bluuebunny/crossref_metadata_2025 huggingface dataset:
A fast, stochastic threading algorithm for proteins.
Authors: Crawford O H
Published: July 2002

MOTIVATION: Sequences for new proteins are being determined at a rapid rate, as a result of the Human Genome Project, and related genome research. The ability to predict the three-dimensional structure of proteins from sequence alone would be useful in discovering and understanding their function. Threading, or fold recognition, aims to predict the tertiary structure of a protein by aligning its amino acid sequence with a large number of structures, and finding the best fit. This approach depends on obtaining good performance from both the scoring function, which simulates the free energy for given trial alignments, and the threading algorithm, which searches for the lowest-score alignment. It appears that current scoring functions and threading algorithms need improvement. RESULTS: This paper presents a new threading algorithm. Numerical tests demonstrate that it is more powerful than two popular approximate algorithms, and much faster than exact methods.
Local sequence alignments with monotonic gap penalties.
Authors: Mott R
Published: July 2002

MOTIVATION: Sequence alignments obtained using affine gap penalties are not always biologically correct, because the insertion of long gaps is over-penalised. There is a need for an efficient algorithm which can find local alignments using non-linear gap penalties. RESULTS: A dynamic programming algorithm is described which computes optimal local sequence alignments for arbitrary, monotonically increasing gap penalties, i.e. where the cost g(k) of inserting a gap of k symbols is such that g(k) >/= g(k-1). The running time of the algorithm is dependent on the scoring scheme; if the expected score of an alignment between random, unrelated sequences of lengths m, n is proportional to log mn, then with one exception, the algorithm has expected running time O(mn). Elsewhere, the running time is no greater than O(mn(m+n)). Optimisations are described which appear to reduce the worst-case run-time to O(mn) in many cases. We show how using a non-affine gap penalty can dramatically increase the probability of detecting a similarity containing a long gap. AVAILABILITY: The source code is available to academic collaborators under licence.
Some Comments on Free‐Riding in Leontief Partnerships
Authors: Hvide Hans K.
Published: July 2002

Holmström (Bell Journal of Economics, 13, 1982, 324–40) showed that free‐riding is inevitable in partnerships where inputs are substitutes. Legros and Matthews (Review of Economic Studies, 68, 1993, 599–611) and Vislie (Journal of Economics Behavior and Organization, 23, 1994, 83–91) showed that when inputs are strict complements (Leontief technology), free‐riding can be avoided with a linear sharing rule. This paper considers the robustness and some extensions of the positive result of these articles. First, I show that Legros and Matthews's and Vislie's results are not robust to the introduction of participation constraints and limited liability. However, I construct a novel rule that mitigates that problem. Second, I perturb the (deterministic) model of the other authors. It turns out that free‐riding is avoidable with noise added to joint output and is inevitable when noise is added to individual productivity.
Quantitative trait loci modulate neutrophil infiltration in the liver during LPS‐induced inflammation
Authors: Matesic Lydia E. , Niemitz Emily L. , De Maio Antonio , Reeves Roger H.
Published: July 2002

A crucial aspect of the inflammatory response is the recruitment of activated neutrophils (PMN) to the site of damage. Lytic enzymes and oxygen radicals released by PMN are important in clearing an infection or cellular debris, but can also produce host tissue damage. Failure to properly regulate the inflammatory response contributes to a variety of human diseases like sepsis and multiple organ dysfunction syndrome, the leading cause of morbidity and mortality in surgical intensive care units. Many aspects of human disease pathology, including hepatic PMN infiltration, can be recapitulated in mice using an endotoxic shock model. Six quantitative trait loci that predispose to high infiltration of PMN in hepatic sinusoids after high‐dose endotoxin administration were provisionally identified. Two of these loci, Hpil and Hpi2 on mouse chromosomes 5 and 13, were mapped to the significant and highly significant level using a low‐resolution genome scan on 122 intercross animals. These loci interact epistatically to produce a high degree of PMN infiltration. Intercross and recombinant inbred strain mice with a specific genotype at these loci always had a high infiltration response, indicating that genotype analysis at just these two loci can accurately predict a high PMN infiltration response. Genetic predisposition to the degree of PMN infiltration in the inflammatory response in mice suggests that analogous genetic mechanisms occur in human beings that could be used for diagnostic purposes.— Matesic, L. E., Niemitz, E. L., De Maio, A., Reeves, R. H. Quantitative trait loci modulate neutrophil infiltration in the liver during LPS‐induced inflammation. FASEB J. 14, 2247–2254 (2000)
Regulation of ATR substrate selection by Rad17-dependent loading of Rad9 complexes onto chromatin
Authors: Zou Lee , Cortez David , Elledge Stephen J.
Published: July 2002

