bluuebunny/crossref_metadata_2025 huggingface dataset:
Threshold adjusted calcium scoring using CT is less susceptible to cardiac motion and more accurate
Authors: Groen J. M. , Dijkstra H. , Greuter M. J. W. , Oudkerk M.
Published: February 2009

The purpose of this paper is to investigate calcium scoring on computed tomography (CT) using an adjusted threshold depending on the maximum Hounsfield value within the calcification . The volume of 19 calcifications was retrospectively determined on 64‐slice multidetector CT and dual source CT (DSCT) at different thresholds and the threshold associated with the physical volume was determined. In addition, approximately computer simulations were done simulating the same process for calcifications with mixed density. Using these data a relation between the and the threshold could be established. Hereafter, this relation was assessed by scanning six calcifications in a phantom at 40–110 beats per minute using DSCT. The influence of motion was determined and the measured calcium scores were compared to the physical volumes and mass. A positive linear correlation was found between the scoring threshold and the of the calcifications both for the phantom measurements as for the computer simulations. Using this relation the individual threshold for each calcification could be calculated. Calcium scores of the moving calcifications determined with an adjusted threshold were approximately 30% less susceptible to cardiac motion compared to standard calcium scoring. Furthermore, these scores approximated the physical volume and mass at least 10% better than the standard calcium scores. The threshold in calcium scoring should be adjusted for each individual calcification based on the of the calcification. Calcium scoring using an adjusted threshold is less susceptible to cardiac motion and more accurate compared to the physical values.
Deformable registration of abdominal kilovoltage treatment planning CT and tomotherapy daily megavoltage CT for treatment adaptation
Authors: Yang Deshan , Chaudhari Summer R. , Goddu S. Murty , Pratt David , Khullar Divya , Deasy Joseph O. , El Naqa Issam
Published: February 2009

In adaptive radiation therapy the treatment planning kilovoltage CT (kVCT) images need to be registered with daily CT images. Daily megavoltage CT (MVCT) images are generally noisier than the kVCT images. In addition, in the abdomen, low image contrast, differences in bladder filling, differences in bowel, and rectum filling degrade image usefulness and make deformable image registration very difficult. The authors have developed a procedure to overcome these difficulties for better deformable registration between the abdominal kVCT and MVCT images. The procedure includes multiple image preprocessing steps and a two deformable registration steps. The image preprocessing steps include MVCT noise reduction, bowel gas pockets detection and painting, contrast enhancement, and intensity manipulation for critical organs. The first registration step is carried out in the local region of the critical organs (bladder, prostate, and rectum). It requires structure contours of these critical organs on both kVCT and MVCT to obtain good registration accuracy on these critical organs. The second registration step uses the first step results and registers the entire image with less intensive computational requirement. The two‐step approach improves the overall computation speed and works together with these image preprocessing steps to achieve better registration accuracy than a regular single step registration. The authors evaluated the procedure on multiple image datasets from prostate cancer patients and gynecological cancer patients. Compared to rigid alignment, the proposed method improves volume matching by over 60% for the critical organs and reduces the prostate landmark registration errors by 50%.
Accuracy in the localization of thoracic and abdominal tumors using respiratory displacement, velocity, and phase
Authors: Langner U. W. , Keall P. J.
Published: February 2009

Current four dimensional (4D) computed tomography (CT) reconstruction techniques are retrospectively created based on either the phase or displacement of the respiratory signal. Both techniques have known limitations which cause clinically significant motion artifacts in 4D CT images. These artifacts, which appear as undefined or irregular boundaries in the 4D CT images, cause systematic errors in patient contouring and dose calculations. The aim of this work was to evaluate the reproducibility of tumor position as a function of displacement, phase, and velocity of the respiratory signal, respectively, in order to determine the optimum parameter or combination of parameters to use in order to minimize artifacts in 4D CT images or to accurately deliver radiation to relevant structures during treatment. Method and materials: Estimated tumor centroid position and respiratory signal data were acquired with the Cyberknife Synchrony system for 26 thoracic radiotherapy patients (52 fractions). A reference respiratory cycle was calculated for each patient. Displacement, phase, and velocity of ten data points were calculated from this reference respiratory cycle, where each point represents an image bin. The corresponding tumor position was then sorted into these image bins if the phase, displacement, simultaneous displacement and phase, or simultaneous displacement and velocity of the respiratory signal were within tolerances of for displacement and for velocity, respectively, from the corresponding data of the reference cycle for each image bin. Results: The mean of the standard deviations of tumor positions over all bins and all fractions for the superior‐inferior direction were for phase sorting, for displacement sorting, for simultaneous displacement and phase sorting, and for simultaneous displacement and velocity sorting, with maximum deviations of 43.0, 16.1, 15.5, and for each scenario, respectively. The same trend was observed for the anterior‐posterior and left‐right directions. A linear dependence was observed between the mean of the standard deviations of tumor positions over all fractions as a function of the velocity of the respiratory signal at each bin for all the sorting scenarios. A substantially larger gradient for the phase sorting scenario, compared to the other scenarios, suggests that tumor localization will become increasingly less accurate as the velocity of the tumor increases during a breathing cycle, e.g., if the amplitude of motion increases while the period of the respiratory cycle stays constant or during mid inhale or exhale phases of the respiratory cycle. Conclusion: This study illustrates that position of a tumor can be determined more accurately if displacement and velocity are used simultaneously as sorting parameters for 4D CT images or during treatment. A real‐time displacement and velocity based 4D CT image sorting method may therefore produce fewer and smaller artifacts in 4D CT images than current retrospective sorting methods.
Optimizing the dynamic range extension of a radiochromic film dosimetry system
Authors: Devic Slobodan , Tomic Nada , Soares Christopher G. , Podgorsak Ervin B.
Published: February 2009

