Penumbral Effects Research Articles

Transferring Tsyganenko 1989 magnetospheric modulation for cosmic radiation cutoff rigidity values to different epochs of the International geomagnetic reference field

A technique has been developed that will transfer the Tsyganenko 1989 modulation for cutoff rigidity values to different epochs of the International Geomagnetic Reference Field (IGRF). The technique separates the effect of the Tsyganenko external currents on cutoff rigidity values previously calculated at 450 km using the 1995 IGRF such that these effects can be applied to other models of the IGRF. The Tsyganenko modulation effects were then applied to the 2020 internal field model. Two improvements to the original set of calculations include a three-point longitudinal smoothing to reduce the unevenness of the penumbral effects and a replacement of the original −100 Dst values for Kp values > 5 to −50 Dst increments. These changes have extended the original library of 88 world grids to a total of 128 world grids of cutoff rigidity values that are applicable for all Kp values (0–9 + ) and Dst values to −500 nT.

SU-E-T-421: 6MV Radiation Small Field Dose: Off Axis and Penumbra Effects, a Study with Radiochromic Film.

To accurately determine relative dose factor scaling, tissue depth relation, and penumbra details of small 6MV radiation fields with an emphasis on far-off-central-axis fields for various MLC designs -subject matter important in the context of SRS, FSRT, SBRT, and IMRT. Radiochromic film designed for quantitative dosimetry, EBT3™ film (Ashland Inc.) can be reliably used as a relative dosimeter to within 1% accuracy if one controls for both film and flatbed scanner variation. This enabled a study of the fine detail of small radiation fields and the impact upon these small dose profiles by different MLC designs both on central axis and off. In particular, the impact of leaf-end penumbra from bifocal MLC Optifocus (Siemens) and Synergy Beam Modulator (Elekta) is examined.EBT3 films were placed at 100cm SAD in solid water phantom and irradiated to 200 MU at various depths 1.5cm - 20cm and for various off-axis locations. The films were analysed to give the detailed dosimetric profile of each small field and relate them to both a standard reference field (10 × 10)cm2 and to a broadfield flatness/symmetry irradiation (20 × 20)cm2 . Films were scanned at ∼20 hours post irradiation on an Epson 2000 flatbed scanner and analysed using FilmQAPro and in-house methods. As small field size decreases, the field center falls within the penumbra of each MLC edge for megavoltage photon energies. The penumbral effects cause the conventional dosimetric quantity SP to sharply depart from the smooth fieldsize dependence for larger conventional fields. Careful characterization of small field dose and leaf end modeling within a TPS is crucial in both predicting accurate small field dosimetry and off-axis dosimetry.

The value of new imaging technology for target volume delineation

Cross sectional imaging is central to modern radiotherapy. The use of multimodality imaging can confer advantages for radiotherapy treatment planning by providing complimentary data as well as functional or biological information. The obvious use of imaging is to stage the extent of the disease and subsequently to assess response to therapy. This process is also important for radiotherapy. The intent of radiotherapy and plan design can alter substantially if there is involvement of the regional nodes or if there is local invasion into neighbouring surrounding normal structures. It is difficult to define the extent of microscopic or subclinical nodal disease spread and it cannot be visualised using morphological cross sectional imaging methods. Often it has to be estimated using clinical experience and knowledge of the disease. The use of functional and biological imaging studies can be useful in this respect and its utility is being investigated for radiotherapy. Apart from positron emission tomography (PET), which can provide functional and biological information, there are now functional studies using computed tomography (CT) and magnetic resonance imaging (MRI). Although there are systemic modes of irradiation, such as radioisotopes, the majority of irradiation is given locally to a specified anatomical site or organ. This is administered using either external beam or brachytherapy (interstitial or intracavitary therapy). As such, the radiation source needs to be aimed or placed at the tumour or target volume. Therefore, it is crucial to appropriately identify tumour volumes or target volume(s) so that the planning of radiotherapy and its delivery can be optimised. There is a set of pre-defined nomenclature for radiotherapy planning volumes. The International Commission for Radiation Units (ICRU) Report 50 and ICRU 62 (Table 1) define a series of volumes which take into consideration factors such as the subclinical extent of disease, internal organ motion due to physiological activity, patient movement, treatment set-up variability and penumbral effects of the treating beam [1,2]. A further update for this nomenclature is expected shortly to

