Coordination Of Exit Research Articles

Investigation of the effect of air gap size on the spatial resolution in proton- and helium radio- and tomography

PurposeProton computed (transmission) tomography (pCT) refers to the process of imaging an object by letting protons pass through it, while measuring their energy after, and their position and (optionally) direction both before and after their traversal through that object. The so far experimental technique has potential to improve treatment planning of proton therapy by enabling the direct acquisition of a proton stopping power map of tissue, thus removing the need to obtain it by converting X-ray CT attenuation data and thereby eliminating uncertainties which arise in the mentioned conversion process. The image reconstruction in pCT requires accurate estimates of the proton trajectories. In experimental pCT detector setups where the direction of the protons is not measured, the air gap between the detector planes and the imaged object worsens the spatial resolution of the image obtained. In this work we determined the mean proton paths and the corresponding spatial uncertainty, taking into account the presence of the air gap. MethodsWe used Monte Carlo simulations of radiation transport to systematically investigate the effect of the air gap size between detector and patient on the spatial resolution of proton (ion) computed tomography for protons with an energy of 200MeV and 250MeV as well as for helium ions (He-4) with an energy of 798MeV. For the simulations we used TOPAS which itself is based on Geant4. ResultsFor all particles, which are detected at the same entrance and exit coordinate, the average ion path and the corresponding standard deviation was computed. From this information, the dependence of the spatial resolution on the air gap size and the angular confusion of the particle beam was inferred. ConclusionThe presence of the airgap does not pose a problem for perfect fan beams. In realistic scenarios, where the initial angular confusion is around 5mrad and for typical air gap sizes up to 10cm, using an energy of 200MeV a spatial resolution of about 1.6mm can be achieved. Using protons with E=250MeV a spatial resolution of about 1.1mm and using helium ions (He-4) with E=798MeV even a spatial resolution below 0.7mm respectively is attainable.

Implementation of streamline simulation based on finite element method in FEniCS

Understanding flow and transport behaviors in subsurface porous rock is extremely important for various applications such as oil/gas reservoirs, groundwater, geothermal energy explorations, and CO2 sequestration. However, the relevant calculation is never an easy task because of the large areal extend and high heterogeneity of subsurface system. Although streamline-based simulations are suitable for solving large, geologically complex, and heterogeneous systems, they still present difficulties while working with the finite element method. This paper introduces a new algorithm to trace streamlines based on the underlying flow field in unstructured triangular mesh systems. The new solution procedure only requires velocities on each element vertex but not on cell facets, and therefore it does not rely on conservative fluxes. The algorithm is based on coupled ODEs derived from shape functions as well as the transformation formula between the actual and master elements. The analytical solution describing the trajectory of a streamline segment in the master element is derived so that the time-of-flight and exit coordinate (leaving the element) can be determined accurately and efficiently. Additionally, the presented algorithm allows the implementation of streamline-based methods in the programming framework, FEniCS, so as to extend the technique to other research areas.

Insights into interplay between rexinoid signaling and myogenic regulatory factor-associated chromatin state in myogenic differentiation.

While skeletal myogenesis is tightly coordinated by myogenic regulatory factors including MyoD and myogenin, chromatin modifications have emerged as vital mechanisms of myogenic regulation. We have previously established that bexarotene, a clinically approved agonist of retinoid X receptor (RXR), promotes the specification and differentiation of skeletal muscle lineage. Here, we examine the genome-wide impact of rexinoids on myogenic differentiation through integral RNA-seq and ChIP-seq analyses. We found that bexarotene promotes myoblast differentiation through the coordination of exit from the cell cycle and the activation of muscle-related genes. We uncovered a new mechanism of rexinoid action which is mediated by the nuclear receptor and largely reconciled through a direct regulation of MyoD gene expression. In addition, we determined a rexinoid-responsive residue-specific histone acetylation at a distinct chromatin state associated to MyoD and myogenin. Thus, we provide novel molecular insights into the interplay between RXR signaling and chromatin states pertinent to myogenic programs in early myoblast differentiation.

Technical Note: Spatial resolution of proton tomography: Impact of air gap between patient and detector

Proton radiography and tomography were investigated since the early 1970s because of its low radiation dose, high density resolution, and ability to image directly proton stopping power. However, spatial resolution is still a limiting factor. In this note, preliminary results of the impact of an air gap between detector system and patient on spatial resolution are presented. Spatial resolution of proton radiography and tomography is governed by multiple Coulomb scattering (MCS) of the protons in the patient. In this note, the authors employ Monte Carlo simulations of protons traversing a 20 cm thick water box. Entrance and exit proton coordinate measurements were simulated for improved spatial resolution. The simulations were performed with and without a 5 cm air gap in front of and behind the patient. Loss of spatial resolution due to the air gap was studied for protons with different initial angular confusion. It was found that spatial resolution is significantly deteriorated when a 5 cm air gap between the position sensitive detector and the patient is included. For a perfect parallel beam spatial resolution worsens by about 40%. Spatial resolution is getting worse with increasing angular confusion and can reach 80%. When proton radiographies are produced by measuring the entrance and exit coordinates of the protons in front of and behind the patient the air gap between the detector and the patient can significantly deteriorate the spatial resolution of the system by up to 80%. An alternative would be to measure in addition to the coordinates also the exit and entrance angles of each proton. In principle, using the air gap size and proton angle, images can be reconstructed with the same spatial resolution than without air gap.