The authors present a radiochromic film dosimetry protocol for a multicolor channel radiochromic film dosimetry system consisting of the external beam therapy (EBT) model GAFCHROMIC™ film and the Epson Expression 1680 flat‐bed document scanner. Instead of extracting only the red color channel, the authors are using all three color channels in the absorption spectrum of the EBT film to extend the dynamic dose range of the radiochromic film dosimetry system. By optimizing the dose range for each color channel, they obtained a system that has both precision and accuracy below 1.5%, and the optimized ranges are for the red channel, for the green channel, and above for the blue channel.
Maximum proton kinetic energy and patient‐generated neutron fluence considerations in proton beam arc delivery radiation therapy
Authors: Sengbusch E. , Pérez‐Andújar A. , DeLuca P. M. , Mackie T. R.
Published: February 2009

Several compact proton accelerator systems for use in proton therapy have recently been proposed. Of paramount importance to the development of such an accelerator system is the maximum kinetic energy of protons, immediately prior to entry into the patient, that must be reached by the treatment system. The commonly used value for the maximum kinetic energy required for a medical proton accelerator is , but it has not been demonstrated that this energy is indeed necessary to treat all or most patients eligible for proton therapy. This article quantifies the maximum kinetic energy of protons, immediately prior to entry into the patient, necessary to treat a given percentage of patients with rotational proton therapy, and examines the impact of this energy threshold on the cost and feasibility of a compact, gantry‐mounted proton accelerator treatment system. One hundred randomized treatment plans from patients treated with IMRT were analyzed. The maximum radiological pathlength from the surface of the patient to the distal edge of the treatment volume was obtained for 180° continuous arc proton therapy and for 180° split arc proton therapy (two 90° arcs) using CT# profiles from the Pinnacle™ (Philips Medical Systems, Madison, WI) treatment planning system. In each case, the maximum kinetic energy of protons, immediately prior to entry into the patient, that would be necessary to treat the patient was calculated using proton range tables for various media. In addition, Monte Carlo simulations were performed to quantify neutron production in a water phantom representing a patient as a function of the maximum proton kinetic energy achievable by a proton treatment system. Protons with a kinetic energy of , immediately prior to entry into the patient, were needed to treat 100% of patients in this study. However, it was shown that 90% of patients could be treated at , and 95% of patients could be treated at . Decreasing the proton kinetic energy from decreases the total neutron energy fluence produced by stopping a monoenergetic pencil beam in a water phantom by a factor of 2.3. It is possible to significantly lower the requirements on the maximum kinetic energy of a compact proton accelerator if the ability to treat a small percentage of patients with rotational therapy is sacrificed. This decrease in maximum kinetic energy, along with the corresponding decrease in neutron production, could lower the cost and ease the engineering constraints on a compact proton accelerator treatment facility.
Compression and smart coding of offset and gain maps for intraoral digital x‐ray sensors
Authors: Frosio I. , Borghese N. A.
Published: February 2009

The response of indirect x‐ray digital imaging sensors is often not homogenous on the entire surface area. In this case, calibration is needed to build offset and gain maps, which are used to correct the sensor output. The sensors of new generation are equipped with an on‐board memory, which serves to store these maps. However, because of its limited dimension, the maps have to be compressed before saving them. This step is critical because of the extremely high compression rate required. The authors propose here a novel method to achieve such a high compression rate, without degrading the quality of the sensor output. It is based on quad tree decomposition, which performs an adaptive sampling of the offset and gain maps, matched with a RBF‐based interpolation strategy. The method was tested on a typical intraoral radiographic sensor and compared with traditional compression techniques. Qualitative and quantitative results show that the method achieves a higher compression rate and produces images of superior quality. The method can be adopted also in different fields where a high compression rate is required.
Direct two‐dimensional reconstruction algorithm for an inverse‐geometry CT system
Authors: Baek Jongduk , Pelc Norbert J.
Published: February 2009

