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.
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
We report on developments of an intraoperative probe, capable of functioning in real time with high spatial resolution
and high sensitivity. This probe combines two novel technologies and is based on an electron multiplying charge
coupled device (EMCCD) bonded to a high spatial resolution microcolumnar CsI(Tl) scintillator via a flexible fiberoptic
cable. Our data demonstrates that the probe can be used with such beta-emitting radiolabels as 18F, 131I, and 32P. The
basic design of the probe and its evaluation using standard clinical phantoms is presented. In addition, the operational
data obtained on swine models is included to demonstrate the probe's efficacy in practical procedures.
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.
Although conventional mammography is currently the best modality to detect early breast cancer, it is limited in that the recorded image represents the superposition of a 3D object onto a 2D plane. As an alternative, cone-beam CT breast imaging with a CsI based flat-panel imager (CTBI) has been proposed with the ability to provide 3D visualization of breast tissue. To investigate possible improvements in lesion detection accuracy using CTBI over digital mammography (DM), a computer simulation study was conducted using simulated lesions embedded into a structured 3D breast model. The computer simulation realistically modeled x-ray transport through a breast model, as well as the signal and noise propagation through the flat-panel imager. Polyenergetic x-ray spectra of W/Al 50 kVp for CTBI and Mo/Mo 28 kVp for DM were modeled. For the CTBI simulation, the intensity of the x-ray spectra for each projection view was determined so as to provide a total mean glandular dose (MGD) of 4 mGy, which is approximately equivalent to that given in a conventional two-view screening mammography study. Since only one DM view was investigated here, the intensity of the DM x-ray spectra was defined to give 2 mGy MGD. Irregular lesions were simulated by using a stochastic growth algorithm providing lesions with an effective diameter of 5 mm. Breast tissue was simulated by generating an ensemble of backgrounds with a power law spectrum. To evaluate lesion detection accuracy, a receiver operating characteristic (ROC) study was performed with 4 observers reading an ensemble of images for each case. The average area under the ROC curves (Az) was 0.94 for CTBI, and 0.81 for DM. Results indicate that a 5 mm lesion embedded in a structured breast phantom can be detected by CT breast imaging with statistically significant higher confidence than with digital mammography.
In recent years, there has been interest in exploring the feasibility of CT breast imaging using flat-panel digital detectors in a truncated cone-beam geometry. Preliminary results are promising and it appears as if 3D tomographic imaging of the breast has great potential for reducing the masking effect of superimposed parenchymal structure typically observed with conventional mammography. In this study, a mathematical framework used for determining optimal design and acquisition parameters for such a CT breast imaging system is described. The ideal observer SNR is used as a figure-of-merit, under the assumptions that the imaging system is linear and shift-invariant. Computation of the ideal observer SNR used a parallel-cascade model to predict signal and noise propagation through the detector, as well as a realistic model of the lesion detection task in breast imaging. For all optimizations discussed here, the total mean glandular dose for a CT breast imaging study is constrained to be approximately equivalent to that of a two-view conventional mammography study. The framework presented is used to explore the affect of the specific task on the optimal exposure technique of flat-panel CT breast imaging. In particular, it is observed that modeling the normal mammographic structure in the projection images can sometimes impact the optimal kVp settings.
The development of new digital mammography techniques such as dual-energy imaging, tomosynthesis and CT mammography often requires investigating optimal camera design parameters and imaging techniques. One tool that is useful for this purpose is Monte Carlo simulation. This paper presents a methodology for generating simulated images from a CsI-based, flat-panel imager by using the Geant 3 Monte Carlo code to model x-ray transport and absorption within the CsI scintillator, and the DETECT-II code to track optical photon spread within a columnar model of the CsI scintillator. The Monte Carlo modeling of x-ray transport and absorption within the CsI was validated by comparing to previously published values for the probability of a K-shell interaction, the fluorescent yield, the probability of a K-fluorescent emission, and the escape fraction describing the probability of a K x-ray escaping the scintillator. To validate the combined (Geant coupled with DETECT-II) Monte Carlo approach to form simulated images, comparison of modulation transfer functions (MTFs) and system sensitivity (electrons/mR/pixel) obtained from simulations were compared to empirical measurements obtained with different x-ray spectra and imagers with varying CsI thicknesses. By varying the absorption and reflective properties of the columnar CsI used in the DETECT-II code, good agreement between simulated MTFs and system sensitivity and empirically measured values were observed.