bluuebunny/crossref_metadata_2025 huggingface dataset:
A One-Dimensional Viscous-Inviscid Strong Interaction Model for Flow in Indented Channels With Separation and Reattachment
Authors: Kalse S. G. C. , Bijl H. , van Oudheusden B. W.
Published: June 2003

A new one-dimensional model is presented for the calculation of steady and unsteady flow through an indented two-dimensional channel with separation and reattachment. It is based on an interactive boundary layer approach, where the equations for the boundary layer flow near the channel walls and for an inviscid core flow are solved simultaneously. This approach requires no semi-empirical inputs, such as the location of separation and reattachment, which is an advantage over other existing one-dimensional models. Because of the need of an inviscid core alongside the boundary layers, the type of inflow as well as the length of the channel and the value of the Reynolds number poses some limitations on the use of the new model. Results have been obtained for steady flow through the indented channel of Ikeda and Matsuzaki. In further perspective, it is discussed how the present model, in contrast to other one-dimensional flow models, can be extended to calculate the flow in nonsymmetrical channels, by considering different boundary layers on each of the walls.
Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation. Report of the AAPM Radiation Therapy Committee Task Group 63
Authors: Reft Chester , Alecu Rodica , Das Indra J. , Gerbi Bruce J. , Keall Paul , Lief Eugene , Mijnheer Ben J. , Papanikolaou Nikos , Sibata Claudio , Van Dyk Jake
Published: June 2003

This document is the report of a task group of the Radiation Therapy Committee of the AAPM and has been prepared primarily to advise hospital physicists involved in external beam treatment of patients with pelvic malignancies who have high atomic number (Z) hip prostheses. The purpose of the report is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and, finally, to provide recommendations for treatment planning and delivery. Some of the data and recommendations are also applicable to patients having implanted high‐Z prosthetic devices such as pins, humeral head replacements. The scientific understanding and methodology of clinical dosimetry for these situations is still incomplete. This report is intended to reflect the current state of scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high‐Z hip prostheses.
CT evaluation of patient deep inspiration self‐breath‐holding: How precisely can patients reproduce the tumor position in the absence of respiratory monitoring devices?
Authors: Onishi Hiroshi , Kuriyama Kengo , Komiyama Takafumi , Tanaka Shiho , Ueki Junko , Sano Naoki , Araki Tsutomu , Ikenaga Satoshi , Tateda Yoshihito , Aikawa Yoshihito
Published: June 2003

The aim of the present study was to evaluate the reproducibility of tumor position under patient deep inspiration self‐breath‐holding in the absence of respiratory monitoring devices, as well as to compare the reproducibility of deep inspiration self‐breath‐holding on the verbal command of a radiation technologist (Passive mode) with that initiated by patients' own estimation (Active mode). Twenty patients with lung cancer were shown how the tumor and diaphragm move during the respiration cycle. Patients were instructed to hold their breath during deep inspiration and reproduce identical tumor position as well as possible either by the Active mode or by the Passive mode. After patients had practiced self‐breath‐holding during deep inspiration, a set of three CT scans was obtained for each of the two modes of self‐breath‐holding (6 CT scans total) to obtain randomly timed images of 2 mm thickness in the vicinity of the tumor. The first three scans were performed during breath‐hold using the Active mode, and next three scans were using the Passive mode. Maximum difference in tumor position for the three CT scans was then calculated along three axes: cranial‐caudal (C‐C); anterior‐posterior (A‐P); and right‐left (R‐L). In the 20 patients who underwent analysis of self‐breath‐holding, mean maximum difference in tumor position obtained under breath‐hold using the Active and the Passive modes were: 2.2 and 3.1 mm along the C‐C axis; 1.4 and 2.4 mm along the A‐P axis; and 1.3 and 2.2 mm along the R‐L axis, respectively. These differences in all axes were significantly smaller for the Active mode than for the Passive mode. Most tumors displayed maximal respiratory movement along the C‐C axis, and minimal movement along the R‐L axis, but tumors located in the upper lung displayed maximal movement along the A‐P axis. Significant correlation was observed between differences along three axes in either mode of breath‐hold. In conclusion, the reproducibility of tumor position under self‐breath‐holding by patients during deep inspiration after sufficient practice and in the absence of respiratory monitoring devices was satisfactorily accurate, and differences in tumor position were smaller under breath‐holding using the Active mode than using the Passive mode. We believe this new technique is likely to prove extremely useful for the irradiation of lung tumors with a small internal margin and for reduced proportion of high‐dose irradiated normal lung to total lung volume.
Noninvasive temperature estimation based on the energy of backscattered ultrasound
Authors: Arthur R. Martin , Straube William L. , Starman Jared D. , Moros Eduardo G.
Published: June 2003

