bluuebunny/crossref_metadata_2025 huggingface dataset:
Implementation of a Monte Carlo based inverse planning model for clinical IMRT with MCNP code
Authors: He Tongming Tony
Published: March 2003

Inaccurate dose calculations and limitations of optimization algorithms in inverse planning introduce systematic and convergence errors to treatment plans. This work was to implement a Monte Carlo based inverse planning model for clinical IMRT aiming to minimize the aforementioned errors. The strategy was to precalculate the dose matrices of beamlets in a Monte Carlo based method followed by the optimization of beamlet intensities. The MCNP 4B (Monte Carlo N‐Particle version 4B) code was modified to implement selective particle transport and dose tallying in voxels and efficient estimation of statistical uncertainties. The resulting performance gain was over eleven thousand times. Due to concurrent calculation of multiple beamlets of individual ports, hundreds of beamlets in an IMRT plan could be calculated within a practical length of time. A finite‐sized point source model provided a simple and accurate modeling of treatment beams. The dose matrix calculations were validated through measurements in phantoms. Agreements were better than 1.5% or 0.2 cm. The beamlet intensities were optimized using a parallel platform based optimization algorithm that was capable of escape from local minima and preventing premature convergence. The Monte Carlo based inverse planning model was applied to clinical cases. The feasibility and capability of Monte Carlo based inverse planning for clinical IMRT was demonstrated. Systematic errors in treatment plans of a commercial inverse planning system were assessed in comparison with the Monte Carlo based calculations. Discrepancies in tumor doses and critical structure doses were up to 12% and 17%, respectively. The clinical importance of Monte Carlo based inverse planning for IMRT was demonstrated.
Is axial wall stress compressive in certain arteries?
Authors: Doriot P.‐A. , Dorsaz P.‐A.
Published: March 2003

In mathematical–physical models of blood vessels, the “zero‐stress state” of the vessel wall is usually defined with reference to the atmospheric pressure Due to this conventional choice, axial and circumferential stresses generated by the (positive) transmural pressure over the radial wall depth can only be positive (in absence of residual stresses) and thus, by definition, only tensile. If the zero‐stress state were defined “unconventionally” with reference to vacuum pressure the isotropic compressive stress generated by the atmospheric pressure everywhere in the wall would, however, be included in the stress values, and negative stresses would become formally possible. Since materials submitted only to compressions do not need to have the same resistive properties as materials which may also experience tractions, the question whether axial stress (and perhaps also circumferential stress) might be permanently compressive in vessels under physiologic conditions may therefore be important for investigations of the relationship between wall stresses on one side and wall structures, vessel growth, vessel damages, or vessel adaptation processes on the other side. In the present study, radial, circumferential, and axial wall stresses were calculated conventionally and “unconventionally” for three representative “vessel examples.” The results clearly suggest that axial wall stress might well be compressive in many vessels. Furthermore, relative differences between conventional and unconventional stress values are quite considerable, and ratios between stresses calculated in the same manner appear to be strongly dependent on the chosen zero‐stress state definition.
Influence of phantom diameter, kVp and scan mode upon computed tomography dose index
Authors: Nickoloff Edward L. , Dutta Ajoy K. , Lu Zheng F.
Published: March 2003

The computed tomography (CT) radiation dose to pediatric patients has received considerable attention recently. Moreover, it is important to be able to determine CT radiation doses for various patient sizes ranging from infants to large adults. The current AAPM protocol only measures CT radiation dose using a 16 cm acrylic phantom to represent an adult head and a 32 cm acrylic phantom to represent an adult body. The goal of this paper is to study the dependence of the computed tomography dose index (CTDI) upon the size of the phantom, the kVp selected and the scan mode employed. Our measurements were done on phantom sizes ranging from 6 cm to 32 cm. The x‐ray tube potential ranged from 80 to 140 kVp. The scan modes utilized for the measurements included: consecutive axial scans, single‐slice helical scans with variable pitch and multislice helical scans with variable pitch. The results were consolidated into simplified equations which related the phantom diameter and kVp to the measured CTDI. Some generalizations were made about the relationship between the scan modes of the various CT units to the measured radiation doses. The CTDI appears to be an exponential function of phantom diameter. For the same kVp and mAs, the radiation doses for smaller phantoms are much greater than for larger sizes. The derived relationship can be used to estimate the radiation doses for a variety of scan conditions and modes from measurements with the two standard reference phantoms. A method was also given for converting axial CT dose measurements to appropriate MSAD values for helical CT scans.
Using the volumetric effect of a finite‐sized detector for routine quality assurance of multileaf collimator leaf positioning
Authors: Yang Yong , Xing Lei
Published: March 2003

