Purpose
To assess image quality and to quantify the accuracy of relative electron densities (ρe) and effective atomic numbers (Zeff) for three dual‐energy computed tomography (DECT) scanners: a novel ...single‐source split‐filter (i.e., twin‐beam) and two dual‐source scanners.
Methods
Measurements were made with a second generation dual‐source scanner at 80/140Sn kVp, a third‐generation twin‐beam single‐source scanner at 120 kVp with gold (Au) and tin (Sn) filters, and a third‐generation dual‐source scanner at 90/150Sn kVp. Three phantoms with tissue inserts were scanned and used for calibration and validation of parameterized methods to extract ρe and Zeff, whereas iodine and calcium inserts were used to quantify Contrast‐to‐Noise‐Ratio (CNR). Spatial resolution in tomographic images was also tested.
Results
The third‐generation scanners have an image resolution of 6.2, ~0.5 lp/cm higher than the second generation scanner. The twin‐beam scanner has low imaging contrast for iodine materials due to its limited spectral separation. The parameterization methods resulted in calibrations with low fit residuals for the dual‐source scanners, yielding values of ρe and Zeff close to the reference values (errors within 1.2% for ρe and 6.2% for Zeff for a dose of 20 mGy, excluding lung substitute tissues). The twin‐beam scanner presented overall higher errors (within 3.2% for ρe and 28% for Zeff, also excluding lung inserts) and also larger variations for uniform inserts.
Conclusions
Spatial resolution is similar for the three scanners. The twin‐beam is able to derive ρe and Zeff, but with inferior accuracy compared to both dual‐source scanners.
To investigate the feasibility of using dual-energy CT (DECT) for tissue segmentation and kilovolt (kV) dose calculations in pre-clinical studies and assess potential dose calculation accuracy gain.
...Two phantoms and an ex-vivo mouse were scanned in a small animal irradiator with two distinct energies. Tissue segmentation was performed with the single-energy CT (SECT) and DECT methods. A number of different material maps was used. Dose calculations were performed to verify the impact of segmentations on the dose accuracy.
DECT showed better overall results in comparison to SECT. Higher number of DECT segmentation media resulted in smaller dose differences in comparison to the reference. Increasing the number of materials in the SECT method yielded more instability. Both modalities showed a limit to which adding more materials with similar characteristics ceased providing better segmentation results, and resulted in more noise in the material maps and the dose distributions. The effect was aggravated with a decrease in beam energy. For the ex-vivo specimen, the choice of only one high dense bone for the SECT method resulted in large volumes of tissue receiving high doses. For the DECT method, the choice of more than one kind of bone resulted in lower dose values for the different tissues occupying the same volume. For the organs at risk surrounded by bone, the doses were lower when using the SECT method in comparison to DECT, due to the high absorption of the bone. SECT material segmentation may lead to an underestimation of the dose to OAR in the proximity of bone.
The DECT method enabled the selection of a higher number of materials thereby increasing the accuracy in dose calculations. In phantom studies, SECT performed best with three materials and DECT with seven for the phantom case. For irradiations in preclinical studies with kV photon energies, the use of DECT segmentation combined with the choice of a low-density bone is recommended.
During precision irradiation of a preclinical lung tumour model, the tumour is subject to breathing motion and it can partially move out of the irradiation field. This work aimed to perform a ...quantitative analysis of the impact of respiratory motion on a mouse lung tumour irradiation with small fields.
A four-dimensional digital mouse whole body phantom (MOBY) with a virtual 4-mm spherical lung tumour at different locations in both lungs is used to simulate a breathing anaesthetized mouse in different breathing phases representing a full breathing cycle. The breathing curve is determined by fluoroscopic imaging of an anaesthetized mouse. Each MOBY time frame is loaded in a dedicated treatment planning system (small animal radiotherapy-Plan) and is irradiated by a full arc with a 5-mm circular collimator. Mean and time-dependent organ doses are calculated for the tumour, heart and spinal cord.
Depending on the location of the lung tumour, an overestimation of the mean tumour dose up to 11% is found. The mean heart dose could be both overestimated or underestimated because the heart moves in or out of the irradiation field depending on the beam target location. The respiratory motion does not affect the mean spinal cord dose. A dose gradient is visible in the time-dependent tumour dose distribution.
In the future, new methods need to be developed to track the lung tumour motion before preclinical irradiation to adjust the irradiation plan. Margins, collimator diameter and target dose could be changed easily, but they all have their drawbacks. State-of-the-art clinical techniques such as respiratory gating or motion tracking may offer a solution for the cold spots in the time-dependent tumour dose. Advances in knowledge: A suitable method is found to quantify changes in organ dose due to respiratory motion in mouse lung tumour image-guided precision irradiation.