An inverse‐geometry computed tomography (IGCT) system uses a large source array opposite a smaller detector array. A previously described IGCT reconstruction algorithm uses gridding, but this gridding step produces blurring in the reconstructed image. In this article, the authors describe a two‐dimensional (2D) IGCT reconstruction algorithm without gridding. In the transverse direction, the raw data of the IGCT system can be viewed as being composed of many fan beams. Because the spacing between source spots is larger than the spot width, each fan beam has undersampled projection data, but the missing samples are effectively provided by other undersampled fan beam views. In the proposed method, a direct fan beam reconstruction algorithm is used to process each undersampled fan beam. Initial images with this method showed ring artifacts caused by nonuniform sampling in the radial direction as compared to an ideal fan beam. A new method for correcting this effect was developed. With this correction, high quality images were obtained. The noise performance of the proposed 2D IGCT reconstruction algorithm was investigated, and it was comparable to that of the fan beam system. A MTF study showed that the proposed method achieves better resolution than the gridding method.
Optical observation of energy loss distribution and practical range of positrons from a water solution in a water‐equivalent phantom
Authors: Hasegawa Tomoyuki , Yoshida Eiji , Shibuya Kengo , Murayama Hideo
Published: February 2009

The energy loss distribution of particles is closely related to their maximum penetration depth distribution and annihilation point distribution. The latter is of practical importance for positron emission tomography. Experimental data related to the energy loss distribution are important for comprehensive validation of physics and simulation models of interactions. In this paper the authors present a new experimental approach that allows them to visually observe the energy loss distribution of a solution of nuclear medicine radioisotopes in a plastic scintillator using an optical camera. The authors also report a set of the first experimental results. A water solution of was localized in a small hole in a plastic scintillator (BC430). Optical imaging of the scintillator yielded visual images of the energy loss distribution with a submillimeter resolution. The radial dependence in the energy distribution was quantitatively measured by analysis of the images, and exponential fitting parameters were obtained. The authors observed that the results of Monte Carlo simulation with EGS5 (version 1.0.2) and GEANT4 (version 4.9.01.p01) were consistent with those obtained experimentally. The results of the Monte Carlo simulation indicated that for a linear scale, the energy loss distribution in the scintillator was approximately the same as that in water, and the relative shape of the energy loss distribution was close to those of the maximum penetration depth distribution and annihilation point distribution. This paper also presents discussions about the further possibilities of this optical imaging approach. Thus, optical observation of the energy loss distribution in a scintillator is a promising technique for visual and quantitative experimental studies of emission from a solution of radioisotopes that are used in nuclear medicine.
An investigation on the capabilities of the PENELOPE MC code in nanodosimetry
Authors: Bernal M. A. , Liendo J. A.
Published: February 2009

The Monte Carlo (MC) method has been widely implemented in studies of radiation effects on human genetic material. Most of these works have used specific‐purpose MC codes to simulate radiation transport in condensed media. PENELOPE is one of the general‐purpose MC codes that has been used in many applications related to radiation dosimetry. Based on the fact that PENELOPE can carry out event‐by‐event coupled electron‐photon transport simulations following these particles down to energies of the order of few tens of eV, we have decided to investigate the capacities of this code in the field of nanodosimetry. Single and double strand break probabilities due to the direct impact of rays originated from and isotopes and characteristic x‐rays, from Al and C K‐shells, have been determined by use of PENELOPE. Indirect damage has not been accounted for in this study. A human genetic material geometrical model has been developed, taking into account five organizational levels. In an article by Friedland et al. [Radiat. Environ. Biophys. 38, 39–47 (1999)], a specific‐purpose MC code and a very sophisticated DNA geometrical model were used. We have chosen that work as a reference to compare our results. Single and double strand‐break probabilities obtained here underestimate those reported by Friedland and co‐workers by 20%–76% and 50%–60%, respectively. However, we obtain RBE values for , and radiations in agreement with those reported in previous works [Radiat. Environ. Biophys. 38, 39–47 (1999)] and [Phys. Med. Biol. 53, 233–244 (2008)]. Some enhancements can be incorporated into the PENELOPE code to improve its results in the nanodosimetry field.
Nonrigid registration of three‐dimensional ultrasound and magnetic resonance images of the carotid arteries
Authors: Nanayakkara Nuwan D. , Chiu Bernard , Samani Abbas , Spence J. David , Samarabandu Jagath , Parraga Grace , Fenster Aaron
Published: February 2009

Atherosclerosis at the carotid bifurcation can result in cerebral emboli, which in turn can block the blood supply to the brain causing ischemic strokes. Noninvasive imaging tools that better characterize arterial wall, and atherosclerotic plaque structure and composition may help to determine the factors which lead to the development of unstable lesions, and identify patients at risk of plaque disruption and stroke. Carotid magnetic resonance (MR) imaging allows for the characterization of carotid vessel wall and plaque composition, the characterization of normal and pathological arterial wall, the quantification of plaque size, and the detection of plaque integrity. On the other hand, various ultrasound (US) measurements have also been used to quantify atherosclerosis, carotid stenosis, intima‐media thickness, total plaque volume, total plaque area, and vessel wall volume. Combining the complementary information provided by 3D MR and US carotid images may lead to a better understanding of the underlying compositional and textural factors that define plaque and wall vulnerability, which may lead to better and more effective stroke prevention strategies and patient management. Combining these images requires nonrigid registration to correct the nonlinear misalignments caused by relative twisting and bending in the neck due to different head positions during the two image acquisition sessions. The high degree of freedom and large number of parameters associated with existing nonrigid image registration methods causes several problems including unnatural plaque morphology alteration, high computational complexity, and low reliability. Thus, a “twisting and bending” model was used with only six parameters to model the normal movement of the neck for nonrigid registration. The registration technique was evaluated using 3D US and MR carotid images at two field strengths, 1.5 and , of the same subject acquired on the same day. The mean registration error between the segmented carotid artery wall boundaries in the target US image and the registered MR images was calculated using a distance‐based error metric after applying a “twisting and bending” model based nonrigid registration algorithm. An average registration error of was obtained for MR and for MR, when registered with 3D US images using the nonrigid registration technique presented in this paper. Visual inspection of segmented vessel surfaces also showed a substantial improvement of alignment with this nonrigid registration technique compared to rigid registration.
Atlas‐based functional radiosurgery: Early results
Authors: Stancanello J. , Romanelli P. , Pantelis E. , Sebastiano F. , Modugno N.
Published: February 2009