Hyperthermia has been used as a cancer treatment in which tumors are elevated to cytotoxic temperatures to aid in their control. A noninvasive method for volumetrically determining temperature distribution during treatment would greatly enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. We investigated changes in backscattered energy (CBE) from pulsed ultrasound with temperature. Our predicted changes in backscattered energy were matched by in vitro measurements in samples of bovine liver, turkey breast, and pork rib muscle. We studied CBE in tissue regions with multiple scatterers, of isolated individual scatterers, and in collections of individual scatterers. The latter appears to have the most potential. We measured the CBE with a focused circular transducer with a center frequency of 7.5 MHz. The standard deviation of the CBE of 75–125 scattering regions from 0.3 to 0.5 cm3 volumes increased nearly monotonically from 37 °C to 50 °C in each tissue type. Although the slopes were different, the curve for each type of tissue was well matched by a second‐degree polynomial, with a correlation coefficient of 0.99 in each case. Thus the use of the CBE of ultrasound for temperature estimation may have clinical promise with a convenient, low cost modality. Because our approach exploits the inhomogeneities present in tissue, we believe that if it is successful in vitro, it holds promise for in vivo application.
Dosimetric characteristics of Thermo‐Shield material for orthovoltage photon beams
Authors: Bahmaid Mohammad , Kim Siyong , Liu Chihray R. , Palta Jatinder R.
Published: June 2003

Conventionally, lead has been used for field shaping in orthovoltage radiation therapy. Recently, a compensator material named Thermo‐Shield was presented for field shaping in electron beams. Thermo‐Shield is composed of nontoxic, high atomic weight metal particles dispersed in a thermoplastic matrix. It is manually moldable and conforms to human anatomy or any shape at temperatures of It is reusable and can be continuously reshaped to better fit the treatment field. Dosimetric characteristics of thermoplastic material were studied for Philips RT250 orthovoltage photon beams ranging from 75 to 250 kVp. It was found that Thermo‐Shield should be four to five times thicker than lead to achieve the same transmission (less than 5%). However, it did not cause significant degradation in penumbra. Clinical procedures for use are discussed.
GAF film dosimetry of a tandem positioned β‐emitting intravascular brachytherapy source train
Authors: Iftimia Ileana , Devlin Phillip M. , Chin Lee M. , Baron J. M. , Cormack R. A.
Published: June 2003

Coronary artery brachytherapy may require treatment of lesions longer than a single source length. A treatment option is tandem positioning of the single source. This study presents relative dosimetric measurements of a cardiovascular brachytherapy source and the dosimetric characteristics in the junction region of tandem treatments. Measurements were carried out using a Novoste Beta Cath™ 40 mm β source in a plastic water phantom. Radiochromic MD‐55‐2 film, calibrated using both 6 MV photon and 6 MeV electron beams from a linear accelerator, was used as the dosimeter. Dose distributions around a single source and in the junction region of tandem irradiation were measured. Measurements of the near field dose as close as 1.2 mm from the source are presented. Significant over‐ or underdoses in the junction region of tandem irradiation were quantified. At a radial distance of 2 mm from the longitudinal axis of the source, the dose value in the middle of the junction region, normalized to the dose at 2 mm midline single source, was about 182% for a 2‐seed overlap and 16% for a 2‐seed gap, respectively. Dose distributions in the junction region as a function of source overlap and radial distance have fairly high gradients and exhibit characteristic patterns. The fraction of prescription dose was found to have a sigmoidal dependence on overlap size, for radial distances ranging between 1.2 and 3 mm. The parameters of these sigmoids, quantified as functions of radial distance, could be used to provide quick and reasonable over/underdose estimates, given any potential overlap or gap in the junction area, with an uncertainty within 10%.
Automated lung nodule classification following automated nodule detection on CT: A serial approach
Authors: Armato Samuel G. , Altman Michael B. , Wilkie Joel , Sone Shusuke , Li Feng , Doi Kunio , Roy Arunabha S.
Published: June 2003

We have evaluated the performance of an automated classifier applied to the task of differentiating malignant and benign lung nodules in low‐dose helical computed tomography (CT) scans acquired as part of a lung cancer screening program. The nodules classified in this manner were initially identified by our automated lung nodule detection method, so that the output of automated lung nodule detection was used as input to automated lung nodule classification. This study begins to narrow the distinction between the “detection task” and the “classification task.” Automated lung nodule detection is based on two‐ and three‐dimensional analyses of the CT image data. Gray‐level‐thresholding techniques are used to identify initial lung nodule candidates, for which morphological and gray‐level features are computed. A rule‐based approach is applied to reduce the number of nodule candidates that correspond to non‐nodules, and the features of remaining candidates are merged through linear discriminant analysis to obtain final detection results. Automated lung nodule classification merges the features of the lung nodule candidates identified by the detection algorithm that correspond to actual nodules through another linear discriminant classifier to distinguish between malignant and benign nodules. The automated classification method was applied to the computerized detection results obtained from a database of 393 low‐dose thoracic CT scans containing 470 confirmed lung nodules (69 malignant and 401 benign nodules). Receiver operating characteristic (ROC) analysis was used to evaluate the ability of the classifier to differentiate between nodule candidates that correspond to malignant nodules and nodule candidates that correspond to benign lesions. The area under the ROC curve for this classification task attained a value of 0.79 during a leave‐one‐out evaluation.
In vivo continuous‐wave optical breast imaging enhanced with Indocyanine Green
Authors: Intes Xavier , Ripoll Jorge , Chen Yu , Nioka Shoko , Yodh A. G. , Chance Britton
Published: June 2003

