bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrogenic Sodium–Sodium Exchange Carried Out by Na,k -Atpase Containing the Amino Acid Substitution Glu779ala
Authors: Peluffo R. Daniel , Argüello José M. , B Lingrel Jerry , Berlin Joshua R.
Published: July 2002

Na,K -ATPase containing the amino acid substitution glutamate to alanine at position 779 of the α subunit (Glu779Ala) supports a high level of Na-ATPase and electrogenic Na+–Na+ exchange activityin the absence of K +. In microsomal preparations of Glu779Ala enzyme, the Na+ concentration for half maximal activation of Na-ATPase activity was 161 ± 14 mM (n = 3). Furthermore, enzyme activity with 800 mM Na+ was found to be similar in the presence and absence of 20 mM K +. These results showed that Na+, with low affinity, could stimulate enzyme turnover as effectively as K +. To gain further insight into the mechanism of this enzyme activity, HeLa cells expressing Glu779Ala enzyme were voltage clamped with patch electrodes containing 115 mM Na+ during superfusion in K +-free solutions. Electrogenic Na+–Na+ exchange was observed as an ouabain-inhibitable outward current whose amplitude was proportional to extracellular Na+ (Na+o) concentration. At all Na+o concentrations tested (3–148 mM), exchange current was maximal at negative membrane potentials (VM), but decreased as VM became more positive. Analyzing this current at each VM with a Hill equation showed that Na+–Na+ exchange had a high-affinity, low-capacity component with an apparent Na+o affinity at 0 mV (K 00.5) of 13.4 ± 0.6 mM and a low-affinity, high-capacity component with a K 00.5 of 120 ± 13 mM (n = 17). Both high- and low-affinity exchange components were VM dependent, dissipating 30 ± 3% and 82 ± 6% (n = 17) of the membrane dielectric, respectively. The low-affinity, but not the high-affinity exchange component was inhibited with 2 mM free ADP in the patch electrode solution. These results suggest that the high-affinity component of electrogenic Na+–Na+ exchange could be explained by Na+o acting as a low-affinity K + congener; however, the low-affinity component of electrogenic exchange appeared to be due to forward enzyme cycling activated by Na+o binding at a Na+-specific site deep in the membrane dielectric. A pseudo six-state model for the Na,K -ATPase was developed to simulate these data and the results of the accompanying paper (Peluffo, R.D., J.M. Argüello, and J.R. Berlin. 2000. J. Gen. Physiol. 116:47–59). This model showed that alterations in the kinetics of extracellular ion-dependent reactions alone could explain the effects of Glu779Ala substitution on the Na,K -ATPase.
On a transformation and re-transformation technique for constructing an affine equivariant multivariate median
Authors: Chakraborty Biman , Chaudhuri Probal
Published: July 2002

An affine equivariant version of multivariate median is introduced. The proposed median is easy to compute and has some appealing geometric features that are related to the configuration of a multivariate data cloud. The transformation and re-transformation approach used in the construction of the median has some fundamental connection with the data driven co-ordinate system considered by Chaudhuri and Sengupta (1993, Journal of the American Statistical Association). Large sample statistical properties of the median are discussed and finite sample performance is investigated using Monte Carlo simulations.
Hilbert 𝐶*-modules in which all closed submodules are complemented
Authors: Magajna Bojan
Published: July 2002

Let

B
B

be a C

∗

−

^*-

algebra. If there exists a full Hilbert

B
−

B-

module

X
X

such that

X
=
Y
⊕

Y
⊥

X=Y\oplus Y_\perp

for each closed submodule

Y
Y

, then

B
B

is *-isomorphic to a C

∗

−

^*-

algebra of (not neccesarily all) compact operators on a Hilbert space.
Using imperfect secondary structure predictions to improve molecular structure computations.
Authors: Chen C C , Singh J P , Altman R B
Published: July 2002

MOTIVATION: Until ab initio structure prediction methods are perfected, the estimation of structure for protein molecules will depend on combining multiple sources of experimental and theoretical data. Secondary structure predictions are a particularly useful source of structural information, but are currently only approximately 70% correct, on average. Structure computation algorithms which incorporate secondary structure information must therefore have methods for dealing with predictions that are imperfect. EXPERIMENTS PERFORMED: We have modified our algorithm for probabilistic least squares structural computations to accept 'disjunctive' constraints, in which a constraint is provided as a set of possible values, each weighted with a probability. Thus, when a helix is predicted, the distances associated with a helix are given most of the weight, but some weights can be allocated to the other possibilities (strand and coil). We have tested a variety of strategies for this weighting scheme in conjunction with a baseline synthetic set of sparse distance data, and compared it with strategies which do not use disjunctive constraints. RESULTS: Naive interpretations in which predictions were taken as 100% correct led to poor-quality structures. Interpretations that allow disjunctive constraints are quite robust, and even relatively poor predictions (58% correct) can significantly increase the quality of computed structures (almost halving the RMS error from the known structure). CONCLUSIONS: Secondary structure predictions can be used to improve the quality of three-dimensional structural computations. In fact, when interpreted appropriately, imperfect predictions can provide almost as much improvement as perfect predictions in three-dimensional structure calculations.
Application of distance geometry to 3D visualization of sequence relationships.
Authors: Forster M , Heath A , Afzal M
Published: July 2002