Functional disorders of the brain, such as dystonia and neuropathic pain, may respond poorly to medical therapy. Deep brain stimulation (DBS) of the globus pallidus pars interna (GPi) and the centromedian nucleus of the thalamus (CMN) may alleviate dystonia and neuropathic pain, respectively. A noninvasive alternative to DBS is radiosurgical ablation [internal pallidotomy (IP) and medial thalamotomy (MT)]. The main technical limitation of radiosurgery is that targets are selected only on the basis of MRI anatomy, without electrophysiological confirmation. This means that, to be feasible, image‐based targeting must be highly accurate and reproducible. Here, we report on the feasibility of an atlas‐based approach to targeting for functional radiosurgery. In this method, masks of the GPi, CMN, and medio‐dorsal nucleus were nonrigidly registered to patients’ T1‐weighted MRI (T1w‐MRI) and superimposed on patients’ T2‐weighted MRI (T2w‐MRI). Radiosurgical targets were identified on the T2w‐MRI registered to the planning CT by an expert functional neurosurgeon. To assess its feasibility, two patients were treated with the CyberKnife using this method of targeting; a patient with dystonia received an IP ( prescribed to the 65% isodose) and a patient with neuropathic pain received a MT ( to the 77% isodose). Six months after treatment, T2w‐MRIs and contrast‐enhanced T1w‐MRIs showed edematous regions around the lesions; target placements were reevaluated by DW‐MRIs. At post‐treatment steroids for radiation‐induced edema and medications for dystonia and neuropathic pain were suppressed. Both patients experienced significant relief from pain and dystonia‐related problems. Fifteen months after treatment edema had disappeared. Thus, this work shows promising feasibility of atlas‐based functional radiosurgery to improve patient condition. Further investigations are indicated for optimizing treatment dose.
Measurement of the x‐ray tube anodes’ surface profile and its effects on the x‐ray spectra
Authors: Erdélyi M. , Lajkó M. , Kákonyi R. , Szabó G.
Published: February 2009

An experimental study—involving measurements with an optical microscope, a profilometer, and a scanning electron microscope—for determination of the surface profile of x‐ray tube anodes is presented. The islands on the “mud‐flatting” surface are separated by approximately deep cracks. The surface roughness on the island is typically below , and the area ratio of cracks to the total surface is higher on the more extensively used regions (anode aging). A simple model was proposed to calculate the spectrum modification introduced by the rough surface. Loss of x‐ray intensity of 4% was predicted using the roughest surface at a small emission angle.
Statistical characteristics of streak artifacts on CT images: Relationship between streak artifacts and mA s values
Authors: Imai Kuniahru , Ikeda Mitsuru , Enchi Yukihiro , Niimi Takanaga
Published: February 2009

The purpose of this study is to investigate how streak artifacts on computed tomography (CT) images vary with reduction in radiation doses by assessing the quantitative relationship between the streak artifacts and milliampere‐time product (mA s) values. A commercially available chest phantom was used to measure the streak artifacts on the CT images obtained using a 4‐ and 16‐multidetector‐row helical CT scanners with various mA s values at a constant tube voltage of . The cardiac slice image was employed as a target image for evaluating the streak artifacts on the CT image. Eighty parallel line segments with a length of were placed perpendicular to numerous streak artifacts on the cardiac slice image, and the largest difference between adjacent CT values in each of the 80 CT‐value profiles of these line segments was employed as a feature variable of streak artifacts; these feature variables have been analyzed by the extreme value theory. The largest difference between adjacent CT values in each CT‐value profile can be statistically modeled by a Gumbel distribution. Further, the maximum level of streak artifacts on CT images that will be tolerated for clinical use and low‐dose CT screening examination was expected to be estimated using the location parameter in the Gumbel distribution.
Considerations and limitations of fast Monte Carlo electron transport in radiation therapy based on precalculated data
Authors: Jabbari Keyvan , Keall Paul , Seuntjens Jan
Published: February 2009

