Irradiation Geometry Research Articles

MO‐D‐110‐04: Imaging Symposium

Patients who undergo medical imaging procedures such as fluoroscopy are exposed to ionizing radiations. The quantification of the amount of radiation used to perform a given examination is achieved by the Kerma Area Product (KAP) that is incident on the patient, together with appropriate information on the beam quality (i.e., Half Value Layer). A formal methodology will be presented to show how Kerma Area Product (KAP) in fluoroscopy, as well as the corresponding patient effective dose, can be converted into individual patient radiation risk provided appropriate account is taken of the irradiation geometry, patient size and demographics. It is concluded that patient specific risks in medical imaging are essential for identifying indicated examinations, where the patient benefit exceeds any radiation risk. Of equal importance is the fact that current scientific knowledge may be employed to enable practitioners to compute patient risks for any type of fluoroscopy based examination.

SU‐D‐BRB‐02: Monte Carlo Modeling of the Gamma Knife Perfexion

Purpose: For dosimetric and research irradiation studies, we have implemented a Monte Carlo (MC) dose computation model based on the physical and radiological characteristics of the Gamma Knife Perfexion (GK PFX) using the Penelope MC dosimetry codes. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles. Methods: Vendor proprietary information facilitated our modeling of the GK PFX irradiation geometry, which was mathematically defined within Penelope. MC simulations were carried out on a Linux cluster. 3D dose distributions were analyzed using Matlab. A 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. Detector volume varied from 33 to 590 mm̂3 to study detector volume effects. A single source per row was modeled for five rows for each collimator (15 beams modeled). Single‐source dose distributions were rotated about the z‐axis of the axially symmetric geometry and summed to simulate all 192 sources. Results: Good agreement is found for row‐ and total‐output factors (greatest deviation <2% for the 4 mm collimator) compared to reference values. Simulated and measured full‐width at half‐ max values of 3D dose distribution profiles show sub‐millimeter differences (0.4 mm, 4 and 8 mm collimators; 0.9 mm, 16 mm collimator). There is excellent agreement for integrated profile shapes. Conclusions: Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values are dependent on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile shapes compared to reference values.Acknowledgement: Non‐disclosure agreement for proprietary information with Elekta AB. No financial contribution.

Evaluation of neutron sources for ISAGE—in-situ-NAA for a future lunar mission

For a future Moon landing, a concept for an in-situ NAA involving age determination using the (40)Ar-(39)Ar method is developed. A neutron source (252)Cf is chosen for sample irradiation on the Moon. A special sample-in-source irradiation geometry is designed to provide a homogeneous distribution of neutron flux at the irradiation position. Using reflector, the neutron flux is likely to increase by almost 200%. Sample age of 1Ga could be determined. Elemental analysis using INAA is discussed.

DEVELOPMENT OF NEW TWO-DOSIMETER ALGORITHM FOR EFFECTIVE DOSE IN ICRP PUBLICATION 103

The two-dosimeter method, which employs one dosimeter on the chest and the other on the back, determines the effective dose with sufficient accuracy for complex or unknown irradiation geometries. The two-dosimeter method, with a suitable algorithm, neither significantly overestimates (in most cases) nor seriously underestimates the effective dose, not even for extreme exposure geometries. Recently, however, the definition of the effective dose itself was changed in ICRP Publication 103; that is, the organ and tissue configuration employed in calculations of effective dose, along with the related tissue weighting factors, was significantly modified. In the present study, therefore, a two-dosimeter algorithm was developed for the new ICRP 103 definition of effective dose. To that end, first, effective doses and personal dosimeter responses were calculated using the ICRP reference phantoms and the MCNPX code for many incident beam directions. Next, a systematic analysis of the calculated values was performed to determine an optimal algorithm. Finally, the developed algorithm was tested by applying it to beam irradiation geometries specifically selected as extreme exposure geometries, and the results were compared with those for the previous algorithm that had been developed for the effective dose given in ICRP Publication 60.