SUMMARY: We describe the application of distance geometry methods to the three-dimensional visualization of sequence relationships, with examples for mumps virus SH gene cDNA and prion protein sequences. Sequence-sequence distance measures may be obtained from either a multiple sequence alignment or from sets of pairwise alignments. AVAILABILITY: C/Perl code and HTML/VRML files from http://www.nibsc.ac.uk/dg3dseq/
WESTERN RANGELANDS REFORM: AN ANALYSIS OF THE 1996 SENATE VOTE ON FEDERAL GRAZING FEES
Authors: Fennemore Gerald A. , Nelson Jon P.
Published: July 2002

Grazing of livestock on public lands is a controversial environmental issue, despite a long history of federal regulation of this activity. Environmentalists and economists have argued that grazing fees charged by the Bureau of Land Management subsidize livestock operators and that higher fees would help bring rangeland usage in line with environmental and economic goals. In 1996, the U.S. Senate voted to determine whether higher fees would be charged for use of federally owned land. Based on a theoretical model, probit regressions are used to analyze this vote with respect to the influences of the general electorate, special interests, party affiliation, senatorial preferences, and electoral security. We find that political competition among producers utilizing federal lands was a crucial factor in determining voting decisions. However, environmental interests had a smaller direct impact on the grazing fee vote.
COMPLEMENTARY MEASURES TO FOREIGN AID: TAIWAN AND THE PRC UNDER DIPLOMATIC RIVALRY
Authors: Lin Richard C.
Published: July 2002

The competition on foreign aid, as a means for securing foreign policy and other benefits by the donor, can lead to diplomatic rivalry. This article utilizes a game theoretic model to analyze the existing rivalry for foreign aid and international status between Taiwan and the People's Republic of China. Similar to the role of political contributions in the public choice theory, foreign aid is taken a means of international lobbying, to realize the donor's objective. Research results, based on the policy effect consideration, indicates that the equilibrium aid policy, which is endogenously determined, renders an inability on the part of the donors, which affects the recipient's optimal foreign policy. How does a donor obtain favorable outcomes when such rivalry exists? Strengthening diplomatic endeavors to realize the access effect has been proved an effective method. Analytical results have shown that this method not only complements aid rivalry but also increases the welfare of the donor.
The human uncoupling protein‐3 gene promoter requires myod and is induced by retinoic acid in muscle cells
Authors: Solanes Gemma , Pedraza Neus , Iglesias Roser , Giralt Marta , Villarroya Francesc
Published: July 2002

The uncoupling protein‐3 (UCP‐3) gene encodes for a mitochondrial protein expressed preferentially in skeletal muscle. UCP‐3 mRNA is expressed in cultured muscle cells (C2C12 or L6E9) only when differentiated, at which stage UCP‐3 is highly induced by all‐trans retinoic acid (RA). Here we report that human UCP‐3 promoter activity is dependent on MyoD and inducible by all trans‐RA. The action of all trans‐RA is increased by co‐transfection with RA receptor (RAR). We have characterized the RA response element that controls the induction by RA in the 5′ noncoding region of the UCP‐3 gene. Deletion and point‐mutation analysis of the hUCP‐3 promoter led us to identify a direct‐repeat element with one base‐pair spacing (DR1) at position −71/−59 responsible for the induction by RA of the activity of the promoter. This DR1 element bound a nuclear protein complex from muscle cells that contain RAR and retinoid X receptor (RXR). In the absence of this element, the promoter became unresponsive to RA, but it was still dependent on MyoD. In conclusion, it has been established that UCP‐3 gene promoter activity is dependent on MyoD, and the first regulatory pathway for UCP‐3 gene promoter regulation has been recognized by identifying RA as a transcriptional activator of the gene.
Mutual genetic antagonism involving GLI3 and dHAND prepatterns the vertebrate limb bud mesenchyme prior to SHH signaling
Authors: te Welscher Pascal , Fernandez-Teran Marian , Ros Marian A. , Zeller Rolf
Published: July 2002

The bHLH transcription factor dHAND is required for establishment of SHH signaling by the limb bud organizer in posterior mesenchyme, a step crucial to development of vertebrate paired appendages. We show that the transcriptional repressor GLI3 restricts dHANDexpression to posterior mesenchyme prior to activation of SHH signaling in mouse limb buds. dHAND, in turn, excludes anterior genes such asGli3 and Alx4 from posterior mesenchyme. Furthermore, genetic interaction of GLI3 and dHAND directs establishment of the SHH/FGF signaling feedback loop by restricting the BMP antagonist GREMLIN posteriorly. These interactions polarize the nascent limb bud mesenchyme prior to SHH signaling.
CPP calculation of multiple scattering distributions for charged particles penetrating compounds or mixtures
Authors: Papiez L. , Sandison G. , Ning X. , Lu X.
Published: July 2002