Intensity modulated radiation therapy (IMRT) is an advanced form of radiation therapy and promises to improve dose conformation while reducing the irradiation to the sensitive structures. The modality is, however, more complicated than conventional treatment and requires much more stringent quality assurance (QA) to ensure what has been planned can be achieved accurately. One of the main QA tasks is the assurance of positioning accuracy of multileaf collimator (MLC) leaves during IMRT delivery. Currently, the routine quality assurance of MLC in most clinics is being done using radiographic films with specially designed MLC leaf sequences. Besides being time consuming, the results of film measurements are difficult to quantify and interpret. In this work, we propose a new and effective technique for routine MLC leaf positioning QA. The technique utilizes the fact that, when a finite‐sized detector is placed under a leaf, the relative output of the detector will depend on the relative fractional volume irradiated. A small error in leaf positioning would change the fractional volume irradiated and lead to a deviation of the relative output from the normal reading. For a given MLC and detector system, the relation between the relative output and the leaf displacement can be easily established through experimental measurements and used subsequently as a quantitative means for detecting possible leaf positional errors. The method was tested using a linear accelerator with an 80‐leaf MLC. Three different locations, including two locations on central plane (X1=X2=0) and one point on an off‐central plane location (X1=−7.5, X=7.5), were studied. Our results indicated that the method could accurately detect a leaf positional change of ∼0.1 mm. The method was also used to monitor the stability of MLC leaf positioning for five consecutive weeks. In this test, we intentionally introduced two positional errors in the testing MLC leaf sequences: −0.2 mm and 1.2 mm. The technique was found to be robust and could detect the positional inaccuracy in each week's test. The influence of other possible error sources, including the ion chamber placement, jaw settings, gantry and collimator angle read‐outs, and the positioning errors of the adjacent leaves, on detection accuracy were also investigated. The principle of our method is independent of the types of the MLC and the detector and may have significant practical implications in facilitating routine MLC QA for IMRT delivery.
A virtual photon energy fluence model for Monte Carlo dose calculation
Authors: Fippel Matthias , Haryanto Freddy , Dohm Oliver , Nüsslin Fridtjof , Kriesen Stephan
Published: March 2003

The presented virtual energy fluence (VEF) model of the patient‐independent part of the medical linear accelerator heads, consists of two Gaussian‐shaped photon sources and one uniform electron source. The planar photon sources are located close to the bremsstrahlung target (primary source) and to the flattening filter (secondary source), respectively. The electron contamination source is located in the plane defining the lower end of the filter. The standard deviations or widths and the relative weights of each source are free parameters. Five other parameters correct for fluence variations, i.e., the horn or central depression effect. If these parameters and the field widths in theXandYdirections are given, the corresponding energy fluence distribution can be calculated analytically and compared to measured dose distributions in air. This provides a method of fitting the free parameters using the measurements for various square and rectangular fields and a fixed number of monitor units. The next step in generating the whole set of base data is to calculate monoenergetic central axis depth dose distributions in water which are used to derive the energy spectrum by deconvolving the measured depth dose curves. This spectrum is also corrected to take the off‐axis softening into account. The VEF model is implemented together with geometry modules for the patient specific part of the treatment head (jaws, multileaf collimator) into theXVMCdose calculation engine. The implementation into other Monte Carlo codes is possible based on the information in this paper. Experiments are performed to verify the model by comparing measured and calculated dose distributions and output factors in water. It is demonstrated that open photon beams of linear accelerators from two different vendors are accurately simulated using the VEF model. The commissioning procedure of the VEF model is clinically feasible because it is based on standard measurements in air and water. It is also useful for IMRT applications because a full Monte Carlo simulation of the treatment head would be too time‐consuming for many small fields.
Automatic quantitative low contrast analysis of digital chest phantom radiographs
Authors: Kwan Alexander L. C. , Filipow Larry J. , Le Lawrence H.
Published: March 2003

Up to the present, the majority of low contrast object evaluations performed on phantom images have been accomplished in a subjective fashion. This is mainly due to the time and effort required to manually measure each radiograph with a densitometer to obtain quantitative results. However, with the development of digital radiographic systems, it has become feasible to automate the detection and computation processes. In this work, a method that can automatically detect and compute the subject‐to‐noise ratio of the low contrast disks inside a geometric chest phantom is examined. This algorithm has the ability to locate the low contrast objects to an accuracy of less than one pixel, and provides results that are consistent with the understanding of subject‐to‐noise ratio. This algorithm should simplify the task of quantitative evaluation of contrast detail phantoms.
How should we describe the radiobiologic effect of extracranial stereotactic radiosurgery: Equivalent uniform dose or tumor control probability?
Authors: Kavanagh B. D. , Timmerman R. D. , Benedict S. H. , Wu Q. , Schefter T. E. , Stuhr K. , McCourt S. , Newman F. , Cardinale R. M. , Gaspar L. E.
Published: March 2003

Extracranial stereotactic radiosurgery (ESR) is now undergoing clinical investigation at numerous institutions as a treatment for solitary malignant lesions. Because there is no standard ESR technique, the same minimum dose might be applied through widely variable target dose–volume histograms. For multicenter trials of ESR or interinstitutional comparisons, a reliable index of radiobiological dose equivalency might facilitate the evaluation of dose–response relationships. Equivalent uniform dose (EUD) and tumor control probability (TCP) were considered for this application. While EUD appears more robust for the prospective description of ESR, TCP is expected to remain more valuable for a post hoc estimation of radiosensitivity parameters.
Experimental investigation of the dose and image quality characteristics of a digital mammography imaging system
Authors: Huda Walter , Sajewicz Anthony M. , Ogden Kent M. , Dance David R.
Published: March 2003