A distance to dose difference tool for estimating the required spatial accuracy of a displacement vector field

To introduce a tool, termed distance to dose difference (DTD), which estimates the required spatial accuracy of displacement vector fields (DVFs) used for mapping four dimensional dose values. Dose mapping maps dose values from an irradiated geometry to a reference geometry. DVF errors result in dose being mapped from the wrong spatial location in the irradiated geometry, with a dose error equal to the dose difference between the error-free and sampled spatial locations. The DTD, defined as the distance to observe a given dose difference in the irradiated geometry, quantifies the permitted DVF error to ensure a prespecified desired dose mapping accuracy is achieved. To demonstrate the DTD, a treatment plan is generated with a 5 mm internal target volume-to-planning target volume margin for an intensity modulated radiation therapy lung patient. The DTD is evaluated for mapping dose from the end of inhale image with a dose error tolerance of 3.30 Gy, which equals 5% of the 66 Gy prescription dose. The DTD is loaded into the treatment planning system to visualize positional dependencies of permissible DVF errors overlaid on the patient's anatomy and DTD-volume-histograms are generated. DTD values vary with location in the patient anatomy. For the test case, DTD analysis indicates that accurate DVFs (approximately 1 mm) are required in high dose gradient regions while large DVF errors (>20 mm) are acceptable in low dose gradient regions. Within the clinical target volume (CTV), tolerated DVF uncertainties range from 1 to 12 mm, depending on location. Ninety percent of the CTV volume had DTD values less than 4 mm. The DVF spatial accuracy required to meet a dose mapping accuracy tolerance depends on the spatial location within the dose distribution. For dose mapping, DVFs accuracy must be highest in dose gradient regions, while less accurate DVFs can be tolerated in uniform dose regions. The DTD tool provides a useful first estimate of DVF required spatial accuracy.

Effective dose evaluation for BNCT treatment in the epithermal neutron beam at THOR

This paper aims to evaluate the effective dose as well as equivalent doses of several organs of an adult hermaphrodite mathematical phantom according to the definition of ICRP Publication 60 for BNCT treatments of brain tumors in the epithermal neutron beam at THOR. The MCNP5 Monte Carlo code was used for the calculation of the average absorbed dose of each organ. The effective doses for a typical brain tumor treatment with a tumor treatment dose of 20 Gy-eq were evaluated to be 0.59 and 0.35 Sv for the LLAT and TOP irradiation geometries, respectively. In addition to the stochastic effect, it was found that it is also likely to produce deterministic effects, such as cataracts and depression of haematopoiesis.

Does photoelectrocatalysis by TiO2 work?

AbstractBACKGROUND: This paper examines TiO2 photoelectrocatalysis (PEC), a process that increases the efficiency of TiO2 photocatalysis (PC) by applying a potential to separate the UV‐generated charge carriers whose recombination typically limits photonic efficiencies of conventional photocatalysis.RESULTS: Four representative photoelectrocatalytic reactions, nitrophenol oxidation, oxalate degradation, E. coli inactivation and dye decolouration were considered. For all four, a small applied potential raised the rate of pollutant removal by TiO2 electrodes. Because the improvements were probably insufficient to make PEC technologically viable except in niche applications, rates of pollutant removal by PEC and by PC using TiO2 particle dispersions were directly compared. PEC rates were not significantly larger than rates of PC by dispersions.CONCLUSION: Discussions of the implications of these conclusions focus on whether PEC is currently limited by reactor design (irradiation geometry, or mass transfer) or by electrode materials. It is inferred that the performance of present electrodes is not limited significantly by mass transfer constraints. Since the choice of electrode materials (sol–gel or thermal electrodes) has been shown to influence PEC efficiency, recent results on titania nanotubes (TNT) are reviewed. It is concluded that the enhancement factors—the PEC:PC ratio—of TNT electrodes are no higher than those of conventional materials. Copyright © 2011 Society of Chemical Industry

Spectroscopic Response of CZT Detectors Obtained by the Boron Oxide Encapsulated Vertical Bridgman Method

Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as the basic, medical, industrial, and space research. The purpose of this work is to present the spectroscopic proper ties of some CZT crystals grown by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR. By this technique the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented, thus allowing larger single grains with lower dislocation density to be obtained. Several detectors were realized about 4 mm x 4 mm x 1 mm in size, with two planar gold contacts on both the surfaces realized by an electroless technique. The behavior of these detectors was studied as a function of the bias voltage, irradiation geometry and energy of the interacting photons. Good electron charge collection properties have been demonstrated and electric field distribution has been investigated using the Pockels effect.

Organ dose conversion coefficients based on a voxel mouse model and MCNP code for external photon irradiation

A set of conversion coefficients from kerma free-in-air to the organ absorbed dose for external photon beams from 10 keV to 10 MeV are presented based on a newly developed voxel mouse model, for the purpose of radiation effect evaluation. The voxel mouse model was developed from colour images of successive cryosections of a normal nude male mouse, in which 14 organs or tissues were segmented manually and filled with different colours, while each colour was tagged by a specific ID number for implementation of mouse model in Monte Carlo N-particle code (MCNP). Monte Carlo simulation with MCNP was carried out to obtain organ dose conversion coefficients for 22 external monoenergetic photon beams between 10 keV and 10 MeV under five different irradiation geometries conditions (left lateral, right lateral, dorsal-ventral, ventral-dorsal, and isotropic). Organ dose conversion coefficients were presented in tables and compared with the published data based on a rat model to investigate the effect of body size and weight on the organ dose. The calculated and comparison results show that the organ dose conversion coefficients varying the photon energy exhibits similar trend for most organs except for the bone and skin, and the organ dose is sensitive to body size and weight at a photon energy approximately <0.1 MeV.