We investigate the uptake of a nontargeted contrast agent by breast tumors using a continuous wave diffuse optical tomography apparatus. The instrument operates in the near‐infrared spectral window and employs 16 sources and 16 detectors to collect light in parallel on the surface of the tumor‐bearing breast (coronal geometry). In our protocol an extrinsic contrast agent, Indocyanine Green (ICG), was injected by bolus. Three clinical scenarios with three different pathologies were investigated. A two‐compartment model was used to analyze the pharmacokinetics of ICG and preprocess the data, and diffuse optical tomography was used for imaging. Localization and delineation of the tumor was achieved in good agreement with a priori information. Moreover, different dynamical features were observed for differing pathologies. The malignant cases exhibited slower rate constants (uptake and outflow) compared to healthy tissue. These results provide further evidence that in vivo pharmacokinetics of ICG in breast tumors may be a useful diagnostic tool for differentiation of benign and malignant pathologies.
An EGSnrc investigation of cavity theory for ion chambers measuring air kerma
Authors: Buckley Lesley A. , Kawrakow I. , Rogers D. W. O.
Published: June 2003

The EGSnrc system is used to compare the response of an aluminum‐walled thimble chamber to that of a graphite‐walled thimble chamber for a beam. When compared to previous experimental results, the EGSnrc values of the ratios of chamber response differ by as much as 0.7% from the experiment. However, it is shown that this difference can be more than accounted for by switching from using the graphite mean excitation energy of 78 eV used in dosimetry protocols to the value of 86.8 eV suggested by more recent stopping‐power experiments. This suggests that the uncertainty analysis of Monte Carlo results must be done more carefully, by taking into account uncertainties in the underlying basic data such as the electron and photon cross sections. In comparison to Spencer–Attix cavity theory for a thick‐walled ion chamber, the Monte Carlo calculated values of the chamber response differ from the expected ones by 0.15% and 0.01% for the graphite and aluminum chambers, respectively, which are comparable to previously reported values for the Spencer–Attix correction factors. EGSnrc is also used to investigate the effect on the chamber response of thin dag layers on the inside of the aluminum wall. There is good agreement between the calculated and measured changes in chamber response versus the thickness of the dag. The results are compared to the predictions of the Almond–Svensson extension of cavity theory and show that the theory does not correctly predict the chamber response in the presence of thin dag layers. This finding is in agreement with previously reported experimental results. It is demonstrated that the values of the fraction of ionizations in the gas arising from electrons generated in the dag layer, used in the theory, are not the source of the disagreement.
A patch source model for treatment planning of ruthenium ophthalmic applicators
Authors: Astrahan Melvin A.
Published: June 2003

Beta‐ray emitting Ru‐106/Rh‐106 ophthalmic applicators have been used for close to 4 decades in the treatment of choroidal melanoma. The form factor of these applicators is a spherically concave silver bowl with an inner radius of curvature between 12 and 14 mm, and a total shell thickness of 1 mm. The radioactive nuclide is deposited in a layer 0.1 mm below the concave surface of the applicator. Calculation of dose distributions for clinical treatment planning purposes is complicated by the concave nature of the distributed source, the asymmetric shape of the active region of some applicators, imperfections in the manufacturing process which can result in an inhomogeneous distribution of activity across the active surface, and absorption and scatter in the 0.1 mm layer of silver which seals and protects the radioactive layer. A semi‐empirical method of calculating dose distributions for these applicators is described which is fundamentally compatible with treatment planning systems that use the AAPM TG43 brachytherapy formalism. Dose to water is estimated by summing a “patch source” dose function over a discrete number of overlapping patches uniformly distributed over the active surface of the applicator. The patch source dose function differs conceptually from a point source dose function in that it is intended to represent the macroscopic behavior of a small, disk‐like region of the applicator. The patch source dose function includes an anisotropy term to account for angular variation in absorption and scatter as particles traverse the 0.1 mm silver window. It geometrically models the nearfield of a patch with properties akin to both a small disk and infinite plane, and models the farfield as if the patch were a point. This allows a manageable number of discrete patches (300 to 1000) to provide accuracy appropriate for clinical treatment planning. This approach has the advantages of using familiar concepts and data structures, it is computationally quick, and it readily adapts to asymmetric applicator shapes and inhomogeneities in the radionuclide distribution. A method for optimizing the patch source dose function parameters is presented, and the dosimetric calculations are compared with published Monte Carlo calculations and measurements.
Phase‐contrast imaging of tissue using near‐infrared diffusing light
Authors: Jiang Huabei , Xu Yong
Published: June 2003

