Comparison between a position sensitive germanium detector and a taper optics CCD 'FRELON' camera for diffraction enhanced imaging

摘要

Diffraction Enhanced Imaging (DEI) can significantly improve the expressiveness of radiology. The contrast mechanism of DEI, in addition to absorption contrast, exploits the differences in X-ray refraction properties, which are sensed by a perfect crystal placed between the sample and the detector. DEI needs a monochromatic collimated X-ray source, which is available for instance from synchrotrons. The X-ray beam is laminar and the sample is vertically scanned for projection imaging or is rotated for CT. Detectors should match the beam characteristics and should also accomplish the other two main requirements for DEI mammography: high spatial resolution and high Detective Quantum Efficiency (DQE) in a large energy range (20―60 keV). The first permit to exploit the edge contrast enhancement obtained with the DEI technique, for example the improved visualization of microcalcifications in mammographic imaging. The second allows minimizing the dose needed for a radiograph without sacrificing spatial resolution. Apart from this, a dynamic range as good as possible is required (typically from 14 to 16 bits) as well as a high readout speed, which is particularly important for CT. These specifications are difficult to be all condensed in a single detector. At the medical beamline of the ESRF two devices have been utilized for DEI radiography: a linear germanium detector (432 pixels, 350 microns pitch), which had been developed for angiography and cerebral CT and a 2048 x 2048 CCD camera with taper optics which has been built at the ESRF. The first detector shows an excellent DQE at zero frequency in a large energy range (～ 90% from 20 keV up to 50 keV) but limited spatial resolution. In the latter a better compromise for DEI in the 20―30 keV range has been realized: a pixel size of 47 μm and a DQE(0) from 0.5 to 0.6 has been achieved. The performances of the two detectors will be presented here in detail and discussed.