CBCT Iterative Image Reconstruction Method Using Energy Spectrum Information for Adaptive Proton Therapy

Reactive sites on the surface of polycyclic aromatic hydrocarbon clusters: A numerical study

Abstract. Absolute profiles of atomic oxygen number densities with high vertical resolution have been determined in the mesosphere–lower thermosphere (MLT) region from in situ measurements by several rocket-borne solid electrolyte sensors. The amperometric sensors were operated in both controlled and uncontrolled modes and with various orientations on the foredeck and aft deck of the payload. Calibration was based on mass spectrometry in a molecular beam containing atomic oxygen produced in a microwave discharge. The sensor signal is proportional to the number flux onto the electrodes, and the mass flow rate in the molecular beam was additionally measured to derive this quantity from the spectrometer reading. Numerical simulations provided aerodynamic correction factors to derive the atmospheric number density of atomic oxygen from the sensor data. The flight results indicate a preferable orientation of the electrode surface perpendicular to the rocket axis. While unstable during the upleg, the density profiles measured by these sensors show an excellent agreement with the atmospheric models and photometer results during the downleg of the trajectory. The high spatial resolution of the measurements allows for the identification of small-scale variations in the atomic oxygen concentration.

Comparison between the characteristics of the plasmas generated by laser on solid and molten steels

Determination of effective atomic number and electron density of heavy metal oxide glasses

The time-independent ion distributions of variable composition laser-produced Sn plasmas are studied for a wide range of electron temperatures and atomic number densities, the purpose of which is to elucidate the effect that varying the number density of Sn within a mixed species plasma has upon the steady state populations of Sn and its ions. Particular emphasis will be placed on binary mixtures of Sn with Li, C, O or Sm and more specifically the charge states Sn8+ to Sn13+ within these mixed plasmas, where it will be assumed that the plasma is optically thin. It is found that using these composites has relatively little effect upon the Sn ion population distributions for plasma atomic number densities of less than approximately 1019.5 cm−3. However, for greater values of number densities the Sn ion populations can be shifted by as much as 10–15 eV for Li mixtures. These results are of particular relevance to current research being carried out on extreme ultraviolet lithographic technologies for the optimization of XUV sources in the 13.5 nm wavelength region, which include composite target investigations.

Purpose: Accurate assessment of electron density (ρe‐[electrons/cm3]) is essential for radiotherapy treatment planning, especially for protons. Measurements of ρe‐ are typically performed with kilovoltage CT (kVCT) and its calibration assumes that object is tissue equivalent. In this work, we demonstrate that kVCT presents confusing information when objects have varying atomic number (Z). At the same time, we demonstrate that megavoltage CT (MVCT) may be preferred over kVCT in treatment planning since its calibrated pixel value, or CT number, is proportional to ρe‐, independent of atomic number(ignoring pair production), and is suitable for non‐tissue equivalent materials(e.g. hip prostheses). Method and Materials: A dual‐energy kV & MVCT benchtop is used to image a tissue‐equivalent phantom as well as samples with various combinations of ρe‐ and Z. Results: The plot of CT number to ρe‐ for the tissue equivalent phantom is non‐linear for the kVCT, but linear for the MVCT. For the non‐tissue equivalent samples, the MVCT number increases linearly with ρe‐ for all the samples, however the kV CT number to electron density is complicated by the two material components of the kV attenuation: atomic number and electron density Conclusion: For all the samples presented the MVCT number increases with the electron density of the sample. The kVCT number assessment of electron density is only reliable for low atomic number materials. Beam hardening affects the kVCT number even for relatively small samples. Beam hardening effects for MVCT is slight and only apparent in large patients. Ignoring image‐quality to dose considerations, MVCT should be preferred for electron density assessment. Conflict of Interest: This work was partially supported by grant from the NIH (PO1 CA088960) and a contract from TomoTherapy Inc.

In this work, we investigated the effective atom number, the effective electron density, the mean free path, the tenth-value layer, the half-value layer, and the mass attenuation coefficient for some stainless steels: AISI 302, AISI 303, AISI 304, AISI 304L, AISI 310, AISI 316, AISI 321, and AISI 410. The mass attenuation coefficients were determined using the WinXCom computer program in the energy region 1keV- 100 GeV. The effective atom number and effective electron density have been calculated using two different methods, the direct method, and the interpolation method. The results reveal that the values of effective atomic numbers and effective electron numbers are greatly influenced by the atomic number of elements in the alloy and the interaction photon energy. The effective atom numbers grew as the atomic number of the constituents in the alloys increased. The effective atomic number and effective electron density values for all steels were found to have the highest values at 0–0.1 MeV energy and the lowest values in the 0.5–6 MeV energy range. The shielding properties of the steels produced close results, but AISI 304L provided the best protection while AISI 410 provided the least. The results obtained with both methods were also compared. The result of the present study may provide new and helpful knowledge about stainless steel for gamma-ray shielding applications.