We describe a new model‐based image reconstruction method that can extract the spatial variation of tissue refractive index with near‐infrared diffusing light. The model of light propagation is a diffusion approximation to the radiative transfer equation for highly scattering media with spatially varying refractive index. The spatial map of refractive index is recovered using a finite element‐based Newton‐type reconstruction algorithm. The imaging method is validated using several tissue‐like phantom experiments. Initial in vivo imaging of breast tissue is also presented, demonstrating the ability of this new imaging method for tumor detection.
The effects of tumor motion on planning and delivery of respiratory‐gated IMRT
Authors: Hugo Geoffrey D. , Agazaryan Nzhde , Solberg Timothy D.
Published: June 2003

The purpose of this study is to investigate the effects of object motion on the planning and delivery of IMRT. Two phantoms containing objects were imaged using CT under a variety of motion conditions. The effects of object motion on axial CT acquisition with and without gating were assessed qualitatively and quantitatively. Measurements of effective slice width and position for the CT scans were made. Mutual information image fusion was adapted for use as a quantitative measure of object deformation in CT images. IMRT plans were generated on the CT scans of the moving and gated object images. These plans were delivered with motion, with and without gating, and the delivery error between the moving deliveries and a nonmoving delivery was assessed using a scalable vector‐based index. Motion during CT acquisition produces motion artifact, object deformation, and object mispositioning, which can be substantially reduced with gating. Objects that vary in cross section in the direction of motion exhibit the most deformation in CT images. Mutual information provides a useful quantitative estimate of object deformation. The delivery of IMRT in the presence of target motion significantly alters the delivered dose distribution in relation to the planned distribution. The utilization of gating for IMRT treatment, including imaging, planning, and delivery, significantly reduces the errors introduced by object motion.
The role of advisory organizations in ionizing radiation protection science and policy: A proposal
Authors: Mossman Kenneth L.
Published: June 2003

Funding for radiation protection science and policy has been in decline for more than a decade. Agencies that set policies and standards for radiation protection depend on external expert groups for advice, and unless the funding situation is stabilized, the ability of these advisory organizations to provide timely advice will be compromised. This paper examines the history of radiation protection policy in the United States, reviews the funding patterns of international and national radiation protection advisory bodies, and suggests recommendations for assuring that radiological and radiation protection science remains an important part of the nation's public health policy agenda. Five major advisory organizations are the focus of this paper—ICRP, ICRU, NCRP, National Research Council BEIR Committees, and UNSCEAR. The recommendations developed in this paper address the following issues: (1) the need to coordinate activities among national and international advisory bodies in order to minimize overlap of work scope and ensure comprehensive coverage of major radiation protection issues; (2) the need to reevaluate activities and operations of advisory groups in the context of an ever‐changing radiation protection landscape; and (3) the need to establish the NCRP as the major federal advisory organization for radiation protection in the United States and to stabilize funding through Congressional appropriations.
Use of BANG® polymer gel for dose measurements in a 68 MeV proton beam
Authors: Heufelder J. , Stiefel S. , Pfaender M. , Lüdemann L. , Grebe G. , Heese J.
Published: June 2003

BANG polymer gel dosimetry using magnetic resonance imaging (MRI) was applied to an ophthalmologic 68 MeV proton beam. The object was to examine the use of BANG gel for the verification of proton fields in eye tumor therapy and to explore the applicability of polymer gel dosimetry in proton therapy under practical aspects. The gel phantoms were irradiated with monoenergetic and modulated proton beams. MRI analysis was carried out at clinical 1.5 and 3 T MR scanners. At constant LET, results show a linear relationship between spin–spin relaxation rates and dose. However, depth dose curves in BANG gel reveal a quenching of the Bragg maximum due to LET effects. The dose response of the gel for monoenergetic protons and spread‐out depth dose distributions can be calculated based on ionization chamber measurements. Experiment and calculations show good agreement and indicate that BANG polymer gels might become a valuable tool in proton therapy quality assurance.
Inductively coupled rf coils for examinations of small animals and objects in standard whole‐body MR scanners
Authors: Graf Hansjörg , Martirosian Petros , Schick Fritz , Grieser Marco , Bellemann Matthias E.
Published: June 2003

Inductively coupled solenoid coils fitting to objects in the size of mice or rats were developed to adapt modern whole‐body MR scanners featuring sufficient gradient strength for animal examinations with high spatial resolution. Homogenous receiver characteristics is achievable over almost the whole inner region of the solenoid coils. The SNR can be increased by a factor 2 to 6 with the adapting coils for examinations using the head coil as connected receiver. Standard sequences on clinical 1.5 T scanners can be applied with adapted transmitter voltages. For example, a SNR value of about 30 is achievable in a mouse liver after 10 minutes measuring time using a 2‐D spin echo imaging sequence and a size of for the picture elements.
The role of water desorption on optical clearing of biotissue: Studied with near infrared reflectance spectroscopy
Authors: Xu Xiangqun , Wang Ruikang K.
Published: June 2003

