Microcolumnar CsI:Tl scintillator screens have been the gold standard in X-ray imaging for many years due to their high density, high atomic number, and scintillation efficiency. The structured screens provide an improvement in performance by channeling the light to the detector, improving detection efficiency and spatial resolution. We have taken this concept a step further by laser-machining the CsI:Tl scintillator to provide pixels that match the detector pixels. This allows for still thicker CsI:Tl layers up to 700 μm pixelated with pitch of 100 μm to match CMOS flat panel pixels, thus improving X-ray absorption and resolution. We are investigating the applications of CMOS detectors with pixelated scintillators for imaging of bone microarchitecture on diagnostic Cone Beam CT (CBCT) systems to provide improved quantitative metrics for diagnosis of osteoporosis and osteoarthritis.
The scintillator design includes reflective coatings applied to the laser-cut grooves to improve optical isolation between pixels. Such coatings are created by atomic layer deposition (ALD), a unique approach, which permits formation of reflectors over inter-pixel grooves with aspect ratios as high as 140:1. Here we present initial results quantifying performance gains in CMOS detector resolution and their impact on the quality of bone microstructure segmentation. We demonstrate 77% gain in spatial resolution at 2 lp/mm and extension of the limiting resolution from 3 lp/mm to 4.5 lp/mm for the CMOS detector with a pixelated screen compared to a commercial sensor. In a bench-top CBCT study emulating diagnostic systems for orthopedic applications (extremity CBCT), we achieved >0.75 correlations in metrics of trabecular microarchitecture between pixelated CsI:Tl based CBCT and gold-standard micro-CT. The pixelated scintillator is expected to have significant impact for many other applications including mammography and digital radiography, where resolution and dose efficiency (DQE) of the detector are of critical importance.
Thallium-doped cesium iodide (CsI:Tl) single crystal is a well-known scintillator that has found many applications in nuclear science, radiography, and active interrogation of cargo in transit. It has relatively high density, high light yield, light emission matched with photodetectors and is less hygroscopic than sodium iodide. On the other hand, it has been hampered by a persistent afterglow, attributed to thermal ionization of trapped electrons (Tl0) followed by radiative recombination with trapped holes, which causes pulse pile-up in high count- rate applications. However, codoping by an appropriate modifier ion, especially divalent samarium and europium, has been found to be quite successful in suppressing this afterglow. But this effect had not yet been demonstrated in the crystal sizes and excitation energies relevant to real-time scanning of cargo. It is the purpose of this work to address this issue.
In this work, codoping of CsI single crystal with Sm2+ or Eu2+ was carried out using the vertical Bridgman technique. Large diameter CsI crystals, in the range of 1 to 3 inches, were grown for application in sentry portal detector. The crystals were cut in the form of pillars suitable to use in the detector module and tested for after-glow. At 2 ms after excitation cut-off, the codoped CsI:Tl crystal pillars showed afterglow on the order of 0.5-0.8 % compared to 2% for CsI doped with Tl alone. In scaling up the crystal growth process for larger diameter, it was observed that Eu2+-codoped crystals had much better reproducibility than those codoped with Sm2+.
High frame-rate imaging is a valuable tool for non-destructive evaluation (NDE) as well as for ballistic impact studies (terminal ballistics), in-flight projectile imaging, studies of exploding ordnance and characterization of other high-speed phenomena. Current imaging systems exist for these studies, however, none have the ability to do in-barrel characterization (in-bore ballistics) to image kinetics of the moving projectile BEFORE it exits the barrel.
The system uses an intensified high-speed CMOS camera coupled to a specially designed scintillator to serve as the X-ray detector. The X-ray source is a sequentially fired portable pulsed unit synchronized with the detector integration window and is able to acquire 3,600 frames per second (fps) with mega-pixel spatial resolution and up to 500,000 fps with reduced pixel resolution. This paper will discuss our results imaging .30 caliber bullets traveling at ~1,000 m/s while still in the barrel. Information on bullet deformation, pitch, yaw and integrity are the main goals of this experimentation. Planned future upgrades for imaging large caliber projectiles will also be discussed.
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.
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.
