Recent advances in the domain of energy-resolved semiconductor detectors stimulate research in X-ray computed
tomography (CT). However, the imperfections of these detectors induce errors that should be considered for further
applications. Charge sharing and pile-up effects due to high photon fluxes can degrade image quality or yield wrong
material identification. Basis component decomposition provides separate images of principal components, based on the
energy related information acquired in each energy bin. The object is typically either decomposed in photoelectric and
Compton physical effects or in basis materials functions.
This work presents a simulation study taking into account the properties of an energy-resolved CdTe detector with
flexible energy thresholds in the context of materials decomposition CT. We consider the effects of a first order pile-up
model with triangular pulses of a non-paralyzable detector and a realistic response matrix. We address the problem of
quantifying mineral content in bone based on a polynomial approach for material decomposition in the case of two and
three energy bins. The basis component line integrals are parameterized directly in the projection domain and a
conventional filtered back-projection reconstruction is performed to obtain the material component images. We use
figures of merit such as noise and bias to select the optimal thresholds and quantify the mineral content in bone. The
results obtained with an energy resolved detector for two and three energy bins are compared with the ones obtained for
the dual-kVp technique using an integrating-mode detector with filters and voltages optimized for bone densitometry.
Cone-Beam Computed Tomography (CBCT) enables three-dimensional imaging with isotropic resolution. X-ray scatter estimation is a big challenge for quantitative CBCT imaging: even in the presence of anti-scatter grid, the scatter level is significantly higher on cone beam systems compared to collimated fan beam systems. The effects of this scattered radiation include cupping artifacts, streaks, and quantification inaccuracies. In this paper, a scatter management process for tomographic projections, without supplementary on-line acquisition, is presented. The scattered radiation is corrected using a method based on scatter calibration through off-line acquisitions. This is combined with on-line analytical transformation based on physical equations, to perform an estimation adapted to the object observed. This approach has been previously applied to a system without anti-scatter grid. The focus of this paper is to show how to combine this approach with an anti-scatter grid. First, the interest of the grid is evaluated in terms of noise to signal ratio and scatter rejection. Then, the method of scatter correction is evaluated by testing it on an anthropomorphic phantom of thorax. The reconstructed volume of the phantom is compared to that obtained with a strongly collimated conventional multi-slice CT scanner. The new method provides results that closely agree with the conventional CT scanner, eliminating cupping artifacts and significantly improving quantification.
In order to obtain accurate quantitative results, flat panel detectors require specific calibration and correction of acquisitions. Main artefacts are due to bad pixels, variations of photodiodes characteristics and inhomogeneity of X-rays sensitivity of the scintillator layer. Other limitations for quantification are the non-linearity of the detector due to charge trapping in the transistors and the scattering generated inside the detector, called detector scattering. Based on physical models of artefacts generation, this paper presents an unified framework for the calibration and correction of these artefacts. The following specific algorithms have been developed to correct them. A new method for correction of deviation to linearity is based on the comparison between experimental and simulated data. A method of detector scattering correction is performed in two steps: off-line characterization of detector scattering by considering its spatial distribution through a convolution model and on-line correction based on a deconvolution approach. Radiographic results on an anthropomorphic thorax phantom imaged with a flat panel detector, that convert X-rays into visible light using scintillator coupled to an amorphous silicon transistor frame for photons to electrons conversion, demonstrate that experimental X-rays attenuation images are significantly improved qualitatively and quantitatively by applying non-linearity correction and detector scattering correction. Results obtained on tomographic reconstructions from pre-processed acquisitions of the phantom are in very good agreement with expected attenuation coefficients values obtained with a multi-slice CT scanner. Thus, this paper demonstrates the efficiency of the proposed pre-processings to perform accurate quantification on radiographies and tomographies.
Cone beam Computed Tomography (CBCT) enables three-dimensional imaging with isotropic resolution. X-rays scatter estimation is a big challenge for quantitative CBCT imaging of thorax: scatter level is significantly higher on cone beam systems compared to collimated fan beam systems. The effects of this scattered radiation are cupping artifacts, streaks, and quantification inaccuracies. In this paper, an original scatter management process on tomographic projections without supplementary on-line acquisitions is presented. The correction method is based on scatter calibration through off-line acquisitions, combined to an on-line analytical transformation issued from physical equations to adapt calibration to the observed object. Evaluations of the method were performed on an anthropomorphic thorax phantom. First, tomographic acquisitions were performed with a flat panel detector. Reconstructed volume obtained with the proposed scatter correction method has been compared with the one obtained through a classical beam stops method. Secondly, reconstructed volume has been compared with the one obtained through a fan beam system (Philips multi slice CT scanner). The new method provided results in good agreement with the beam stops approach and with the multi slice CT scanner, suppressing cupping artifacts and improving quantification significantly. Compared to the beam stops method, lower X-rays doses (divided by a factor 9) and shorter acquisition times were needed.
KEYWORDS: Monte Carlo methods, Sensors, Solid modeling, Computer aided design, Computer simulations, 3D modeling, Optical simulations, Data modeling, Lead, Photon transport
The use of focused anti-scatter grids on digital radiographic systems with two-dimensional detectors produces acquisitions with a decreased scatter to primary ratio and thus improved contrast and resolution. Simulation software is of great interest in optimizing grid configuration according to a specific application. Classical simulators are based on complete detailed geometric descriptions of the grid. They are accurate but very time consuming since they use Monte Carlo code to simulate scatter within the high-frequency geometric description of the grid. We propose a new practical method which couples an analytical simulation of the grid interaction with a radiographic system simulation program. First, a two dimensional matrix of probability depending on the grid is created offline, in which the first dimension represents the angle of impact with respect to the normal to the grid lines and the other the energy of the photon. This matrix of probability is then used by the Monte Carlo simulation software in order to provide the final x-rays scatter flux image. To evaluate the gain of CPU time, we define the increasing factor as the increase of CPU time of the simulation with as or without the grid. Increasing factors were calculated with the new model and with classical methods representing the grid with a Computed-Aided Designed (CAD) model. With this new method, increasing factors are shortened by three to four orders of magnitude.
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