In order to understand the role of water desorption, i.e., the opposite of water adsorption, in optical clearing of biological tissue created by the application of hyperosmotic agents, dynamics of water loss in fresh porcine muscle tissue administrated with glycerol or ethylene glycol was investigated with the near‐infrared reflectance spectroscopy. It is found that there exist three stages of water desorption with the elapse of time within the tissue after the application of agents: i.e., the rapid stage during the first minute; at the second stage (1–10 min) water loss increases linearly at the rate slower than at the first stage; at the third stage (10–30 min) water content decreases exponentially at the slowest rate and tends to equilibrate. The progress of optical clearing measured by the optical coherence tomography system and spectrophotometer, respectively, corresponds very well with the three stages of water desorption. It indicates that optical clearing induced by hyperosmotic agents is strongly correlated with dehydration. The overall water loss in fresh porcine muscle tissue at 30 min after the application of 80% glycerol and ethylene glycol are approximately 39% and 23% respectively. The second derivative spectra show that glycerol is able to make bulk and bound water desorb. As a consequence, due to its high refractive index and high dehydration capability, glycerol is a more effective agent in optical clearing than ethylene glycol.
Incorporation of a combinatorial geometry package and improved scoring capabilities in the EGSnrc Monte Carlo Code system
Authors: Fragoso Margarida , Seco Joao , Nahum Alan E. , Verhaegen Frank
Published: June 2003

A description is given of a generic EGSnrc Monte Carlo user code, GenUC, which was developed as an attempt to simplify and optimize the geometry and scoring coding of EGSnrc user codes. GenUC was developed using the methodology of combinatorial geometry that allows a straightforward implementation of complicated geometric setups with intersecting boundaries, where subsequent modifications to the geometry are easily performed. Presently, GenUC has five elemental volumes that can be defined in any position in space: spheres, ellipsoids, parallelepipeds, and circular cylinders and cones. The mortran macro‐based implementation of the combinatorial geometry package allows an easy definition/extension of any other elemental volume, e.g., elliptical cylinders and cones. The scoring of the relevant parameters and the output of the results in GenUC are performed with two CERN data analysis packages, which permit the generation of nonplanar phase space distribution files and can also be used for geometry verification, among many other capabilities. GenUC has been successfully applied to complex geometric setups, e.g., intracavitary brachytherapy applicators and was also benchmarked against the EGSnrc user code, DOSRZnrc.
The effect of interobserver differences in post‐implant prostate CT image interpretation on dosimetric parameters
Authors: Han Ben H. , Wallner Kent , Merrick Gregory , Badiozamani Kas , Butler Wayne
Published: June 2003

The purpose of this study was to clarify where observers differ in their interpretation of CT scans, and to relate those differences to clinically relevant dosimetric parameters. Twenty unselected patients treated with I‐125 or Pd‐103 brachytherapy at the Veterans Affairs Puget Sound Health Care System (VAPSHCS) in 2001 were studied. Patients were implanted with I‐125 (7 patients, 0.87 mCi/source) or Pd‐103 (13 patients, 2.54 U/source). The number of I‐125 sources implanted ranged from 52 to 78. The number of Pd‐103 sources implanted ranged from 58–144. Post‐implant 3 mm CT images were imported into a laptop running Varian Variseed™ and sent to the four physician investigators, who outlined the prostate independently. Investigators were not coached specifically for this study, beyond their having read prior reports regarding prostate volume determinations. There was moderate interobserver variability in CT volume determination, with the standard deviations as a percent of the mean ranging from 9% to 29% (median: 17%). An average of 14% of implants (range: 5%–20%) would have been judged inadequate based on a minimum V100 of 80%, versus 24% of implants (range: 5%–45%) being judged inadequate based on a minimum D90 of 90% of prescription dose. The greatest variability was seen in prostate length (median standard deviation: 0.57 cm), due to vagaries in base and apical localization. However, the prostatic width and thickness also varied substantially between observers, with median standard deviations of 0.24 and 0.32 cm, respectively. Treatment margin variability was greatest at the anterior border, with a median standard deviation of 0.21 cm±0.10. We believe that CT‐based dosimetry, while influenced by CT interpretation, still provides useful general dosimetric calculations, that are likely to be reproducible enough to provide clinically useful information between institutions. The V100 and TMs are less influenced by interobserver CT interpretation variability than is the D90, and may be better suited as interinstitutional quality indices.
A method of calculating a lung clinical target volume DVH for IMRT with intrafractional motion
Authors: Kung J. H. , Zygmanski P. , Choi N. , Chen G. T. Y.
Published: June 2003