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
The europium-doped lutetium oxide (Lu2O3:Eu) transparent optical ceramic has excellent scintillation properties, namely
very high density (9.5 g/cm3), high effective atomic number (67.3), light output comparable to thallium-doped cesium
iodide (CsI:Tl), and emission wavelength (610 nm) for which silicon-based detectors have a very high quantum efficiency.
If microcolumnar films of this material could be fabricated, it would find widespread use in a multitude of highspeed
imaging applications. However, the high melting point of over 2400°C makes it extremely challenging to make
microcolumnar films of this material. We have recently fabricated and characterized microcolumnar films of Lu2O3:Eu.
These results are presented in this paper.
While a wide variety of new scintillators are now available, new cerium-doped lanthanide halide scintillators have shown
a strong potential to move beyond their familiar role in conventional gamma ray spectroscopy, toward fulfilling the
needs of highly demanding applications such as radioisotope identification at room temperature, homeland security, and
quantitative molecular imaging for medical diagnostics, staging and research. Despite their extraordinary advantages,
however, issues related to reliable, large volume manufacturing of these high light yield materials in a rapid and
economic manner have not been resolved or purposefully addressed. Also, if microcolumnar films of this material could
be fabricated, it would find widespread use in a multitude of high-speed imaging/nuclear medicine applications. Here
we report on synthesizing LaBr3:Ce scintillators using a thermal evaporation technique, which permits the fabrication of
high spatial resolution microcolumnar films and holds a potential to synthesize large volumes of high quality material in
a time efficient and cost effective manner. Performance evaluation of the fabricated films and their application for
SPECT imaging are also discussed.
The limitations of current CCD-based microCT X-ray imaging systems arise from two important factors. First, readout
speeds are curtailed in order to minimize system read noise, which increases significantly with increasing readout rates.
Second, the afterglow associated with commercial scintillator films can introduce image lag, leading to substantial
artifacts in reconstructed images, especially when the detector is operated at several hundred frames/second (fps). For
high speed imaging systems, high-speed readout electronics and fast scintillator films are required. This paper presents
an approach to developing a high-speed CT detector based on a novel, back-thinned electron-multiplying CCD
(EMCCD) coupled to various bright, high resolution, low afterglow films. The EMCCD camera, when operated in its
binned mode, is capable of acquiring data at up to 300 fps with reduced imaging area. CsI:Tl,Eu and ZnSe:Te films,
recently fabricated at RMD, apart from being bright, showed very good afterglow properties, favorable for high-speed
imaging. Since ZnSe:Te films were brighter than CsI:Tl,Eu films, for preliminary experiments a ZnSe:Te film was
coupled to an EMCCD camera at UC Davis Medical Center. A high-throughput tungsten anode X-ray generator was
used, as the X-ray fluence from a mini- or micro-focus source would be insufficient to achieve high-speed imaging. A
euthanized mouse held in a glass tube was rotated 360 degrees in less than 3 seconds, while radiographic images were
recorded at various readout rates (up to 300 fps); images were reconstructed using a conventional Feldkamp cone-beam
reconstruction algorithm. We have found that this system allows volumetric CT imaging of small animals in
approximately two seconds at ~110 to 190 μm resolution, compared to several minutes at 160 μm resolution needed for
the best current systems.
