The authors introduce an algorithm to estimate the spatial dose distributions in computed tomography (CT)
images. The algorithm calculates dose distributions due to the primary and scattered photons separately. The
algorithm only requires the CT data set that includes the patient CT images and the scanner acquisition parameters.
Otherwise the scanner acquisition parameters are extracted from the CT images. Using the developed
algorithm, the dose distributions for head and chest phantoms are computed and the results show the excellent
agreements with the dose distributions obtained using a commercial Monte Carlo code. The developed algorithm
can be applied to a patient-specific CT dose estimation based on the CT data.
The scatter effect on detective quantum efficiency (DQE) of digital mammography is investigated using the
cascaded-systems model. The cascaded-systems model includes a scatter-reduction device as a binomial selection
stage. Quantum-noise-limited operation approximates the system DQE into the multiplication form of the
scatter-reduction device DQE and the conventional detector DQE. The developed DQE model is validated in
comparisons with the measured results using a CMOS flat-panel detector under scatter environments. For various
scatter-reduction devices, the slot-scan method shows the best scatter-cleanup performance in terms of DQE,
and the scatter-cleanup performance of the conventional one-dimensional grid is rather worse than the air gap.
The developed model can also be applied to general radiography and will be very useful for a better design of
We have developed a novel sandwich-style single-shot (single-kV) detector by stacking two indirect-conversion flat-panel detectors for preclinical mouse imaging. In the sandwich detector structure, extra noise due to the direct x-ray absorption in photodiode arrays is inevitable. We develop a simple cascaded linear-systems model to describe signal and noise propagation in the flat-panel sandwich detector considering direct x-ray interactions. The noise-power spectrum (NPS) and detective quantum efficiency (DQE) obtained from the front and rear detectors are analyzed by using the cascaded-systems model. The NPS induced by the absorption of direct x-ray photons that are unattenuated within the photodiode layers is white in the spatial-frequency domain like the additive readout noise characteristic; hence that is harmful to the DQE at higher spatial frequencies at which the number of secondary quanta lessens. The model developed in this study will be useful for determining the optimal imaging techniques with sandwich detectors and their optimal design.
We revisit the doubly-layered sandwich detector configuration for single-shot dual-energy x-ray imaging. In order to understand its proper operation, we investigated the contrast-to-noise performance in terms of the x-ray beam setup using the Monte Carlo methods. Using a pair of active photodiode arrays coupled to phosphor screens, we have built a sandwich detector. For better spectral separation between the projection images obtained from the front and rear detectors during a single x-ray exposure, we inserted a copper sheet between two detectors. We have successfully obtained soft tissue- and bone-enhanced images for a postmortem mouse with the developed sandwich detector using weighted logarithmic subtraction, and the image quality was comparable to those achieved by the conventional kVp-switching technique. Although some problems to be mitigated for the optimal and practical use, for example, the scatter effect and image registration, are still left, the performance of the sandwich detector for single-shot dual-energy x-ray imaging is promising. We expect that the active sandwich detector will provide motion-artifact-free dual-energy images with a reasonable image quality.
Single-photon counting (SPC) x-ray imaging has the potential to improve image quality and enable new advanced energy-dependent methods. Recently, cascaded systems analysis (CSA) has been extended to the description of the detective quantum efficiency (DQE) of SPC detectors. In this article we apply the new CSA approach to the description of the DQE of hypothetical direct-conversion selenium (Sc) and cadmium zinc telluride (CdZnTc) detectors including the effects of poly-energetic x-ray spectra, stochastic conversion of x-ray energy to electron hole (c-h) pairs, depth-dependent collection of e-h pairs using the Hecht relation, additive electronic noise, and thresholding. Comparisons arc made to an energy-integrating model. For this simple model, with the exception of thick (1- 10 mm) Sc-bascd convertors, we found that the SPC DQE was 5-20 %greater than that of the energy integrating model. This trend was tnw even when additive noise was included in the SPC model and excluded from the energy-integrating model. However, the DQE of SPC detectors with poor collection efficiency (such as thick (<1 mm) Sc detectors) and high levels of additive noise can be degraded by 40-90 % for all energies and x-ray spectra considered. vVhile photon-counting approaches arc not yet ready for routine diagnostic imaging, the available DQE is equal to or higher than that of conventional energy-integrating detectors under a wide range of x-ray energies and convertor thickness. However, like energy-integrating detectors, the DQE of SPC detectors will be degraded by the combination of poor collection efficiency and high levels of additive noise.