The motion of lung tumors from respiration has been reported in the literature to be as large as 1–2 cm. This motion requires an additional margin between the Clinical Target Volume (CTV) and the Planning Target Volume (PTV). In Intensity Modulated Radiotherapy (IMRT), while such a margin is necessary, the margin may not be sufficient to avoid unintended high and low dose regions to the interior on moving CTV. Gated treatment has been proposed to improve normal tissues sparing as well as to ensure accurate dose coverage of the tumor volume. The following questions have not been addressed in the literature: (a) what is the dose error to a target volume without a gated IMRT treatment? (b) What is an acceptable gating window for such a treatment. In this study, we address these questions by proposing a novel technique for calculating the three‐dimensional (3‐D) dose error that would result if a lung IMRT plan were delivered without a gated linac beam. The method is also generalized for gated treatment with an arbitrary triggering window. IMRT plans for three patients with lung tumors were studied. The treatment plans were generated with HELIOS for delivery with 6 MV on a CL2100 Varian linear accelerator with a 26 pair MLC. A CTV to PTV margin of 1 cm was used. An IMRT planning system searches for an optimized fluence map for each port, which is then converted into a dynamic MLC file (DMLC). The DMLC file contains information about MLC subfield shapes and the fractional Monitor Units (MUs) to be delivered for each subfield. With a lung tumor, a CTV that executes a quasiperiodic motion does not receive but rather an Effective Incident Fluence We numerically evaluate the from a given DMLC file by a coordinate transformation to the Target's Eye View (TEV). In the TEV coordinate system, the CTV itself is stationary, and the MLC is seen to execute a motion that is superimposed on the DMLC motion. The resulting is input back into the dose calculation engine to estimate the 3‐D dose to a moving CTV. In this study, we model respiratory motion as a sinusoidal function with an amplitude of 10 mm in the superior–inferior direction, a period of 5 s, and an initial phase of zero.
Dual‐energy digital mammography: Calibration and inverse‐mapping techniques to estimate calcification thickness and glandular‐tissue ratio
Authors: Kappadath S. Cheenu , Shaw Chris C.
Published: June 2003

Breast cancer may manifest as microcalcifications in x‐ray mammography. Small microcalcifications, essential to the early detection of breast cancer, are often obscured by overlapping tissue structures. Dual‐energy imaging, where separate low‐ and high‐energy images are acquired and synthesized to cancel the tissue structures, may improve the ability to detect and visualize microcalcifications. Transmission measurements at two different kVp values were made on breast‐tissue‐equivalent materials under narrow‐beam geometry using an indirect flat‐panel mammographic imager. The imaging scenario consisted of variable aluminum thickness (to simulate calcifications) and variable glandular ratio (defined as the ratio of the glandular‐tissue thickness to the total tissue thickness) for a fixed total tissue thickness—the clinical situation of microcalcification imaging with varying tissue composition under breast compression. The coefficients of the inverse‐mapping functions used to determine material composition from dual‐energy measurements were calculated by a least‐squares analysis. The linear function poorly modeled both the aluminum thickness and the glandular ratio. The inverse‐mapping functions were found to vary as analytic functions of second (conic) or third (cubic) order. By comparing the model predictions with the calibration values, the root‐mean‐square residuals for both the cubic and the conic functions were ∼50 μm for the aluminum thickness and ∼0.05 for the glandular ratio.
Automated generation of a four‐dimensional model of the liver using warping and mutual information
Authors: Brock K. M. , Balter J. M. , Dawson L. A. , Kessler M. L. , Meyer C. R.
Published: June 2003

The use of mutual information (MI) based alignment to map changes in liver shape and position from exhale to inhale was investigated. Inhale and exhale CT scans were obtained with intravenous contrast for six patients. MI based alignment using thin‐plate spine (TPS) warping was performed between each inhale and exhale image set. An expert radiation oncologist identified corresponding vessel bifurcations on the exhale and inhale CT image and the transformation for identified points was determined. This transformation was then used to determine the accuracy of the MI based alignment. The reproducibility of the vessel bifurcation identification was measured through repeat blinded vessel bifurcation identification. Reproducibility [standard deviation (SD)] in theanddirections was 0.11, 0.09, and 0.14 cm, respectively. The average absolute difference between the transformation obtained using MI based alignment and the vessel bifurcation in theanddirections was 0.13 cm (SD=0.10 cm), 0.15 cm (SD=0.12 cm), and 0.15 cm (SD‐0.14 cm), respectively. These values are comparable to the reproducibility of bifurcation identification, indicating that MI based alignment using TPS warping is accurate to within measurement error and is a reliable tool to aid in describing deformation that the liver undergoes from the exhale to inhale state.
Normalized sensitometric curves for the verification of hybrid IMRT treatment plans with multiple energies
Authors: Georg Dietmar , Kroupa Bernhard , Winkler Peter , Pötter Richard
Published: June 2003