MO‐D‐AUD B‐03: An Enabling Technology for Creating Sculpted Brachytherapy Dose Patterns with the Xoft Axxent™ System

Purpose: Study a potential means of partially attenuating X‐rays from the Xoft Axxent™ system over controlled spatial areas, while minimizing changes to depth‐dose characteristics. This would be the basis of an enabling technology to sculpt brachytherapy dose patterns to, for example, spare critical structures such as skin in breast brachytherapy treatments. Method and Materials: Measurements of output were made from the Xoft 50 kVp source attenuated by thin (0.001″) dot‐shaped layers of silver, with diameters of several mm. Measurements were made with azimuthal scans around the source at distances from 2 to 7 cm. The dots were attached to the source cooling catheter, approximately 2.6 mm from the source center. Attenuation calculations and Monte Carlo studies of the effect were performed using EGSnrc. Results: Partially attenuating dots of silver create shadows in measured dose that agree with attenuation calculations in proportion, and in the critical behavior of how the attenuation varies with distance. Monte Carlo studies were consistent with measured results. Materials other than silver (or elements nearby in atomic number) will harden the beam, and so the attenuation will lessen with distance. Silver has much less of a hardening effect, because the K absorption edge reduces the higher energy portion of the spectrum at a rate similar to the losses in the lower energy region. Conclusion: It is possible to create predictable, directed shadows in dose around the Xoft 50 kVp x‐ray source using practical thicknesses of silver foils. The shadows have soft edges owing to penumbral effects when the dots are placed on the cooling catheter, within a few mm of the source. Such shadows could be used in future applications to spare healthy tissue during brachytherapy. In a simulated breast treatment plan, using a simple model of the attenuation, isodose lines were shifted by several mm.

SU‐FF‐T‐273: Investigation of the Beam Penumbra Effect On IMRT Dose Conformity and Uniformity

Purpose: To investigate the effect of beam penumbra and intensity variation on IMRT dose distributions and to determine an optimal beam penumbra for IMRT dose delivery. Method and Materials: Differences in accelerator/MLC geometry may have a significant effect on IMRT dose conformity and uniformity. The beam penumbra is affected by photon energy, source size and the distance from the collimator to the patient. In this work, we have investigated the beam penumbral effect on IMRT dose distributions based on ten real IMRT cases (6 prostate, 2 breast and 2 head and neck) originally planned with the Corvus inverse planning system. Monte‐Carlo dose calculations were performed with different penumbra settings, which correspond to three accelerator models, Siemen‐Primus, Primart and Varian‐21Ex. A variety of beam penumbra settings were considered, including the primary and extrafocal source size and the distance from the MLC to the patient. Film measurements were performed to verify a penumbra effect model and the Monte Carlo calculations. Results: The results show that the dose for 95% of the target volume is not significantly affected by the difference in the beam penumbra. Siemen‐Primus and Primart have slightly larger geometrical beam penumbra and result in slightly larger doses to nearby critical structures. The Varian‐21Ex has smaller MLC leaves (5mm) and smaller beam penumbra but gives more heterogeneous target dose distributions (hot and cold spots). A beam overlapping model is used to explain these results. Conclusions: Smaller MLC leaves may improve dose conformity but introduce more target dose heterogeneity. The hot spots can be partially explained by the overlapping of MLC segments while larger beam penumbra can smear out the overlapping effect. An optimal beam penumbra can lead to the best target dose uniformity.

Effects of hydraulic architecture and spatial variation in light on mean stomatal conductance of tree branches and crowns

