We have developed microstructured Lu2O3:Eu scintillator films that provide spatial resolution on the order of micrometers for hard X-ray imaging. In addition to their outstanding resolution, Lu2O3:Eu films also exhibits both high absorption efficiency for 20 to 100 keV X-rays, and bright 610 nm emission whose intensity rivals that of the brightest known scintillators. At present, high spatial resolution of such a magnitude is achieved using ultra-thin scintillators measuring only about 1 to 5 μm in thickness, which limits absorption efficiency to ~3% for 12 keV X-rays and less than 0.1% for 20 to 100 keV X-rays; this results in excessive measurement time and exposure to the specimen. But the absorption efficiency of Lu2O3:Eu (99.9% @12 keV and 30% @ 70 keV) is much greater, significantly decreasing measurement time and radiation exposure. Our Lu2O3:Eu scintillator material, fabricated by our electron-beam physical vapor deposition (EB-PVD) process, combines superior density of 9.5 g/cm3, a microcolumnar structure for higher spatial resolution, and a bright emission (48000 photons/MeV) whose wavelength is an ideal match for the underlying CCD detector array. We grew thin films of this material on a variety of matching substrates, measuring some 5–10μm in thickness and covering areas up to 1 x 1 cm2, which can be a suitable basis for microtomography, digital radiography as well as CT and hard X-ray Micro-Tomography (XMT). The microstructure and optical transparency of such screens was optimized, and their imaging performance was evaluated in the Argonne National Laboratory’s Advanced Photon Source. Spatial resolution and efficiency were also characterized.
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.
To achieve high spatial resolution required in nuclear imaging, scintillation light spread has to be controlled. This has
been traditionally achieved by introducing structures in the bulk of scintillation materials; typically by mechanical
pixelation of scintillators and fill the resultant inter-pixel gaps by reflecting materials. Mechanical pixelation however, is
accompanied by various cost and complexity issues especially for hard, brittle and hygroscopic materials. For example
LSO and LYSO, hard and brittle scintillators of interest to medical imaging community, are known to crack under thermal
and mechanical stress; the material yield drops quickly with large arrays with high aspect ratio pixels and therefore the
pixelation process cost increases.
We are utilizing a novel technique named Laser Induced Optical Barriers (LIOB) for pixelation of scintillators that
overcomes the issues associated with mechanical pixelation. In this technique, we can introduce optical barriers within the
bulk of scintillator crystals to form pixelated arrays with small pixel size and large thickness. We applied LIOB to LYSO
using a high-frequency solid-state laser. Arrays with different crystal thickness (5 to 20 mm thick), and pixel size (0.8×0.8
to 1.5×1.5 mm2) were fabricated and tested. The width of the optical barriers were controlled by fine-tuning key parameters
such as lens focal spot size and laser energy density.
Here we report on LIOB process, its optimization, and the optical crosstalk measurements using X-rays. There are
many applications that can potentially benefit from LIOB including but not limited to clinical/pre-clinical PET and SPECT
systems, and photon counting CT detectors.
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.