Structured CsI(Tl) scintillators for X-ray imaging applications

Nagarkar, Vivek V.; Gupta, T. K.; Miller, Stuart; Klugarman, Y.; Squillante, M.R.; Entine, G.

doi:10.1109/23.682433

Cited by 261 publications

(149 citation statements)

References 9 publications

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“…Initially, this result would seem to have no practical impact, since back-screens are not currently used clinically due to the glass substrate of the thin-film transistor (TFT) array for digital signal readout. 25,27 If these detectors were operated as back-screens, the high attenuation of glass ($0.7 mm thick) would prevent a large percentage of x-rays from reaching the phosphor itself, 19 and hence both QDE and DQE would be compromised. Based on new research on flexible organic light-emitting diode (OLED) displays, however, TFT arrays may soon be manufactured on a plastic substrate.…”

Section: Discussionmentioning

confidence: 99%

“…47 Another extension of this work would be to model detector pixilation due to the TFT array in which the phosphor is placed in optical contact. [25][26][27] In this setting, the PSF is the convolution of the phosphor blurring function with the detector element sampling function. [48][49][50] An additional subtlety that may be modeled is the blurring of the focal spot.…”

Section: Discussionmentioning

confidence: 99%

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Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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Purpose: In digital breast tomosynthesis (DBT), a volumetric reconstruction of the breast is generated from a limited range of x-ray projections. One trade-off of DBT is resolution loss in the projections due to non-normal (i.e., oblique) x-ray incidence. Although degradation in image quality due to oblique incidence has been studied using empirical data and Monte Carlo simulations, a theoretical treatment has been lacking. The purpose of this work is to extend Swank's calculations of the transfer functions of turbid granular phosphors to oblique incidence. The model is ultimately used as a tool for optimizing the design of DBT detectors. Methods: A quantum-limited system and 20 keV x-rays are considered. Under these assumptions, the modulation transfer function (MTF) and noise power spectra (NPS) are derived using the diffusion approximation to the Boltzmann equation to model optical scatter within the phosphor. This approach is applicable to a nonstructured scintillator such as gadolinium oxysulfide doped with terbium (Gd 2 O 2 S:Tb), which is commonly used in breast imaging and which can reasonably approximate other detector materials. The detective quantum efficiency (DQE) is then determined from the Nishikawa formulation, where it is written as the product of the x-ray quantum detection efficiency, the Swank factor, and the Lubberts fraction. Transfer functions are calculated for both front-and back-screen configurations, which differ by positioning the photocathode at the exit or entrance point of the x-ray beam, respectively. Results: In the front-screen configuration, MTF and DQE are found to have considerable angular dependence, while NPS is shown to vary minimally with projection angle. As expected, the high frequency MTF and DQE are degraded substantially at large angles. By contrast, all transfer functions for the back-screen configuration have the advantage of significantly less angular dependence. Using these models, we investigated the possibility for optimizing the design of DBT detectors. As an example optimization strategy, the phosphor thickness which maximizes the DQE at a fixed frequency is analyzed. This work demonstrates that the optimal phosphor thickness for the frontscreen is angularly dependent, shifting to lower thickness at higher angles. Conversely, the backscreen is not optimized by a single thickness but instead attains reasonably high DQE values over a large range of thicknesses. Although the back-screen configuration is not suited for current detectors using a glass substrate, it may prove to be preferred in future detectors using newly proposed plastic thin-film transistor (TFT) substrates. Conclusions: Using the diffusion approximation to the Boltzmann equation to model the spread of light in a scintillator, this paper develops an analytical model of MTF, NPS, and DQE for a phosphor irradiated obliquely. The model is set apart from other studies on oblique incidence in being derived from first principles. This work has applications in the optimization of DBT detector design.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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“…The crystal itself has a complex columnar structure that directs the optical photons down the thickness of the crystal and improves the detector resolution as compared with unstructured crystals. [1][2][3] This columnar structure also affects the point response function ͑PRF͒ of the detector in a complex manner that is difficult to model. The PRF fully describes the blurring behavior of the scintillator and is simply the image that results from x rays that are incident on the detector at a single, infinitesimally small point.…”

Section: Introductionmentioning

confidence: 99%

Experimental validation of Monte Carlo (MANTIS) simulated x‐ray response of columnar CsI scintillator screens