In a Pinus taeda L. (loblolly pine) plantation, we investigated whether the response to vapour pressure deficit (D) of canopy average stomatal conductance (G(S)) calculated from sap flux measured in upper and lower branches and main stems follows a hydraulically modelled response based on homeostasis of minimum leaf water potential (Psi(L)). We tested our approach over a twofold range of leaf area index (L; 2-4 m(2) m(-2)) created by irrigation, fertilization, and a combination of irrigation and fertilization relative to untreated control. We found that G(S) scaled well from leaf-level porometery [porometry-based stomatal conductance (g(s))] to branch-estimated and main stem-estimated G(S). The scaling from branch- to main stem-estimated G(S) required using a 45 min moving average window to extract the diurnal signal from the large high-frequency variation, and utilized a light attenuation model to weigh the contribution of upper and lower branch-estimated G(S). Our analysis further indicated that, regardless of L, lower branch-estimated G(S) represented most of the main stem-estimated G(S) in this stand. We quantified the variability in both upper and lower branch-estimated G(S) by calculating the SD of the residuals from a moving average smoothed diurnal. A light model, which incorporated penumbral effects on vertical distribution of direct light, was employed to estimate the variability in light intensity at each canopy level in order to explain the increasing SD of both upper and lower branch-estimated G(S) with light. The results from the light model showed that the upper limit of the variability in individual branch-estimated G(S) could be attributed to incoming light, but not the variation below that upper limit. A porous medium model of water flow in trees produced a pattern of variation below the upper limit that was consistent with the observed variability in branch-estimated G(S). Our results indicated that stems acted to buffer leaf- and branch-level variation and might transmit a less-variable water potential signal to the roots.

Importance of needle age and shoot structure on canopy net photosynthesis of balsam fir (Abies balsamea): a spatially inexplicit modeling analysis

We have developed a spatially inexplicit model of canopy photosynthesis for balsam fir (Abies balsamea (L.) Mill.) that accounts for key processes of light-shoot interaction including irradiance interception by the shoot, spatial aggregation of shoots into branches and crowns, the differential propagation of diffuse and direct light within the canopy, and an ideal representation of penumbra. Also accounted for in the model are the effects of the average radiative climate and shoot age on needle retention, light interception, and photosynthetic capacity. We used reduced versions of this model to quantify the effects of simplifying canopy representation on modeled canopy net photosynthesis. Simplifications explored were the omission of direct beam transformation into penumbral light and the use of different constant shoot properties throughout the canopy. The model was parameterized for a relatively dense balsam fir stand (leaf area index of 5.8) north of Québec City, Canada, and run using hourly meteorological data obtained at the site. The overall performance of the complete model was satisfactory, with maximum values of canopy net photosynthesis of 23 micromol (m(2) ground)(-1) s(-1) (83 mmol m(-2) h(-1)), and a near-saturation of the canopy at a photosynthetically active radiation photon flux density of about 750 micromol m(-2) s(-1) (2.7 mol m(-2) h(-1)). The omission of penumbral effects through the use of unattenuated direct (beam) radiation at all layers of the canopy, as used for broad-leaved species, reduced canopy net photosynthesis by 3.7%. Analysis of the results show that the small impact of penumbra on canopy net photosynthesis stems from the high proportion of diffuse radiation (73%) estimated from our meteorological data set; single-hour results under clear sky conditions approach theoretical bias values of about 30%. Use of mean shoot photosynthetic, light capture and light transmission properties throughout the canopy biased canopy net photosynthesis by less than 3%. However, simulations carried out based on properties of 1-year-old shoots throughout the canopy overestimated canopy net photosynthesis by 9%. Use of the shoot as our smallest functional unit was a potential source of bias because the differential absorption of direct and diffuse radiation within the shoot could not be factored into the model. Other sources of potential bias are discussed.

Penumbral and foliage distribution effects on Pinus sylvestris canopy gas exchange

Tree canopy water use and foliage net CO2 uptake (NPP) were simulated for a 31-year-old Pinus sylvestris (Scots pine) plantation near Hartheim, in the Upper Rhine Valley, Germany with a mechanistically-based, three-dimensional stand gas-exchange model (STANDFLUX) for a ten-day period during spring 1992. STANDFLUX was formulated to include the effects of penumbra caused by the fine structure of the needles on light distribution within crowns. Good correspondence was found between simulated rates of tree canopy water use when including penumbral effects and eddy-covariance ET and sap flow transpiration measurements. Water use was 8–13% lower and NPP was 10–17% lower in simulations for the ten-day period when penumbral effects were not included.