The purpose of this work is to revisit the impediments and characteristics of fast Monte Carlo techniques for applications in radiation therapy treatment planning using new methods of utilizing pregenerated electron tracks. The limitations of various techniques for the improvement of speed and accuracy of electron transport have been evaluated. A method is proposed that takes advantage of large available memory in current computer hardware for extensive generation of precalculated data. Primary tracks of electrons are generated in the middle of homogeneous materials (water, air, bone, lung) and with energies between 0.2 andusing the EGSnrc code. Secondary electrons are not transported, but their position, energy, charge, and direction are saved and used as a primary particle. Based on medium type and incident electron energy, a track is selected from the precalculated set. The performance of the method is tested in various homogeneous and heterogeneous configurations and the results were generally within 2% compared to EGSnrc but with a 40–60 times speed improvement. In a second stage the authors studied the obstacles for further increased speed‐ups in voxel geometries by including ray‐tracing and particle fluence information in the pregenerated track information. The latter method leads to speed increases of about a factor of 500 over EGSnrc for voxel‐based geometries. In both approaches, no physical calculation is carried out during the runtime phase after the pregenerated data has been stored even in the presence of heterogeneities. The precalculated data are generated for each particular material and this improves the performance of the precalculated Monte Carlo code both in terms of accuracy and speed. Precalculated Monte Carlo codes are accurate, fast, and physics independent and therefore applicable to different radiation types including heavy‐charged particles.
An energy fluence‐convolution model for amorphous silicon EPID dose prediction
Authors: Greer Peter B. , Cadman Patrick , Lee Christopher , Bzdusek Karl
Published: February 2009

In this work, an amorphous silicon electronic portal imaging device (a‐Si EPID) dose prediction model based on the energy fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI) is developed. An energy fluence matrix at very high resolution is used to incorporate multileaf collimator (MLC) leaf effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the energy fluence using experimentally derived radially dependent EPID interaction coefficients. Separate coefficients are used for the open beam energy fluence component and the component of the energy fluence transmitted through closed MLC leaves to each EPID pixel. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition, central axis EPID backscatter, and optical glare is convolved with the primary dose. The kernel is further optimized to give accurate EPID penumbra prediction and EPID scatter factor with changing MLC field size. An EPID calibration method was developed to reduce the effect of nonuniform backscatter from the support arm (E‐arm) in a calibrated EPID image. This method removes the backscatter component from the pixel sensitivity (flood field) correction matrix retaining only field‐specific backscatter in the images. The model was compared to EPID images for jaw and MLC defined open fields and eight head and neck intensity modulated radiotherapy (IMRT) fields. For the head and neck IMRT fields with 2%, criteria (mean standard deviation) of points passed with a gamma index less than 1, and for 3%, of points were within the criteria. For these fields, the 2%, pass score reduced to when backscatter was present in the pixel sensitivity correction matrix. The model incorporates the effect of MLC leaf transmission, EPID response to open and MLC leakage dose components, and accurately predicts EPID images of IMRT fields. Removing the backscatter component of the pixel sensitivity matrix correction reduces the effect of nonuniform E‐arm backscatter.
Comparison of patient size‐based methods for estimating quantum noise in CT images of the lung
Authors: Li Xiang , Samei Ehsan
Published: February 2009

The authors explored four methods for estimating quantum noise in CT images of the lung, each based on a different definition of patient size (water‐equivalent diameter) and the relationship between noise and diameter determined in water phantoms. The accuracies of the four methods were evaluated using an image‐subtraction method as a gold standard. Noise estimates based on patient sizes derived from chest area, water‐equivalent area, non‐lung area, and water‐equivalent path length had maximum errors of 229%, 93%, 34%, and 57%, respectively. Considering the magnitude of noise variation across the lung volume , noise estimate based on non‐lung area was reasonably accurate.
An efficient pre‐object collimator based on an x‐ray lens
Authors: Fredenberg Erik , Cederström Björn , Åslund Magnus , Nillius Peter , Danielsson Mats
Published: February 2009

A multiprism lens (MPL) is a refractive x‐ray lens with one‐dimensional focusing properties. If used as a pre‐object collimator in a scanning system for medical x‐ray imaging, it reduces the divergence of the radiation and improves on photon economy compared to a slit collimator. Potential advantages include shorter acquisition times, a reduced tube loading, or improved resolution. We present the first images acquired with a MPL in a prototype for a scanning mammography system. The lens showed a gain of flux of 1.32 compared to a slit collimator at equal resolution, or a gain in resolution of 1.31–1.44 at equal flux. We expect the gain of flux in a clinical setup with an optimized MPL and a custom‐made absorption filter to reach 1.67, or 1.45–1.54 gain in resolution.
Depth dependence of electron backscatter: An energy spectral and dosimetry study using Monte Carlo simulation
Authors: Chow James C. L. , Owrangi Amir M.
Published: February 2009