Freed

Miller²,

Tang

et al. 2009

Medical Physics

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Purpose: MANTIS is a Monte Carlo code developed for the detailed simulation of columnar CsI scintillator screens in x-ray imaging systems. Validation of this code is needed to provide a reliable and valuable tool for system optimization and accurate reconstructions for a variety of x-ray applications. Whereas previous validation efforts have focused on matching of summary statistics, in this work the authors examine the complete point response function ͑PRF͒ of the detector system in addition to relative light output values. Methods: Relative light output values and high-resolution PRFs have been experimentally measured with a custom setup. A corresponding set of simulated light output values and PRFs have also been produced, where detailed knowledge of the experimental setup and CsI:Tl screen structures are accounted for in the simulations. Four different screens were investigated with different thicknesses, column tilt angles, and substrate types. A quantitative comparison between the experimental and simulated PRFs was performed for four different incidence angles ͑0°, 15°, 30°, and 45°͒ and two different x-ray spectra ͑40 and 70 kVp͒. The figure of merit ͑FOM͒ used measures the normalized differences between the simulated and experimental data averaged over a region of interest. Results: Experimental relative light output values ranged from 1.456 to 1.650 and were in approximate agreement for aluminum substrates, but poor agreement for graphite substrates. The FOMs for all screen types, incidence angles, and energies ranged from 0.1929 to 0.4775. To put these FOMs in context, the same FOM was computed for 2D symmetric Gaussians fit to the same experimental data. These FOMs ranged from 0.2068 to 0.8029. Our analysis demonstrates that MANTIS reproduces experimental PRFs with higher accuracy than a symmetric 2D Gaussian fit to the experimental data in the majority of cases. Examination of the spatial distribution of differences between the PRFs shows that the main reason for errors between MANTIS and the experimental data is that MANTIS-generated PRFs are sharper than the experimental PRFs. Conclusions: The experimental validation of MANTIS performed in this study demonstrates that MANTIS is able to reliably predict experimental PRFs, especially for thinner screens, and can reproduce the highly asymmetric shape seen in the experimental data. As a result, optimizations and reconstructions carried out using MANTIS should yield results indicative of actual detector performance. Better characterization of screen properties is necessary to reconcile the simulated light output values with experimental data.

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“…In this specific example the recorded data showed that the initial alignment (red squares/purple dotted line, 0.94 mm [18 ] radial precession) did not meet the requirement (0.52 mm [10 ]). After additional adjustments were made, the PSF measurements at different roll positions were repeated (green circles/dashed line, 0.42 mm [8 ] radial precession) and used to verify that the precession tolerance met specifications. The two data sets show that after the re-adjustment, the alignment was improved and fully met the NuSTAR calibration requirements † .…”

Section: Optics Installation and Alignmentmentioning

confidence: 99%

“…Right:Cross-sectional SEM view of microcolumnar CsI(Tl) film which was deposited on a suitable substrate. 8 The column diameter is about 10 µm on average.…”

Section: Introductionmentioning

confidence: 99%

Application of an EMCCD camera for calibration of hard X-ray telescopes

Vogel

Pivovaroff²,

Nagarkar

et al. 2012

SPIE Proceedings

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Recent technological innovations make it feasible to construct efficient hard x-ray telescopes for space-based astronomical missions. Focusing optics are capable of improving the sensitivity in the energy range above 10 keV by orders of magnitude compared to previously used instruments. The last decade has seen focusing optics developed for balloon experiments and they are implemented in approved space missions such as the Nuclear Spectroscopic Telescope Array (NuSTAR). The full characterization of x-ray optics for astrophysical missions, including measurement of the point spread function (PSF) as well as scattering and reflectivity properties of substrate coatings, requires a large area detector with very high spatial resolution and sensitivity, photon counting and energy discriminating capability. Novel back-thinned Electron Multiplying Charge-Coupled Devices (EMCCDs) are suitable detectors for ground-based calibrations if combined with a scintillating material. This optical coupling of the EMCCD chip to a microcolumnar CsI(Tl) scintillator can be achieved via a fiberoptic taper. Not only does this detector system exhibit low noise and high spatial resolution inherent to CCDs, but the EMCCD is also able to handle high frame rates. Additionally, thick CsI(Tl) yields high detection efficiency for x-rays. In this paper, we discuss the advantages of using an EMCCD to calibrate hard x-ray optics. We will illustrate the promising features of this detector solution using examples of data obtained during the ground calibration of the NuSTAR telescopes performed at Columbia University during 2010/2011. Finally, we give an outlook on latest development and optimizations.

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Structured CsI(Tl) scintillators for X-ray imaging applications

Cited by 261 publications

References 9 publications

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Experimental validation of Monte Carlo (MANTIS) simulated x‐ray response of columnar CsI scintillator screens

Application of an EMCCD camera for calibration of hard X-ray telescopes

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