Our purpose in this study was to investigate the image quality and absorbed dose characteristics of a digital mammography imaging system with a CsI scintillator, and to identify an optimal x‐ray tube voltage for imaging simulated masses in an average size breast with 50% glandularity. Images were taken of an ACR accreditation phantom using a LORAD digital mammography system with a Mo target and a Mo filter. In one experiment, exposures were performed at 80 mAs with x‐ray tube voltages varying between 24 and 34 kVp. In a second experiment, the x‐ray tube voltage was kept constant at 28 kVp and the technique factor was varied between 5 and 500 mAs. The average glandular dose at each x‐ray tube voltage was determined from measurements of entrance skin exposure and x‐ray beam half‐value layer. Image contrast was measured as the fractional digital signal intensity difference for the image of a 4 mm thick acrylic disk. Image noise was obtained from the standard deviation in a uniformly exposed region of interest expressed as a fraction of the background intensity. The measured digital signal intensity was proportional to the mAs and to the kVp5.8. Image contrast was independent of mAs, and dropped by 21% when the x‐ray tube voltage increased from 24 to 34 kVp. At a constant x‐ray tube voltage, image noise was shown to be approximately proportional to (mAs)−0.5, which permits the image contrast to noise ratio (CNR) to be modified by changing the mAs. At 80 mAs, increasing the x‐ray tube voltage from 24 to 34 kVp increased the CNR by 78%, and increased the average glandular dose by 285%. At a constant lesion CNR, the lowest average glandular dose value occurred at 27.3 kVp. Increasing or decreasing the x‐ray tube voltage by 2.3 kVp from the optimum kVp increased the average glandular dose values by 5%. These results show that imaging simulated masses in a 4.2 cm compressed breast at ∼27 kVp with a Mo/Mo target/filter results in the lowest average glandular dose.
Digital tomosynthesis of hand joints for arthritis assessment
Authors: Duryea J. , Dobbins J. T. , Lynch J. A.
Published: March 2003

The two principal forms of hand arthritis, rheumatoid arthritis (RA) and osteoarthritis (OA) have large clinical and economic costs. Radiography has been shown to be a useful tool to assess the condition of the disease. A hand radiograph, however, is a two‐dimensional projection of a three‐dimensional object. In this report we present the results of a study that applied digital tomosynthesis to hand radiography in order to extract three‐dimensional outcome measures that should be more sensitive to arthritis progression. The study was performed using simulated projection radiographs created using micro computed tomography (μCT) and a set of five dry‐bone hand skeletons. These simulated projection images were then reconstructed into tomographic slices using the matrix inversion tomosynthesis (MITS) algorithm. The accuracy of the tomosynthesis reconstruction was evaluated by comparing the reconstructed images to a gold standard created using the μCT data. A parameter from image registration science, normalized mutual information, provided a quantifiable figure of merit. This study examined the effects of source displacement, number of reconstructed planes, number of acquisitions, noise added to the gray scale images, and errors in the location of a fiducial marker. We also optimized the reconstruction as a function of two variables k and α, that controlled the mixing of MITS with conventional shift‐and‐add tomosynthesis. A study using hand delineated joint margins demonstrated that MITS images provided a better measurement of average joint space width. We found good agreement between the MITS slices and the true planes. Both joint margins and trabecular structure were visible and the reconstructed slices showed additional structures not visible with the standard projection image. Using hand‐delineated joint margins we compared the average joint space width of the gold standard slices to the MITS and projection images. A root‐mean square deviation (RMSD), calculated for this comparison, gave and for the projection and MITS images, respectively. We have demonstrated the potential of digital tomosynthesis for imaging of the hand to assess arthritic changes. We have also developed a methodology that can be used to optimize the technique and have studied the issues that will control the feasibility of clinical implementation.
Magnetic susceptibility quantitation with MRI by solving boundary value problems
Authors: Li Lin , Wang Zhiyue J.
Published: March 2003

Magnetic susceptibility measurement is of considerable research interest in MRI and MRS. A rigorous method was previously developed to quantify the susceptibility of an arbitrarily shaped uniform object in an inhomogeneous external field. However, it requires using the field distribution information on a spherical surface or shell in the surrounding homogeneous medium enclosing the object. In this work, a new approach was developed through solving the boundary value problems of the Laplace equation, which has an advantage that the boundary providing the necessary field distribution information can have an arbitrary shape. This method has been validated on rectangular boundaries with both numerical simulation as well as experimental data. It has also been realized that MRI provides an experimental means of solving some boundary value problems of partial differential equations, if proper boundary condition can be set up.
Computerized scheme for determination of the likelihood measure of malignancy for pulmonary nodules on low‐dose CT images
Authors: Aoyama Masahito , Li Qiang , Katsuragawa Shigehiko , Li Feng , Sone Shusuke , Doi Kunio
Published: March 2003