Calibration of grey values of hemispherical photographs for image analysis

The hemispherical photography method is widely used for the estimation of solar radiation in forests or agricultural crops. However, calibration problems repeatedly arise concerning the grey-value information of the photographs. This article introduces a method which may help to solve these calibration problems. The method takes into consideration two information transport media (black-and-white film and digital image analysis). This method is based on the exposure of the photographs to the zenith luminance of an overcast sky and the determination of the gamma function of the film by means of an optical density wedge photograph. By taking these characteristics into account, it is possible to obtain reproducible and bias-free results from the diffuse site factor even with different film materials. Moreover, the modelling of penumbral effects is shown. Limitations are discussed.

Automatic detection of single MLC leaf positions with corrections for penumbral effects and portal imager dose rate characteristics

A method has been developed to determine the individual settings of the MLC leaves from the image of the beam portal. The portal image is analysed by a two-step approach: (i) The general orientation of the collimator leaves is determined. (ii) Next, a one-dimensional edge operator (generalized Laplacian) is applied in a scanline fashion to extract an array holding the image coordinates of the field edge. The prescribed position of each leaf is then compared with the position of the steepest gradient behind that leaf as determined from the portal image. To obtain adequate precision in the determination of the leaf positions, the dose rate dependency of the electronic image detector, the increased transmission through the leaf tips and flanges, and the properties of the edge detector used for image processing were investigated. From these studies, a set of correction terms were obtained that are used to correctly calculate the position of the field edge from the prescribed MLC leaf settings. If not corrected, a leaf position-dependent discrepancy in the range -0.5 to 1.0 mm will arise. The results from tests of this method, applying the correction terms, show that the leaf positions can be determined with a precision better than 0.1 mm (one standard deviation). The reproducibility of the leaf positions was found to be better than 0.1 mm.

Penumbra in within-shoot and between-shoot shading in conifers and its significance for photosynthesis

Simulations of the statistical distribution of direct solar irradiance in a Scots pine ( Pinus sylvestris L.) crown were used to evaluate the effect of a penumbra (a partial shading of the solar disc) on the rate of photosynthesis. Three different situations were considered: the rate of photosynthesis was estimated at points on needles subjected to (a) shading from the same shoot, (b) shading from another shoot, and (c) both within- and between-shoot shading. Results showed that the penumbral effect of within-shoot shading was small, i.e., the assumption of parallel solar beam geometry within a shoot did not lead to serious underestimates of the rate of photosynthesis. In contrast, shading from another shoot (needle layer) situated further away than ca. 25 cm from the target point could be better characterised as diffuse shading. It is proposed that, in modelling the penetration of direct radiation in a coniferous stand, shading from a short distance (“within-shoot shading”) and that from a longer distance (“between-shoot shading”) be treated separately. A method is presented to estimate mean rates of photosynthesis based on this approach. This method was found to be more accurate than two commonly used methods to estimate photosynthesis, one of which is based on the assumption of parallel solar beam geometry (no penumbra) while the other assumes no stochastic variation in the direct solar component.

ガス注入型プラズマＸ線源

A high brightness, compact plasma x-ray source for x-ray lithography employing a gas-puff z-pinch plasma is developed. This source is designed to direct the x-ray downward into the atmosphere by connecting a diode with a condenser bank through a parallel transmission line and to easily link to a conventional aligner. High x-ray output power is attained by high-repetition-rate discharge (3Hz), developing a fast-acting gas valve. When pinched plasma is formed, the x-rays are emitted from the pinched columnar plasma region (20mm in length and 2mm in diameter). Axial x-ray exposure is achieved by a newly developed x-ray extraction method to reduce the blur in pattern replication caused by the penumbral effect. The x-rays are efficiently extracted through a window highly transparent to x-rays. With this x-ray source, 9-14 A x-rays with 200 J per pulse (600W average power) are obtained using Ne gas plasma in a 3Hz operation. The x-rays through x-ray extraction window (15-μm-thick Be) and x-ray mask (1-μm-thick SiN) have a peak intensity at 12 A wavelengths. The x-ray wavelength is longer than that of a conventional electron impact source. In addition, hard x-rays are not emitted by this plasma x-ray source. High contrast sufficient to replicate fine patterns can be obtained using a mask with a thin absorber, such as 0.4-μm-thick Ta. A 0.2μm resist pattern can be easily replicated.

