bluuebunny/crossref_metadata_2025 huggingface dataset:
Normal and Mutant Rhodopsin Activation Measured with the Early Receptor Current in a Unicellular Expression System
Authors: Shukla Pragati , Sullivan Jack M.
Published: July 2002

The early receptor current (ERC) represents molecular charge movement during rhodopsin conformational dynamics. To determine whether this time-resolved assay can probe various aspects of structure–function relationships in rhodopsin, we first measured properties of expressed normal human rhodopsin with ERC recordings. These studies were conducted in single fused giant cells containing on the order of a picogram of regenerated pigment. The action spectrum of the ERC of normal human opsin regenerated with 11-cis-retinal was fit by the human rhodopsin absorbance spectrum. Successive flashes extinguished ERC signals consistent with bleaching of a rhodopsin photopigment with a normal range of photosensitivity. ERC signals followed the univariance principle since millisecond-order relaxation kinetics were independent of the wavelength of the flash stimulus. After signal extinction, dark adaptation without added 11-cis-retinal resulted in spontaneous pigment regeneration from an intracellular store of chromophore remaining from earlier loading. After the ERC was extinguished, 350-nm flashes overlapping metarhodopsin-II absorption promoted immediate recovery of ERC charge motions identified by subsequent 500-nm flashes. Small inverted R2 signals were seen in response to some 350-nm flashes. These results indicate that the ERC can be photoregenerated from the metarhodopsin-II state. Regeneration with 9-cis-retinal permits recording of ERC signals consistent with flash activation of isorhodopsin. We initiated structure–function studies by measuring ERC signals in cells expressing the D83N and E134Q mutant human rhodopsin pigments. D83N ERCs were simplified in comparison with normal rhodopsin, while E134Q ERCs had only the early phase of charge motion. This study demonstrates that properties of normal rhodopsin can be accurately measured with the ERC assay and that a structure–function investigation of rapid activation processes in analogue and mutant visual pigments is feasible in a live unicellular environment.
Severed Channels Probe Regulation of Gating of Cystic Fibrosis Transmembrane Conductance Regulator by Its Cytoplasmic Domains
Authors: Csanády László , Chan Kim W. , Seto-Young Donna , Kopsco David C. , Nairn Angus C. , Gadsby David C.
Published: July 2002

Opening and closing of a CFTR Cl− channel is controlled by PKA-mediated phosphorylation of its cytoplasmic regulatory (R) domain and by ATP binding, and likely hydrolysis, at its two nucleotide binding domains. Functional interactions between the R domain and the two nucleotide binding domains were probed by characterizing the gating of severed CFTR channels expressed in Xenopus oocytes. Expression levels were assessed using measurements of oocyte conductance, and detailed functional characteristics of the channels were extracted from kinetic analyses of macroscopic current relaxations and of single-channel gating events in membrane patches excised from the oocytes. The kinetic behavior of wild-type (WT) CFTR channels was compared with that of split CFTR channels bearing a single cut (between residues 633 and 634) just before the R domain, of split channels with a single cut (between residues 835 and 837) just after the R domain, and of split channels from which the entire R domain (residues 634–836) between those two cut sites was omitted. The channels cut before the R domain had characteristics almost identical to those of WT channels, except for less than twofold shorter open burst durations in the presence of PKA. Channels cut just after the R domain were characterized by a low level of activity even without phosphorylation, strong stimulation by PKA, enhanced apparent affinity for ATP as assayed by open probability, and a somewhat destabilized binding site for the locking action of the nonhydrolyzable ATP analog AMPPNP. Split channels with no R domain (from coexpression of CFTR segments 1–633 and 837–1480) were highly active without phosphorylation, but otherwise displayed the characteristics of channels cut after the R domain, including higher apparent ATP affinity, and less tight binding of AMPPNP at the locking site, than for WT. Intriguingly, severed channels with no R domain were still noticeably stimulated by PKA, implying that activation of WT CFTR by PKA likely also includes some component unrelated to the R domain. As the maximal opening rates were the same for WT channels and split channels with no R domain, it seems that the phosphorylated R domain does not stimulate opening of CFTR channels; rather, the dephosphorylated R domain inhibits them.
𝐴_{𝑝} weights for nondoubling measures in 𝑅ⁿ and applications
Authors: Orobitg Joan , Pérez Carlos
Published: July 2002

We study an analogue of the classical theory of

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p

(
μ
)

A_p(\mu )

weights in

R

n

\mathbb {R}^n

without assuming that the underlying measure

μ
\mu

is doubling. Then, we obtain weighted norm inequalities for the (centered) Hardy-Littlewood maximal function and corresponding weighted estimates for nonclassical Calderón-Zygmund operators. We also consider commutators of those Calderón- Zygmund operators with bounded mean oscillation functions (