Cells respond to DNA damage by activating a network of signaling pathways that control cell cycle progression and DNA repair. Genetic studies in yeast suggested that several checkpoint proteins, including the RFC-related Rad17 protein, and the PCNA-related Rad1–Rad9–Hus1 protein complex might function as sensors of DNA damage. In this study, we show that the human Rad17 protein recruits the Rad9 protein complex onto chromatin after damage. Rad17 binds to chromatin prior to damage and is phosphorylated by ATR on chromatin after damage but Rad17's phosphorylation is not required for Rad9 loading onto chromatin. The chromatin associations of Rad17 and ATR are largely independent, which suggests that they localize to DNA damage independently. Furthermore, the phosphorylation of Rad17 requires Hus1, suggesting that the Rad1–Rad9–Hus1 complex recruited by Rad17 enables ATR to recognize its substrates. Our data are consistent with a model in which multiple checkpoint protein complexes localize to sites of DNA damage independently and interact to trigger the checkpoint-signaling cascade.
Development of a prototype stereotactic radiotherapy—Radiosurgery unit (in English)
Authors: Theodorou Kiki
Published: July 2002

In this thesis the design, construction, and installation of a new linac‐based stereotactic radiotherapy unit is described. The design requirements were a noninvasive head fixation providing accurate patient repositioning with high reproducibility during multiple fractions. The noninvasive immobilization frame we constructed is used for both localization and for treatments. The immobilization is achieved using three stable locations: the upper dentition, the bridge of the nose, and the back of the neck. This fixation ensures adequate immobilization. The maximum relocation error measured on 22 patients in more than 200 mountings was 1.2 mm. The beam collimation is achieved by eight nondivergent–parallel–circular collimators (diameters between 1–4 cm) and the photon energy used is 6 MV. Concerning dosimetry, a methodology has been developed for dose measurements at the beam central axis. The methodology consists of the irradiation of an ionization chamber with its logitudinal axis parallel to the beam central axis. A correction factor is applied that includes the beam dose profile averaged over the chamber volume. The applied methodology proved to be compatible with film and TLD dosimetry and it has been tested successfully to 4 stereotactic units at different European centers. Furthermore, a new design of head phantom accommodates various dosimeters—ion chambers, TLDs, and films. A three‐dimensional treatment planning system, the X‐STING, was developed. It is dedicated for stereotactic radiotherapy. In addition to standard features, it includes innovative algorithms for image correlation (between CT and angiographic images with volumetric accuracy of 3%) and a complete model of the treatment machine.
Monte Carlo dosimetry of a tandem positioned beta‐emitting intravascular brachytherapy source train
Authors: Wallace Steven A. , Schumer Wendy , Horrigan Mark
Published: July 2002

Prevention of restenosis following interventional coronary procedures with catheter based beta‐emitting sources is currently under clinical trial investigations. Systems utilizing fixed length source trains limit the clinician's ability to increase the radiation source length as required. A technique known as “pull back” is used when the segment of artery requiring radiation is longer than the available fixed length source train. In this instance, tandem positioning of the fixed length source is used to treat the longer length of artery. The aim of this study was to examine the dosimetry of the junction region associated with pull back treatments using a commercially available catheter based intravascular brachytherapy source train. Dose profiles were calculated, using the Monte Carlo code MCNP4B, at radial distances of 1.5, 2.0, and 2.5 mm for pull back techniques using 2.5 mm overlapping, abutting, and 2.5 mm spaced source trains. Results at the protocol prescription radius of 2 mm showed a junction dose elevated 61% above prescription for 2.5 mm overlapping source trains. For 2.5 mm spaced trains, this figure falls to 64% below prescription dose. In contrast, abutted source trains exhibited only a 1% depression below prescription dose in the junction region. The reference point dose rate per unit activity of this source was found to be consistent with previous studies.
Design considerations for a computer controlled multileaf collimator for the Harper Hospital fast neutron therapy facility
Authors: Maughan Richard L. , Yudelev Mark , Aref Amr , Chuba Paul J. , Forman Jeffrey D. , Blosser Emanuel J. , Horste Timothy
Published: July 2002

The neutron beam from the Harper Hospital superconducting cyclotron is collimated using a unique multirod collimator (MRC). A computer controlled multileaf collimator (MLC) is being designed to improve efficiency and allow for the future development of intensity modulated radiation therapy with neutrons. For the current study the use of focused or unfocused collimator leaves has been studied. Since the engineering effort associated with the leaf design and materials choice impacts significantly on cost, it was desirable to determine the clinical impact of using unfocused leaves in the MLC design. The MRC is a useful tool for studying the effects of using focused versus unfocused beams on beam penumbra. The effects of the penumbra for the different leaf designs on tumor and normal tissue DVHs in two selected sites (prostate and head and neck) was investigated. The increase in the penumbra resulting from using unfocused beams was small (∼1.5 mm for a field and ∼7.6 mm for a field at 10 cm depth) compared to the contribution of phantom scatter to the penumbra width (5.4 and 20 mm for the small and large fields at 10 cm depth, respectively). Comparison of DVHs for tumor and critical normal tissue in a prostate and head and neck case showed that the dosimetric disadvantages of using an unfocused rather than focused beam were minimal and only significant at shallow depths. For the rare cases, where optimum penumbra conditions are required, a MLC incorporating tapered leaves and, thus, providing focused collimation in one plane is necessary.
A leaf sequencing algorithm to enlarge treatment field length in IMRT
Authors: Xia Ping , Hwang Andrew B. , Verhey Lynn J.
Published: July 2002