An automated computerized scheme has been developed for determination of the likelihood measure of malignancy of pulmonary nodules on low‐dose helical CT (LDCT) images. Our database consisted of 76 primary lung cancers (147 slices) and 413 benign nodules (576 slices). With this automated computerized scheme, the location of a nodule was first indicated by a radiologist. The outline of the nodule was segmented automatically by use of a dynamic programming technique. Various objective features on the nodules were determined by use of outline analysis and image analysis, and the likelihood measure of malignancy was determined by use of linear discriminant analysis (LDA). The effect of many different combinations of features and the performance of LDA in distinguishing benign nodules from malignant ones were evaluated by means of receiver operating characteristic (ROC) analysis. The Az value (area under the ROC curve) obtained by the computerized scheme in distinguishing benign nodules from malignant ones was 0.828 when a single slice was employed for each of the nodules. However, the Az value was improved to 0.846 when multiple slices were used for determination of the likelihood measure of malignancy. The Az values obtained by the computerized scheme on LDCT images were significantly greater than the Az value of 0.70, which was obtained from our previous observer studies by radiologists in distinguishing benign nodules from malignant ones on LDCT images. The automated computerized scheme for determination of the likelihood measure of malignancy would be useful in assisting radiologists to distinguish between benign and malignant pulmonary nodules on LDCT images.
Tomographic mammography using a limited number of low‐dose cone‐beam projection images
Authors: Wu Tao , Stewart Alexander , Stanton Martin , McCauley Thomas , Phillips Walter , Kopans Daniel B. , Moore Richard H. , Eberhard Jeffrey W. , Opsahl‐Ong Beale , Niklason Loren , Williams Mark B.
Published: March 2003

A method is described for using a limited number (typically 10–50) of low‐dose radiographs to reconstruct the three‐dimensional (3D) distribution of x‐ray attenuation in the breast. The method uses x‐ray cone‐beam imaging, an electronic digital detector, and constrained nonlinear iterative computational techniques. Images are reconstructed with high resolution in two dimensions and lower resolution in the third dimension. The 3D distribution of attenuation that is projected into one image in conventional mammography can be separated into many layers (typically 30–80 1‐mm‐thick layers, depending on breast thickness), increasing the conspicuity of features that are often obscured by overlapping structure in a single‐projection view. Schemes that record breast images at nonuniform angular increments, nonuniform image exposure, and nonuniform detector resolution are investigated in order to reduce the total x‐ray exposure necessary to obtain diagnostically useful 3D reconstructions, and to improve the quality of the reconstructed images for a given exposure. The total patient radiation dose can be comparable to that used for a standard two‐view mammogram. The method is illustrated with images from mastectomy specimens, a phantom, and human volunteers. The results show how image quality is affected by various data‐collection protocols.
Optimization of technique factors for a silicon diode array full‐field digital mammography system and comparison to screen‐film mammography with matched average glandular dose
Authors: Berns Eric A. , Hendrick R. Edward , Cutter Gary R.
Published: March 2003

Contrast‐detail experiments were performed to optimize technique factors for the detection of low‐contrast lesions using a silicon diode array full‐field digital mammography (FFDM) system under the conditions of a matched average glandular dose (AGD) for different techniques. Optimization was performed for compressed breast thickness from 2 to 8 cm. FFDM results were compared to screen‐film mammography (SFM) at each breast thickness. Four contrast‐detail (CD) images were acquired on a SFM unit with optimal techniques at 2, 4, 6, and 8 cm breast thicknesses. The AGD for each breast thickness was calculated based on half‐value layer (HVL) and entrance exposure measurements on the SFM unit. A computer algorithm was developed and used to determine FFDM beam current (mAs) that matched AGD between FFDM and SFM at each thickness, while varying target, filter, and peak kilovoltage (kVp) across the full range available for the FFDM unit. CD images were then acquired on FFDM for kVp values from 23–35 for a molybdenum–molybdenum (Mo–Mo), 23–40 for a molybdenum–rhodium (Mo–Rh), and 25–49 for a rhodium–rhodium (Rh–Rh) target‐filter under the constraint of matching the AGD from screen‐film for each breast thickness (2, 4, 6, and 8 cm). CD images were scored independently for SFM and each FFDM technique by six readers. CD scores were analyzed to assess trends as a function of target–filter and kVp and were compared to SFM at each breast thickness. For 2 cm thick breasts, optimal FFDM CD scores occurred at the lowest possible kVp setting for each target–filter, with significant decreases in FFDM CD scores as kVp was increased under the constraint of matched AGD. For 2 cm breasts, optimal FFDM CD scores were not significantly different from SFM CD scores. For 4–8 cm breasts, optimum FFDM CD scores were superior to SFM CD scores. For 4 cm breasts, FFDM CD scores decreased as kVp increased for each target–filter combination. For 6 cm breasts, CD scores decreased slightly as kVp increased for Mo–Mo, but did not change significantly as a function of kVp for either Mo–Rh or Rh–Rh. For 8 cm breasts, Rh/Rh FFDM CD scores were superior to other target–filter combinations and increased significantly as kVp increased. These results indicate that low‐contrast lesion detection was optimized for FFDM by using a softer x‐ray beam for thin breasts and a harder x‐ray beam for thick breasts, when AGD was kept constant for a given breast thickness. Under this constraint, optimum low‐contrast lesion detection with FFDM was superior to that for SFM for all but the thinnest breasts.
Application of TACT® to the evaluation of total joint arthroplasty
Authors: Fahey Frederic H. , Webber Richard L. , Chew Felix S.‐K. , Dickerson Bryon A.
Published: March 2003