B
M
O

BMO

), extending the main result from R. Coifman, R. Rochberg, and G. Weiss, Factorization theorems for Hardy spaces in several variables, Ann. of Math. 103 (1976), 611–635. Finally, we study self–improving properties of Poincaré–B.M.O. type inequalities within this context; more precisely, we show that if

f
f

is a locally integrable function satisfying
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1

μ
(
Q
)

∫

Q

|

f
−

f

Q

|

d
μ
≤
a
(
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)

\frac {1}{\mu (Q)}\int _{Q} |f-f_{Q}| d\mu \le a(Q)

for all cubes

Q
Q

, then it is possible to deduce a higher

L
p

L^p

integrability result for

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f

, assuming a certain simple geometric condition on the functional

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a

.
Impact of early Polynesian occupation on the land snail fauna of Henderson Island, Pitcairn group (South Pacific)
Authors: Preece R. C.
Published: July 2002

Henderson Island, an uninhabited raised coral atoll in the Pitcairn group, has recently been designated a World Heritage Site because of its unique and relatively undisturbed ecosystem. The island is believed to have been uplifted and subaerially exposed during the last 275 kyr. This therefore provides the maximum age for the terrestrial biota that includes several endemic taxa. Henderson today supports 16 strictly terrestrial species of snails, about half of which are endemic. Analyses of sediments beneath Polynesian occupation horizons dated between the 11th and 17th centuries AD, have yielded 11 species of land snail present in the modern fauna, together with at least six (and possibly as many as eight) further species that no longer occur on the island. These extinct taxa are illustrated and formal descriptions provided for five (Pleuropoma hendersoni,Orobophana carinacosta,Minidonta macromphalus,Philonesia pyramidalis,P. weisleri); a sixth, known only from broken shells, appears to belong to the genusHiona. The two remaining taxa are ‘tornatellinids’ that have not been recognized among the modern fauna. Radiocarbon dates from bones of associated extinct land birds confirm their occurrence on Henderson before the first signs of Polynesian settlement. The extinction of these taxa seems to coincide with the Polynesian occupation and evidence for large-scale burning, at least around parts of the plateau margin, suggests that their demise can be linked with habitat destruction. At least three species,Gastrocopta pediculus,Lamellidea oblongaandPupisoma orcula, first appear in Polynesian occupation horizons. Their status as prehistoric introductions is therefore confirmed butG. pediculusno longer lives on Henderson.Pacificella variabilis,Tornatellides oblongus parvulusandElasmiassp., all previously thought to have been other prehistoric introductions to Henderson, were recovered from pre-Polynesian levels and are therefore native.
Mutualism, parasitism and competition in the evolution of coviruses
Authors: Nee Sean
Published: July 2002

Coviruses are viruses with the property that their genetic information is divided up among two or more different viral particles. I model the evolution of coviruses using information on both viral virulence and the interactions between viruses and molecules that parasitize them: satellite viruses, satellite RNAs and defective interfering viruses. The model ultimately, and inevitably, contains within it single–species dynamics as well as mutualistic, parasitic, cooperative and competitive relationships. The model shows that coexistence between coviruses and the self–sufficient viruses that spawned them is unlikely, in the sense that the quantitative conditions for coexistence are not easy to satisfy. I also describe an abrupt transition from mutualistic two–species to single–species dynamics, showing a new sense in which questions such as ‘Is a lichen one species or two?’ can be given a definite answer.
Relatedness and the fraternal major transitions
Authors: Queller David C.
Published: July 2002

Many of the major transitions in evolution involved the coalescence of independent lower–level units into a higher organismal level. This paper examines the role of kinship, focusing on the transitions to multicellularity in animals and to coloniality in insects. In both, kin selection based on high relatedness permitted cooperation and a reproductive division of labour. The higher relatedness of haplodiploid females to their sisters than to their offspring might not have been crucial in the origin of insect societies, and the transition to multicellularity shows that such special relationships are not required. When multicellular forms develop from a single cell, selfish conflict is minimal because each selfish mutant obtains only one generation of within–individual advantage in a chimaera. Conditionally expressed traits are particularly immune to within–individual selfishness because such mutations are rarely expressed in chimaeras. Such conditionally expressed altruism genes lead easily to the evolution of the soma, and the germ line might simply be what is left over. In most social insects, differences in relatedness ensure that there will be potential conflicts. Power asymmetries sometimes lead to such decisive settlements of conflicts that social insect colonies can be considered to be fully organismal.
Motor learning through the combination of primitives
Authors: Mussa–Ivaldi F.A. , Bizzi E.
Published: July 2002