With the clinical implementation of time‐variable dose patterns and intensity modulated radiotherapy (IMRT) film dosimetry has regained popularity. Films are currently the most frequently used dosimetric means for patient specific quality assurance in IMRT. A common method is to verify a so‐called hybrid IMRT plan, which is the patient specific treatment plan with unmodified fluence patterns recalculated in a dedicated phantom. For such applications the sensitometric curve, i.e., the relation between optical density (OD) and absorbed dose, should not depend critically on beam energy, field size and depth, or film orientation. In order to minimize the influence of all these variables a normalization of sensitometric curves is performed at various photon beam energies (6 MV, 10 MV, 25 MV). By doing so one unique sensitometric curve can be used for these three beam qualities. This holds for both film types investigated: Kodak X‐Omat V films and EDR‐2 films. Additionally, the influence of field size, depth and film orientation on a normalized sensitometric curve is determined for both film types. For doses smaller than 0.8 Gy for X‐Omat V and doses smaller than 3 Gy for EDR‐2 films the field size variation of normalized sensitometric curves is much smaller than 3% for fields up to 20×20 For X‐Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15 cm are smaller than 3%. For EDR‐2 films deviations larger than 3% are only observed at low net OD smaller than 0.25. The dependence of film orientation (parallel versus perpendicular) on a normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20% for sensitometric curves derived from films of the same batch but using different film processors. When normalizing sensitometric curves to the dose value necessary to obtain a net OD=1 for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. This becomes especially important for the verification of hybrid IMRT plans with multiple energies. Additionally, such a normalization minimizes other influences such as field size, depth, and film orientation. This method is generally applicable to both Kodak X‐Omat V and EDR‐2 films. In order to achieve the highest accuracy level an upper dose limit of 0.8 Gy for X‐Omat V films and 3 Gy for EDR‐2 films should be taken into account. However, these dose limits may vary with film reading instrument and film processor.
Bose–Einstein condensation, fluctuations, and recurrence relations in statistical mechanics
Authors: Mullin W. J. , Fernández J. P.
Published: June 2003

We calculate certain features of Bose–Einstein condensation in the ideal gas by using recurrence relations for the partition function. The grand canonical ensemble gives inaccurate results for certain properties of the condensate that are accurately provided by the canonical ensemble. Calculations in the latter can be made tractable for finite systems by means of the recurrence relations. The ideal one-dimensional harmonic Bose gas provides a particularly simple and pedagogically useful model for which detailed results are easily derived. An analysis of the Bose system via permutation cycles yields insight into the physical meaning of the recurrence relations.
Direct visualization of evanescent optical waves
Authors: Papathanassoglou Dimitri A. , Vohnsen Brian
Published: June 2003

The existence of evanescent optical waves is usually demonstrated by the observation of transmitted light in frustrated total-internal-reflection experiments that make use of two closely spaced prisms. The main characteristic of a monochromatic evanescent plane wave is the exponential decay of its amplitude in the direction perpendicular to its surface of generation. This decay, however, is not what is seen in the usual experiments when the gap between the prisms is small. Only when the gap is sufficiently large does it gradually approach the exponential dependence. We use a different technique that uses a local probe to reveal the presence of an evanescent wave. The results come closer to the ideal of the exponential decay of the wave amplitude, and the presence of the evanescent wave can be seen directly, making it a suitable demonstration for pedagogical purposes.
Resource Letter: BEC-1: Bose–Einstein condensates in trapped dilute gases
Authors: Hall David S.
Published: June 2003

This Resource Letter provides a guide to the literature on Bose–Einstein condensation in trapped dilute gases. Journal articles and books are cited for the following topics: history, technological advances, condensates as quantum fluids, effects of interatomic interactions, condensates as matter waves, condensate optics, multiple condensates, lower dimensions, spectroscopy and precision measurement, entanglement, and cosmology.
Modeling heat flow in a thermos
Authors: Karls Michael A. , Scherschel James E.
Published: June 2003

One of the first mathematical models that students encounter is that of the cooling of a cup of coffee. A related, but more complicated, problem is how the temperature in a thermos full of ice-cold water changes as a function of both time and position in the thermos. We use the approach developed by Fourier for the heating of an insulated rod to establish a model for a thermos. We verify the model by comparing it to data recorded with a calculator-based laboratory.
Algebraic approach to the radioactive decay equations
Authors: Moral L. , Pacheco A. F.
Published: June 2003

The radioactive decay equations are solved using an algebraic approach that is simple and easily extensible to branching systems. Two examples are included to show the nature of the approach.
Spacetime physics with geometric algebra
Authors: Hestenes David
Published: June 2003

This is an introduction to spacetime algebra (STA) as a unified mathematical language for physics. STA simplifies, extends, and integrates the mathematical methods of classical, relativistic, and quantum physics while elucidating geometric structure of the theory. For example, STA provides a single, matrix-free spinor method for rotational dynamics with applications from classical rigid body mechanics to relativistic quantum theory—thus significantly reducing the mathematical and conceptual barriers between classical and quantum mechanics. The entire physics curriculum can be unified and simplified by adopting STA as the standard mathematical language. This would enable early infusion of spacetime physics and give it the prominent place it deserves in the curriculum.
Variable mass oscillator
Authors: Flores José , Solovey Guillermo , Gil Salvador
Published: June 2003

We studied the motion of a variable mass oscillator. The mass used is a container full of sand that loses sand at a constant rate and hangs from a spring. The spring was suspended from a force sensor connected to a data acquisition system that let us study the evolution of the system. In the underdamped regime we identified three distinct types of behavior for the system, depending on the relation between the energy loss due to the exit of mass and the energy loss through friction. The experimental results are well described by both the numerical solution to the equations of motion and our model, which makes it simple to predict the different types of behavior and to assess the relevant physical parameters involved in the dynamics of this system.
A representation of the Schrödinger and Klein–Gordon equations obtained using simple hyperbolic numbers
Authors: Bracken Paul , Hayes James
Published: June 2003