Three‐dimensional imaging may be valuable in the evaluation of total joint arthroplasty. The partial‐volume effect and streak artifacts from the metallic components of these devices limit the applicability of computed tomography (CT) to this application. Tuned‐aperture computed tomography® (TACT®) is a tomographic approach that has been successfully used in other medical applications. In TACT, the acquisition geometry is inferred from the localization of fiducial markers in a series of projection images. The flexibility and robustness of TACT, as well as the fact that through an appropriate choice of reconstruction algorithms it can suppress streak artifacts, potentially makes it an appropriate approach for evaluating total joint arthroplasty. A simple computer‐simulated model of a total knee replacement (TKR) was generated and used to evaluate the accuracy and artifacts associated with three‐dimensional (3‐D) renderings produced using TACT. A knee specimen from a cadaver that had received a TKR was used to investigate further the potential of TACT for this application. In both tests, TACT provided high‐quality 3‐D representations of the object. Both simple back‐projection and minimum pixel back‐projection were used to reconstruct the data. Minimum pixel back‐projection provided high‐contrast images that appeared to be relatively free of tomosynthetic artifacts. In summary, the potential of TACT in the evaluation of total joint arthroplasty was demonstrated. Future investigations will study TACT's ability to quantify the spatial relationship between the metallic components of these devices as well as TACT's ability to identify bony changes of diagnostic consequence.
A method for fast evaluation of neutron spectra for BNCT based on in‐phantom figure‐of‐merit calculation
Authors: Martı́n Guido
Published: March 2003

In this paper a fast method to evaluate neutron spectra for brain BNCT is developed. The method is based on an algorithm to calculate dose distribution in the brain, for which a data matrix has been taken into account, containing weighted biological doses per position per incident energy and the incident neutron spectrum to be evaluated. To build the matrix, using the MCNP 4C code, nearly monoenergetic neutrons were transported into a head model. The doses were scored and an energy‐dependent function to biologically weight the doses was used. To find the beam quality, dose distribution along the beam centerline was calculated. A neutron importance function for this therapy to bilaterally treat deep‐seated tumors was constructed in terms of neutron energy. Neutrons in the energy range of a few tens of kilo‐electron‐volts were found to produce the best dose gain, defined as dose to tumor divided by maximum dose to healthy tissue. Various neutron spectra were evaluated through this method. An accelerator‐based neutron source was found to be more reliable for this therapy in terms of therapeutic gain than reactors.
The volume effect of detectors in the dosimetry of small fields used in IMRT
Authors: Laub Wolfram U. , Wong Tony
Published: March 2003

In this study we investigate the effect of detector size in the dosimetry of small fields and steep dose gradients with a particular emphasis on IMRT measurements. Comparisons of calculated and measured cross‐profiles and absolute dose values of IMRT treatment plans are presented. As a consequence of the finite size of the detector that was used for the commissioning of the IMRT tool, local discrepancies of more than 10% are found between calculated cross‐profiles of intensity modulated beams and intensity modulated profiles measured with film. Absolute dose measurements of intensity modulated fields with a 0.6 cm3 Farmer chamber show significant differences of more than 6% between calculated and measured dose values at the isocenter of an IMRT treatment plan. Differences of not more than 2% are found in the same experiment for dose values measured with a 0.015 cm3 pinpoint ion chamber. A method to correct for the spatial response of finite‐sized detectors and to obtain the “real” penumbra width of cross‐profiles from measurements is introduced. Output factor measurements are performed with different detectors and are presented as a function of detector size for a 1×1 cm2 field. Because of its high spatial resolution and water equivalence, a diamond detector is found to be suitable as an alternative to other detectors used for small field dosimetry as there are photographic and photochromic film, TLDs, or water‐equivalent scintillation detectors.
Automated detection of lung nodules in CT scans: Effect of image reconstruction algorithm
Authors: Armato Samuel G. , Altman Michael B. , La Rivière Patrick J.
Published: March 2003

We have investigated the effect of computed tomography (CT) image reconstruction algorithm on the performance of our automated lung nodule detection method. Commercial CT scanners offer a choice of several algorithms for the reconstruction of projection data into transaxial images. Different algorithms produce images with substantially different properties that are apparent not only quantitatively, but also through visual assessment. During some clinical thoracic CT examinations, patient scans are reconstructed with multiple reconstruction algorithms. Thirty‐eight such cases were collected to form two databases: one with patient projection data reconstructed with the “standard” reconstruction algorithm and the other with the same patient projection data reconstructed with the “lung” reconstruction algorithm. The automated nodule detection method was applied to both databases. This method is based on gray‐level‐thresholding techniques to segment the lung regions from each CT section to create a segmented lung volume. Further gray‐level‐thresholding techniques are applied within the segmented lung volume to identify a set of lung nodule candidates. Rule‐based and linear discriminant classifiers are used to differentiate between lung nodule candidates that correspond to actual nodules and those that correspond to non‐nodules. The automated method that was applied to both databases was exactly the same, except that the classifiers were calibrated separately for each database. For comparison, the classifier then was trained on one database and tested independently on the other database. When applied to the databases in this manner, the automated method demonstrated overall a similar level of performance, indicating an encouraging degree of robustness.
Automatic verification of step‐and‐shoot IMRT field segments using portal imaging
Authors: Woo M. K. , Lightstone A. W. , Shan G. , Kumaraswamy L. , Li Y.
Published: March 2003