With MLC‐based IMRT, the maximum usable field size is often smaller than the maximum field size for conventional treatments. This is due to the constraints of the overtravel distances of MLC leaves and/or jaws. Using a new leaf sequencing algorithm, the usable IMRT field length (perpendicular to the MLC motion) can be mostly made equal to the full length of the MLC field without violating the upper jaw overtravel limit. For any given intensity pattern, a criterion was proposed to assess whether an intensity pattern can be delivered without violation of the jaw position constraints. If the criterion is met, the new algorithm will consider the jaw position constraints during the segmentation for the step and shoot delivery method. The strategy employed by the algorithm is to connect the intensity elements outside the jaw overtravel limits with those inside the jaw overtravel limits. Several methods were used to establish these connections during segmentation by modifying a previously published algorithm (areal algorithm), including changing the intensity level, alternating the leaf‐sequencing direction, or limiting the segment field size. The algorithm was tested with 1000 random intensity patterns with dimensions of 21×27 cm2, 800 intensity patterns with higher intensity outside the jaw overtravel limit, and three different types of clinical treatment plans that were undeliverable using a segmentation method from a commercial treatment planning system. The new algorithm achieved a success rate of 100% with these test patterns. For the 1000 random patterns, the new algorithm yields a similar average number of segments of 36.9±2.9 in comparison to 36.6±1.3 when using the areal algorithm. For the 800 patterns with higher intensities outside the jaw overtravel limits, the new algorithm results in an increase of 25% in the average number of segments compared to the areal algorithm. However, the areal algorithm fails to create deliverable segments for 90% of these patterns. Using a single isocenter, the new algorithm provides a solution to extend the usable IMRT field length from 21 to 27 cm for IMRT on a commercial linear accelerator using the step and shoot delivery method.
Microdosimetric considerations in the use of the boron neutron capture reaction in radiation therapy
Authors: Kota Chandrasekhar
Published: July 2002

This thesis attempts to further the understanding of the dosimetric and microdosimetric aspects of the boron neutron capture reaction (BNCR) in two distinct radiation therapy modalities, the conventional BNCT and boron neutron capture enhanced fast neutron therapy (BNCEFNT). The absorbed dose in these two modalities consists of contributions from gamma rays, neutrons and the neutron capture reactions in tissue including the BNCR. The use of low pressure tissue equivalent proportional counters for the dual purpose of dosimetry and microdosimetry of the complex radiation fields used in BNCT and BNCEFNT has been thoroughly investigated. Commercially obtained, paired 1/2″ Rossi type LET counters (with and without boron‐10 in the A‐150 TEP wall) were used in this work. The counters were dose calibrated by positioning the proton edge in the measured spectra at a lineal energy of 150 keV/micrometer. Using this paired counter technique, the gamma, neutron and BNCR dose components in different radiation fields were resolved. Experimental results indicate that this technique is well suited for the dosimetry of BNCT and BNCEFNT fields. The uncertainties in these dose measurements are smaller than, or comparable to those achieved using conventional techniques. The microdosimetric spectra measured by the proportional counters have been found to be useful for evaluating the quality of these radiation fields.
Recombination processes of ions in dielectric liquids. A study of their influence on dosimetry and RBE determination with liquid ionization chambers
Authors: Johansson Bengt
Published: July 2002

The design and properties of liquid ionization chambers for absorbed dose measurements in radiation therapy fields have been analyzed. The results show that liquid ionization chambers with a liquid layer of 1 mm and a sensitive volume of 7 can be used for measurements at absorbed dose rates down to 50–100 mGy/min with a reproducibility better than 1% at 1 min measurement time. The liquid ionization chambers can be designed with good geometrical resolution, negligible directional dependency, good long term calibration stability, at most 5% change in response in the photon energy interval 30–140 keV and negligible perturbation of the radiation fluence in tissuelike materials. General recombination experiments show that the general collection efficiency for the liquids can be calculated by the general collection efficiency equations derived for gases in continuous and pulsed radiation respectively, using the general recombination rate constant and ion mobility determined in the experiments for each of the liquids. These properties make liquid ionization chambers suitable for measurements of the absorbed dose distribution in water around small interstitial radiotherapy sources with intermediate photon energies.
Lithium borate TLD for determining the backscatter factors for low‐energy x rays: Comparison with chamber‐based and Monte Carlo derived values
Authors: Coudin Dany , Marinello Ginette
Published: July 2002

