X-ray detection with luminescent screens requires optical signal transfer as an intermediate step between x-ray detection and conversion to an electronic signal. Luminescent screens may be granular (phosphor screens), structured (e.g. CsI) or transparent (scintillators). The optical signal is imaged with lenses, fibre optics, electron optics or by proximity focussing to an electronic detector. Poor focussing or poor optical contact may degrade the signal and noise transfer characteristics, i.e. modulation transfer function (MTF) and detective quantum efficiency (DQE).
The case when x-rays are detected with granular luminescent screens, imaged onto flat panel electronic detectors is considered here. The detector assembly often requires layers of glue or protective thin films creating optical gaps, in which light is spread, hence spatial resolution is degraded. The noise spectrum is not necessarily changed the same way. Its exact shape depends on the dominant noise sources in a given detector configuration under the specific operating conditions: The noise of the primary x-ray quanta, noise aliasing and direct x-ray detection by the electronic detection layer are the main contributions in this investigation.
Especially at high spatial frequencies small optical gaps in conjunction with white quantum noise from direct x-ray absorption of the electronic imager degrade DQE: A gap of 40 μm between luminescent screen and detector reduces the DQE by 33% at the Nyquist frequency. This was demonstrated with an a-Si imager of 143-μm pixel size and a Lanex Fine luminescent screen operated at 100 kV.
The FDA has approved the SenoScan slot-scanning Full-field Digital Mammography system. A high power Tungsten-target x- ray tube enables breast imaging with 0.22 s effective exposure time. A 21-cm X 29-cm area is scanned in less than 6 seconds, at a typical clinical technique of 30 kVp, 170 mA. The detector comprises a Thalium-doped Cesium Iodide scintillator coupled to a combination of four CCDs abutted along their narrow dimension to from a 10-mm by 210-mm slot. With the CCDs operated in time-delay-and-integration mode along the narrow dimension, the system functions in a continuous scanning mode. The MTF in the standard and high- resolution modes extend to 10-cycles/mm and beyond 14 cycles/mm respectively. The Detective Quantum Efficiency curve starts at 50 percent at DC and extends to 10 cycles/mm in Standard model. Accordingly the SenoScan system enables screening and diagnostic breast imaging with a limiting resolution approaching that of film-based systems. The overall system design and intrinsic scatter rejection efficiency directly translate in high DQE characteristics that enable screening at a significantly reduced patient dose.
Amorphous silicon flat panel x-ray detectors (A-Si FXD) are expected eventually to replace traditional x-ray image intensifier systems (XRII) in medical radiography in the long term. The advantages of FXD's are their large detection area, no distortion, no sensitivity to magnetic fields, low weight and compactness. However, they do not provide the high sensitivity of specific optimized systems based on image intensifiers, which approach the sensitivity of single x-ray photon counting in an appropriate configuration whereas the noise equivalent number of photons for an a-Si imager is typically several photons at medical energies. That is, the detective quantum efficiency of an XRII at low dose is expected to be higher.
An x-ray camera for imaging with a spatial resolution in the micrometer and sub-micrometer range has been developed. The camera consists of a scintillator, light microscopy optics and a cooled charged-coupled device (CCD). A transparent scintillator converts the x-ray field into a visible light image which is projected onto the CCD by the light optics. A resolution of 0.8 micrometer fwhm was achieved using 12-keV x rays and a 5-micrometer thick commercially available Y3Al5O12:Ce (YAG:Ce) scintillator. The detective quantum efficiency (DQE) of the camera is mainly limited by the low absorption of x rays in the thin layer of the scintillator. To increase the absorption, Lu3Al5O12 (LAG) scintillators have been grown by liquid phase epitaxy (LPE). The characteristics of LAG activated with Eu or Tb have been investigated, in particular spectral emission, efficiency of x-ray to light conversion, and time response. Three-dimensional computed x-ray microtomography (3D-CMT) images of mouse bone samples have been recorded with this camera using a 5-micrometer thick YAG:Ce screen. The 3D-CMT system uses a parallel monochromatic x-ray beam extracted from the synchrotron radiation. A series of 2D x-ray projection images at different angles were recorded, and processed numerically to yield the 3D image of the bone structure with a voxel size of 1.8 micrometer. Image features smaller than 3 micrometer are clearly visible in the reconstructed tomogram. The quality of the images allows the analysis of the trabecular bone structure that is important for the understanding of the mechanisms of osteoporosis.
3D computed tomographic images with micrometer resolution were made in phase-contrast mode with high energy x-rays at a third generation synchrotron source. The phase-contrast technique enables one to obtain information not only about the amplitude of the wave field behind the object and thus about the absorption, but also about the refractive index distribution inside the sample. Increasing the x-ray energy from the soft x-ray region up to 10-60 keV simplifies the experimental setup and opens the possibility to study organic samples at room-temperature and under normal pressure conditions. The projection data is recorded with a fast, high-resolution x-ray camera consisting of a 5 micrometers thin YAG scintillator crystal, a visible light microscope optics and a slow scan 1k X 1k CCD camera. The spatial resolution of phase-contrast microtomography is currently limited by the resolution of the x-ray detector to about 1-2 micrometers . First applications in biology and geophysics are shown.
Serial readout of cooled, linear Si-photodiode arrays or CCDs offer a high dynamic range but only at moderate pixel readout rates. Recording spectroscopic information, i.e. 1D images, requires only a few sensitive lines of a 2D CCD array. A camera is presented where the unexposed part of the CCD is used as a buffer to store successive spectra with high frame rates. In this 'streak mode' the time resolution for data acquisition depends only on the line shift time of the CCD and no longer on the slow pixel readout time. A frame rate of 10 kHz for X-ray absorption spectra has been achieved. The CCD is part of an X-ray camera consisting of a scintillating screen lens-coupled to the CCD. The camera provides a high dynamic range of 17 bit, a spatial resolution of 60 micrometers (FWHM) and a high detective quantum efficiency of > 40% for x- ray energies between 4 keV and 25 keV. The camera is used for time- resolved energy-dispersive XAFS (X-ray Absorption Fine Structure) experiments. This technique permits the study of time-dependent variations of electronic properties and the local environment of atoms under induced external perturbation.
Absolute displacement sensing by white-light interferometry is
demonstrated without mechanically moving parts in the receiving
interferoineter. Its characteristics are: Michelson interferomneter with
tilted mirrors, photodiode array of 1024 elements, lead-insensitivity
up to 9 dB loss, displacement range 75 /.m, resolution 0.02 pm, read-
out time < 7 ins, and multiplexing of several transducers.
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