This study investigated the depth dependence of electron backscatter from a layer of lead (Pb) for clinical electron beams. The change in the electron backscatter with variation in the water depth above the Pb was determined. Electron energy spectra and relative depth doses as a function of depth in water over the Pb layer were calculated using a Monte Carlo simulation and studied. Phase‐space files for 4 and 9 MeV electron beams ( applicator and cutout) based on the Varian 21 EX linear accelerator were generated using the EGSnrc‐based BEAMNRC code. 3 mm of Pb, at depths of 0.5 and 1 cm in water, was irradiated with electrons. The source‐to‐surface distance is equal to 100 cm. Electron energy spectra and relative depth doses with and without the presence of the Pb layer at different depths in water were determined using the BEAMNRC code. For the 4 MeV electron energy spectra at a depth of 0.5 cm in water, electron backscatter was found to originate at the Pb‐water interface and extend to 0.5 cm above the Pb insert. However, at a depth of 1 cm in water, electron backscatter almost disappeared at 0.5 and 1 cm above th ePb insert. This is due to the increased attenuation of the incident 4 MeV electron beam in a thicker layer of water as well as increased attenuation of the electron backscatter above the Pb. This resulted in a 23% decrease in relative dose at a measurement point of 0.5 cm depth, when the depth of the Pb insert was changed from 1 to 0.5 cm. For the electron energy spectra of the 9 MeV beams with a 0.5 cm depth of water, only a small amount of electron backscatter was observed. However, more electron backscatter was found when the water depth was increased to 1 cm. This is because the electron beam energy was decreased more due to the increase in attenuation from the increased depth of water compared to 0.5 cm. Since the electron energy spectrum and relative depth dose above the Pb layer vary with depth of water on top of the Pb, the electron backscatter depends significantly on the thickness of water, or water equivalent bolus, or critical tissue over the Pb shield in electron radiotherapy.
Determination of correction factors for a 2D diode array device in MV photon beams
Authors: Zhou Xiaodong , Both Stefan , Zhu Timothy C.
Published: February 2009

The purpose of this study is to evaluate the dosimetric characteristics of a 2D diode array device (MAPCHECK™) and provide correction factors for clinically relevant conditions. Measurements were performed for various collimator settings , SSDs (80–110 cm), and depths (5–20 cm) for 6 and 15 MV photon beams. The correction factor was determined as the ratio of dose measured by an ionization chamber in a large full scatter water phantom to the dose measured by the MAPCHECK with solid water phantom. The total correction factor was separated into three components. The phantom scatter factor was evaluated by taking the ratio of dose measured by MAPCHECK with backscatter material and phantom to that by the ionization chamber in an infinite large water phantom renormalized to the reference field size . This was also compared to the theoretical calculation using ratios of phantom scatter factor between an infinitely large water phantom and solid water phantoms. The backscatter effect was determined by taking the ratio of the MAPCHECK measurements with and without 10 cm thick solid water backscatter material also normalized to the reference field size at the depth of measurement. The extra correction factor is defined as the ratio of dose measured by the MAPCHECK (without backscatter) to that measured by the ionization chamber in a large water phantom at depth for the reference field size normalized to the reference depth (10 cm) and reference field size. The total correction factors for 6 and 15 MV photon beams were up to 16% and 11.5%, respectively, due to the difference in phantom scatter in MAPCHECK measurements when the field sizes are larger than 30 cm. The result was consistent with theoretical analysis with phantom scatter alone. The backscatter correction factor varied by up to 2% and increased with increasing field size. The extra correction factor decreases with depths and is for all depths. The authors have verified that correction factors for MAPCHECK are a function of the scaled field size and depth only and are independent of the SSD. Results were verified for off‐axis points measured by MAPCHECK.
Measurement of compressed breast thickness by optical stereoscopic photogrammetry
Authors: Tyson Albert H. , Mawdsley Gordon E. , Yaffe Martin J.
Published: February 2009

The determination of volumetric breast density (VBD) from mammograms requires accurate knowledge of the thickness of the compressed breast. In attempting to accurately determine VBD from images obtained on conventional mammography systems, the authors found that the thickness reported by a number of mammography systems in the field varied by as much as 15 mm when compressing the same breast or phantom. In order to evaluate the behavior of mammographic compression systems and to be able to predict the thickness at different locations in the breast on patients, they have developed a method for measuring the local thickness of the breast at all points of contact with the compression paddle using optical stereoscopic photogrammetry. On both flat (solid) and compressible phantoms, the measurements were accurate to better than 1 mm with a precision of 0.2 mm. In a pilot study, this method was used to measure thickness on 108 volunteers who were undergoing mammography examination. This measurement tool will allow us to characterize paddle surface deformations, deflections and calibration offsets for mammographic units.
Accurate estimation of compressed breast thickness in mammography
Authors: Mawdsley Gordon E. , Tyson Albert H. , Peressotti Chris L. , Jong Roberta A. , Yaffe Martin J.
Published: February 2009

