The radiation record of extraterrestrial rocks provides important insights into their thermal and radiation history. For meteorites this relates to their orbits, thermal history, terrestrial age, preatmospheric size and shape, and possibly cosmic ray exposure age. For meteorites from the Moon and Mars, the radiation record allows insights into transit times. For Martian surface samples, the radiation record enables estimates of their sedimentary age. Despite this, there is a growing tendency to artificially expose these samples to large radiation doses by the use of X‐ray computed tomography (
- Award ID(s):
- 1917069
- NSF-PAR ID:
- 10325111
- Date Published:
- Journal Name:
- Physics of Plasmas
- Volume:
- 29
- Issue:
- 5
- ISSN:
- 1070-664X
- Page Range / eLocation ID:
- 053506
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract CT ) imaging, often as part of their initial examination. In order to understand the effect of synchrotron microCT on meteorites, we placed samples of the Bruderheim L6 chondrite in theCT imaging port of the Advanced Photon Source at the Argonne National Laboratory, Argonne, Illinois. Monoenergetic X‐ray beams of 25 and 46 keV and a high flux broad spectrum beam were used. The synchrotronCT procedure exposed the samples to radiation doses significantly higher than the natural doses observed for meteorites (1670 to ~10,000 Gyr, compared to ~1000 Gyr for natural samples). It is clear thatCT imaging, whether using a laboratory system as in our previous report or using the synchrotron source, makes measurement of the natural radiation record of the samples impossible. Samples should not be placed in aCT scanner without due consideration of the loss of unique information for these valuable extraterrestrial samples. -
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Abstract Background Spectral CT material decomposition provides quantitative information but is challenged by the instability of the inversion into basis materials. We have previously proposed the constrained One‐Step Spectral CT Image Reconstruction (cOSSCIR) algorithm to stabilize the material decomposition inversion by directly estimating basis material images from spectral CT data. cOSSCIR was previously investigated on phantom data.
Purpose This study investigates the performance of cOSSCIR using head CT datasets acquired on a clinical photon‐counting CT (PCCT) prototype. This is the first investigation of cOSSCIR for large‐scale, anatomically complex, clinical PCCT data. The cOSSCIR decomposition is preceded by a spectrum estimation and nonlinear counts correction calibration step to address nonideal detector effects.
Methods Head CT data were acquired on an early prototype clinical PCCT system using an edge‐on silicon detector with eight energy bins. Calibration data of a step wedge phantom were also acquired and used to train a spectral model to account for the source spectrum and detector spectral response, and also to train a nonlinear counts correction model to account for pulse pileup effects. The cOSSCIR algorithm optimized the bone and adipose basis images directly from the photon counts data, while placing a grouped total variation (TV) constraint on the basis images. For comparison, basis images were also reconstructed by a two‐step projection‐domain approach of Maximum Likelihood Estimation (MLE) for decomposing basis sinograms, followed by filtered backprojection (MLE + FBP) or a TV minimization algorithm (MLE + TVmin) to reconstruct basis images. We hypothesize that the cOSSCIR approach will provide a more stable inversion into basis images compared to two‐step approaches. To investigate this hypothesis, the noise standard deviation in bone and soft‐tissue regions of interest (ROIs) in the reconstructed images were compared between cOSSCIR and the two‐step methods for a range of regularization constraint settings.
Results cOSSCIR reduced the noise standard deviation in the basis images by a factor of two to six compared to that of MLE + TVmin, when both algorithms were constrained to produce images with the same TV. The cOSSCIR images demonstrated qualitatively improved spatial resolution and depiction of fine anatomical detail. The MLE + TVminalgorithm resulted in lower noise standard deviation than cOSSCIR for the virtual monoenergetic images (VMIs) at higher energy levels and constraint settings, while the cOSSCIR VMIs resulted in lower noise standard deviation at lower energy levels and overall higher qualitative spatial resolution. There were no statistically significant differences in the mean values within the bone region of images reconstructed by the studied algorithms. There were statistically significant differences in the mean values within the soft‐tissue region of the reconstructed images, with cOSSCIR producing mean values closer to the expected values.
Conclusions The cOSSCIR algorithm, combined with our previously proposed spectral model estimation and nonlinear counts correction method, successfully estimated bone and adipose basis images from high resolution, large‐scale patient data from a clinical PCCT prototype. The cOSSCIR basis images were able to depict fine anatomical details with a factor of two to six reduction in noise standard deviation compared to that of the MLE + TVmintwo‐step approach.