PurposeThe aim of this study was to improve the image quality of cone-beam computed tomography (CBCT) mounted on the gantry of a linear accelerator used in radiation therapy based on the image information provided by planning multi-detector CT (MDCT).MethodsMDCT-based shading correction for CBCT and virtual monochromatic CT (VMCT) synthesized using the dual-energy method were performed. In VMCT, the high-energy data were obtained from CBCT, while the low-energy data were obtained from MDCT. An electron density phantom was used to investigate the efficacy of shading correction and VMCT on improving the target detectability, Hounsfield unit (HU) accuracy and variation, which were quantified by calculating the contrast-to-noise ratio (CNR), the percent difference (%Diff) and the standard deviation of the CT numbers for tissue equivalent background material, respectively. Treatment plan studies for a chest phantom were conducted to investigate the effects of image quality improvement on dose planning.ResultsFor the electron density phantom, the mean value of CNR was 17.84, 26.78 and 34.31 in CBCT, shading-corrected CBCT and VMCT, respectively. The mean value of %Diff was 152.67%, 11.93% and 7.66% in CBCT, shading-corrected CBCT and VMCT, respectively. The standard deviation within a uniform background of CBCT, shading-corrected CBCT and VMCT was 85, 23 and 15 HU, respectively. With regards to the chest phantom, the monitor unit (MU) difference between the treatment plan calculated using MDCT and those based on CBCT, shading corrected CBCT and VMCT was 6.32%, 1.05% and 0.94%, respectively.ConclusionsEnhancement of image quality in on-board CBCT can contribute to daily patient setup and adaptive dose delivery, thus enabling higher confidence in patient treatment accuracy in radiation therapy. Based on our results, VMCT has the highest image quality, followed by the shading corrected CBCT and the original CBCT. The research results presented in this study should be able to provide a route to reach a high level of image quality for CBCT imaging in radiation oncology.

BackgroundRotational beam delivery enables concurrent acquisition of cone-beam CT (CBCT), thereby facilitating further geometric verification of patient setup during radiation treatment. However, it is challenging to acquire CBCT during stereotactic body radiation therapy (SBRT) using flattening-filter free X-ray beams, in which a high radiation dose is delivered. This study presents quantitative evaluation results of the image quality in four-dimensional (4D) in-treatment CBCT acquired during SBRT delivery.MethodsThe impact of megavoltage (MV) scatter and acquisition parameters on the image quality was evaluated using Catphan 503 and XSight lung tracking phantoms. The in-treatment CBCT images of the phantoms were acquired while delivering 16 SBRT plans. The uniformity, contrast, and contrast-to-noise ratio (CNR) of the in-treatment CBCT images were calculated and compared to those of CBCT images acquired without SBRT delivery. Furthermore, the localizing accuracy of the moving target in the XSight lung phantom was evaluated for 10 respiratory phases.ResultsThe CNR of the 3D-reconstucted Catphan CBCT images was reduced from 6.3 to 2.6 due to the effect of MV treatment scatter. Both for the Catphan and XSight phantoms, the CBCT image quality was affected by the tube current and monitor units (MUs) of the treatment plan. The lung target in the XSight tracking phantom was most visible for extreme phases; the mean CNRs of the lung target in the in-treatment CBCT images (with 40 mA tube current) across the SBRT plans were 3.2 for the end-of-exhalation phase and 3.0 for the end-of-inhalation phase. The lung target was localized with sub-millimeter accuracy for the extreme respiratory phases.ConclusionsFull-arc acquisition with an increased tube current (e.g. 40 mA) is recommended to compensate for degradation in the CBCT image quality due to unflattened MV beam scatter. Acquiring in-treatment CBCT with a high-MU treatment beam is also suggested to improve the resulting CBCT image quality.

Studies on mass attenuation coefficient, effective atomic number and electron density of some thermoluminescent dosimetric compounds

This work aims to calculate the effective atomic number and electron density by Monte Carlo method. In previous studies, the most widely used solution is to use the transmission method with the narrow gamma-ray beam. In the approach of this work, the gamma-ray beam after going through material is uncollimated to recording by NaI(Tl) detector. To do this, the inner diameter of detector collimator was enlarged with the aim of decreasing the strengthen of radioactive source. The obtained results were compared with NIST data and the experimental values which yield the maximum deviation of 9.05% and 3.43%, respectively. These results show the promising approach in determining the features of material.

Attenuation-density anomalous relationship of lead alkali borosilicate glasses

Determination of mass attenuation coefficients and effective atomic numbers for Inconel 738 alloy for different energies obtained from Compton scattering

Mass attenuation coefficient, μm, atomic cross-section, σi, electronic cross-section, σe, effective atomic number, Zeff and effective electron density, Nel, were determined experimentally and theoretically for some vitamins (retinol, beta-carotene, thiamine, riboflavin, niacinamide, pantothenic acid, pyridoxine, biotin, folic acid, cyanocobalamin, ascorbic acid, cholecalciferol, alpha-tocopherol, ketamine, hesperidin) at 30.82, 59.54, 80.99, 356.61, 661.66 and 1,408.01 keV photon energies using a NaI(Tl) scintillation detector. The theoretical mass attenuation coefficients were estimated using mixture rules. The calculated values were compared with the experimental values for all vitamins.

Dual-energy computed tomography (DECT) is a promising technology that has shown a number of clinical advantages over conventional X-ray CT, such as improved material identification, artifact suppression, etc. For proton therapy treatment planning, besides material-selective images, maps of effective atomic number (Z) and relative electron density to that of water (ρ_e) can also be achieved and further employed to improve stopping power ratio accuracy and reduce range uncertainty. In this work, we propose a one-step iterative estimation method, which employs multi-domain gradient L₀-norm minimization, for Z and ρ_e maps reconstruction. The algorithm was implemented on GPU to accelerate the predictive procedure and to support potential real-time adaptive treatment planning. The performance of the proposed method is demonstrated via both phantom and patient studies.