In this paper we discuss a new perspective on how the central nervous system (CNS) represents and solves some of the most fundamental computational problems of motor control. In particular, we consider the task of transforming a planned limb movement into an adequate set of motor commands. Tocarry out this task the CNS must solve a complex inverse dynamic problem. This problem involves the transformation from a desired motion to the forces that are needed to drive the limb. The inverse dynamic problem is a hard computational challenge because of the need to coordinate multiple limb segments and because of the continuous changes in the mechanical properties of the limbs and of the environment with which they come in contact. A number of studies of motor learning have provided support for the idea that the CNS creates, updates and exploits internal representations of limb dynamics in order to deal with the complexity of inverse dynamics. Here we discuss how such internal representations are likely to be built by combining the modular primitives in the spinal cord as well as other building blocks found in higher brain structures. Experimental studies on spinalized frogs and rats have led to the conclusion that the premotor circuits within the spinal cord are organized into a set of discrete modules. Each module, when activated, induces a specific force field and the simultaneous activation of multiple modules leads to the vectorial combination of the corresponding fields. We regard these force fields as computational primitives that are used by the CNS for generating a rich grammar of motor behaviours.
Effects of metapopulation processes on measures of genetic diversity
Authors: Pannell John R. , Charlesworth Brian
Published: July 2002

Many species persist as a metapopulation under a balance between the local extinction of subpopulations or demes and their recolonization through dispersal from occupied patches. Here we review the growing body of literature dealing with the genetic consequences of such population turnover. We focus our attention principally on theoretical studies of a classical metapopulation with a ‘finite–island’ model of population structure, rather than on ‘continent–island’ models or ‘source–sink’ models. In particular, we concern ourselves with the subset of geographically subdivided population models in which it is assumed that all demes are liable to extinction from time to time and that all demes receive immigrants. Early studies of the genetic effects of population turnover focused on population differentiation, such as measured byFST. A key advantage ofFSTover absolute measures of diversity is its relative independence of the mutation process, so that different genes in the same species may be compared. Another advantage is thatFSTwill usually equilibrate more quickly following perturbations than will absolute levels of diversity. However, becauseFSTis a ratio of between–population differentiation to total diversity, the genetic effects of metapopulation processes may be difficult to interpret in terms ofFSTon its own, so that the analysis of absolute measures of diversity in addition is likely to be informative. While population turnover may either increase or decreaseFST, depending on the mode of colonization, recurrent extinction and recolonization is expected always to reduce levels of both within–population and specieswide diversity (πSand πT, respectively). One corollary of this is that πScannot be used as an unbiased estimate of the scaled mutation rate, θ, as it can, with some assumptions about the migration process, in species whose demes do not fluctuate in size. The reduction of π in response to population turnover reflects shortened mean coalescent times, although the distribution of coalescence times under extinction–colonization equilibrium is not yet known. Finally, we review current understanding of the effect of metapopulation dynamics on the effective population size.
The Functional Magnetic Resonance Imaging Data Center (fMRIDC): the challenges and rewards of large–scale databasing of neuroimaging studies
Authors: Van Horn John D. , Grethe Jeffrey S. , Kostelec Peter , Woodward Jeffrey B. , Aslam Javed A. , Rus Daniela , Rockmore Daniel , Gazzaniga Michael S.
Published: July 2002

The Functional Magnetic Resonance Imaging Data Center (fMRIDC) (http://www.fmridc.org) was established in the Autumn of 1999 with the objective of creating a mechanism by which members of the neuroscientific community may more easily share functional neuroimaging data. Examples in other sciences offer proof of the usefulness and benefit that sharing data provides through encouraging growth and development in those fields. By building a publicly accessible repository of raw data from peer–reviewed studies, the Data Center hopes to create a similarly successful environment for the neurosciences.

In this article, we discuss the continuum of data–sharing efforts and provide an overview of the scientific and practical difficulties inherent in managing various fMRI data–sharing approaches. Next, we detail the organization, design and foundation of the fMRIDC, ranging from its current capabilities to the issues involved in the submitting and requesting of data. We discuss how a publicly accessible database enables other fields to develop relevant tools that can aid in the growth of understanding of cognitive processes. Information retrieval and meta–analytic techniques can be used to search, sort and categorize study information with a view towards subjecting study data to secondary ‘meta–’ and ‘mega–analyses’. In addition, we detail the technical and policy challenges that have had to be addressed in the formation of the Data Center. Among others, these include: human subject confidentiality issues; ensuring investigator's rights; heterogeneous data description and organization; development of search tools; and data transfer issues. We conclude with comments concerning the future of the fMRIDC effort, its role in promoting the sharing of neuroscientific data, and how this may alter the manner in which studies are published.
Effect of scattered radiation on image noise in cone beam CT
Authors: Endo Masahiro , Tsunoo Takanori , Nakamori Nobuyuki , Yoshida Katsuya
Published: July 2002