Charged particle multiple scattering distributions may be constructed from individual atomic scattering events on the basis of compound Poisson process (CPP) theory. We present a CPP method for computing multiple scattering transition probability densities from charged particles penetrating compounds and mixtures. Water as a scattering medium provides here an example of the calculation method which is applicable to compounds or mixtures. Electrons are chosen as examples of charged particle beams. The Rutherford single scattering cross section and a partial wave analysis single scattering cross section are chosen as example cross sections. Transition probability densities predicted on the basis of CPP theory can be calculated with great accuracy for the improvement of radiation dose calculations. The advantages of the CPP method are (a) an effective atomic number need not be defined for the scattering medium, (b) it can be applied in both spherical and planar coordinate systems, and (c) it does not require any specific form for the single scattering cross section.
Experimental kerma coefficients of biologically important materials at neutron energies below 75 MeV
Authors: Meulders J. P. , Benck S. , Slypen I. , Corcalciuc V.
Published: July 2002

The present work summarizes our results already published on cross sections and partial kerma coefficients for hydrogen, carbon, and oxygen and then applies them for determining experimental partial and total kerma coefficients of composite biologically important materials. Double‐differential cross sections for light‐charged particle production (proton, deuteron, triton, and alpha particle) induced by fast neutrons on hydrogen, carbon, and oxygen have been experimentally measured at several incident energies from 25 to 75 MeV. The measurements covered the laboratory angular range 20° to 160° and were extended to very forward and very backward angles by using a reliable extrapolation procedure. Energy‐differential, angle‐differential, and total production cross sections were derived from the measured data. The experimental methods and data reduction procedures are briefly presented here. The experimental cross sections were compared to existing data in the literature for nucleon‐induced reactions and against prediction of nuclear models. Partial and total elemental kerma coefficients were deduced on the basis of the measured cross sections. Procedures for extrapolating the partial kerma coefficients down to the reaction threshold energies for each of the measured ejectile species have been applied to carbon and oxygen. A simple‐to‐use analytical formula to describe the experimental hydrogen kerma coefficients was proposed which provides the recoil kerma coefficients in the incident neutron energy range 0.3 to 100 MeV. The present article reports for the first time experimental partial kerma coefficients for composite materials of biological interest. Resulting total kerma coefficients are compared to theoretical predictions and to other experimental data.
Fast contrast‐enhanced imaging with projection reconstruction
Authors: Peters Dana C.
Published: July 2002

The use of contrast agents has lead to great advances in magnetic resonance angiography (MRA). Here we present the first application of projection reconstruction (PR) to contrast‐enhanced MRA. In this research the limited angle projection reconstruction trajectory is implemented to acquire higher resolution images per unit time than with conventional Fourier transform (FT) imaging. It is well known that as the field of view (FOV) is reduced in conventional FT imaging, higher resolution per unit time can be obtained, but aliasing may appear as a replication of outside material within the FOV. The limited angle PR acquisition also produces aliasing artifacts. This method produced artifacts that were unacceptable in x‐ray CT but that appear to be tolerable in MR angiography. Resolution throughout the FOV is determined by the projection readout resolution and not by the number of projections. As the number of projections is reduced, the resolution is unchanged, but low‐intensity artifacts appear. This thesis presents the outcome of using limited angle PR for contrast‐enhanced angiography. 3‐D volume images of the renal arteries, carotic arteries and pulmonary vessels were obtained with 512 readout resolution, about 128 projections and 32 slice encodings. This provided in‐plane resolution of 0.5×0.5 mm in a 24 s scan for the carotic arteries; resolution of 0.8×0.8 mm in a 17 s breath‐hold for the pulmonary vessels; and resolution of 0.8×0.8 mm in a 24 s breath‐hold for the renal arteries. These images have about four times greater resolution than conventional FT images acquired in equal scan time.
High resolution gel‐dosimetry by optical‐CT and MR scanning
Authors: Oldham Mark , Siewerdsen Jeffrey H. , Shetty Anil , Jaffray David A.
Published: July 2002