The performance measurement of hypervelocity projectiles in flight is critical in ensuring proper projectile operation, for
designing new long-range missile systems with improved accuracy, and for assessing damage to the target upon impact
to determine the projectile's lethality. We are developing a modular, low cost, digital X-ray imaging system to measure
hypervelocity projectile parameters with high precision and to almost instantaneously map its trajectory in 3D space to
compute its pitch, yaw, displacement from its path, and velocity. The preliminary data suggest that this system can
render an accuracy of 0.25° in measuring pitch and yaw, an accuracy of 0.03" in estimating displacement from the
centerline, and a precision of ±0.0001% in measuring velocity, which is well beyond the capability of any existing
Despite its obvious advantages, well known CsI:Tl scintillator has two characteristic properties that undermine its use in
clinical and high speed imaging: the presence of an afterglow component in its scintillation decay, and a hysteresis effect
that causes drift in the scintillation yield after exposure to high radiation doses. We have previously reported that the
addition of a second dopant, Sm2+, to the CsI:Tl crystals, significantly suppresses both afterglow and hysteresis. Here we
report on the fabrication and characterization of the Sm co-doped CsI:Tl microcolumnar films to examine if these
properties are preserved in films as well. Our preliminary data suggests that the Sm co-doped CsI:Tl films significantly
improve temporal response relative to their CsI:Tl counterpart, and that the newly developed films demonstrate excellent
spatial resolution. Various aspects of these effects and their consequences for imaging performance are discussed in this
While a wide variety of new scintillators are now available, CsI:Tl remains a highly desired material for medical and
industrial applications due to its excellent properties, low cost, and easy availability. Despite its advantages, however,
its use in high-speed imaging applications has been hindered by an undesirably high afterglow component in its scintillation
decay. To address this specific issue and to make the material suitable for applications such as volumetric CT and
high-speed radiography, we have discovered that codoping the material with certain dipositive rare earth ions is particularly effective for such afterglow suppression. We have extensively studied the manner in which one such ion, Eu2+,
alters the spectroscopic and kinetic properties of the scintillation, and have developed a coherent mathematical model
consistent with the experimental results.
Unfortunately, the beneficial effect of Eu2+ appears to be restricted only to relatively short times (say ≤200 ms) after
the end of the excitation pulse. At longer times, the carriers whose diversion into deep traps is responsible for suppression
of the short-term afterglow begin to escape those traps, resulting in enhancement of the low-level persistence on a
time scale of seconds or minutes. What is needed is to provide some nonradiative means to annihilate the trapped carriers
before their escape can enhance the low-level long-term emission. And, as predicted by the mathematical model,
this is exactly what Sm2+ does.
In this paper we compare the respective effects of the two additives on the afterglow and hysteresis characteristics of
the host CsI:Tl material system. We find that while Eu begins to exert its afterglow-suppressive effect sooner after termination
of excitation, the influence of Sm lasts much longer. Moreover, the suppressive effect of the latter is always
found, regardless of the conditions of excitation, and becomes more profound the greater the duration of the exciting
pulse. Various aspects of these effects and some their consequences for imaging performance are also discussed.
The development of new small animal imaging techniques such as high-speed microCT and low-dose microCT often
requires investigating optimal detector parameters and imaging techniques. This paper presents an approach to develop
a low-dose microCT detector based on a novel, back-thinned, back-illuminated electron multiplying CCD (EMCCD)
coupled to a high performance microcolumnar CsI scintillator via a fiberoptic taper. Our goal is to achieve high
DQE(0) for X-ray energies typically used in small animal imaging (40 to 80 kVp), providing high quality imaging at
substantially reduced dose.
Towards achieving this goal we have developed a novel EMCCD camera fitted with a fiberoptic window. To
enhance the imaging area we fabricated additional fiberoptic tapers measuring 3:1 and 6:1 in demagnification ratio,
mechanically coupled to the EMCCD.
The high sensitivity and internal gain of the EMCCD is further exploited in our system design by the use of a
thick CsI screen. These screens not only provide higher absorption for 40 to 80 kVp X-rays, but even at ~200 &mgr;m
thickness maintain a high resolution of up to 11 lp/mm.
This paper outlines the quantitative performance of each detector component and the detector as a whole.
While the detector demonstrated the potential for achieving the targeted DQE performance, it also showed that
mechanical coupling of the tapers to the CCD results in unacceptable light loss, and that direct CCD-to-taper bonding
and using new versions of large-area EMCCD chips would be better options.
Columnar CsI(Tl) screens are now routinely used in indirect digital x ray imaging detectors. The CsI(Tl) scintillator provides high density, high atomic number, and high scintillation efficiency. These properties, coupled with the fact that CsI(Tl) can be grown in columnar form, provide excellent spatial resolution, high x-ray absorption, and low noise resulting in detectors with high overall detective quantum efficiency (DQE(f)). While such screens are now commercially available, developments leading to further improvements in scintillator performance are ongoing at RMD. Here we report on the recent progress in developing very thin (10 μm) to very thick (~3 mm) columnar screens and discuss their application potential in digital radiology and nuclear medicine.