The determination of volumetric breast density (VBD) from mammograms requires an accurate knowledge of the thickness of the compressed breast. Previously, the authors described a technique for measuring local thicknesses using optical stereoscopic photogrammetry [A. H. Tyson, G. E. Mawdsley, and M. J. Yaffe, “Measurement of compressed breast thickness by optical stereoscopic photogrammetry,” Med. Phys. (), 569–576 (2009)]. Here, the authors describe the use of this tool to guide the development of a simpler, more practical field technique for the estimation of breast thickness and test its accuracy. Phantoms were constructed having similar shapes and compression characteristics to breasts of different sizes. These phantoms were compressed at different forces on several types of mammography units and their thickness under compression was measured using optical stereoscopic photogrammetry at many points of contact with the compression plate. A prediction equation was developed that uses the readout of compressed thickness and compression force provided by the mammography system to estimate local breast thickness. Using this approach, systems can be calibrated to an accuracy of better than 5 mm in thickness using a simple test object compared to an error of up to 15 mm associated with using only the thickness readout of the mammography machine. On the systems tested, the estimated value of VBD obtained using this method is significantly reduced from that determined using the constant thickness reported by the mammography machine.
Radiation Oncology: A Physicist's‐Eye View
Authors: Thomadsen Bruce
Published: February 2009

Radiation Oncology: A Physicist's‐Eye View Goitein Michael Springer‐Science + Business Media, LLC, New York, NY, 2008. Hardcover 330 pp. Price: $129.00. ISBN: 9780387726441.
LOCAL EFFECT LEADS TO UNCERTAINTY IN OPINION DYNAMICS
Authors: HSU JIANN-WIEN , HUANG DING-WEI
Published: February 2009

We study the interweaving of deterministic and stochastic mechanisms in opinion dynamics. Based on the three-choice Galam model, we introduce a theoretical local-mode featuring with a nearest-neighbor network structure to investigate the effects of local connections. We present both numerical and analytical results for the local and the original global modes. We find that operating in the local mode leads to a much slower but richer process to achieve the opinion consensus. It is shown that in the local-mode dynamics, the stochasticity becomes more effective while the correlations between the initial configurations and final results turn out to be less predictive.
EVACUATION MODEL FOR EMERGENCY VEHICLES IN HIGHWAYS
Authors: MOUSSA NAJEM
Published: February 2009

In this paper, we propose a cellular automaton model for the evacuation of an emergency vehicle (EV) in highways. To facilitate the evacuation of the EV, some additional lane-changing rules (pull over of ordinary vehicles) are introduced in a two-lane cellular automata model for traffic flow in highway. We find that this pulling over of ordinary vehicles promotes a faster moving of the EV in traffic flow and minimize the EV's impact on the overall traffic.
A MARKOV-PROCESS INSPIRED CA MODEL OF HIGHWAY TRAFFIC
Authors: WANG FA , LI LI , HU JIAN-MING , JI YAN , MA RUI , JIANG RUI
Published: February 2009

To provide a more accurate description of the driving behaviors especially in car-following, namely a Markov-Gap cellular automata model is proposed in this paper. It views the variation of the gap between two consequent vehicles as a Markov process whose stationary distribution corresponds to the observed gap distribution. This new model provides a microscopic simulation explanation for the governing interaction forces (potentials) between the queuing vehicles, which cannot be directly measurable for traffic flow applications. The agreement between empirical observations and simulation results suggests the soundness of this new approach.
EMERGENT 1D ISING BEHAVIOR IN AN ELEMENTARY CELLULAR AUTOMATON MODEL
Authors: KASSEBAUM PAUL G. , IANNACCHIONE GERMANO S.
Published: February 2009

The fundamental nature of an evolving one-dimensional (1D) Ising model is investigated with an elementary cellular automaton (CA) simulation. The emergent CA simulation employs an ensemble of cells in one spatial dimension, each cell capable of two microstates interacting with simple nearest-neighbor rules and incorporating an external field. The behavior of the CA model provides insight into the dynamics of coupled two-state systems not expressible by exact analytical solutions. For instance, state progression graphs show the causal dynamics of a system through time in relation to the system's entropy. Unique graphical analysis techniques are introduced through difference patterns, diffusion patterns, and state progression graphs of the 1D ensemble visualizing the evolution. All analyses are consistent with the known behavior of the 1D Ising system. The CA simulation and new pattern recognition techniques are scalable (in both dimension, complexity, and size) and have many potential applications such as complex design of materials, control of agent systems, and evolutionary mechanism design.
HIDDEN COLOR CHANNEL EFFECTS TO THE NONSTRANGE SIX-QUARK SYSTEMS
Authors: PANG H. R. , PING J. L. , CHENG L. Z.
Published: February 2009

A transformation table between physical bases including hidden color channels and symmetry bases for nonstrange six-quark systems is provided by using fractional parentage expansion method. By solving a coupled-channel equation, in the framework of the resonating group method (RGM), the effect of hidden color channels to all nonstrange six-quark systems is examined. It is shown that the overall effects from hidden color channels would reduce the energies of systems. For ΔΔ systems, the greatest decrease is about 20 MeV. Effects of hidden color on the masses of NN systems have been found to be about 1 MeV. However, we should emphasize that such a minor change for NN system is not negligible. We also give the most favorable hidden color channels for NN system.
ASSOCIATED PRODUCTION OF THE CHARGED HIGGS BOSON AND SINGLE TOP QUARK AT THE LHC
Authors: LIU YAO-BEI , SHEN JIE-FEN
Published: February 2009