Recently, new backscatter factors for low‐energy x rays derived from Monte Carlo calculations have been recommended in the UK code of practice for kilovoltage dosimetry published by (IPEMB). As these data, presented as a function of half‐value layer, do not take account of the variation of the x‐ray spectra for a given HVL, we have undertaken an experimental study in order to determine BSF for the beam qualities provided by a Darpac 2000 therapy unit. A RTL detector such as and parallel‐plate ion chambers specially designed for low‐energy x‐ray dosimetry have been used. The results obtained show very good agreement between the TLD and the Monte Carlo calculations, confirming values obtained by other authors with lithium borate TLD. On the contrary, the results obtained with plane‐parallel ion chambers show discrepancies up to 9% that are discussed.
Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies
Authors: Wickman Göran , Johansson Bengt , Bahar‐Gogani Jalil , Holmström Thord , Grindborg Jan Erik
Published: July 2002

Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane‐parallel configurations, are presented. They are designed to be suitable for high‐precision measurements of absorbed dose‐to‐water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 (plane‐parallel) and 20 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane tetramethylsilane (Si or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about C for absorbed dose measurements in water in an x‐ray radiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dose measurements can be done with good reproducibility at dose‐rates as low as 50 Gy 3%). The long‐term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose‐rates up to about 100 mGy . The temperature dependence at room temperature is 0.5% per C. The response is almost independent of the direction of the radiation for the plane‐parallel LIC.
Design for an accelerator‐based orthogonal epithermal neutron beam for boron neutron capture therapy
Authors: Allen D. A. , Beynon T. D. , Green S.
Published: July 2002

This paper is concerned with the proposed Birmingham accelerator‐based epithermal neutron beam for boron neutron capture therapy (BNCT). In particular, the option of producing a therapy beam at an orthogonal direction to the incoming protons is considered. Monte Carlo radiation transport simulations, both with and without a head phantom, have shown that an orthogonal beam geometry is not only acceptable but is indeed beneficial, in terms of a lower mean neutron energy and an enhanced therapeutic ratio for the same useful neutron fluence in the therapy beam. Typical treatment times for various beam options have been calculated, and range from 20 to 48 min with a 5 mA beam of 2.8 MeV protons, if the maximum photon‐equivalent dose delivered to healthy tissue is to be 12.6 Gy Eq. The effects of proton beam diameter upon the therapy beam parameters have also been considered.
The importance of computed tomography slice thickness in radiographic patient positioning for radiosurgery
Authors: Murphy Martin J.
Published: July 2002

A new radiographic patient positioning technique developed for radiosurgery has been analyzed to show the effect of computed tomography (CT) slice thickness on the precision of target localization during treatment. The positioning technique establishes the pose of the patient's anatomy during treatment by comparing treatment room radiographs with digitally reconstructed radiographs derived from a CT study. The measured pose is then used to align the x‐ray therapy beam with the treatment site, without resorting to mechanical fixation. The technique has been found to be sensitive to submillimeter changes in skull position, which is the level of precision desired for radiosurgery. In this report it is shown that the precision of head localization improves by a factor of 2 when the CT slice thickness is reduced from 3.0 to 1.5 mm. This indicates that, in radiosurgical applications, image‐guided beam alignment can be significantly influenced by the spatial resolution of the reference CT study. This result is relevant to all high‐precision radiographic positioning techniques that utilize CT images.
Computational techniques for fast hyperthermia temperature optimization
Authors: Das Shiva K. , Clegg Scott T. , Samulski Thaddeus V.
Published: July 2002

Hyperthermia temperature optimization involves arriving at a temperature distribution which minimizes a stated goal function, the goal function having a biological basis in maximizing tumor cell kill while not exceeding normal tissue toxicity. This involves the computationally intensive process of multiple evaluations of the temperature goal function, requiring repeated evaluations of the power deposition and its corresponding temperature distribution. Two computational schemes are proposed to expedite the temperature optimization process: (1) temperature distribution evaluation by superpositioning precomputed distributions, and (2) using representative tissue groups (rather than every point in the domain) to evaluate the goal function. The application of these schemes is illustrated with a typical optimization problem, as applied to symmetric and asymmetric, heterogeneous models. Application of these schemes reduced the optimization time on a DEC Alpha 1000 4/266 (Alpha is a registered trademark of Digital Equipment Corporation.) from several h to min, with little difference in results. The computational schemes, though demonstrated in the context of electromagnetic hyperthermia, are generally applicable to other forms of nonionizing radiation employed in hyperthermia therapy.
Optimization of beam‐orientations in conformal radiotherapy treatment planning
Authors: Rowbottom Carl Graham
Published: July 2002