The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of the European Space Agency's XMM-Newton observatory. Recent calculations of the scattering of ...axion-conversion X-rays suggest that the sunward magnetosphere could be an observable source of 0.2-10 keV photons. For XMM-Newton, any conversion X-ray intensity will be seasonally modulated by virtue of the changing visibility of the sunward magnetic field region. A simple model of the geomagnetic field is combined with the ephemeris of XMM-Newton to predict the seasonal variation of the conversion X-ray intensity. This model is compared with stacked XMM-Newton blank sky datasets from which point sources have been systematically removed. Remarkably, a seasonally varying X-ray background signal is observed. The European Photon Imaging Camera count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11... (pn), 4... (MOS1) and 5... (MOS2). After examining the distribution of the constituent observations spatially, temporally and in terms of the accepted representation of the cosmic X-ray background, we conclude that this variable signal is consistent with the conversion of solar axions in the Earth's magnetic field, assuming the resultant photons are not strictly forward-directed, and enter the field of view of XMM-Newton. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung- and Compton-like processes, distinct from a Primakoff spectrum, i.e. axion-electron coupling dominates over axion-photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2 x 10... GeV... is derived for the product of the axion-photon and axion-electron coupling constants, for an axion mass in the ...eV range. Comparisons, e.g., with limits derived from white dwarf cooling may not be applicable, as these refer to axions in the ~0.01 eV range. Preliminary results are given of a search for axion-conversion X-ray lines, in particular the predicted narrow features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from ...Fe. (ProQuest: ... denotes formulae/symbols omitted.)
The Mercury Imaging X-ray Spectrometer (MIXS) on the BepiColombo Mercury Planetary Orbiter (MPO) will measure fluorescent X-ray emission from the surface of Mercury in the energy range 0.5–7.5
keV, ...which is induced by incident solar X-rays and solar wind electrons and protons. These X-rays will reveal the elemental composition of the surface of Mercury and aid the determination of the planet's evolution.
MIXS is a two component instrument. A collimated channel (MIXS-C) provides measurements on scales of 70–270
km, sufficient to separate the major Mercurian terrains. A second channel (MIXS-T) is the first imaging X-ray telescope for planetary remote sensing and will make measurements on spatial scales of less than 10
km for major elements during solar flares, sufficient to isolate surface landforms, such as craters and their internal structures. The spatial resolution achieved by MIXS-T is made possible by novel, low mass microchannel plate X-ray optics, in a Wolter type I optical geometry.
MIXS measurements of surface elemental composition will help determine rock types, the evolution of the surface and ultimately a probable formation process for the planet. In this paper we present MIXS and its predicted performance at Mercury as well as discussing the role that MIXS measurements will play in answering the major questions about Mercury.
To analyse the effect of different image reconstruction techniques on image quality and dual energy CT (DECT) imaging metrics.
A software platform for pre-clinical cone beam CT X-ray image ...reconstruction was built using the open-source reconstruction toolkit. Pre-processed projections were reconstructed with filtered back-projection and iterative algorithms, namely Feldkamp, Davis, and Kress (FDK), Iterative FDK, simultaneous algebraic reconstruction technique (SART), simultaneous iterative reconstruction technique and conjugate gradient. Imaging metrics were quantitatively assessed, using a quality assurance phantom, and DECT analysis was performed to determine the influence of each reconstruction technique on the relative electron density (ρ
) and effective atomic number (Z
) values.
Iterative reconstruction had favourable results for the DECT analysis: a significantly smaller spread for each material in the ρ
-Z
space and lower Z
and ρ
residuals (on average 24 and 25% lower, respectively). In terms of image quality assurance, the techniques FDK, Iterative FDK and SART provided acceptable results. The three reconstruction methods showed similar geometric accuracy, uniformity and CT number results. The technique SART had a contrast-to-noise ratio up to 76% higher for solid water and twice as high for Teflon, but resolution was up to 28% lower when compared to the other two techniques.
Advanced image reconstruction can be beneficial, but the benefit is small, and calculation times may be unacceptable with current technology. The use of targeted and downscaled reconstruction grids, larger, yet practicable, pixel sizes and GPU are recommended.
An iterative CBCT reconstruction platform was build using RTK.
The aim of this work was to investigate whether quantitative dual-energy CT (DECT) imaging is feasible for small animal irradiators with an integrated cone-beam CT (CBCT) system.
The optimal imaging ...protocols were determined by analyzing different energy combinations and dose levels. The influence of beam hardening effects and the performance of a beam hardening correction (BHC) were investigated. In addition, two systems from different manufacturers were compared in terms of errors in the extracted effective atomic numbers (Z
) and relative electron densities (ρ
) for phantom inserts with known elemental compositions and relative electron densities.
The optimal energy combination was determined to be 50 and 90 kVp. For this combination, Z
and ρ
can be extracted with a mean error of 0.11 and 0.010, respectively, at a dose level of 60 cGy.
Quantitative DECT imaging is feasible for small animal irradiators with an integrated CBCT system. To obtain the best results, optimizing the imaging protocols is required. Well-separated X-ray spectra and a sufficient dose level should be used to minimize the error and noise for Z
and ρ
. When no BHC is applied in the image reconstruction, the size of the calibration phantom should match the size of the imaged object to limit the influence of beam hardening effects. No significant differences in Z
and ρ
errors are observed between the two systems from different manufacturers. Advances in knowledge: This is the first study that investigates quantitative DECT imaging for small animal irradiators with an integrated CBCT system.
Status of large area CsI photocathode developments Schyns, E
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2002, Letnik:
494, Številka:
1
Journal Article
Recenzirano
The development of large area reflective CsI photocathodes for RICH detectors is reviewed in the context of their production process and their performance. The quantum efficiencies of six large ...photocathodes recently produced by ALICE are compared with results available from the large photocathodes at HADES and STAR and with earlier R&D results obtained on small area samples. The CsI coating parameters and new developments in the domain of the production and the preparation of substrates are discussed in relation with the photocathode performance.
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