Intracavity system design for IR multiphoton dissociation

An intracavity system for the infrared multiple photon dissociation (IRMPD) of molecules with high dissociation energy threshold has been designed and implemented. The system design based on a TEA CO2 laser with a cavity folded in V-shape included the analysis of its stability varying the cavity dimensions as well as the analysis of the positions of the beam waists and of the beam size at them. The intracavity energy as a function of the total sample pressure has been measured and the laser-operation threshold has been determined. Intracavity IRMPD has been compared to traditional IRMPD performed in an irradiation geometry in which the radiation is focused into a photoreactor placed outside the laser cavity. Dissociation volumes in intracavity irradiation have resulted an order of magnitude larger than those obtained in experiments performed with the photoreactor outside the laser cavity.

Fluence-to-dose conversion coefficients for aircrew dosimetry based on the new ICRP Recommendations

The estimation of the fluence-to-effective dose conversion coefficients used especially for aircrew dosimetry is an important issue, since the geometrical conditions of aircrew exposure are considered not to be represented by the standard irradiation geometries such as the anterior-to-posterior (AP) and isotropic (ISO) geometries. We therefore calculated the dose conversion coefficients for neutrons and protons based on the ICRP 2007 Recommendations for three additional irradiation geometries: semi-isotropic irradiation from the upper hemisphere, and geometries closely representing the geometrical situations of aircrew exposure for two different flight conditions. The calculation was performed by the PHITS code coupled with the ICRP/ICRU adult reference computational phantoms. It was found from the calculation that (1) the conversion coefficients for the irradiation geometries closely representing the geometrical situations of aircrew exposure agree better with the corresponding data for the isotropic irradiation than those for the semi-isotropic irradiation; (2) the error associated with the assumption of the isotropic irradiation in the aircrew dosimetry is less than only 1% for conventional flight conditions. These findings indicate the adequacy of the use of dose conversion coefficients for the isotropic irradiation geometry in aircrew dosimetry.

Direct Monte Carlo Dose Calculation Using Polygon-surface Computational Human Model

In the present study, a voxel-type computational human model was converted to a polygon-surface model, after which it was imported directly to the Geant4 code without using a voxelization process, that is, without converting back to a voxel model. The original voxel model was also imported to the Geant4 code, in order to compare the calculated dose values and the computational speed. The average polygon size of the polygon-surface model was ~0.5 cm, whereas the voxel resolution of the voxel model was 1.981 × 1.981 × 2.0854 mm. The results showed a good agreement between the calculated dose values of the two models. The polygon-surface model was, however, slower than the voxel model by a factor of 6–9 for the photon energies and irradiation geometries considered in the present study, which nonetheless is considered acceptable, considering that direct use of the polygon-surface model does not require a separate voxelization process..

The influence of vegetation and building morphology on shadow patterns and mean radiant temperatures in urban areas: model development and evaluation

The solar and longwave environmental irradiance geometry (SOLWEIG) model simulates spatial variations of 3-D radiation fluxes and mean radiant temperature (Tmrt) as well as shadow patterns in complex urban settings. In this paper, a new vegetation scheme is included in SOLWEIG and evaluated. The new shadow casting algorithm for complex vegetation structures makes it possible to obtain continuous images of shadow patterns and sky view factors taking both buildings and vegetation into account. For the calculation of 3-D radiation fluxes and Tmrt, SOLWEIG only requires a limited number of inputs, such as global shortwave radiation, air temperature, relative humidity, geographical information (latitude, longitude and elevation) and urban geometry represented by high-resolution ground and building digital elevation models (DEM). Trees and bushes are represented by separate DEMs. The model is evaluated using 5 days of integral radiation measurements at two sites within a square surrounded by low-rise buildings and vegetation in Goteborg, Sweden (57°N). There is good agreement between modelled and observed values of Tmrt, with an overall correspondence of R2 = 0.91 (p < 0.01, RMSE = 3.1 K). A small overestimation of Tmrt is found at locations shadowed by vegetation. Given this good performance a number of suggestions for future development are identified for applications which include for human comfort, building design, planning and evaluation of instrument exposure.