The increased intricacy of Intensity‐Modulated‐Radiation‐Therapy (IMRT) delivery has created the need for a high‐resolution 3D‐dosimetry (three‐dimensional) system capable of measuring and verifying the complex delivery. Present clinical methods are inadequate being restricted to single points (e.g., ion‐chambers) or to 2D planes (e.g., film), and are labor intensive. In this paper we show that gel‐dosimetry in conjunction with optical‐CT scanning can yield maps of dose that are of sufficient accuracy, resolution and precision to allow verification of complex radiosurgery deliveries, and by extension IMRT deliveries. The radiosurgery dose‐distribution represents the most challenging case encountered in external beam therapy by virtue of the steep dose‐gradients and high resolution of delivery. We characterize the stringent radiosurgery requirements by the RTAP (Resolution‐Time‐Accuracy‐Precision) criteria defined as ⩽1 mm3 spatial resolution, ⩽1 hour imaging time, accurate to within 3%, and within ⩽1% precision. The RTAP criteria is applied to an in‐house laser‐based optical‐CT scanning system presented here, and evaluated using gel‐flasks containing BANG3™ gel. The same gel flasks were subsequently imaged using the MR imaging protocol recommended by the gel manufacturer, but modified to match as closely as possible the RTAP. The resulting dose‐maps demonstrate the high precision (< 1.3% noise at high dose) achievable with optical CT scanning while preserving high spatial resolution (<1 mm3). Using the sequence above, the MR gel‐dose maps were found to have poorer precision by a factor of 5, under the strict conditions of the RTAP. The optical CT gel‐dosimetry system was further evaluated for the verification of a complex 3‐isocenter radiosurgery delivery. In conclusion, this work demonstrates that gel‐dosimetry and optical‐CT scanning approach an important long‐term goal of radiation dosimetry, as specified by the RTAP criteria, and have potential to impact the clinic by improving and facilitating clinical dose verification for the most complex external beam radiation treatments.
Dosimetric study of the new InterSource125 iodine seed
Authors: Reniers B. , Vynckier S. , Scalliet P.
Published: July 2002

The use of low energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumors, has significantly increased these last few years. New seed models for have been recently introduced. The aim of this study is to determine the dosimetric parameters as recommended by the AAPM in the TG43 formalism for a new iodine seed design: the InterSource125 (Furnished by IBt, Seneffe, Belgium). Measurements are made with LiF thermoluminescent dosimeters (size of 1 mm3) in solid water phantoms to obtain the dose constant, the radial dose function, and the anisotropy function. The TLDs were calibrated at 6 MV and an energy correction factor of 1.41 has been applied. The same dose parameters are also obtained by Monte Carlo calculations (MCNP4B) in solid water and in liquid water. The radial function was measured at 1, 1.5, 2, 3, 4, 5, 6, and 7 cm and calculated between 0.3 and 7 cm. The anisotropy functions were measured at 2, 3, and 5 cm and calculated between 0.3 and 7 cm. The calculated and the measured TG43 functions for solid water are in excellent agreement. We have then calculated these functions in liquid water to obtain the dosimetric information for clinical applications as per TG43 recommendations. In WT1, the calculated dose rate constant is and the measured value is The calculated value for water is In conclusion, the dosimetric functions for the new iodine seed InterSource125 have been determined. They are quite different from the data of the well‐known model 6711 from Amersham due to the absence of silver in the new seed. The characteristics are very similar to those of model 6702.
Communication and sampling rate limitations in IMRT delivery with a dynamic multileaf collimator system
Authors: Xia Ping , Chuang Cynthia F. , Verhey Lynn J.
Published: July 2002

The delivery of an intensity modulated radiation field with a dynamic multileaf collimator (MLC) requires precise correlation between MLC positions and cumulative monitor units (MUs). The purpose of this study is to investigate the precision of this correlation as a function of delivered MUs and dose rate. A semi‐Gaussian shaped intensity profile and a simple geometric intensity pattern consisting of four square segments were designed to deliver a total of 1, 4, 16, 64, and 100 MUs at three different dose rates of 100, 400, and 600 MU/min. The semi‐Gaussian intensity pattern was delivered using both sliding window and step and shoot techniques. The dose profiles of this intensity pattern were measured with films. The four square intensity pattern was delivered using step and shoot and conventional delivery techniques for comparison. Because of geometrical symmetry, the dose to each segment in this intensity pattern is expected to be the same when the same MU is assigned to each segment. An ionization chamber was used to measure the dose in the center of each of the four square segments. For the semi‐Gaussian shaped profile, significant artifacts were observed when the profile was delivered with small MUs and/or at a high dose rate. For the four square intensity pattern, the dose measured in each segment presented a large variation when delivered with small MUs and a high dose rate. The variation increases as the MU/segment decreases and as the dose rate increases. These MU and dose rate dependencies were not observed when the intensity pattern was delivered using a conventional delivery technique. The observed distortion of the semi‐Gaussian profile and dose variations among the segments of the four square intensity pattern are explained by considering the sampling rate and the communication time lag between the control systems. Finally, clinical significance is discussed.
Comparing calibration methods of electron beams using plane‐parallel chambers with absorbed‐dose to water based protocols
Authors: Stewart K. J. , Seuntjens J. P.
Published: July 2002

Recent absorbed‐dose‐based protocols allow for two methods of calibrating electron beams using plane‐parallel chambers, one using the for a plane‐parallel chamber, and the other relying on cross‐calibration of the plane‐parallel chamber in a high‐energy electron beam against a cylindrical chamber which has an factor. The second method is recommended as it avoids problems associated with the correction factors at for plane‐parallel chambers which are used in the determination of the beam quality conversion factors. In this article we investigate the consistency of these two methods for the PTW Roos, Scanditronics NACP02, and PTW Markus chambers. We processed our data using both the AAPM TG‐51 and the IAEA TRS‐398 protocols. Wall correction factors in beams and absorbed‐dose beam quality conversion factors for 20 MeV electrons were derived for these chambers by cross‐calibration against a cylindrical ionization chamber. Systematic differences of up to 1.6% were found between our values of and those from the Monte Carlo calculations underlying AAPM TG‐51, and up to 0.6% when comparing with the IAEA TRS‐398 protocol. The differences in translate directly into differences in the beam quality conversion factors in the respective protocols. The relatively large spread in the experimental data of and consequently the absorbed‐dose beam quality conversion factor, confirms the importance of the cross‐calibration technique when using plane‐parallel chambers for calibrating clinical electron beams. We confirmed that for well‐guarded plane‐parallel chambers, the fluence perturbation correction factor at is not significantly different from the value at For the PTW Markus chamber the variation in the latter factor is consistent with published fits relating it to average energy at depth.
Reconstruction of electron spectra using singular component decomposition
Authors: Chvetsov Alexei V. , Sandison George A.
Published: July 2002

