical challenges had to be met in the framework of the project:

- Disponibility of large size CdTe/CZT crystals: Continuous large size CdTe crystals growth by THM method has been carried out, material characterisation and quality enhancement methods, crystals selection methods and wafers preparation for further pixelised detectors production.
- Due to the very small pixel size, the uniformity of the wafers has become an important parameter: Improving uniformity of the materials' properties has been undertaken through adequate thermal annealing procedures by both partners, each with his specific analytical tools in a concerted programme.
- The preparation of the semiconductor surface as well as the nature and structure of the metallic contacts is playing an important role since on one hand these contacts determine the electric performance of the device, i.e. the noise level and on the other hand the aptitude to be bonded in a stable manner with the IC chip.
- The experience gained on the bulk wafers in both surface processing and bonding, added to the software processing is of greatest interest for the development of the large size thick film materials for large imagers.
- Fabrication of full functional pixel detector module made of single crystal detector: Application of planar technology (metallisation, bump deposition and flip-chip bonding).
- Software adjustment of Medipix to medical applications: The software has to fulfil the requirements of clinical standards and commercial applications. The clinical standards have been applied according to Eurorad's directives as the company has a good knowledge and long-term experience of software development for medical applications. A suitable solution for recording large images is the use of special algorithms to tile images. This is a reliable solution if the detector efficiency is very high and the dose of radiation exposure is quite small even for several images. Such an algorithm has been optimised to be used with Medipix electronics and the highly efficient sensor material.
- Production of detector grade thick films and characterisation, fabrication of X-ray detection modules based on Medipix2 and CdTe crystals and films. Improvement of the film and investigations of stability of the Medipix detector system. The improvements of the polycrystalline films needed detailed characterisation of the deposited CdTe films regarding transport properties and homogeneity of signal response.
- Application of Medipix2 and newest evolution Medipix3 based detector systems for energy selective 'coloured' X-ray imaging: High resolution and low contrast X-ray imaging with silicon and spectroscopic imaging with Medipix3 and CdTe.
- Large size detectors testing. The images in multi-colour mode have been registered. The quality of these images demonstrates the excellent quality of the Medipix-CdTe detector as well as the optimum of the tiling algorithm to record objects with a large FOV for tiling (e.g. 28 x 42 mm2).
- Medipix detector application in computer tomography (CT). For the first time, very high quality images have been demonstrated with the newly developed Medipix3 detector. The quality of these images demonstrates the excellent quality of the Medipix-CdTe detector as well as the optimum of the tiling algorithm for recording objects with a large FOV for tiling. The images were recorded with Medipix2-Silicon and Medipix3-CdTe detectors. This is a very important feature for future medical application and an advantage in view of reducing the dose
- Transfer of economy knowledge from Eurorad to FMF. The commercial background of Eurorad and its position in the medical market will give an important push to the technological know-how of FMF.
- The application for CXI or spectroscopic X-ray imaging is rapidly growing. The possible areas of this new X-ray method are in medical diagnostics like radiography and CT and security applications.

The applications which can greatly benefit from these novel imagers include mammography, planar radiography, and CT. CXI opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different 'colours' can be used to discern materials in the object.

The present day best performances and our objectives within the LACX project are described below. The objectives within the LACX project have all been achieved.

- TFT arrays based on scintillators coupled to silicon thin film transistors:
Minimum pixel size : 127 micrometers
Spectroscopy: Not yet (noise)
Single photon counting: No

- LACX project objectives based on pixelised semiconductors coupled to integrated chips
Minimum pixel size: 55 micrometers
Spectroscopy :Yes
Single photon counting: Yes

The project which lasted 48 months consisted of four different work-packages (WPs):

WP1: Planar technology
- Preparation of single crystals from Eurorad using technology for efficient passivation of CdTe/CZT detector modules
- Hybridisation of CdTe/CZT detectors with photon counting read-out electronics by flip-chip bonding.

WP2: Electronics

- The use of photon counting electronics (e.g.Medipix) for medical applications
- Adjustment of existing software for medical applications
- Introduction of pixel electronics for CXI.

WP3: Thick film deposition
- Development of technology for the production of detector grade thick film CdTe/CZT using Eurorad expertise for detector performance
- Growth of thick films CdTe/CZT on readout-electronics directly

WP4: Applications
- Transfer of economic knowledge
- Search for new application for large area X-ray detectors.

In the future, the evolution of a medical gamma-camera will go in the same direction as the photography camera , when the film was replaced by a high density of small sized pixels (CMOS). Even its size will come close to that of photographic camera and its weight below 1 kg.

Until now, the pinhole cameras would not allow both high space resolution and high efficiency because the unit pixel or scintillator size was by far too large, i.e. 1.2 - 2.0 mm, in the best situation down to a few µm in our project. Now sub-millimeter spatial resolution will become possible with strongly increased efficiency (factor ten).

These innovations will have other very important consequences, especially:

- to observe in real time moving parts of the body (heart, lumb, ...) since the acquisition time can be drastically reduced,
- to develop small dedicated cameras, much cheaper than the present large instruments,
- the small size of the pixel detector combined to the pinhole collimator will drastically reduce the overall weight of the camera.