Cone beam CT has a capability for the 3‐dimensional imaging of large volumes with isotropic resolution, and has a potentiality for 4‐dimensional imaging (dynamic volume imaging), because cone beam CT acquires data of a large volume with one rotation of an x‐ray tube‐detector pair. However, one of the potential drawbacks of cone beam CT is a larger amount of scattered x‐rays, which may enhance the noise in reconstructed images, and thus affect the low‐contrast detectablity. Our aim in this work was to estimate the scatter fractions and effects of scatter on image noise, and to seek methods of improving image quality in cone beam CT. First we derived a relationship between the noise in a reconstructed image and in an x‐ray intensity measurement. Then we estimated the scatter to primary ratios in x‐ray measurements using a Monte‐Carlo simulation. From these we estimated the image noise under relevant clinical conditions. The results showed that the scattered radiation made a substantial contribution to the image noise. However, focused collimators could improve it by decreasing the scattered radiation drastically while keeping the primary radiation at nearly the same level. A conventional grid also improved the image noise, though the improvement was less than that of focused collimators.
Detective quantum efficiency of a direct‐detection active matrix flat panel imager at megavoltage energies
Authors: Lachaine M. , Fourkal E. , Fallone B. G.
Published: July 2002

The use of an amorphous selenium (a‐Se) based direct‐detection active matrix flat‐panel imager (AMFPI) is studied for megavoltage imaging. The detector consists of a 1.2 mm copper front plate and 200 μma‐Se layer, and has a 85 μm pixel pitch. The Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) are measured for 6 and 15 MV photon beams. A theoretical expression for the DQE is derived using a recently developed formalism for nonelementary cascade stages. A comparison of theory with experiment is good for the 6 and 15 MV beams. The model is used to explore the DQE for more typical pixel sizes. The results indicate that with proper modifications, such as a largera‐Se thickness, a direct flat‐panel AMFPI is a very promising detector for megavoltage imaging.
A Monte Carlo study of radiation transport through multileaf collimators
Authors: Kim Jong Oh , Siebers Jeffrey V. , Keall Paul J. , Arnfield Mark R. , Mohan Radhe
Published: July 2002

Due to the significant increase in the number of monitor units used to deliver a dynamic IMRT treatment, the total MLC leakage (transmission plus scatter) can exceed 10% of the maximum in‐field dose. To avoid dosimetric errors, this leakage must be accurately accounted for in the dose calculation and conversion of optimized intensity patterns to MLC trajectories used for treatment delivery. In this study, we characterized the leaf end transmission and leakage radiation for Varian 80‐ and 120‐leaf MLCs using Monte Carlo simulations. The complex geometry of the MLC, including the rounded leaf end, leaf edges (tongue‐and‐groove and offset notch), mounting slots, and holes was modeled using MCNP4b. Studies were undertaken to determine the leakage as a function of field size, components of the leakage, electron contamination, beam hardening and leaf tip effects. The leakage radiation with the MLC configured to fully block the field was determined. Dose for 6 and 18 MV beams was calculated at 5 cm depth in a water phantom located at 95 cm SSD, and normalized to the dose for an open field. Dose components were scored separately for radiation transmitted through and scattered from the MLC. For the 80‐leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.7%, 1.8%, and 1.9% forandfields, respectively. For the 120‐leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.6%, 1.7%, and 1.9% for the same field sizes. Measured leakage values for the 120‐leaf MLC agreed with calculated values to within 0.1% of the open field dose. The increased leakage with field size is attributed to MLC scattered radiation. The fractional electron contamination for a blocked MLC field is greater than that for an open field. The MLC attenuation significantly affects the photon spectrum, resulting in an increase in percent depth dose at 6 MV, however, little effect is observed at 18 MV. Both phantom scatter and the finite source size contribute to the leaf tip profile observed in phantom. The results of this paper can be applied to fluence‐to‐trajectory and trajectory‐to‐fluence calculations for IMRT.
Relative effectiveness of polyacrylamide gel dosimeters applied to proton beams: Fourier transform Raman observations and track structure calculations
Authors: Jirasek A. , Duzenli C.
Published: July 2002