Reconstruction of electron spectra of medical accelerators from measured depth dose distributions is an attractive tool for commissioning of a Monte Carlo treatment planning system. However, the reconstruction method is an inverse radiation transport problem which is poorly conditioned, in the sense it may become unstable due to small perturbations in the input data. Predicting the sharp (delta‐like) peak in the electron spectrum provides an additional challenge for the numerical reconstruction technique. To improve efficiency and robustness of the reconstruction technique, we developed an algorithm based on a separation of the electron spectrum into singular and regular components. We approximate the singular peak of the spectrum by a narrow weighted Gaussian function. The parameters of this Gaussian function are sought using only the fall‐off and toe regions of the depth‐dose curve. Analytical representation of the spectral peak by a Gaussian has benefit since only one weight and the mean and variance must be derived from the depth‐dose curve instead of multiple spectra weights. The regular part of the spectrum is reconstructed from the residual depth‐dose distribution using a variational method combined with a regularization technique to avoid the nonphysical oscillations. The effectiveness of the method is demonstrated by comparing predictions to “benchmark” spectra and depth‐dose distributions from Monte Carlo simulation of medical accelerators.
On the implementation of dose‐volume objectives in gradient algorithms for inverse treatment planning
Authors: Hristov D. , Stavrev P. , Sham E. , Fallone B. G.
Published: July 2002

A method that allows a straightforward implementation of dose‐volume constraints in gradient algorithms for inverse treatment planning is presented. The method is consistent with the penalty function approach, which requires the formulation of an objective function with penalty terms proportional to the magnitudes of constraint violations. Dose constraints with respect to minimum and maximum target dose levels are incorporated in quadratic, dose‐penalty terms. Analogously, quadratic volume‐penalty terms in the objective function reflect the violation of dose‐volume constraints imposing limits on the fractions of healthy organ volumes that can be irradiated above specified dose levels. It has been demonstrated that within the framework of this formulation neither modified objective functions nor finite difference gradient calculations are necessary for the incorporation of gradient minimization algorithms. As an example, a simple steepest descent algorithm is presented along with its application to illustrate prostate and lung cases.
The multielectrode current‐source interstitial hyperthermia system
Authors: van der Koijk John F.
Published: July 2002

The multielectrode current‐source interstitial hyperthermia (MECS‐IHT) system uses metallic electrodes inserted into plastic catheters and connected to 27 MHz sources. The heat deposition (SAR) along the catheter track is controlled by using several independent electrodes in a single catheter. The MECS probes incorporate multi‐sensor thermocouple thermometry. The planning system contains modules for the calculation of the power deposition in tissue, heat transport calculation, incorporating discrete vasculature, electrode power control, analysis of temperature distributions, implant planning and the visualisation of temperature distributions and of the implant setup. The ideal current source approximation used in the treatment planning/SAR model is validated and it is shown that tissue inhomogeneities do not pose major electrical problems. The temperatures measured in the catheter can be used for power control during treatment. The temperature distributions during treatment were investigated using models of idealized anatomies with and without discrete vasculature. Even with high contrasts in electrical and thermal conductivity in the implant it remains possible to obtain satisfactory temperature distributions with the MECS system. Inhomogeneities in the implant volume lead to inferior temperature distributions if no longitudinal SAR control is available. The spatial SAR control of the MECS‐IHT system, combined with the temperature feedback, is essential for optimizing the temperature control. Treatments using the system are technically successful.
Test objects for MRI quality assurance based on polymer gels
Authors: Gore J. C. , Maryanski M. J. , Schulz R. J.
Published: July 2002

Radiation‐sensitive polymer gels have been adapted for making test objects that can be used to assess the imaging characteristics of magnetic resonance imaging (MRI) systems. The polymer gels contain acrylic monomers within a gel matrix, and when irradiated with x rays the constituents polymerize to produce highly cross‐linked microparticles that dramatically affect water NMR relaxation rates where they form. The size of these effects depends on the radiation dose and composition of the mixture irradiated, while the spatial pattern of relaxation time changes can be precisely controlled by spatial modulation of the x‐ray exposure. This permits the manufacture of complex test patterns free of susceptibility or edge effects, and overall image performance can be assessed by constructing contrast‐detail diagrams using a singly irradiated gel containing areas of different sizes and contrasts. Polymer gels are stable and a variety of different tests objects can be constructed inexpensively. Such materials and test phantoms may find widespread application in diagnostic MRI quality assurance and testing programs.
Primary beam exposure outside the fluoroscopic field of view
Authors: Granger William E. , Bednarek Daniel R. , Rudin Stephen
Published: July 2002