Spatial resolution of proton tomography: Methods, initial phase space and object thickness

PurposeProton radiography and tomography was investigated since the early 1970s because of its low radiation dose, high density resolution and ability to image directly proton stopping power. However, spatial resolution is still a limiting factor and as a consequence experimental methods and image reconstruction should be optimized to improve position resolution. MethodsSpatial resolution of proton radiography and tomography is given by multiple Coloumb scattering (MCS) of the protons in the patient. In this paper we employ an improved MCS model to study the impact of various proton tomographic set-ups on the spatial resolution, such as different combinations of entrance and exit coordinate and angle measurements, respectively, initial particle energy and angular confusion of the incident proton field. ResultsIt was found that best spatial resolution is obtained by measuring in addition to the entrance and exit coordinates also the entrance and exit angles. However, by applying partial backprojection and by using a perfect proton fan beam a sufficient spatial resolution can be achieved with less experimental complexity (measuring only exit angles). It was also shown that it is essential to use the most probable proton trajectory to improve spatial resolution. A simple straight line connection for image reconstruction results in a spatial resolution which is not clinically sufficient. The percentage deterioration of spatial resolution due to the angular confusion of the incident proton field is less than the phase space in mrad. A clinically realistic proton beam with 10 mrad angular confusion results in a less than 10% loss of spatial resolution. ConclusionsClinically sufficient spatial resolution can be either achieved with a full measurement of entrance and exit coordinates and angles, but also by using a fan beam with small angular confusion and an exit angle measurement. It is necessary to use the most probable proton path for image reconstruction. A simple straight line connection is in general not sufficient. Increasing proton energy improves spatial resolution of an object of constant size. This should be considered in the design of proton therapy facilities.

TH-E-220-02: Spatial Resolution of Proton Tomography: Methods, Initial Phase Space and Object Thickness

Purpose: Protonradiography and tomography was investigated in the early 1970s because of its low radiation dose, high density resolution and ability to image directly protonstopping power.Spatial resolution is still a limiting factor. therefore experimental methods and image reconstruction should be optimized to improve position resolution. Methods: Spatial resolution of protontomography is given by multiple scattering(MCS) of the protons. We employ an improved MCS model to study the impact of proton tomographic set‐ups on spatial resolution, such as different combinations of entrance and exit coordinate and angle measurements, respectively, particle energy and angular confusion of the proton field. Results: It was found that best spatial resolution is obtained by measuring in addition to the entrance and exit coordinates the entrance and exit angles. By applying partial backprojection and a perfect proton fan beam a sufficient spatial resolution can be achieved with less experimental complexity. It was also shown that it is essential to use the most probable proton trajectory to improve spatial resolution. A simple straight line connection for image reconstruction results in a spatial resolution which is not clinically sufficient. The percentage deterioration of spatial resolution due to the angular confusion of the incident proton field is less than the phase space in mrad. Conclusions: Clinically sufficient spatial resolution can be either achieved with a full measurement of entrance and exit coordinates and angles, but also by using a fan beam with small angular confusion and an exit angle measurement. It is necessary to use the most probable proton path for image reconstruction. A simple straight line connection is in general not sufficient. Increasing proton energy improves spatial resolution of an object of constant size. This should be considered in the design of proton therapy facilities.

SU‐E‐T‐350: Calibration of a Prototype Proton CT Scanner

Purpose: To calibrate a novel proton CT scanner using polystyrene degraders of accurately known thickness and relative stopping power.Methods: A prototype proton CT scanner was constructed and installed on a horizontal proton research beam line for further study of this novel, potentially low‐dose tomographic imaging modality. The proton CT scanner registers individual protons up to a maximum rate of about 10̂5 protons/sec, recording their entry and exit coordinates and directions with high‐resolution silicon strip detectors, as well as their residual energy by stopping them in an augmented 18‐crystal CsI calorimeter. To avoid uncertainties in the mean excitation energy (I) of water when converting energy loss to water equivalent path length (WEPL) via the Bethe‐Bloch equation, the response of the calorimeter was calibrated using polystyrene plates of known water‐equivalent thickness (WET). Stacks of polystyrene plates of variable thickness were placed between the proton CT detectors and about 3 × 10̂5 protons were recorded for each thickness. For each proton event, the calorimeter response was derived from the weighted sum of the response of the 18 crystals, taking into account variations in the sensitivity of each crystal. A Gaussian distribution function was fitted to the response data at each WET, and the mean of each distribution was used to establish the WEPL calibration curve. Results: For protons of 100 MeV and 200 MeV, the mean response of the calorimeter was obtained for 7–8 different WET values (including zero) and the WEPL vs. response calibration curve was fitted with a quadratic polynomial with an excellent goodness of fit. A WEPL measurement accuracy of 1 mm or better can be achieved by measuring the calorimeter response of about 150 protons Conclusions: This work is an important step towards a thorough investigation of proton computed tomography for therapeutic and imaging applications.Funded in part by DOD Contract W81XWH‐08‐1‐0205 and the Department of Radiation Medicine, Loma Linda University Medical Center

Multiple Coulomb scattering and spatial resolution in proton radiography

A simple formula for the spatial resolution of transmission proton radiography is derived for two different methods of measuring the proton coordinates. The effect of multiple Coulomb scattering and energy loss are taken into account. Experimental measurements of the spatial resolution have been done and are compared with the calculations. The technique of measuring entrance and exit coordinates in coincidence for each single proton improves the spatial resolution by a factor of 8 compared to a single coordinate measurement.