Traditionally, human planners have devised the treatment parameters used in radiotherapy treatment plans via a manually iterative process. Computer “optimization” algorithms have been shown to improve treatment plans as they can explore much more of the search space in a relatively short time. This thesis examines beam‐orientation computer “optimization” in radiotherapy treatment planning. New techniques were developed and a comparison was performed between treatment plans with “standard,” fixed beam‐orientations and treatment plans with “optimized” beam‐orientations for patients with cancer of the prostate, esophagus, and brain. A cohort of patients was considered in each group to avoid bias from a specific patient geometry. In the case of the patient cohort with cancer of the prostate, a coplanar beam‐orientation “optimization” scheme led to an average increase in the tumor control probability (TCP) of (5.7±1.4)% compared to the standard plans after the dose to the isocenter had been scaled to produce a rectal normal tissue complication probability (NTCP) of 1%. For the patient cohort with cancer of the esophagus, the beam‐orientation “optimization” scheme reduced the average lung NTCP by (0.7±0.2)% at the expense of a modest increase in the average spinal cord NTCP of (0.1±0.2)%. A noncoplanar beam‐orientation “optimization” scheme was tested using five patients with tumors of the brain. The scheme reduced the mean orbit doses by (19±4)%, when averaged over the patient cohort. The optic‐nerve mean and maximum doses were reduced by (11±5)% and (12±1)%, respectively. In conclusion, “optimization” of coplanar beam‐orientations led to improved treatment plans, but improvements were relatively modest compared to those from the “optimization” of noncoplanar beam‐orientations.
Toward a statistically relevant calibration end point for prostate seed implants
Authors: Rosenzweig Douglas P. , Schell Michael C. , Yu Yan
Published: July 2002

Interstitial brachytherapy for carcinoma of the prostate is achieved through the use of a configuration of radioactive seeds placed in a manner that delivers a customized, reasonably uniform dose to the target volume. Accurate dose delivery depends on both precise seed placement and reliable seed strength in the implanted configuration. This study assumes the independence of the two issues, and quantifies the reduction in the minimum dose to the surface of the gland due only to variability in individual seed strengths. Current AAPM guidelines pertaining to the acceptable limits on seed‐to‐seed variability are prudent for small configurations of seeds, yet are likely to be overly stringent for applications such as prostate seed implantation. In this study we determine the reduction in the minimum peripheral dose (mPD) caused by the introduction of source strength variability, and provide statistical insight into this effect. It is concluded that the current guidelines limit the reduction in mPD to ⩽0.4% relative to the prescription value, for an average configuration, due to the inclusion of strength variability. The maximum observed reduction in mPD would be ⩽1.5%. This value is an order of magnitude lower than the recommendations of the AAPM Task Group 40 for the overall accuracy of brachytherapy procedures, which suggests that seed strength variability is of limited concern and that constraints on this factor should perhaps be reevaluated.
Feasibility of linear arrays for interstitial ultrasound thermal therapy
Authors: Chopra Rajiv , Bronskill Michael J. , Foster F. Stuart
Published: July 2002

The feasibility of linear array transducers for interstitial ultrasound thermal therapy was evaluated. Theoretical acoustic power distributions were used to calculate spatial heating patterns using the bioheat transfer equation. The spatial heating patterns of linear array and single element planar rectangular transducers were compared. Scanned heating with both transducer geometries produced asymmetric heating volumes; however, a more uniform radial temperature profile with a sharper margin was achieved with linear arrays. Single element transducers produced excessive heating near the probe surface. Homogeneous blood flow is predicted to reduce the mean temperature within the heated region, with little effect on the spatial pattern.
Implementation of a Tc‐99m and Ce‐139 scanning line source for attenuation correction in SPECT using a dual opposing detector scintillation camera
Authors: du Raan H. , du Toit P. D. , van Aswegen A. , Lötter M. G. , Herbst C. P. , van der Walt T. N. , Otto A. C.
Published: July 2002

Image degradation during single photon emission computed tomography (SPECT) due to attenuation and Compton scatter of photons can cause clinical image artifacts and will also result in inaccurate quantitative data. Therefore attenuation correction methods recently received wide interest. Transmission imaging can be performed to obtain the attenuation coefficients of a nonhomogeneous attenuating medium accurately. The aim of this study was firstly to evaluate the imaging characteristics of the scanning line source assembly. The results obtained with Tc‐99m and Ce‐139 were compared. Secondly the calculated attenuation coefficients were compared with known values from literature, using Tc‐99m and Ce‐139 as transmission sources. Lastly the method of acquiring simultaneous transmission and emission data was investigated. This study shows that an attenuation coefficient map can be obtained using a scanning line source for transmission imaging with a dual opposing detector camera. The imaging characteristics of Tc‐99m and Ce‐139 as transmission sources are similar. The resolution obtained with the Ce‐139 line source was poorer than that obtained with the Tc‐99m line source. A linear relationship was found between CT numbers and attenuation coefficients for transmission images using both Tc‐99m and Ce‐139 line sources. The attenuation coefficient value for water was underestimated by 1% using the Tc‐99m transmission source and underestimated by 10% using Ce‐139 as transmission source. This underestimation of attenuation coefficient values was also obtained in the human study. A myocardial perfusion study processed without and with attenuation correction clearly demonstrated the effect of the attenuation correction in the inferior myocardial region. The potential of using a scanning line source as transmission source with a dual opposing detector camera has been demonstrated in this study. The transmission source, Ce‐139 was successfully introduced in this investigation for simultaneous acquisition of transmission and emission data.
A new high energy scintillating fiber optic single photon computed tomography detector
Authors: Monroe James I.
Published: July 2002