Fluoroscopic units can irradiate an area outside the displayed field of view (FOV) which provides no additional information to the fluoroscopist and may significantly increase the integral dose to the patient. This unnecessary exposure can be reduced using region of interest (ROI) filtration techniques to attenuate the x rays peripheral to the ROI as well as outside the displayed FOV [Granger et al., Med. Phys. 23, 1059 (1996)]. A survey was conducted to quantitatively determine the deviation between the total exposed area and the displayed FOV on 18 fluoroscopic x‐ray units which have different shaped collimation. A film was taken in fluoroscopic mode and the area of the FOV was determined from the image of markers placed at the edge of the displayed FOV. For each unit, a measurement was made for each magnification mode at various source to image distances. The measured difference between the total exposed area and the area of the displayed FOV ranged between 5% and 32% for units meeting federal compliance standards and between 22% and 48% for units which were found to be out of compliance. The results of the survey and the application of ROI imaging techniques as a possible method to reduce dose due to inexact fluoroscopic collimation are presented.
A light localizer for use at large distances from a radiation source
Authors: Maughan Richard L.
Published: July 2002

In the superconducting cyclotron used for neutron therapy at Harper Hospital, space limitations prevented the use of a simple conventional light localizing system, employing a single mirror and a light source. The superconducting magnet coil is close to the neutron producing target and the vacuum window separating the acceleration chamber from atmospheric pressure is situated from this target. Therefore, the light localizing system must be designed to incorporate lenses, mirrors, an optical fiber light pipe and adjustment motors, in order to make the system compact enough to fit in the available space and to allow easy access for adjustment and alignment. The design of the light localizing system is explained in detail.
Inverse radiosurgery treatment planning through deconvolution and constrained optimization
Authors: Harmon Joseph F. , Bova Frank , Meeks Sanford
Published: July 2002

An inverse radiosurgery treatment planning approach is presented which calculates conformal dose distributions for small, irregularly shaped targets. Two general approaches have been suggested for solution of the inverse radiotherapy problem: explicit and implicit. Explicit methods are typically fast, but generally require geometric and/or dosimetric simplifications. Implicit methods have also been used, but are computationally expensive because they require iterative manipulation of each beam's individual elements. The method presented here incorporates an integrated approach in order to efficiently solve the inverse problem without requiring simplifications which may affect the accuracy of the final result. A deconvolution algorithm (explicit approach) is utilized to determine the intensity modulation function for multiple user‐selected beam's eye views of the desired dose distribution. A simulated annealing algorithm (implicit approach) then optimizes each beam's macroscopic weight. Additionally, this method is fully three‐dimensional and accurately models phantom scatter by incorporating Monte Carlo generated energy deposition kernels into the dosimetry process. Several small target structure examples are presented and applicability of this methodology to larger targets for general radiotherapy cases is addressed.
Viability of an isocentric cobalt‐60 teletherapy unit for stereotactic radiosurgery
Authors: Poffenbarger Brett A. , Podgorsak Ervin B.
Published: July 2002

The potential for radiosurgery with an isocentric teletherapy cobalt unit was evaluated in three areas: (1) the physical properties of radiosurgical beams, (2) the quality of radiosurgical dose distributions obtained with four to ten noncoplanar converging arcs, and (3) the accuracy with which the radiosurgical dose can be delivered. In each of these areas the cobalt unit provides a viable alternative to an isocentric linear accelerator (linac) as a radiation source for radiosurgery. A 10 MV x‐ray beam from a linac used for radiosurgery served as a standard for comparison. The difference between the 80%–20% penumbras of stationary radiosurgical fields in the nominal diameter range from 10 to 40 mm of the cobalt‐60 and 10 MV photon beams is remarkably small, with the cobalt‐60 beam penumbras, on average, only about 0.7 mm larger than those of the linac beam. Differences between the cobalt‐60 and 10 MV radiosurgical treatment plans in terms of dose homogeneity within the target volume, conformity of the prescribed isodose volume to the target volume, and dose falloffs outside the target volume are also minimal, and therefore of essentially no clinical significance. Moreover, measured isodose distributions for a radiosurgical procedure on our Theratron T‐780 cobalt unit agreed with calculated distributions to within the ±1 mm spatial and ±5% numerical dose tolerances, which are generally specified for radiosurgery. The viability of isocentric cobalt units for radiosurgery will be of particular interest to centers in developing countries where cobalt units, because of their relatively low costs, provide the only megavoltage source of radiation for radiotherapy, and could easily and inexpensively be modified for radiosurgery. Of course, the quality assurance protocols and mechanical condition of a particular teletherapy cobalt unit must meet stringent requirements before the use of the unit for radiosurgery can be advocated.
Evidence for a significantly higher than expected depth dependence of wedge transmission factors on the 4 MV beam of a new dual energy accelerator
Authors: Biggs P. J. , Beatty J. , Radloff J.
Published: July 2002