The feasibility of using polyacrylamide gel (PAG) to characterize a clinically relevant 74 MeV proton beam has been considered. Fourier transform Raman spectroscopy has been used to measure the response (i.e., consumption of monomer) of PAG exposed to the central and end portions of a spread out Bragg peak (SOBP), i.e., in two regions with measurable difference in proton linear energy transfer (LET). The response curve in each region was compared with a typical 6 MV x‐ray irradiated gel response curve, thus arriving at a gel “relative effectiveness” (RE) in each of the two regions. In addition, the theory of track structure, which is typically used to calculate a detector RE, is shown to give reasonable agreement when compared with the experimental results. Both experimental and track structure results indicate a decrease in gel response when irradiated with protons, as compared with the x‐ray response. In addition, both sets of results indicate a variation in gel response between the mid and end SOBP regions, thus illuminating the dependence of gel response to proton LET. The physical phenomenon causing a lower proton versus x‐ray gel response may be understood by considering the track structure calculations, which indicate that gel radiosensitive elements close to the track of a proton (i.e., withincm) are saturated. This saturation is due to the high delta‐ray doses deposited in these regions. The track calculations are extended to other situations (e.g., raw BP, different gel compositions) where experimental determination of RE is difficult or time consuming. Results again indicate a gel response dependent on position in the depth dose curve (i.e., LET). Overall, this study illuminates the difficulty in using polyacrylamide gel to extract quantitative dose maps when exposed to proton radiation.
Electron beam collimation with focused and curved leaf end MLCs—Experimental verification of Monte Carlo optimized designs
Authors: Karlsson Magnus G. , Karlsson Mikael
Published: July 2002

In general, electron beams from conventional accelerators using applicators with lead alloy inserts are not suitable for advanced conformal radiation therapy. However, interesting electron treatments have been demonstrated on a few advanced accelerators. These accelerators have been equipped with helium filled treatment heads and computer controlled MLCs that produce clinically useful energy modulated electron beams or mixed photon electron beams in an automated sequence. This study analyzes the characteristics of different MLC designs, curved and focused leaf ends in helium filled treatment heads, with respect to their effect on electron beams. In addition, this study analyzes the effects that different treatment head designs have on the output factor due to collimator scattering and shielding of secondary sources during treatment. The investigation of the different treatment head designs was performed with the Monte Carlo package BEAM and was verified by experimental methods. The results show that the difference between curved leaf ends and focused ends is negligible in most practical cases. The results also show the importance of scattering foil optimization in the optimization of parameters such as penumbra, virtual source position, and in the reduction of the output variation. In all cases, the experimental data verifies the calculations.
Using Monte Carlo methods to commission electron beams: A feasibility study
Authors: Antolak John A. , Bieda Michael R. , Hogstrom Kenneth R.
Published: July 2002

The purpose of this study was to investigate the feasibility of using Monte Carlo methods to assist in the commissioning of electron beams for a medical linear accelerator. The EGS4/BEAM code system was used to model an installed linear accelerator at this institution. Following an initial tuning of the input parameters, dosimetry data normally measured during the machine commissioning was calculated using the Monte Carlo code. All commissioning data was calculated for 6‐ and 12‐MeV electron beams, and a subset of the commissioning data was calculated for the 20‐MeV electron beams. On central axis, calculated percentage depth dose, cross‐beam profiles, cone‐insert ratios, and air‐gap factors were generally within 2% ofor 1 mm of the measured commissioning data; however, calculated open‐cone ratios were not within 2%, in most cases. Calculated off‐axis dose profiles for small fields were generally within the 2% (1‐mm) criteria; however, calculated dose profiles for larger (open cone) fields frequently failed the 2% (1‐mm) criteria. The remaining discrepancies between Monte Carlo calculations and measurement could be due to flaws in the Monte Carlo code, inaccuracies in the simulation geometry, the approximation of the initial source configuration, or a combination of the above. Although agreement between Monte Carlo calculated and measured doses was impressive and similar to previously published comparisons, our results did not prove our hypothesis that Monte Carlo calculations can generate electron commissioning data that is accurate within 2% ofor 0.1 cm over the entire range of clinical treatment parameters. Although we believe that this hypothesis can be proved, it remains a challenge for the medical physics community. We intend to pursue this further by developing systematic methods for isolating causes of these differences.
A Monte Carlo study on internal wedges using BEAM
Authors: van der Zee W. , Welleweerd J.
Published: July 2002