Image quality in diagnostic x-ray detectors is limited by statistical properties governing how, and where, x-ray
energy is deposited in a detector. This in turn depends on the physics of underlying x-ray interactions, and
the development of theoretical models of x-ray interaction physics is therefore a critical step in optimal detector
design and assessment. While cascaded-systems analyses are often used to describe image signal and noise in many systems, it has always been assumed there is only a single element (single Z) with which all x rays interact
even though most commonly used and promising candidates are compound materials. In addition, coherent and
incoherent scattering and their effects on image quality are usually ignored but may be important in some situa-
tion such as in low-Z atoms with high x-ray energies. We present a theoretical model of energy deposition within
a double-Z x-ray detector material that addresses the nature of energy absorption following photoelectric and
incoherent interactions and the effects of coherent scatter prior to energy deposition by photoelectric interactions.
A cascaded systems approach is used to describe the transfer of signal and noise in terms of the modulation
transfer function (MTF), Wiener noise power spectrum (NPS), and detective quantum efficiency (DQE). The
model is validated by comparing Monte Carlo simulation results with CsI and PbI2 double-Z materials. Excellent
agreement is obtained for each metric over the entire diagnostic energy range up to 10 cycles/mm. It is shown
that in all cases tested, a combination of two single-Z models weighted by the atomic density of each atom type
gives equivalent results to the more comprehensive double-Z model within a few percent. This result suggests
the simpler model is adequate and may be preferred for the optimal design of conventional radiography detectors and the estimation of x-ray imaging performance of novel photoconductor materials.
This paper describes the development of an active-pixel sensor (APS) panel, which has a field-of-view of 23.1×17.1 cm
and features 70-μm-sized pixels arranged in a 3300×2442 array format, for digital mammographic applications. The
APS panel was realized on 12-inch wafers based on the standard complementary metal-oxide-semiconductor (CMOS)
technology without physical tiling processes of several small-area sensor arrays. Electrical performance of the developed
panel is described in terms of dark current, full-well capacity and leakage current map. For mammographic imaging, the
optimized CsI:Tl scintillator is experimentally determined by being combined with the developed panel and analyzing im
aging characteristics, such as modulation-transfer function, noise-power spectrum, detective quantum efficiency, image l
ag, and contrast-detail analysis by using the CDMAM 3.4 phantom. With these results, we suggest that the developed
CMOS-based detector can be used for conventional and advanced digital mammographic applications.
We are developing pixel-structured scintillators for the eventual purpose of high-resolution and high-sensitivity x-ray
imaging applications. The pixel-structured scintillators were fabricated by filling Gd<sub>2</sub>O<sub>2</sub>S:Tb phosphor powder into the
silicon micro-well arrays by using a simple sedimentation method. The micro-well arrays having a depth of 180 μm were
fabricated by deep reactive ion etching of silicon wafers. To enhance the optical gain and the Swank noise factor, we
applied reflectance at the inside wall surfaces. Two different
inside-surface treatments were applied; 0.2-μm-thick
titanium which has 70% reflectance and 1-μm-thick silicon dioxide which was grown by thermal oxidation. The imaging
performance was evaluated in terms of modulation-transfer function (MTF), noise-power spectrum (NPS), and detective
quantum efficiency (DQE). Compared with the commercial phosphor screen as a reference, much enhanced MTF results
were measured. However, very low values of the system gain due to trapping of the generated optical photons at the wall
surfaces give rise to the poorer DQE performance rather than that of the reference detector. The theoretical cascaded
model analysis estimates much improved DQE performances with improved design parameters, such as higher
reflectance of 90% at the wall surfaces.