Penumbral Effects on Sunlight Penetration in Plant Communities

When sunlight passes through openings in leaf or overstory plant canopies, strong penumbral effects (e.g., penumbral light-spreading) can result in major changes in the total sunlit area, the irradiance level, and exposure duration at the incident surface. These effects are based on well-known optical principles whereby light that originates from a non-point source (e.g., sunlight) and passes through a circular aperture will project an image composed of a bright center portion (numbra) circumscribed by a shadow-edge (penumbra). A hypothetical opening diameter (G) and distance to the incident surface (D) was expressed as the ratio G/D (= GDR) and used to quantify changes in the projected sun spot diameter (Gp) and area (Gparea), and the relative irradiance levels and durations of the numbra and penumbra. When GDR was 80% of the exposure period. Beyond a GDR of 0.05, pen- umbral effects were negligible. Definitions of a sunfleck, sunpatch, gap, and clearing are proposed based on the specific sunlight regimes generated by the corresponding GDR values. These penumbral and latitudinal effects on sunlight penetration through canopy openings are discussed for a variety of leaf canopy and forest overstory types using rep- resentative gap sizes and vegetation heights.

A numerical model for simulating the radiation regime within a deciduous forest canopy

A numerical model that simulates the radiation regime inside a fully leafed and leafless deciduous forest canopy is discussed and tested. The model incorporates features which account for the vertical variation in leaf inclination angles and penumbral effects; these features have been experimentally shown to significantly influence the radiation regime in a deciduous forest. This version of the model mimics measured values quite well and improves upon estimates of the canopy radiation regime that are based upon the general assumption that the leaf inclination angle distribution is spherical and that the Poisson probability distribution describes the probability of beam penetration. On the other hand, the numerical model assumes first-order scattering, which is inferior to multiple scattering routines.

Analysis of light transmissivity of forest canopies with a telephoto method

A new method using telephotographs was devised to assess the light transmissivity of forest canopies having the penumbral effect, and applied to three types of forests typical of warm temperate Japan. Photographs of forest canopies at various angular altitudes were taken with a 270-mm telephoto lens and enlarged. Each photograph was then superimposed with a transparent sheet having a circle corresponding in size to the apparent angular size of the solar disk on the print. The circle was moved over regularly distributed points, and the fraction unobstructed by the images of plant parts was read at each point and used to determine frequency distributions of the direct beam transmissivity as well as the averaged diffuse site factors. A preponderance of low direct beam transmissivity was found for all the forest canopies studied, indicating that most of the sunflecks on the forest floors receive only a minute part of the direct solar beam incident on the canopies. The average transmissivity decreased with decreasing angular altitude for each type of forest canopy. In the pine forest, the relationship between transmissivity and altitude did not significantly differ from that predicted by Beer's law. However, in the two broadleaved forest canopies, the relationship for lower altitudes differed markedly from that predicted.

A plasma x-ray source for x-ray lithography

Employing a gas-‘‘puff’’ z-pinch plasma, a high brightness and compact plasma x-ray source has been constructed by using a high-repetition-rate discharge and a new x-ray extraction method. The high-repetition-rate discharge (3 Hz) is attained by using a fast-acting gas valve, capable of operating stably under gas plenum pressures below 300 Torr. This low gas pressure discharge makes possible high-conversion efficiency of electrical input to x-rays, highly reliable plasma pinching, and a drastic reduction of debris resulting from electrode vaporization. An axial x-ray exposure is required to reduce the blur in pattern replication caused by the penumbral effect. To carry out the axial exposure, the high-energy particles emitted from the plasma are removed using a plasma reflector plate and magnet. The x rays are efficiently extracted through a window highly transparent to x rays. With this x-ray source, 9–14 Å x rays with 200 J per pulse (600 W average power) are obtained using Ne gas plasma in a 3 Hz operation.

High brightness laser/plasma source for high throughput submicron x-ray lithography

Laser irradiation of solid targets has been demonstrated to produce an extremely intense source of soft x rays. Conversion efficiencies of greater than 10% emission of over 1 keV radiation into 2π sr have been demonstrated with subjoule 1.06 μ laser pulses. This high conversion efficiency is achieved by using subnanosecond pulse lengths, tight focusing, and shaped pulses. A laser capable of producing the requisite high quality laser pulses at 100 Hz repetition rates has been constructed using newly developed slab techniques to maintain good beam quality at high average powers. A windowless target chamber has been designed to mate to x-ray stepper/aligners which are now being built. The present laser will permit average x-ray intensities of 2 mW/cm2 to be realized behind current (boron nitride) x-ray masks with negligible penumbral effects. This is an order of magnitude higher than possible with conventional electron beam impact sources and, with the present or upgraded laser system, should assist the commercialization of x-ray steppers.