Wedge transmission factors have been measured for two sets of physical wedges for the 4 MV beam of a new dual energy linear accelerator as a function of field size and depth. The field size dependence of these factors has been compared with the 4 MV beam from another, single‐energy linear accelerator and a difference of a factor of approximately 2 has been observed in the relative wedge transmission factors between the two machines.
On absorbed dose in narrow gamma‐ray beams and dosimetry of the Gamma Knife
Authors: Nizin Paul S.
Published: July 2002

Using separate analytical functions describing primary dose, collimator scatter, and phantom scatter, an expression for absorbed dose in narrow gamma‐ray beams is developed and each function is quantified:  The absorbed dose is calculated in beams as narrow as 0.2 cm in radius. Analytical and experimental results are compared using measured dose data for the Gamma Knife. Close agreement with experimental data is observed.
Size and positioning reproducibility of an brachytherapy stepping source
Authors: Berndt A. , Rickey D. W. , Rathee S. , Bews J.
Published: July 2002

In this paper we describe techniques for measuring the dimensions and position reproducibility of an brachytherapy stepping source. Measurements were carried out using a 0.25×10×152 mm3 collimator placed in front of a detector of our own design. The brachytherapy source was translated past the collimator in 0.025 mm increments using a stepper motor. The source was found to be 3.58 mm long and 0.69 mm wide, which is in good agreement with the manufacturer's values of 3.5×0.6 mm2. The source position was reproducible to within 0.12 mm.
A language for generating tomographic images of mathematical phantoms
Authors: Mooij Rob B. , Amols H. I.
Published: July 2002

A computer language is presented that can be used to generate image files, as if the images are created with a CT or a MR scanner. The language defines objects in the “scanner's” coordinate system, as sets of quadratic inequalities. Each of these objects, e.g., an ellipsoid or a half‐plane or a cylinder, has its own density. Objects can be superimposed and collections of objects are allowed to translate and rotate. The language allows for a concise way of describing complex objects with precisely defined geometries and densities. An implementation of the language can be used for testing, developing, and analyzing diagnostic software, treatment planning systems, etc. A software module that is based on the language can be made available. The utility of the module for acceptance testing of radiation therapy treatment planning systems is described.
Specification of the physical and biologically effective absorbed dose in radiation therapies utilizing the boron neutron capture reaction
Authors: Burmeister Jay W.
Published: July 2002

This thesis addresses issues concerning the dosimetry of BNCT, offering a method to more accurately obtain the absorbed dose and a framework for predicting its biological effectiveness. A dual miniature tissue‐equivalent proportional counter (TEPC) technique is introduced as a tool for microdosimetry of BNCT. These paired A‐150 and TEPCs with 12.3 mm3 collecting volumes allow measurements in high flux fields and provide excellent spatial resolution. Single event charged particle spectra measured using these TEPCs allow accurate dose component separation in mixed fields as well as an assessment of the radiation quality. Measurements in epithermal neutron beams used for the current clinical trials for BNCT at the Brookhaven National Laboratory and the Massachusetts Institute of Technology are provided. These data represent an intercomparison of the radiation quality and beam characteristics of these two BNCT facilities using a single phantom and measurement technique. Photon and neutron absorbed doses measured using TEPC microdosimetry agree with results from conventional methods. However, TEPC measured BNC doses conflict with those calculated from foil activation measurements. This thesis also investigates the potential enhancement of fast neutron therapy using the BNC reaction. Methods for modification of a fast neutron therapy beam to increase the BNC enhancement are reviewed. Results of these feasibility studies are encouraging, indicating the possibility of therapeutic gains greater than 30% over conventional fast neutron therapy at depths required to treat brain lesions. An ionization chamber‐based system designed for BNC dosimetry of the modified fast neutron beam is also introduced. The advantages of this ionization chamber technique and TEPC microdosimetry technique over conventional methods are reviewed, elucidating the utility of these dosimetry systems for neutron capture therapy.
Improvement of tomographic intensity modulated radiotherapy dose distributions using periodic shifting of arc abutment regions
Authors: Leybovich Leonid B. , Dogan Nesrin , Sethi Anil , Krasin Matthew J. , Emami Bahman
Published: July 2002

Based on the study of treatment arc positioning versus target length, a method that allowed periodic shift of arc abutment regions through the course of intensity modulated radiotherapy (IMRT) was developed. In this method, two treatment plans were developed for the same tumor. The first plan contained the original target (Planning Target Volume as defined by radiation oncologist) and the second one contained a modified target. The modification of the original target consisted of simply increasing its length, adding a small extension to it, or creating a distant pseudo target. These modifications cause arc abutment regions in the second plan to be shifted relative to their positions in the first plan. Different methods of target modification were investigated because in some cases (for instance, when a critical structure might overlap with the target extension) a simple extension of the target would cause an unacceptable irradiation of the sensitive structures. The dose prescribed to the modified portion of the target varied from 10% to 100% of the original target dose. It was found that a clinically significant shift mm) in abutment region locations occurred when the dose prescribed to the extended portion of the target was of the original target dose. On the other hand, the pseudo target required only to 20% of the original target dose to produce the same shift in arc positions. Results of the film dosimetry showed that when a single plan was used for the treatment delivery, the dose nonuniformity was 17% and 25% of the prescribed dose with 0.5 and 1 mm errors in couch indexing, respectively. The dose nonuniformity was reduced by at least half when two plans were used for IMRT delivery.
Ultra high-pass Fourier filtering
Authors: Danailov M. B. , Narayanamurthy C. S.
Published: July 2002