To do calculations for wedged photon beams with the NRC Monte Carlo simulation package BEAM, a new Component Module for wedges called WEDGE has been designed and built. After an initial series of benchmarks using monoenergetic photon beams as well as realistic 6 MV and 10 MV beams, it was found, that the new CM did work fine for the large wedge (maximum field sizeof the Elekta SL‐linac. The next step was to calculate dose distributions and output factors for a range of wedged fields with field size fromtoResults from these simulations have been compared to measurements. Calculated values for thereference wedge transmission factorand therelative wedge transmission factorswere within 1.5% from the measured data. Dose distributions showed an identical behavior; both depth‐dose curves as well as cross profiles were within 1.5% from measured data, usually even better. Despite the increased mean energy, there was no indication that, as a result, thephantom scatter output factorswill change for a 10 MV photon beam. It was found that by adding a wedge the contributions for the different sources of head scatter changed considerably as compared to the open fields, apart from the additional scatter from the wedge. Another consequence of inserting a wedge was an increase in the mean energy of both primary and scattered radiation with 0.3 MV and 0.7 MV, respectively, for all wedged fields with respect to the corresponding open fields. Despite the statistical uncertainty in the calculated data, which is in the same order of magnitude as the effect to be determined, it was possible to derive reliable data for the beam hardening from the calculated dose distributions. Only for the smallest field (field sizea large difference between the measured and calculated beam hardening factor was found due to the relative large voxel size ofcompared to the field size. For a description of the influence of a wedge on a photon beam, the results of this study strongly support the use of areference wedge transmission factor(determined under reference conditions) in combination with arelative wedge transmission factor. The product of these variables should replace thecollimator scatter output factorused in open fields. The influence on the dose distribution should be incorporated by using the (field size dependent) beam hardening. The ultimate solution will be to make this beam hardening depending on the actual position in the radiation field, as the photon energy varies over the field (holds also for open fields).
Monte Carlo dose calculations in homogeneous media and at interfaces: A comparison between GEPTS, EGSnrc, MCNP, and measurements
Authors: Chibani Omar , Li X. Allen
Published: July 2002

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are benchmarked against dose measurements in homogeneous (both low‐ and high‐media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two‐layer media exposed to agamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high‐energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of abrachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy‐indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces.fits more closely the experimental data thanexcept for the case of Be. At low energy (0.05 and 0.1 MeV),dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron‐dose distributions in high‐media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For thesource, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and thebeta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy‐indexing method.
Monte Carlo based phase‐space evolution for electron dose calculation
Authors: Scora Daryl , Faddegon Bruce A.
Published: July 2002

A system of computer codes based on phase‐space evolution is developed and applied to low energy therapeutic electron beams. Monte Carlo (EGS4) is used to pre‐calculate the electron transport and dose deposition in a 0.5 cm width cubic voxel. Dose calculations at larger scales are computed from the pre‐calculated data using phase‐space evolution. This approach has the theoretical accuracy of Monte Carlo with potentially significant speed gains resulting from the pre‐calculation. This study demonstrates the accuracy of this technique while providing a preliminary assessment of the calculation time. For a 4.3 MeV electron beam in water with a 0.5 cm thick slab of either water (homogeneous), air, or aluminum at 1 cm depth, we observe differences relative to Monte Carlo of less thanalong the central axis for a pencil‐beam. For afield we observe a maximum difference on the central axis of 4% in the build‐up region and less than 0.1 cm in the fall‐off region for all three phantoms. Calculation times are disappointing; however, there is high potential for their reduction to values comparable to or better than condensed history Monte Carlo while retaining clinically acceptable accuracy.
Improved modeling of multiple scattering in the Voxel Monte Carlo model
Authors: Kawrakow Iwan
Published: July 2002

The implementation of an improved model for multiple scattering into the Voxel Monte Carlo (VMC) algorithm for fast electron dose calculation in radiation therapy is presented. The model takes into account path‐length corrections (PLC) and lateral displacement in the individual electron steps. The extraction of the scattering power from the available computed‐tomography images is discussed. It is shown that with the improved modeling of multiple electron scattering, theVMCalgorithm is comparable in accuracy withPRESTA, the electron transport algorithm ofEGS4. The problem of double counting of contributions from atomic electrons to the scattering power is considered and a simple solution is found.
Implementation of a Monte Carlo dosimetry method for patient‐specific internal emitter therapy
Authors: Furhang E. E. , Chui C. S. , Kolbert K. S. , Larson S. M. , Sgouros G.
Published: July 2002

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose‐response relationship, as well as to avoid critical organ toxicity. This work describes the implementation of a dosimetry method that accounts for the radionuclide decay spectrum, and patient‐specific activity and density distributions. The dosimetry algorithm is based on a Monte Carlo procedure that simulates photon and electron transport and scores energy depositions within the patient. The necessary input information may be obtained from a registered set of CT and SPECT or PET images. The algorithm provides the absorbed dose rate for the radioactivity distribution provided by the SPECT or PET image. The algorithm was benchmarked by reproducing dosimetric quantities using the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom and was used to calculate absorbed dose distributions for representative case studies.
Correction factors for water‐proofing sleeves in kilovoltage x‐ray beams
Authors: Ma C.‐M. , Seuntjens J. P.
Published: July 2002