The influence of penumbra on canopy photosynthesis I. Theoretical considerations

A theoretical framework for calculating penumbral effects in leaf canopies is developed. The geometry of partial shading of the sun's disk by straight-edge segments and by circular-edge segments of leaves is considered and the respective conditional probability densities with respect to visible solar fraction are derived. It is found convenient to introduce the concepts of augmented and diminished leaf area index, which are extensions, in the case of a finite solar disk, of the concept of leaf area index which is widely used in the calculation of light extinction in leaf canopies irradiated by a perfectly parallel incident beam. Calculation of probability of irradiation by the full sun and probability of complete shading are discussed. The role of apparently intersecting leaf edges in producing umbra and penumbra is discussed.

乳房撮影に関する研究 その2 : XeromammographyとFilmmammographyの画像の比較検討

The xerox 125 system was used to make a comparative image quality study between xeroradiography (XR) and industrial type MR film which is approximately equivalent to Kodak type AA film. The comparison of the image quality was made by objective and subjective methods at the visual threshold, since diagnostic details in actual mammograms are often marginably detectable. 1) Dependence of the output density distribution on edge gradient is studied in the spatial and the spatial frequency domain. The test objects employed are aluminum rods having square and round cross-sections of 200 μ which simulate calcification in X-ray absorption. The infinite edge gradient of the square alumiunm rod narrows the width at the image with a wide deletion of the powder outside the boundary, while the alumiunm cylinder increases its image width by attracting extra powder inside the boundary. Density distributions of the alminum test objects are converted to spatial frequency spectra by means of Fourier transformation. The use of a square bar results in an excessive decrease of the output amplitude at low spatial frequencies though greater in amplitude in the intermediate to high spatial frequencies than the cylindrical object. 2) RMS granularity σ(D) is determined by a scanning microdensitometer with a 10×150 μ wide scanning slit adjusted with respect to eye's line spread function. Ganularity of the 125 system is dramatically improved than other xeroradiographic systems which have been seen. It may be due to plastic coating on the transfer paper and smaller toner particle size. However, XR is grainiest among the four imaging systems tested. The density fluctuation of the XR is 4 times greater than MR film. This greater density fluctuation may be a result of the high grain pattern contrast since the effective grain sizes are as small as MR film. 3) The concept of "signal-to-noise ratio" is applied to the imaging systems in order to obtain results from objective methods correlated with subjective results or visibility. The output density distributions of alumiunm cylinders ranging from 100 to 1,000 μ in diameter are taken as the inputs to the visual system, and σ(D), as noise. Physiological contrast D/ΔD min. is also taken into consideration for XR in which the surrounding density of an object varies with the magnitude of the edge-effect. The result reveals that XR provides a higher ratio than MR film for aluminum cylinders smaller than 1,000 μ in diameter. XR differs the most from MR film in the radio at 700 μ diameter or so where XR is considered to show the best visibility of calcifications. 4) Visibility is defined here as the adiliity to detect minute calcification in mammary carcinoma ranging 90〜400 μ in diameter, and comparison of visibility between XR and film is made with following factors ; exposure time, radiographic density and geometrical condition. As a whole, XR is superior to film in visibility at both 30 kVp providing additional detectability for smaller calcification by roughly 20 μ in diameter. When visibility of XR is related to varing exposure time, it is far less sensitive than film, but on the contrary more sensitive than film to the radiographic density. Optimum density for XR is very low as 0.3 were the XR print provides the maximum visibility, while 1.5 for film at 7,000 lux rear illumination. XR is less affected by penumbral effects than film, since the visibility of XR depends much more on the contrast than the shapness for the object larger than 100 μ in diameter. It is concluded that the 125 system satisfied an indespensable necessity of mammography for finding small objects in the breast which a patient or her doctor could not find.

Forest Cover as a Solar Camera: Penumbral Effects in Plant Canopies

Forest Cover as a Solar Camera: Penumbral Effects in Plant Canopies