We demonstrate a spatial Fourier filtering experiment that goes beyond the conventional low- and high-pass filtering and is not described in the Fourier optics books. Blocking nearly all the light in the Fourier plane of a 4-f filtering system is shown to produce at the output plane an image that is substantially different from the one corresponding to high-pass filtering. It is verified that the presented effect is due to the very high spatial frequency components created by the film emulsion scattering. This experiment can be performed in any undergraduate level optics laboratory with the same setup normally used for the Fourier filtering.
The influence of an historically oriented course on students’ content knowledge in optics evaluated by means of facets-schemes analysis
Authors: Galili Igal , Hazan Amnon
Published: July 2002

We report on an experimental course in geometrical optics which heavily incorporates historical models accounting for light, vision, optical images, and others. The design and contents of the course were guided by previously elicited knowledge of high school students regarding optical phenomena. We utilized the course in a year-long experiment. The content knowledge of students expressed in a facets-scheme structure was compared with the same under regular instruction. We made qualitative and quantitative assessments based upon facets-scheme frequencies. Clear differences found in students’ conceptual knowledge may support the adopted rationale and teaching approach: using appropriately selected historical materials that address knowledge issues relevant for the students can significantly promote meaningful learning of the subject matter.
Recording depth of the heterodyne laser interferometer for cochlear vibration measurement
Authors: Ren Tianying , Nuttall Alfred L.
Published: July 2002

Measurement of the cochlear partition vibration as a function of the optical-axis (z-axis) position in the gerbil cochlea showed that the velocity distributes over a range of more than 300 μm, which is larger than the thickness of the cochlear partition. This finding suggests that the recording depth (RD) of the heterodyne interferometer probably is not as small as reported in the literature. In the current experiment, the RD of the heterodyne laser interferometer was studied by measuring the velocity of a vibrating mirror as a function of the z-axis position. Results demonstrate that the optical sectioning characteristic, measured by the intensity of the reflected laser beam as a function of the z-axis position, is not able to correctly estimate the RD of the heterodyne interferometer: the RD is much larger than optical sectioning, indicating a poor spatial resolution along the z axis.
Psychoacoustical evaluation of the pitch-synchronous overlap-and-add speech-waveform manipulation technique using single-formant stimuli
Authors: Kortekaas Reinier W. L. , Kohlrausch Armin
Published: July 2002

This article presents two experiments dealing with a psychoacoustical evaluation of the pitch synchronous overlap-and-add (PSOLA) technique. This technique has been developed for modification of duration and fundamental frequency of speech and is based on simple waveform manipulations. Both experiments were aimed at deriving the sensitivity of the auditory system to the basic distortions introduced by PSOLA. In experiment I, manipulation of fundamental frequency was applied to synthetic single-formant stimuli under minimal stimulus uncertainty, level roving, and formant-frequency roving. In experiment II, the influence of the positioning of the so-called “pitch markers” was studied. Depending on the formant and fundamental frequency, experimental data could be described reasonably well by either a spectral intensity-discrimination model or a temporal model based on detecting changes in modulation of the output of a single auditory filter. Generally, the results were in line with psychoacoustical theory on the auditory processing of resolved and unresolved harmonics.
Geodetic position estimation of underwater acoustic sensors
Authors: Vincent Harold T. , Hu Sau-Lon James
Published: July 2002

An improved survey method for estimating the geodetic position of acoustic sensors placed at fixed but unknown locations on the seafloor is developed. Bottom-mounted sensors are surveyed using an extension of the classic trilateration terrestrial survey technique, i.e., making ranging measurements from reference points to the point to be surveyed. For acoustic sensors, these ranging measurements are obtained by transmitting an acoustic signal from a near surface projector and making corresponding timing and/or position measurements. Historically, sensors have been surveyed either relatively or geodeticly. A crucial calculation required for both cases is the effective sound speed (ESP). To avoid systematic (nonrandom) errors in the ESP calculation, an unbiased measurement for the sound speed profile (SSP) is a prerequisite. However, a typical measured SSP may have a 2-m/s bias error. Consequently, the traditional methods may suffer significant errors on estimating the sensor coordinates. The newly developed method considers the timing and sound speed biases, in addition to the sensor coordinates, as parameters to be estimated. In modeling this way, the bias errors are precluded from propagating to the sensor coordinates. Results with both simulated and actual field data show that this new method has significant improvement over traditional methods.