We write the Bohm–Landau wave function in terms of simple hyperbolic numbers and apply this representation to the Schrödinger and Klein–Gordon equations. It is shown that the Schrödinger equation can be separated in one space dimension. The radial part of the wave function and the derivatives of the radial function can be related to the potential and its derivative. We also apply this representation to the Klein–Gordon equation.
V1 Neurons Signal Acquisition of an Internal Representation of Stimulus Location
Authors: Sharma Jitendra , Dragoi Valentin , Tenenbaum Joshua B. , Miller Earl K. , Sur Mriganka
Published: June 2003

A fundamental aspect of visuomotor behavior is deciding where to look or move next. Under certain conditions, the brain constructs an internal representation of stimulus location on the basis of previous knowledge and uses it to move the eyes or to make other movements. Neuronal responses in primary visual cortex were modulated when such an internal representation was acquired: Responses to a stimulus were affected progressively by sequential presentation of the stimulus at one location but not when the location was varied randomly. Responses of individual neurons were spatially tuned for gaze direction and tracked the Bayesian probability of stimulus appearance. We propose that the representation arises in a distributed cortical network and is associated with systematic changes in response selectivity and dynamics at the earliest stages of cortical visual processing.
The Neural Basis of Economic Decision-Making in the Ultimatum Game
Authors: Sanfey Alan G. , Rilling James K. , Aronson Jessica A. , Nystrom Leigh E. , Cohen Jonathan D.
Published: June 2003

The nascent field of neuroeconomics seeks to ground economic decisionmaking in the biological substrate of the brain. We used functional magnetic resonance imaging of Ultimatum Game players to investigate neural substrates of cognitive and emotional processes involved in economic decision-making. In this game, two players split a sum of money;one player proposes a division and the other can accept or reject this. We scanned players as they responded to fair and unfair proposals. Unfair offers elicited activity in brain areas related to both emotion (anterior insula) and cognition (dorsolateral prefrontal cortex). Further, significantly heightened activity in anterior insula for rejected unfair offers suggests an important role for emotions in decision-making.
Holocene Forcing of the Indian Monsoon Recorded in a Stalagmite from Southern Oman
Authors: Fleitmann Dominik , Burns Stephen J. , Mudelsee Manfred , Neff Ulrich , Kramers Jan , Mangini Augusto , Matter Albert
Published: June 2003

A high-resolution oxygen-isotope record from a thorium-uranium–dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4to 0.4thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After ∼8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity.
Selective Charging of tRNA Isoacceptors Explains Patterns of Codon Usage
Authors: Elf Johan , Nilsson Daniel , Tenson Tanel , Ehrenberg Måns
Published: June 2003

We modeled how the charged levels of different transfer RNAs (tRNAs) that carry the same amino acid (isoacceptors) respond when this amino acid becomes growth-limiting. The charged levels will approach zero for some isoacceptors (such as

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{tRNA}_{2}^{\mathrm{Leu}}\) \end{document}

) and remain high for others (such as

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{tRNA}_{4}^{\mathrm{Leu}}\) \end{document}

), as determined by the concentrations of isoacceptors and how often their codons occur in protein synthesis. The theory accounts for (synonymous) codons for the same amino acid that are used in ribosome-mediated transcriptional attenuation, the choices of synonymous codons in trans-translating transfermessenger RNA, and the overrepresentation of rare codons in messenger RNAs for amino acid biosynthetic enzymes.
The Interface Phase and the Schottky Barrier for a Crystalline Dielectric on Silicon
Authors: McKee R. A. , Walker F. J. , Nardelli M. Buongiorno , Shelton W. A. , Stocks G. M.
Published: June 2003

The barrier height for electron exchange at a dielectric-semiconductor interface has long been interpreted in terms of Schottky's theory with modifications from gap states induced in the semiconductor by the bulk termination. Rather, we show with the structure specifics of heteroepitaxy that the electrostatic boundary conditions can be set in a distinct interface phase that acts as a “Coulomb buffer.” This Coulomb buffer is tunable and will functionalize the barrier-height concept itself.
Fusion of Cells by Flipped SNAREs
Authors: Hu Chuan , Ahmed Mahiuddin , Melia Thomas J. , Söllner Thomas H. , Mayer Thomas , Rothman James E.
Published: June 2003

The SNARE (soluble
N
-ethylmaleimide–sensitive factor attachment protein receptor) hypothesis suggests that pairs of proteins known as vesicle (v-) SNAREs and target membrane (t-) SNAREs interact specifically to control and mediate intracellular membrane fusion events. Here, cells expressing the interacting domains of v- and t-SNAREs on the cell surface were found to fuse spontaneously, demonstrating that SNAREs are sufficient to fuse biological membranes.