A new detector for imaging annihilation radiation is designed as an auxiliary unit for single photon computed tomography (SPECT) systems. Increased interest in higher energy medical imaging creates a demand for a low cost device that can be used with existing SPECT units. Key design features are: a custom high‐resolution collimator, a scintillator formed from plastic optical fibers, and a position‐sensitive photomultiplier. The optical fiber core is a ternary scintillator: Polystyrene (solvent) with solutes PPO and BS–MSB. High‐energy resolution (7.5 mm full‐width at half max (FWHM) at a source‐to‐collimator distance of 10 cm), improved total efficiency at a SCD of 10 cm), and low cost are hallmarks of the new detector. The FWHM is more than twice that available imaging 511 keV gammas with conventional SPECT cameras. Images are formed using Compton electrons, which are limited to one interaction inside the detector as opposed to detection by photoelectric interactions in conventional cameras. Computer simulations with the radiation transport code EGS4 verified the basic imaging concept and were then used to optimize the detector and collimator designs. A working prototype detector was manufactured, tested, and characterized. Positron emitting Germanium‐68 (Ge‐68) was used as a laboratory point source of annihilation radiation for testing. A larger area detector (4×4 array of 16 identical modules, each 5 cm ×5 cm ×20 cm with an expected planar sensitivity of specifically for augmented breast cancer diagnosis was then proposed and simulated as a possible application of this technology.
Model for determination of protective shielding needed for diagnostic radiology rooms (in Portuguese)
Authors: Costa Paulo R.
Published: July 2002

The objective of the research was the development and evaluation using modern techniques to calculate radiation protection barriers in clinical radiographic facilities. Our methodology uses realistic primary and scattered spectra. The primary spectra were computer simulated using a wave form generalization and a semiempirical model (the TBC model). The scattered spectra were obtained from published data. An analytical function was used to produce attenuation curves from polychromatic radiation for specified wave form, and filtration. The results of this analytical function are given in ambient dose equivalent units. The attenuation curves were obtained by application of Archer's model to the computer simulation data. We determined the parameters that gave the best fit to our model using primary and secondary radiation data from different radiographic procedures. These resulted in an optimized model for the shielding calculation for any radiographic room. The method was used to determine the shielding needed for two different radiographic room designs using typical workloads and spectra. The shielding costs were about 50% lower than those calculated using the traditional calculation based on NCRP Report No. 49.
A visible extended cavity diode laser for the undergraduate laboratory
Authors: Conroy R. S. , Carleton A. , Carruthers A. , Sinclair B. D. , Rae C. F. , Dholakia K.
Published: July 2002

We demonstrate how to construct a simple single-frequency extended cavity diode laser (ECDL) for the undergraduate laboratory using mainly standard opto-mechanical components. This ECDL is operated with both 635 and 670 nm laser diodes. We present three experiments that can be performed using this ECDL, namely spectroscopic studies of iodine, second harmonic generation, and an optical heterodyne experiment using the ECDL with a helium–neon laser.
A general elementary proof for the momentum–energy relationship, p=E/c, for a pulse on a loss-free transmission line
Authors: Whatley M. C.
Published: July 2002

The impulse delivered to the resistively terminated end of a loss-free transmission line is found, independently of the geometry of the line, using elementary physics, with an elementary application of the method of virtual work. The results correspond to the momentum–energy association p=E/c.
“Observation, Experiment, and the Future of Physics” John G. King’s acceptance speech for the 2000 Oersted Medal presented by the American Association of Physics Teachers, 18 January 2000
Authors: King John G.
Published: July 2002

Looking at our built world, most physicists see order where many others see magic. This view of order should be available to all, and physics would flourish better in an appreciative society. Despite the remarkable developments in the teaching of physics in the last half century, too many people, whether they’ve had physics courses or not, don’t have an inkling of the power and value of our subject, whose importance ranges from the practical to the psychological. We need to supplement people’s experiences in ways that are applicable to different groups, from physics majors to people without formal education. I will describe and explain an ambitious program to stimulate scientific, engineering, and technological interest and understanding through direct observation of a wide range of phenomena and experimentation with them. For the very young: toys, playgrounds, kits, projects. For older students: indoor showcases, projects, and courses taught in intensive form. For all ages: more instructive everyday surroundings with outdoor showcases and large demonstrations.
The nature of discovery in physics
Authors: Osheroff Douglas D.
Published: July 2002