Cellular effects of energetic heavy ions: from DNA breaks to chromosomal rearrangements

Risk from exposure to energetic heavy ions is considered one of the main problems for human space exploration. Late stochastic risk estimates, particularly cancer, are affected by large uncertainties. Basic cell biology studies to elucidate the mechanisms involved in genetic damage are necessary to reduce the uncertainty and eventually design effective countermeasures. To study the influence of nuclear architecture on the formation of chromosomal rearrangements, normal diploid human fibroblasts have been exposed to heavy ions in horizontal and vertical positions. Analysis of chromosomal aberrations by arm-specific mFISH shows that, at the same radiation dose, the yield of chromosomal damage is modified by the irradiation geometry. A clear difference is seen in the fraction of aberrant cells, owing to the different nuclear cross sections.

Monte Carlo simulation of microbeam radiation therapy with an interlaced irradiation geometry and an Au contrast agent in a realistic head phantom

In this study, dose distribution calculations for bidirectional interlaced microbeam radiation therapy (BIMRT) were performed with a detailed head phantom model and the Monte Carlo code MCNPX. Doses were calculated in intracranial targets of dimensions 20 × 6.8 × 20 mm3 and 20 × 20 × 20 mm3 and surrounding tissue for which interlacing arrays are composed of 5 and 15 microbeams, respectively. Simulations were performed with a realistic head phantom and a homogenized head phantom of the same outer shape to study the effects of the structure of the realistic phantom on dose distribution and to show how important it is to use realistic phantoms. Depth–dose profiles and dose falloffs at the edges of the targets were calculated for cases with and without an Au contrast agent deposited in the target region and surrounding tissue. The parallel pattern of the microbeam arrays was preserved through the head phantom which makes it possible to interlace microbeam arrays even at deep seated targets. As the dimensions of the target volume were increased, the valley dose values increased with the number of microbeams. This sets limits on the size and position of the target. The usage of gold as a contrast agent provided a substantial increase in target dose and decreased the skin entrance, maximum skull bone and maximum brain doses inevitable to produce the desired target dose. Short dose falloffs at the edges of the targets were preserved for all cases.

Simulation of Electron-Beam Irradiation of Skin Tissue Model

Monte Carlo simulation of electrons stopping in liquid water was used to model the penetration and quality of electron-beam irradiation incident on the full-thickness EpiDerm™ skin model (EpiDermFT™ MatTek, Ashland, VA). This 3D tissue model has a fully developed basement membrane separating an epidermal layer of keratinocytes in various stages of differentiation from a dermal layer of fibroblasts embedded in collagen. The simulations were motivated by a desire to selectively expose the epidermal layer to low-linear energy transfer (LET) radiation in the presence of a nonirradiated dermal layer. The variable-energy electron microbeam at the Pacific Northwest National Laboratory (PNNL) was used as a model of device characteristics and irradiation geometry. At the highest beam energy available (90 keV), we estimate that no more than a few percent of the beam energy will be deposited in the dermal layer. Energy deposition spectra were calculated for 10-µm-thick layers near the 10th, 50th and 90th percentiles of penetration by the 90 keV electron beam. Bimodal spectra showed an increasing component of "stoppers" with increasing depth, which increases the probability of large energy deposition events. Nevertheless, screening by tissue above the layer of interest is the main factor determining energy deposited at a given depth.

Dose Verification of Stereotactic Radiosurgery Treatment for Trigeminal Neuralgia with Presage 3D Dosimetry System

Achieving adequate verification and quality-assurance (QA) for radiosurgery treatment of trigeminal-neuralgia (TGN) is particularly challenging because of the combination of very small fields, very high doses, and complex irradiation geometries (multiple gantry and couch combinations). TGN treatments have extreme requirements for dosimetry tools and QA techniques, to ensure adequate verification. In this work we evaluate the potential of Presage/Optical-CT dosimetry system as a tool for the verification of TGN distributions in high-resolution and in 3D. A TGN treatment was planned and delivered to a Presage 3D dosimeter positioned inside the Radiological-Physics-Center (RPC) head and neck IMRT credentialing phantom. A 6-arc treatment plan was created using the iPlan system, and a maximum dose of 80Gy was delivered with a Varian Trilogy machine. The delivered dose to Presage was determined by optical-CT scanning using the Duke Large field-of-view Optical-CT Scanner (DLOS) in 3D, with isotropic resolution of 0.7mm3. DLOS scanning and reconstruction took about 20minutes. 3D dose comparisons were made with the planning system. Good agreement was observed between the planned and measured 3D dose distributions, and this work provides strong support for the viability of Presage/Optical-CT as a highly useful new approach for verification of this complex technique.