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50–300 kV). The sleeve effect correction factor,has been calculated using the Monte Carlo method as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated‐sampling variance‐reduction technique. The results show that the sleeve correction factor for a 1‐mm thick PMMA waterproofing sleeve surrounding a Farmer‐type cylindrical chamber at 2 cm depth in water is about 0.991 at 50 kV (1.04 mm Al HVL) and 1.000 at 300 kV (21.5 mm Al HVL). For 1‐mm thick nylon and polystyrene sleeves,varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. Thefactor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of thefactor on field size and source‐surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carried out and show excellent agreement with the Monte Carlo calculations.
Connectivity in human cancellous bone by three‐dimensional magnetic resonance microscopy
Authors: Wessels Mark , Mason Ralph P. , Antich Peter P. , Zerwekh Joseph E. , Pak Charles Y. C.
Published: July 2002

Bone architecture affects strength and resistance to fracture. Trabecular connectivity is now recognized as an important measure of bone quality, and could be useful as an indicator of the osteoporotic condition, as well as a tool for measuring the effectiveness of therapies. We have applied three‐dimensional magnetic resonance imaging microscopy to human cancellous bone biopsies, and report the results of connectivity measurements. Sample heterogeneity was examined on the basis of connectivity density for subvolumes. The choice of examination volume had a significant effect on connectivity density measurements, but sample volumes greater thanwere found to give stable results. Connectivity density was strongly correlated with nodal density, and two‐dimensional estimates of connectivity, but not bone volume fraction. Repeat measurement at constant resolution (69×138×109 μm, signal‐to‐noise ratio of about 35) showed reproducibility of about 5% for connectivity density. Our most recent results have significantly enhanced resolution (69×69×43 μm); bone fraction remained constant, but apparent connectivity density is greater.
Optimization of the scintillation detector in a combined 3D megavoltage CT scanner and portal imager
Authors: Mosleh‐Shirazi M. A. , Swindell W. , Evans P. M.
Published: July 2002

A parametric study is described leading to the optimization of a custom‐made scintillation detector with a relatively high quantum efficiency (QE) for megavoltage photons and light output toward a remote lens. This detector allows low‐dose portal imaging and continuous cone‐beam megavoltage CT acquisition. The EGS4 Monte Carlo code was used to simulate the x‐ray and electron transport in the detector. A Monte Carlo model of optical photon transport in a detector element was devised and used as well as various irradiation experiments on scintillators. Different detector materials and configurations were compared in terms of the optical photon irradiance on the lens from on‐ and off‐axis detector elements and the practical constraints regarding detector construction and weight. Effects of scintillator material, detector element size, crystal coating type, and reflectivity, combinations of different coatings on detector faces, scintillator doping level, and crystal transparency were studied. With scintillator thickness adjusted to give an 18% x‐ray QE at 6 MV, the light output of CsI(Tl) was at least eight times higher thanBGO and NE118 plastic. Further, CsI(Tl) showed the smallest decrease in QE going from 6 to 24 MV. The off‐axis reduction in emittance from the periphery of the detector was relatively small with a slight dependence on the type and reflectivity of the coating and the crystal thickness for a fixed detector element cross section. Light output was more strongly dependent on the reflectivity of lambertian coatings than specular ones. For a fixed detector element cross section, optimum coating type depended on crystal thickness. Typical CsI(Tl) crystals showed a relatively small variation in light output with changes in optical attenuation length. The optimum detector element was found to be CsI(Tl) coated on five faces with‐loaded epoxy resin offering about a ten‐fold improvement in light output per incident photon compared to typical metal/phosphor screens.
Automatic registration of pelvic computed tomography data and magnetic resonance scans including a full circle method for quantitative accuracy evaluation
Authors: van Herk Marcel , de Munck Jan C. , Lebesque Joos V. , Muller Saar , Rasch Coen , Touw Adriaan
Published: July 2002

The purpose of this study is to develop a method for registration of CT and MR scans of the pelvis with minimal user interaction and to obtain a means for objective quantification of the registration accuracy of clinical data without markers. CT scans were registered with proton density MR scans using chamfer matching on automatically segmented bone. A fixed threshold was used to segment CT, while morphological filters were used to segment MR. The method was tested with transverse and coronal MR scans of 18 patients and sagittal MR scans of 8 patients. The registration accuracy was estimated by comparing (triangulating) registrations of a single CT scan with MR in different orientations in a “full circle.” For example, CT is first matched on transverse MR, next transverse MR is matched independently on coronal MR, and finally coronal MR is matched independently on CT. The product of the three transformations is the identity if all matching steps are perfect. Deviations from identity occur both due to random errors and due to some types of systematic errors. MR was registered on MR (to close the “circle”) by minimization of rms voxel value differences. CT‐MR registration takes about 1 min, including user interaction. The random error for CT‐MR registration with transverse or coronal MR was 0.5 mm in translation and 0.4° in rotation (standard deviation) for each axis. A systematic registration error of about 1 mm was demonstrated along the MR frequency encoding direction, which is attributed to the chemical shift. In conclusion, the presented algorithm efficiently and accurately registers pelvic CT and MR scans on bone. The “full circle” method provides an estimate of the registration accuracy on clinical data.
Adaptive streak artifact reduction in computed tomography resulting from excessive x‐ray photon noise
Authors: Hsieh Jiang
Published: July 2002