In step‐and‐shoot IMRT, many individual beam segments are delivered. These segments are generated by the IMRT treatment planning system and subsequently transmitted electronically through computer hardware and software modules before they are finally delivered. Hence, an independent system that monitors the actual field shape during treatment delivery is an added level of quality assurance in this complicated process. In this paper we describe the development and testing of such a system. The system verifies the field shape by comparing the radiation field detected by the built‐in portal imaging system on the linac to the actual field shape planned on the treatment planning system. The comparison is based on a software algorithm that detects the leaf edge positions of the radiation field on the portal image and compares that to the calculated positions. The process is fully automated and requires minimal intervention of the radiation therapists. The system has been tested with actual clinical plan sequences and was able to alert the operator of incorrect settings in real time.
Mothers' Transitions from Welfare to Work and the Well-Being of Preschoolers and Adolescents
Authors: Chase-Lansdale P. Lindsay , Moffitt Robert A. , Lohman Brenda J. , Cherlin Andrew J. , Coley Rebekah Levine , Pittman Laura D. , Roff Jennifer , Votruba-Drzal Elizabeth
Published: March 2003

Results from a longitudinal study of 2402 low-income families during the recent unprecedented era of welfare reform suggest that mothers' transitions off welfare and into employment are not associated with negative outcomes for preschoolers (ages 2 to 4 years) or young adolescents (ages 10 to 14 years). Indeed, no significant associations with mothers' welfare and employment transitions were found for preschoolers, and the dominant pattern was also of few statistically significant associations for adolescents. The associations that did occur provided slight evidence that mothers' entry into the labor force was related to improvements in adolescents' mental health, whereas exits from employment were linked with teenagers' increased behavior problems.
STAT1-Dependent Innate Immunity to a Norwalk-Like Virus
Authors: Karst Stephanie M. , Wobus Christiane E. , Lay Margarita , Davidson John , Virgin Herbert W.
Published: March 2003

Norwalk-like caliciviruses (Noroviruses) cause over 90% of nonbacterial epidemic gastroenteritis worldwide, but the pathogenesis of norovirus infection is poorly understood because these viruses do not grow in cultured cells and there is no small animal model. Here, we report a previously unknown murine norovirus. Analysis of Murine Norovirus 1 infection revealed that signal transducer and activator of transcription 1–dependent innate immunity, but not T and B cell–dependent adaptive immunity, is essential for norovirus resistance. The identification of host molecules essential for murine norovirus resistance may provide targets for prevention or control of an important human disease.
Glacier Surge After Ice Shelf Collapse
Authors: De Angelis Hernán , Skvarca Pedro
Published: March 2003

The possibility that the West Antarctic Ice Sheet will collapse as a consequence of ice shelf disintegration has been debated for many years. This matter is of concern because such an event would imply a sudden increase in sea level. Evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the Larsen Ice Shelf on the Antarctic Peninsula before its collapse in 1995. Satellite images and airborne surveys allowed unambiguous identification of active surging phases of Boydell, Sjögren, Edgeworth, Bombardier, and Drygalski glaciers. This discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves.
Genesis of Suicide Terrorism
Authors: Atran Scott
Published: March 2003

Contemporary suicide terrorists from the Middle East are publicly deemed crazed cowards bent on senseless destruction who thrive in poverty and ignorance. Recent research indicates they have no appreciable psychopathology and are as educated and economically well-off as surrounding populations. A first line of defense is to get the communities from which suicide attackers stem to stop the attacks by learning how to minimize the receptivity of mostly ordinary people to recruiting organizations.
Photolysis of Sulfuric Acid Vapor by Visible Solar Radiation
Authors: Vaida V. , Kjaergaard H. G. , Hintze P. E. , Donaldson D. J.
Published: March 2003

Atmospheric field measurements and models of the stratospheric sulfate aerosol layer led to the suggestion that sulfuric acid (H
2
SO
4
) must photolyze at high altitudes. We propose that excitation of vibrational overtones of H
2
SO
4
and its hydrate in the near-infrared and visible leads to photolysis, forming sulfur trioxide (SO
3
) and water. On the basis of absorption cross sections calculated with ab initio methods calibrated to experimental measurements, we estimated
J
values that are sufficient to explain stratospheric and mesospheric sulfur dioxide (SO
2
) concentrations and the observation of the sulfate layer.
Cassini Imaging of Jupiter's Atmosphere, Satellites, and Rings
Authors: Porco Carolyn C. , West Robert A. , McEwen Alfred , Del Genio Anthony D. , Ingersoll Andrew P. , Thomas Peter , Squyres Steve , Dones Luke , Murray Carl D. , Johnson Torrence V. , Burns Joseph A. , Brahic Andre , Neukum Gerhard , Veverka Joseph , Barbara John M. , Denk Tilmann , Evans Michael , Ferrier Joseph J. , Geissler Paul , Helfenstein Paul , Roatsch Thomas , Throop Henry , Tiscareno Matthew , Vasavada Ashwin R.
Published: March 2003