By their very nature, those discoveries which most change the way we perceive our physical universe are difficult to anticipate. How then, are such discoveries made, and what experimental approaches are most likely to lead to discoveries? In this article I will describe four experiments in which I have participated that have yielded unexpected new physics, and attempt to explain how they came about.
A link between the bounds on relativistic velocities and areas of hyperbolic triangles
Authors: Criado C. , Alamo N.
Published: July 2002

In this paper we show the mathematical equivalence between two well-known facts: the existence of an upper bound for the area in Lobachevskian geometry and the existence of a limit for relativistic velocities. The key point is that the space of relativistic velocities can be interpreted as a Lobachevskian space.
EPR test with photons and kaons: Analogies
Authors: Gisin N. , Go A.
Published: July 2002

We present a unified formalism describing the Einstein–Podolsky–Rosen test using spin 12 particles, photons, and kaons. This facilitates the comparison with existing experiments using photons and kaons. It underlines the similarities between birefringence and polarization-dependent losses that affect experiments using optical fibers and mixing and decay that are intrinsic to the kaons. We also discuss the limitation these two characteristics impose on the testing of Bell’s inequality.
Path integral and the induction law
Authors: Barone F. A. , Farina C.
Published: July 2002

We show how the induction law is correctly used in the path integral computation of the free particle propagator. The way this primary path integral example is treated in most textbooks is a little bit misleading.
How to detect buried structures through electrical measurements
Authors: Osella Ana , Chao Gabriel , Sánchez Federico
Published: July 2002

The experiment reported here, performed by advanced undergraduates as a final laboratory work, was intended as an example of the application of the electricity theory to solve problems related to environmental physics. In particular, the aim of the work was to show how we can get the electrical image of the soil and detect the presence of buried structures from simple geoelectrical measurements. First, we developed scale models in the laboratory to recognize the electrical responses of different layered structures and to evaluate the sensitivity of the method and we interpreted the results using one-dimensional inversion codes. Then we proposed a configuration which permitted simulating a buried pipeline and analyzed the electrical response applying a simple two-dimensional numerical code. Finally, we performed field work in order to compare the results with ones obtained through the laboratory scale models.
Astronomy and particle physics research classes for secondary school students
Authors: Johansson K. E. , Nilsson Ch. , Engstedt J. , Sandqvist Aa.
Published: July 2002

During a 10-day course in astronomy and particle physics, two research classes with 17–18 year old secondary school students studied the differential rotation of the milky way, the intricacies of high energy particle collisions and learned about the origin of the universe. The combination of making their own measurements of the milky way with a small radio telescope, using scientific data from high energy particle collisions, and having the big bang explained by scientists created an attractive and fascinating course in contemporary science.
Resource Letter: G-1: Geomagnetism
Authors: Harrison Christopher G. A.
Published: July 2002

This Resource Letter provides a guide to the literature on the internally generated geomagnetic field. The topics covered include the origin of the geomagnetic field and the energy requirements and core properties necessary for its generation; a description of the geomagnetic field, including its spherical harmonic representation, and its rates of change, as well as models of the secular variation; observations of the field relevant to paleomagnetism and continental drift; information about magnetic anomalies in the oceans, sea floor spreading and plate tectonics; reversals of the geomagnetic field and magnetochronology.
A modern physics laboratory activity: Radio astronomical observations of recombination lines
Authors: McIntosh Gordon
Published: July 2002

The internet has made it possible to control the Massachusetts Institute of Technology Haystack Observatory in Westford, MA, from a distance. This capability allows undergraduate laboratory classes in physics and astronomy to acquire and analyze radio astronomical data that were previously unavailable. This paper describes an activity to observe recombination lines and compare them with theoretical predictions that would be appropriate for a modern physics class.
A more general form for parallel springs
Authors: Folkerts Timothy J.
Published: July 2002

The well-known result for the addition of springs in parallel assumes that the springs are constrained to compress the same amount. We consider the implications of removing this constraint by holding one end of each spring fixed, but allowing the other ends to compress as needed to achieve static equilibrium.
Acceleration for circular motion
Authors: Leff Harvey S.
Published: July 2002

A novel derivation of the instantaneous radial acceleration formula, a=v2/r, for uniform and nonuniform circular motion is given and compared with other approaches, which it can either supplement or replace.
Stars and statistical physics: A teaching experience
Authors: Balian Roger , Blaizot Jean-Paul
Published: July 2002

The physics of stars, their workings and their evolution, is a goldmine of problems in statistical mechanics and thermodynamics. We discuss many examples that illustrate the possibility of deepening student’s knowledge of statistical mechanics by an introductory study of stars. The matter constituting the various stellar objects provides examples of equations of state for classical or quantal and relativistic or non-relativistic gases. Maximum entropy can be used to characterize thermodynamic and gravitational equilibrium which determines the structure of stars and predicts their instability above a certain mass. Contraction accompanying radiation induces either heating or cooling, which explains the formation of stars above a minimum mass. The characteristics of the emitted light are understood from blackbody radiation and more precisely from the Boltzmann–Lorentz kinetic equation for photons. The luminosity is governed by the transport of heat by photons from the center to the surface. Heat production by thermonuclear fusion is determined by microscopic balance equations. The stability of the steady state of stars is controlled by the interplay of thermodynamics and gravitation.
Range of interactions: An experiment in atomic and magnetic force microscopy
Authors: Murphy W. L. , Spalding G. C.
Published: July 2002