For a detector consisting of a phosphor screen and a photodiode array made by complementary metal-oxidesemiconductor
(CMOS) process, we have experimentally re-investigated the long-term stability of the signal and noise
characteristics as a function of the accumulated dose at the entrance surface of the detector in addition to the previous
study [IEEE Trans. Nucl. Sci. 56(3) 1121 (2009)]. The irradiation and analysis were more systematically performed. We
report the aging effect in image quality in terms of dark pixel signal, dynamic range, modulation-transfer function (MTF),
and noise-power spectrum (NPS). Unlike the previous study, the electronic noise was dominantly increased with the total
dose and the other statistical and structural noise sources were nearly independent on the cumulative dose. Similarly, the
increase of dark pixel signal and the related noise gradually reduces the dynamic range as the total dose increases. While
MTF was almost insensitive to the total dose, degradation in NPS was observed. Therefore, preprocessing without
properly updated offset and gain images would underestimate the detective quantum efficiency when performing quality
control of a detector in the field. Restoration of degraded dark signals due to aging is demonstrated by annealing the aged
detector with thermal activation energy. This study provides a motivation that the periodic monitoring of the imagequality
degradation is of great importance for the long-term and healthy use of digital x-ray imaging detectors.
Complementary metal-oxide-semiconductor (CMOS) active pixel sensors (APSs) with high electrical and optical
performances are now being attractive for digital radiography (DR) and dental cone-beam computed tomography
(CBCT). In this study, we report our prototype CMOS-based detectors capable of real-time imaging. The field-of-view
of the detector is 12 × 14.4 cm. The detector employs a CsI:Tl scintillator as an x-ray-to-light converter. The electrical
performance of the CMOS APS, such as readout noise and full-well capacity, was evaluated. The x-ray imaging
characteristics of the detector were evaluated in terms of characteristic curve, pre-sampling modulation transfer function,
noise power spectrum, detective quantum efficiency, and image lag. The overall performance of the detector is
demonstrated with phantom images obtained for DR and CBCT applications. The detailed development description and
measurement results are addressed. With the results, we suggest that the prototype CMOS-based detector has the
potential for CBCT and real-time x-ray imaging applications.
We investigated the potential use of CMOS (complementary
metal-oxide-semiconductor) imaging detectors with a pixel
pitch of 48 μm for mammography. Fundamental imaging characteristics were evaluated in terms of modulation-transfer
function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). The magnitudes of various
image noise sources, such as optical photons, direct x rays unattenuated and scattered x rays from the scintillator, and
additive electronic noise, were measured and analyzed. For the analysis of the measurement results, we applied a model
describing the signal and noise transfer based on the cascaded
linear-systems approach. The direct x-ray was very
harmful to the detector noise performance with white noise characteristics in the spatial frequency domain, and which
significantly degraded the spatial-frequency-dependent DQE at higher frequencies. Although the use of a fiber-optic
plate (FOP) reduces the detector sensitivity and the MTF performance, it enhances the DQE performance by preventing
the direct x-ray photons from the absorption within the photodiode array.
We exploit the development of a pixel-structured scintillator that would match the readout pixel array, such as a
photodiode array. This combination may become an indirect-conversion detector having high x-ray sensitivity without
sacrificing the inherent resolving power defined by the pixel geometry of the photodiodes, because the scintillation
material has a relatively high atomic number and density compared with the photoconductors, and the pixel-structured
design may provide a band-limited modulation-transfer function (MTF) characteristic even with a thicker scintillator. For
the realization of pixel-structured scintillators, two-dimensional (2D) array of pixel-structured wells with a depth of 100μm was prepared by using a deep reactive ion etching (DRIE) process on a silicon wafer. Then, Gd<sub>2</sub>O<sub>2</sub>S:Tb phosphor
powders with organic binders were filled within the well array by using a sedimentation method. Three different pixel
designs of 50, 100 and 200 μm with a wall (or septum) thickness of 10 μm were considered. Each sample size was 20 × 30 mm<sup>2</sup> considering intra-oral imaging. The samples were coupled to the CMOS photodiode array with a pixel pitch of 48 μm and the imaging performances were evaluated in terms of MTF, NPS (noise-power spectrum) and DQE (detective quantum efficiency) at intra-oral imaging conditions. From the measurement results, the sensitivities of the samples with 50, 100 and 200 μm pitch designs were about 12, 25 and 41% of that of the reference commercial phosphor screen (MinR-2000). Hence the DQE performances at 0.2 lp/mm were about 3.7, 9.6, 22.7% of the reference screen. According to the Monte Carlo simulations, the lower sensitivity was due to the loss of optical photons in silicon walls. However, the MTF performance was mainly determined by the designed pixel apertures. If we make pixel-structured scintillators with a deeper depth and provide reflectance on walls, much enhanced DQE performance is expected.