The Cassini Imaging Science Subsystem acquired about 26,000 images of the Jupiter system as the spacecraft encountered the giant planet en route to Saturn. We report findings on Jupiter's zonal winds, convective storms, low-latitude upper troposphere, polar stratosphere, and northern aurora. We also describe previously unseen emissions arising from Io and Europa in eclipse, a giant volcanic plume over Io's north pole, disk-resolved images of the satellite Himalia, circumstantial evidence for a causal relation between the satellites Metis and Adrastea and the main jovian ring, and information on the nature of the ring particles.
Dynamics-Driven Reaction Pathway in an Intramolecular Rearrangement
Authors: Ammal Salai Cheettu , Yamataka Hiroshi , Aida Misako , Dupuis Michel
Published: March 2003

A critical role is traditionally assigned to transition states (TSs) and minimum energy pathways, or intrinsic reaction coordinates (IRCs), in interpreting organic reactivity. Such an interpretation, however, ignores vibrational and kinetic energy effects of finite temperature. Recently it has been shown that reactions do not necessarily follow the intermediates along the IRC. We report here molecular dynamics (MD) simulations that show that dynamics effects may alter chemical reactions even more. In the heterolysis rearrangement of protonated pinacolyl alcohol Me
3
C-CHMe-OH
2
+
(Me, methyl), the MD pathway involves a stepwise route with C-O bond cleavage followed by methyl group migration, whereas the IRC pathway suggests a concerted mechanism. Dynamics effects may lead to new interpretations of organic reactivity.
Reflections of Reflections Without End
Authors: Field Michael
Published: March 2003

Indra's Pearls
The Vision of Felix Klein. David Mumford, Caroline Series, and David Wright. Cambridge University Press, Cambridge, 2002. 416 pp. $50, £27.95. ISBN 0-521-35253-3.

Chaos, fractals, and hyperbolic geometry are among the topics taken up through the authors' computer-aided explorations of the ways in which a pair of Möbius maps can interact.
The Effects of Cenozoic Global Change on Squirrel Phylogeny
Authors: Mercer John M. , Roth V. Louise
Published: March 2003

By modifying habitats and creating bridges and barriers between landmasses, climate change and tectonic events are believed to have important consequences for diversification of terrestrial organisms. Such consequences should be most evident in phylogenetic histories of groups that are ancient, widespread, and diverse. The squirrel family (Sciuridae) is one of very few mammalian families endemic to Eurasia, Africa, and North and South America and is ideal for examining these issues. Through phylogenetic and molecular-clock analyses, we infer that arrival and diversification of squirrels in Africa, on Sunda Shelf islands, across Beringea, and across the Panamanian isthmus coincide in timing and location with multiple well-documented sea-level, tectonic, and paleontological events. These precise correspondences point to an important role for global change in the diversification of a major group of mammals.
Experience Strengthening Transmission by Driving AMPA Receptors into Synapses
Authors: Takahashi Takuya , Svoboda Karel , Malinow Roberto
Published: March 2003

The mechanisms underlying experience-dependent plasticity in the brain may depend on the AMPA subclass of glutamate receptors (AMPA-Rs). We examined the trafficking of AMPA-Rs into synapses in the developing rat barrel cortex. In vivo gene delivery was combined with in vitro recordings to show that experience drives recombinant GluR1, an AMPA-R subunit, into synapses formed between layer 4 and layer 2/3 neurons. Moreover, expression of the GluR1 cytoplasmic tail, a construct that inhibits synaptic delivery of endogenous AMPA-Rs during long-term potentiation, blocked experience-driven synaptic potentiation. In general, synaptic incorporation of AMPA-Rs in vivo conforms to rules identified in vitro and contributes to plasticity driven by natural stimuli in the mammalian brain.
Modification of Ocular Defects in Mouse Developmental Glaucoma Models by Tyrosinase
Authors: Libby Richard T. , Smith Richard S. , Savinova Olga V. , Zabaleta Adriana , Martin Janice E. , Gonzalez Frank J. , John Simon W. M.
Published: March 2003

Mutations in the cytochrome P450 family 1, subfamily B, polypeptide 1 (
CYP1B1
) gene are a common cause of human primary congenital glaucoma (PCG). Here we show that
Cyp1b1
–/–
mice have ocular drainage structure abnormalities resembling those reported in human PCG patients. Using
Cyp1b1
–/–
mice, we identified the tyrosinase gene (
Tyr
) as a modifier of the drainage structure phenotype, with
Tyr
deficiency increasing the magnitude of dysgenesis. The severe dysgenesis in eyes lacking both CYP1B1 and TYR was alleviated by administration of the tyrosinase product dihydroxyphenylalanine (
l
-dopa).
Tyr
also modified the drainage structure dysgenesis in mice with a mutant
Foxc1
gene, which is also involved in PCG. These experiments raise the possibility that a tyrosinase/
l
-dopa pathway modifies human PCG, which could open new therapeutic avenues.
A Supershear Transition Mechanism for Cracks
Authors: Dunham Eric M. , Favreau Pascal , Carlson J. M.
Published: March 2003

