By using the principle of grating interferometry, X-ray phase contrast imaging can now be performed with incoherent radiation from standard X-ray tube. This approach is in stark contrast with imaging methods using coherent synchrotron X-ray sources or micro-focus sources to improve contrast. The gratings interferometer imaging technique is capable of measuring the phase shift of hard X-rays travelling through a sample, which greatly enhances the contrast of low absorbing specimen compared to conventional amplitude contrast images. The key components in this approach are the gratings which consists of alternating layers of high and low Z (atomic number) materials fabricated with high aspect ratios. Here we report on a novel method of fabricating the grating structures using the technique of electron-beam (ebeam) thin film deposition. Alternating layers of silicon (Z=14) and tungsten (Z=74) were deposited, each measuring 100 nm each, on a specially designed echelle substrate, which resulted in an aspect ratio of ~100:1. Fabrication parameters related to the thin film deposition such as geometry, directionality, film adhesion, stress and the resulting scanning electron micrographs will be discussed in detail. Using e-beam method large-area gratings with precise multilayer coating thicknesses can be fabricated economically circumventing the expensive lithography steps.
Preclinical imaging is a cornerstone of translational research, as all therapeutic drugs need to be
tested for efficacy and toxicity on animals prior to human trials. Optical imaging techniques, such as
bioluminescence and multispectral fluorescence imaging, currently dominate preclinical functional
imaging despite their depth dependent limitations on quantitation and sensitivity. Translating drugs
developed with these techniques to clinical models can therefore be difficult. Hence, clinically relevant
nuclear imaging techniques, such as SPECT and PET, are therefore becoming increasingly used in
preclinical imaging. Dedicated preclinical SPECT and PET systems are now available, but for many
preclinical research groups this requires a significant investment in new equipment.
Large penetration depth and weak interaction of high energy X-rays in living organisms provide a non-destructive
way to study entire volumes of organs without the need for sophisticated preparation (injection of contrast material,
radiotracer labels etc.). X-ray computed tomography (CT) is a powerful diagnostic tool allowing 3D image
reconstruction of the complete structure. Using hard X-rays in medical imaging leads to reduced dose received by
the patient. At higher energies, however, the conventional scintillators quickly become the limiting factor. They
must be thin in order to provide reasonable spatial resolution and preserve image quality. Nevertheless, insufficient
thickness introduces the need for long acquisition times due to low stopping power. To address these issues, we
synthesized a new structured scintillator to be integrated into CCD- or photodiode-based CT systems. Europiumdoped
Lu2O3 (Lu2O3:Eu) has the highest density among all known scintillators, very high absorption coefficient for X-rays and a bright red emission matching well to the quantum efficiency of the underlying CCD- and photodiode arrays. When coupled to a suitable detector, this microcolumnar scintillator significantly improves the overall
detective quantum efficiency of the detector. For the first time ever, structured and scintillating film of Lu2O3:Eu
was grown by electron-beam physical vapor deposition. A prototype sensor was produced and evaluated using both
laboratory X-ray sources as well as synchrotron radiation. Comparative performance evaluations of the newly
developed sensor versus commercial grade scintillators were conducted. Such synthesis of high density, microstructured,
scintillating coatings enables the development of high sensitivity X-ray detectors for CT applications.
Columnar, amorphous CsI:Tl scintillators are attractive for biomedical imaging applications because they allow
high spatial resolution with EMCCD detectors and BazookaSPECT. But these scintillators have a serious practical
limitation, they are not useful in thickness larger than 0.5 mm, because of attenuation; this means that detection
efficiencies are very poor for ≥100 keV gamma rays. The development of a new crystalline microcolumnar scintillator
(CMS) of CsI:Tl is described. CMS CsI:Tl has high density and greatly improved light transmission properties. CMS
films were prepared in three thicknesses (0.5 mm, 1.0 mm and 4.0 mm), and test results for these films are described.
The light yield and energy resolution for 0.5 mm, CMS CsI:Tl films was equivalent to that of conventional bulk CsI:Tl
scintillators; while some light attenuation was seen in 4 mm-thick CMS CsI:Tl samples, useable photopeaks were
obtained at 122 keV. The spatial resolution measured for the 0.5 mm and 1.0 mm films in a BazookaSPECT system was
excellent, 150-200 μm. Imaging tests with the 4 mm-thick CMS CsI:Tl films in a BazookaSPECT system with 662 keV
gamma rays showed progressive broadening of the signal clusters on the CCD camera with depth of interaction (DOI),
indicating that DOI determination should be possible with these detectors, i.e. 3D detector operation. Future planned
investigations are described.
Oxygen doped zinc telluride is a bright scintillator with one of the highest X-ray conversion efficiencies. These
properties make it an ideal choice for wide range of X-ray imaging applications in biology and medicine. With an
emission wavelength of 680 nm it is ideally suited for use with silicon imagers such as CCDs. In this paper we report a
new co-evaporation process where the oxygen dopant concentration in the evaporated film is controlled by simultaneous
evaporation of zinc oxide and zinc telluride charge. To date we have fabricated as large as 40 cm2 area films measuring
50 μm to 500 μm in thickness. The fabrication and characterization details of these and other films are discussed in this
paper.
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