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Purpose The purpose of this investigation is to revisit these conclusions in the context of photon beams from modern linacs.
Methods A metric estimating the relative intensity of charged particles emitted in the forward direction, , was proposed using cross‐sections for the photon interactions. The values were calculated for various absorbers using energy spectra of 6 and 10 MV photon beams from a Varian TrueBeam linac. Monte Carlo (MC) simulations were performed using TOPAS MC code to calculate the surface dose for various absorbers. Surface dose measurements were performed with 6 and 10 MV photon beams with tin and lead absorbers.
Results The values were found to decrease as a function of Z for both 6 and 10 MV photon beams indicating that the surface dose from electrons emitted in the forward direction consistently decreases with increasing Z. With the increasing Z of the absorbers, both experimental and MC‐calculated surface dose decreased without exhibiting a minimum at medium‐Z absorbers. The surface dose for lead and tin was determined to be within 1% of each other for both 6 and 10 MV photon beams using MC simulations and experimental measurements. Therefore, no statistically significant difference in surface dose was found between the tin and lead absorbers disproving the presence of any minima in the surface dose versus the Z curve as has been reported in the literature.
Conclusions Surface dose for modern photon beams can be reduced using both medium and high Z absorbers since a consistent decrease in surface dose was found with increasing absorber Z.
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Abstract Purpose The constrained one‐step spectral CT image reconstruction (cOSSCIR) algorithm with a nonconvex alternating direction method of multipliers optimizer is proposed for addressing computed tomography (CT) metal artifacts caused by beam hardening, noise, and photon starvation. The quantitative performance of cOSSCIR is investigated through a series of photon‐counting CT simulations.
Methods cOSSCIR directly estimates basis material maps from photon‐counting data using a physics‐based forward model that accounts for beam hardening. The cOSSCIR optimization framework places constraints on the basis maps, which we hypothesize will stabilize the decomposition and reduce streaks caused by noise and photon starvation. Another advantage of cOSSCIR is that the spectral data need not be registered, so that a ray can be used even if some energy window measurements are unavailable. Photon‐counting CT acquisitions of a virtual pelvic phantom with low‐contrast soft tissue texture and bilateral hip prostheses were simulated. Bone and water basis maps were estimated using the cOSSCIR algorithm and combined to form a virtual monoenergetic image for the evaluation of metal artifacts. The cOSSCIR images were compared to a “two‐step” decomposition approach that first estimated basis sinograms using a maximum likelihood algorithm and then reconstructed basis maps using an iterative total variation constrained least‐squares optimization (MLE+TV). Images were also compared to a nonspectral TV reconstruction of the total number of counts detected for each ray with and without normalized metal artifact reduction (NMAR) applied. The simulated metal density was increased to investigate the effects of increasing photon starvation. The quantitative error and standard deviation in regions of the phantom were compared across the investigated algorithms. The ability of cOSSCIR to reproduce the soft‐tissue texture, while reducing metal artifacts, was quantitatively evaluated.
Results Noiseless simulations demonstrated the convergence of the cOSSCIR and MLE+TV algorithms to the correct basis maps in the presence of beam‐hardening effects. When noise was simulated, cOSSCIR demonstrated a quantitative error of −1 HU, compared to 2 HU error for the MLE+TV algorithm and −154 HU error for the nonspectral TV+NMAR algorithm. For the cOSSCIR algorithm, the standard deviation in the central iodine region of interest was 20 HU, compared to 299 HU for the MLE+TV algorithm, 41 HU for the MLE+TV+Mask algorithm that excluded rays through metal, and 55 HU for the nonspectral TV+NMAR algorithm. Increasing levels of photon starvation did not impact the bias or standard deviation of the cOSSCIR images. cOSSCIR was able to reproduce the soft‐tissue texture when an appropriate regularization constraint value was selected.
Conclusions By directly inverting photon‐counting CT data into basis maps using an accurate physics‐based forward model and a constrained optimization algorithm, cOSSCIR avoids metal artifacts due to beam hardening, noise, and photon starvation. The cOSSCIR algorithm demonstrated improved stability and accuracy compared to a two‐step method of decomposition followed by reconstruction.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0086579