We report on recent advances in the development of powdered Lu2O3:Eu scintillator screens. The Lu2O3:Eu scintillator has excellent intrinsic material properties including high density (9.5 g/cm3), high average atomic number (63), and a peak emission of 610 nm. We have characterized the performance of screens fabricated from this material in comparison with commercial Gd2O2S:Tb screens. This paper presents resolution in terms of MTF(f), noise properties in terms of NPS(f), and overall NEQ(f), obtained by coupling the screens to a CCD detector. We have included sample radiographic images that demonstrate the ability to produce high quality images. These screens are currently under development, and further improvements in performance are expected with optimization of the scintillator and fabrication methods.
We report on a new x-ray converter screen based on the powdered Lu2O3:Eu scintillator. Lu2O3:Eu offers high density (9.4 g/cm3), high average atomic number (63), and a peak emission of 610 nm. The high density of the material and a high packing fraction of the coating provide higher x-ray absorption efficiency, even with thin screens. As a result Lu2O3:Eu screens are expected to provide superior spatial resolution and x-ray stopping power compared to commercial powdered screens. This newly developed screen has excellent imaging performance and offers several practical advantages such as ease of fabrication, low cost, and durability. This paper will discuss preliminary results of the imaging performance of this novel screen.
Columnar CsI(Tl) screens are now routinely used for digital x-ray imaging in a wide variety of applications such as mammography, dental radiography, and non-destructive testing. While commercially available CsI(Tl) screens exhibit excellent properties, it is possible to significantly improve their performance. Here, we report on a new design of a columnar CsI(Tl) screen. Specifically, columnar CsI(Tl) screens were subjected to mechanical pixelation for minimizing the long range spread of scintillation light within the film, thus enhancing spatial and contrast resolution, and increasing the detective quantum efficiency (DQE(f)) of the digital imaging detector. To date we have fabricated up to 200 μm thick pixelated CsI(Tl) screens for mammography, and characterized their performance using a CCD camera. This paper presents a comparison of the new pixelated CsI(Tl) screens, conventional columnar CsI(Tl) screens, and Gd2O2S(Tb) screens. The data show that pixelated screens substantially improve the DQE(f) of the digital imaging system.
Adhesive bonded composites used in naval, aerospace, and automotive technologies require routine nondestructive testing (NDT) to detect flaws and other integrity-reducing anomalies such as porosity, kissing disbonds, and delaminations. We have developed an x-ray radiography/CT system with fast scanning times based on high resolution, high efficiency scintillators coupled to a 1024 x 1024 pixel CCD via a fiberoptic taper. Typical CT systems for NDT use a fan beam x-ray source and a linear array of detectors, with scan times on the order of 10 hours depending on the desired resolution. The prototype CCD-based volumetric imaging system described here is capable of reducing this scan time to less than 1 hour while significantly improving resolution. Additionally, the system is capable of both CT and standard radiographic imaging. We have integrated two different scintillators in the prototype system. One is a structured CsI(Tl) screen, and the other is a new, pixelated, transparent optical ceramic (TOC) scintillator. This unique TOC has a density of 9.5 g/cm3 and a peak emission of 610 nm, particularly suitable for Si readouts. We present here the system design and preliminary results of radiographic imaging and volumetric CT reconstruction.
At RMD we have fabricated structured CsI(Tl) screens tailored for macromolecular x-ray crystallography applications. Diffraction patterns typically consist of several closely spaced Bragg peaks of varying sizes and intensities, and the detection of such features requires screens with high light output, high resolution, and excellent x-ray absorption. Properties of these screens, for example, light output or spatial resolution, were tailored by post deposition treatments to suit the specific needs of the application. Specifically, we have produced up to 45 micrometers thick CsI(Tl) screens with excellent resolution over the spatial frequency range of 0 to 20 lp/mm and very low noise. Imaging characteristics of these screens along with the commercial Gd2O2S (GOS) have been measured using a CCD detector with a fiberoptic taper. Performance of these screens in terms of point spread function (PSF(f)), light output, noise power spectrum (NPS(f)), and the modulation transfer function (MTF(f)) was measured. It is observed that the intrinsic properties of the structured CsI(Tl) screens are heavily influenced by the substrate on which the films are deposited and on the post deposition coatings, thus providing a latitude for modifying the screen properties to match the needs of the application.