The quality of a computed tomography (CT) image is often degraded by streaking artifacts resulting from excessive x‐ray quantum noise. Often, a patient has to be rescanned at a higher technique or at a larger slice thickness in order to obtain an acceptable image for diagnosis. This results in a higher dose to the patient, a degraded cross plane resolution, or a reduced patient throughput. In this paper, we propose an adaptive filtering approach in Radon space based on the local statistical properties of the CT projections. We first model the noise characteristics of a projection sample undergoing important preprocessing steps. A filter is then designed such that its parameters are dynamically adjusted to adapt to the local noise characteristics. Because of the adaptive nature of the filter, a proper balance between streak artifact suppression and spatial resolution preservation is achieved. Phantom and clinical studies have been conducted to evaluate the robustness of our approach. Results demonstrate that the adaptive filtering approach is effective in reducing or eliminating quantum noise induced artifacts in CT. At the same time, the impact on the spatial resolution is kept at a low level.
Study of dosimetry consistency for kilovoltage x‐ray beams
Authors: Ma C.‐M. , Li X. A. , Seuntjens J. P.
Published: July 2002

In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x‐ray beam dosimetry using the “in‐air” method and the in‐phantom measurement has been studied. The procedures for the measurement of the central‐axis depth–dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane–parallel ionization chamber at different depths in a water phantom irradiated by 100–300 kV (2.43 mm Al–3.67 mm Cu half‐value layer) x‐ray beams. The depth–dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100–300 kV tube potential obtained using the “backscatter” method and those using the “in‐phantom” measurement have shown that the “in‐air” method can be equally applied to this energy range if the depth–dose curve can be measured accurately. To this end, measured depth ionization curves require depth‐dependent correction factors.
Intracavitary ultrasound phased arrays for prostate thermal therapies: MRI compatibility andin vivotesting
Authors: Hutchinson E. B. , Hynynen K.
Published: July 2002

A 62 element MRI‐compatible linear phased array was designed and constructed to investigate the feasibility of using transrectal ultrasound for the thermal therapeutic treatment of prostate cancer and benign prostatic hyperplasia. An aperiodic design technique developed in a previous study was used in the design of this array, which resulted in reduced grating lobe levels by using an optimized random distribution of unequally sized elements. The element sizes used in this array were selected to be favorable for both grating lobe levels as determined by array aperiodicity and array efficiency as determined by width to thickness ratios. The heating capabilities and MRI compatibility of the array were tested within vivorabbit thigh muscle heating experiments using MRI temperature monitoring. The array produced therapeutic temperature elevationsin vivoat depths of 3–6 cm and axial locations up to 3 cm off the central axis and increased the size of the heated volume with electronic scanning of a single focus. The ability of this array to be used for ultrasound surgery was demonstrated by creating necrosed tissue lesionsin vivousing short high‐power sonications. The ability of the array to be used for hyperthermia was demonstrated by inducing therapeutic temperature elevations for longer exposures. Based on the acoustic and heating performance of this array, it has the potential to be clinically useful for delivering thermal therapies to the prostate and other target volumes close to body cavities.
Photon dose calculation based on electron multiple‐scattering theory: Practical representation of dose and particle transport integrals
Authors: Jette David
Published: July 2002

In modern photon dose‐calculation algorithms one is frequently called upon to evaluate the integral at various points throughout the irradiated material of a dose or particle transport quantity multiplied by a weighting factor. For example, for a given dose‐calculation point one might be integrating the product of the dose deposited by a monoenergetic beam and the energy distribution of the actual beam, and want to do this throughout the treatment volume. We have developed explicit formulas for replacing such integrations with a weighted sum of two or three functions (of, for example, the point of dose calculation) in order to greatly reduce the calculation time for the algorithm being used. We demonstrate the accuracy of this method of representing dose and particle transport integrals through comparisons with Monte Carlo calculations of dose distributions for two typical problems, in dealing with the energy spectrum of the photon beam and with the energy deposited by all the Compton electrons emerging from a particular interaction point, respectively.