The left–right twin Higgs (LRTH) model predicts the existence of the charged Higgs ϕ±. In this paper, we study the production of the charged Higgs boson ϕ- with single top quark via the process bg → tϕ- at the CERN Large Hadron Collider (LHC). The numerical results show that the production cross-section can reach the level of 10 pb in the reasonable parameter space of the LRTH model. We expect that, as long as it is not too heavy, the possible signatures of the heavy charged Higgs boson ϕ-> might be detected via the decay mode [Formula: see text] at the LHC experiments.
CONSTRAINTS ON LEFT–RIGHT SYMMETRIC AND E6 SUPERSTRING MODELS FROM THE NUMBER OF LIGHT NEUTRINO SPECIES
Authors: GUTIÉRREZ-RODRÍGUEZ A. , HERNÁNDEZ-RUIZ M. A. , PÉREZ M. A. , PÉREZ-VARGAS F.
Published: February 2009

Using the experimental value for the rate [Formula: see text], we derive constraints on the number of light neutrino species with the invisible width method in the framework of a left–right symmetric model (LRSM) and a E6 superstring model as a function of their respective mixing angles for the neutral vector gauge bosons. Using the above LEP result we get the constraints -1.6×10-3≤ ϕ ≤ 1.1 × 10-3 and -1.3 × 10-2 ≤ ϕE6 ≤ 0.4 × 10-2, which are stronger than those obtained in previous studies of these models.
PSEUDODUALITY AND CONSERVED CURRENTS IN SIGMA MODELS
Authors: SARISAMAN MUSTAFA
Published: February 2009

We discuss the current conservation laws in sigma models based on a compact Lie groups of the same dimensionality and connected to each other via pseudoduality transformations in two dimensions. We show that pseudoduality transformations induce an infinite number of nonlocal conserved currents on the pseudodual manifold.
BOUND STATES IN GRAPHENE
Authors: GUPTA KUMAR S. , SEN SIDDHARTHA
Published: February 2009

We present a quantum analysis of the massless excitations in graphene with a charge impurity. When the effective charge exceeds a certain critical value, the spectrum is quantised and unbounded from below. The corresponding eigenstates are square-integrable at infinity and have a rapidly oscillatory behaviour in the short distance, which can be interpreted as a fall to the centre. Using a cutoff regularisation, we show that the effective Coulomb interaction strength is driven to its critical value under the renormalisation group flow. In the subcritical region, we find bound states with imaginary values of the energy for certain range of the system parameters. The physical significance of these bound states with imaginary eigenvalues is discussed.
LIQUID ARGON DETECTORS FOR NEUTRINO PHYSICS
Authors: CURIONI ALESSANDRO
Published: February 2009

In this paper we give a short review of liquid argon detectors for neutrino physics. We will first introduce noble liquid detectors with particular emphasis on liquid argon and detectors of the time projection chamber variety, and we will explain why liquid argon detectors are good for neutrino physics, in particular the measurement of the mixing angle θ13, CP-violating phase δ CP and determination of the mass hierarchy through νe appearance on a νμ beam. We will stress what we consider the main achievements and the main challenges facing liquid argon detectors for neutrino physics.
The Interplay of Catalysis and Toxicity by Amyloid Intermediates on Lipid Bilayers: Insights from Type II Diabetes
Authors: Hebda James A. , Miranker Andrew D.
Published: February 2009

The dynamics, energies, and structures governing protein folding are critical to biological function. Amyloidoses are a class of disease defined, in part, by the misfolding and aggregation of functional protein precursors into fibrillar states. Amyloid fibers contribute to the pathology of many diseases, including type II diabetes, Alzheimer's, and Parkinson's. In these disorders, amyloid fibers are present in affected tissues. However, it has become clear that intermediate states, rather than mature fibers, represent the cytotoxic species. In this review, we focus particularly on lipid bilayer–bound intermediates. Remarkably, the precursors of these fibers are intrinsically disordered, and yet catalysis of β-sheet formation appears to be mediated by the stabilization of α-helical states. On the lipid bilayer, these intermediate species have been implicated as cytotoxic through elimination of ionic homeostasis. Recent advances are enabling insights at a molecular level that promise to provide meaningful targets for the development of therapeutics.
Regulation of Hepcidin and Iron-Overload Disease
Authors: Lee Pauline L. , Beutler Ernest
Published: February 2009

Hepcidin, a 25-amino-acid antimicrobial peptide, is the central regulator of iron homeostasis. Hepcidin transcription is upregulated by inflammatory cytokines, iron, and bone morphogenetic proteins and is downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. The iron transporter ferroportin is the cognate receptor of hepcidin and is destroyed as a result of interaction with the peptide. Except for inherited defects of ferroportin and hepcidin itself, all forms of iron-storage disease appear to arise from hepcidin dysregulation. Studies using multiple approaches have begun to delineate the molecular mechanisms that regulate hepcidin expression, particularly at the transcriptional level. Knowledge of the regulation of hepcidin by inflammation, iron, erythropoiesis, and hypoxia will lead to an understanding of the pathogenesis of primary hemochromatosis, secondary iron overload, and anemia of inflammatory disease.