Dose distribution and mapping with 3D imaging presentation in intraoral and panoramic examinations

Abstract In current medical imaging applications, high quality images not only provide more diagnostic value for anatomic delineation but also offer functional information for treatment direction. However, this approach would potentially subscribe higher radiation dose in dental radiographies, which has been putatively associated with low-birth-weight during pregnancy, which affects the hypothalamus–pituitary–thyroid axis or thereby directly affects the reproductive organs. The aim of this study was to apply the high resolution 3-D image mapping technique to evaluate radiation doses from the following aspects: (1) verifying operating parameters of dental X-ray units, (2) measuring the leakage radiations and (3) mapping dose with 3-D radiographic imaging to evaluate dose distribution in head and neck regions. From the study results, we found that (1) leakage radiation from X-ray units was about 21.31±15.24 mR/h ( Our 3-D dose mapping demonstrated that dose values were relatively lower in soft tissues and higher in bone surfaces compared with other investigations. Multiple causes could contribute to these variations, including irradiation geometry, image equipment and type of technique applied, etc. From the results, we also observed that larger accumulated doses were presented in certain critical organs, such as salivary gland, thyroid gland and bone marrow. Potential biological affects associated with these findings warrant further investigation.

Evaluation of Scattering in Cone-Beam Breast Computed Tomography: A Monte Carlo and Experimental Phantom Study

In this paper Monte Carlo simulations were performed for X-ray irradiations of breast phantoms of various sizes such as PMMA cylinders of different diameters and a hemi-ellipsoidal PMMA phantom. The aim was the evaluation of the 2D distribution of primary and scattered photons and Scatter-to-Primary Ratio (SPR) in projection images in cone-beam breast Computed Tomography (CT). Irradiation geometry and technique factors reproduce the experimental conditions used for validation measurements with a prototype CT breast scanner. Simulations were performed with GEANT4 software. We varied the phantom diameter and shape, the X-ray tube voltage and added filtration. Magnification was 1.31. SPR increases from 0.4 (at 8 cm cylinder diameter) up to 1.5 (14 cm cylinder diameter) at the centre of the phantom. In the same phantom, SPR has lower values toward the bases of the cylinder than at its centre. The scatter component increases by adopting a 50 kVp or higher tube voltages, up to 80 kVp, and by increasing the added filtration. Simulated and measured lateral profiles across a 14 cm cylinder diameter in projection images show a relative deviation of 4%. Simulations show a different distribution of scatter and SPR in a 14 cm diameter cylinder and 14 cm hemi-ellipsoidal phantom, so questioning the use of simple cylindrical geometries when simulating the attenuation of the pendant breast for scatter correction procedures. The strength and the non-uniformity of the SPR inside the cylindrical phantom decrease as the size of the air gap between object and detector increases.

Potential changes in outdoor thermal comfort conditions in Gothenburg, Sweden due to climate change: the influence of urban geometry

AbstractQuantitative information about outdoor thermal comfort, on various temporal and spatial scales, is required to design better cities and mitigate heat problems not only in warm but also in temperate climates. The overall objective of this study is to explore the augmentation of global/regional climate changes by urban features such as geometry in a compact mid‐rise high‐latitude city (Gothenburg). The magnitude of spatial and temporal variations of intra‐urban mean radiant temperatures (Tmrt) is quantified using the SOLWEIG (SOlar and LongWave Environmental Irradiance Geometry) model. Hourly time resolution, statistically downscaled meteorological data, based on the ECHAM5‐GCM under the A1B emission scenario is used to simulate changes in Tmrt and physiologically equivalent temperature (PET) at the 2080–2099 time horizon.Results show that urban geometry causes large intra‐urban differences in Tmrt, on hourly, daytime and yearly time scales. In general, open areas are warmer than adjacent narrow street canyons in summer, but cooler in winter. According to the ECHAM5‐based scenario, the daytime Tmrt will increase by 3.2 °C by the end of this century. This is 0.4° more than simulated increase in air temperature (2.8 °C) and is mainly a result of decreases in summer cloudiness. Occasions of strong/extreme heat stress are expected to triple. This equates to 20–100 h a year, depending on geometry. Conversely, the number of hours with strong/extreme cold stress decreases by 400–450 h. Furthermore, the number of hours with no thermal stress increases by 40–200 h a year.The study confirms the potential for using geometry to mitigate daytime thermal stress. A densely built structure mitigates extreme swings in Tmrt and PET, improving outdoor comfort conditions both in summer and in winter. Furthermore, it highlights the importance of including information on either Tmrt or thermal comfort in climate scenarios to describe the combined effects of changes in multiple climate variables and to more realistically measure the impact on humans. Copyright © 2010 Royal Meteorological Society