Seismic data indicate that fault ruptures follow complicated paths with variable velocity because of inhomogeneities in initial stress or fracture energy. We report a phenomenon unique to three-dimensional cracks: Locally stronger fault sections, rather than slowing ruptures, drive them forward at velocities exceeding the shear wave speed. This supershear mechanism differentiates barrier and asperity models of fault heterogeneity, which previously have been regarded as indistinguishable. High strength barriers concentrate energy, producing potentially destructive pulses of strong ground motion.
(Almost) All About Biodiversity
Authors: Sala Osvaldo E.
Published: March 2003

World Atlas of Biodiversity
Earth's Living Resources in the 21st Century. Brian Groombridge and Martin D. Jenkins. University of California Press, Berkeley, in association with UNEP World Conservation Monitoring Centre, Cambridge, 2002. $54.95, £37.95. ISBN 0-520-23668-8.

The authors, from the United Nations Environment Programme's World Conservation Monitoring Centre in Cambridge, summarize the current state of global biodiversity for readers from fields beyond ecology, conservation biology, and the environmental sciences.
Shock-Induced Localized Amorphization in Boron Carbide
Authors: Chen Mingwei , McCauley James W. , Hemker Kevin J.
Published: March 2003

High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the ballistic performance of boron carbide at high impact rates and pressures. The formation of these amorphous bands is also an example of how shock loading can result in the synthesis of novel structures and materials with substantially altered properties.
Traces of Human Migrations in
Helicobacter pylori
Populations
Authors: Falush Daniel , Wirth Thierry , Linz Bodo , Pritchard Jonathan K. , Stephens Matthew , Kidd Mark , Blaser Martin J. , Graham David Y. , Vacher Sylvie , Perez-Perez Guillermo I. , Yamaoka Yoshio , Mégraud Francis , Otto Kristina , Reichard Ulrike , Katzowitsch Elena , Wang Xiaoyan , Achtman Mark , Suerbaum Sebastian
Published: March 2003

Helicobacter pylori
, a chronic gastric pathogen of human beings, can be divided into seven populations and subpopulations with distinct geographical distributions. These modern populations derive their gene pools from ancestral populations that arose in Africa, Central Asia, and East Asia. Subsequent spread can be attributed to human migratory fluxes such as the prehistoric colonization of Polynesia and the Americas, the neolithic introduction of farming to Europe, the Bantu expansion within Africa, and the slave trade.
The Cosmic Production of Helium
Authors: Jimenez Raul , Flynn Chris , MacDonald James , Gibson Brad K.
Published: March 2003

We estimate the cosmic production rate of helium relative to metals (Δ
Y/
Δ
Z
) using K dwarf stars in the Hipparcos catalog with accurate spectroscopic metallicities. The best fitting value is Δ
Y/
Δ
Z
= 2.1 ± 0.4 at the 68% confidence level. Our derived value agrees with determinations from H
ii
regions and with theoretical predictions from stellar yields with standard assumptions for the initial mass function. The amount of helium in stars determines how long they live and therefore how fast they will enrich the interstellar medium with fresh material.
Role of Chloroplast Protein Kinase Stt7 in LHCII Phosphorylation and State Transition in
Chlamydomonas
Authors: Depège Nathalie , Bellafiore Stéphane , Rochaix Jean-David
Published: March 2003

Photosynthetic organisms adapt to changes in light quality by redistributing light excitation energy between two photosystems through state transition. This reorganization of antenna systems leads to an enhanced photosynthetic yield. Using a genetic approach in
Chlamydomonas reinhardtii
to dissect the signal transduction pathway of state transition, we identified a chloroplast thylakoid–associated serine-threonine protein kinase, Stt7, that has homologs in land plants. Stt7 is required for the phosphorylation of the major light-harvesting protein (LHCII) and for state transition.
Microbial Populations Stimulated for Hexavalent Uranium Reduction in Uranium Mine Sediment
Authors: Suzuki Yohey , Kelly Shelly D. , Kemner Kenneth M. , Banfield Jillian F.
Published: March 2003

Uranium-contaminated sediment and water collected from an inactive uranium mine were incubated anaerobically with organic substrates. Stimulated microbial populations removed U almost entirely from solution within 1 month. X-ray absorption near-edge structure analysis showed that U(VI) was reduced to U(IV) during the incubation. Observations by transmission electron microscopy, selected area diffraction pattern analysis, and energy-dispersive X-ray spectroscopic analysis showed two distinct types of prokaryotic cells that precipitated only a U(IV) mineral uraninite (UO2) or both uraninite and metal sulfides. Prokaryotic cells associated with uraninite and metal sulfides were inferred to be sulfate-reducing bacteria. Phylogenetic analysis of 16S ribosomal DNA obtained from the original and incubated sediments revealed that microbial populations were changed from microaerophilicProteobacteriato anaerobic low-G+C gram-positive sporeforming bacteria by the incubation. Forty-two out of 94 clones from the incubated sediment were related to sulfate-reducingDesulfosporosinusspp., and 23 were related to fermentativeClostridiumspp. The results suggest that, if in situ bioremediation were attempted in the uranium mine ponds,Desulfosporosinusspp. would be a major contributor to U(VI) and sulfate reduction andClostridiumspp. to U(VI) reduction.