We discuss the principles of magnetic force microscopy and present a method of using a relatively inexpensive instructional atomic force microscope in intermediate-level course work. We present calculations and images that give a qualitative feel for the crossover between different dominant forces due to varying ranges of interaction in force microscopy and fundamental physics.
A derivation of the gradient (∇B) drift based on energy conservation
Authors: Cully C. M. , Donovan E. F.
Published: July 2002

We present some basic concepts used to analyze single-particle motion in plasma environments, including the guiding center approximation and simple guiding center drifts. We then use these concepts to derive what is known as the ∇B (“grad B” or “gradient”) drift in a new way. In contrast to the standard approach involving perturbation techniques, we instead use a simple energy conservation law that gives insight into the physical causes of the drift and into some general properties of particle motion in electric and magnetic fields. This is a demonstration of the usefulness of energy conservation in analyzing particle motion and provides a clear example of how charged particles gyrating in an electric field that is not curl free are energized.
A simple calculation of the rate of emission of energy and of linear and angular momentum by a point charge in arbitrary motion
Authors: Napolitano R. , Ragusa S.
Published: July 2002

We calculate directly the rates of emission of energy and linear momentum by a point charge in arbitrary motion using mathematical results that render the calculation of solid-angle integrals very simple. We show that the results of these explicit calculations agree with those based on covariance, illustrating how deeply special relativity is rooted in classical electrodynamics. Then, we use these covariance arguments to calculate the emission of angular momentum, which is a new result. We also indicate how this calculation can be done in the direct, but much longer, way.
Magnetic dipole orientation energetics
Authors: Goedecke G. H. , Wood Roy C. , Nachman Paul
Published: July 2002

We analyze the energies involved in quasistatically changing the orientation of a magnetic dipole with unperturbed magnetic moment μ in the magnetic field B of a stationary source. We define an effective potential energy Veff for the dipole in a stationary orientational state as that part of the Hamiltonian plus the field energy that changes from one such state to another. The detailed energy contributions are quite different for solenoids than for permanent magnets. For a small current-carrying coil (the dipole) in the field of a large one, Veff=+μ⋅B, the interaction field energy; for a permanent magnet dipole in the field of a coil, Veff=0; for a permanent magnet dipole in the field of another magnet, Veff=−μ⋅B, equal to the usual interaction Hamiltonian. The external work required to change the orientation quasistatically from one stationary state to another is of course independent of system structure, and is equal to the change in Veff, plus the incremental changes in battery potential energy and resistive dissipation, if any, caused by the turning process, so all energy is indeed accounted for in all cases.
Multiplexed fingerprint recognition using holography
Authors: Fort Emmanuel , Vach Holger
Published: July 2002

In this paper, we present a fingerprint recognition experiment for undergraduate students using holographic techniques. It provides an inexpensive, enjoyable, and instructive insight into image treatment, optical computing, and its applications. It proves the simplicity and high performance of optical solutions when computing the correlation between two images, especially when a multiplexing scheme is incorporated.
An alternative derivation of the equations of motion of the relativistic (an)harmonic oscillator
Authors: Huang Young-Sea
Published: July 2002

The equations of motion of the relativistic (an)harmonic oscillator are derived based on an alternative Lagrangian formalism of relativistic mechanics using the proper time as the evolution parameter.
Applying covariant versus contravariant electromagnetic tensors to rotating media
Authors: Ridgely Charles T.
Published: July 2002

When the covariant form of Maxwell’s equations are applied to a rotating reference frame, a choice must be made to work with either a covariant electromagnetic tensor Fαβ or a contravariant electromagnetic tensor Fαβ. We argue that which tensor one chooses is ultimately dictated by whether one chooses to express the electric and magnetic fields in terms of a vector basis or in terms of a one-form basis, dual to the vector basis. We explain that when fields are expressed as one-forms, the covariant electromagnetic tensor is used; and when fields are expressed as vectors, the contravariant tensor is used. Using this formalism, we derive general field equations expressed in terms of vector and one-form fields in the rotating and laboratory frames when matter is present. Fields in the presence of matter are then related to those in a vacuum by using a covariant form of Minkowski’s constitutive equations, generalized to noninertial frames. Both vector and one-form field equations are used to derive the fields observed in the reference frame of a polarizable, permeable cylinder that rotates within an axially directed magnetic field. We find that the vector and one-form field equations both lead to predictions consistent with experimental results. We conclude that the choice between working with a covariant or contravariant electromagnetic tensor depends upon whether one chooses to express fields as vectors or as one-forms.