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At least 127 records · Page 7Linked to original sources
PubMed1994

The potential and limitations of the inverse radiotherapy technique.

The objective of the work presented in this paper is to explore the scope of the applicability of the inverse radiotherapy technique for designing optimized intensity distributions to achieve a desired dose distribution. A specified desired uniform dose to the target volume is inverted, subject to constraints on the surrounding normal tissue dose, to produce optimum intensity distributions in a set of beams arranged around the target volume. We employed the inverse technique and software developed by Bortfeld and evaluated results both qualitatively and quantitatively using dose distribution displays, dose-volume histograms and biological indices including tumor control probability and normal tissue complication probabilities. So far we have applied this methodology to prostate and lung treatment plans. For prostate the inverse technique produces satisfactory approximations of the desired dose distributions. However, for lung its performance is considerably inferior. Our investigations point to a number of factors for this difference, the primary ones being differences in the tolerance doses of neighboring normal tissues, magnitudes of volume effect, tissue architectures, and the achievability of the specified desired dose distributions. We conclude that, for certain clinical situations, it is not sufficient to specify the objectives of optimization purely in terms of the desired pattern of the dose. The objectives must also include dose-volume effects and biological indices. Furthermore, the mathematics of optimization must be able to incorporate these factors into the process. We find that the inverse technique is not suitable for situations where dose-volume considerations and biological indices are important and that other methods of optimization of intensity distributions should be explored.

Adenocarcinoma↗
PubMed2006

Paracetamol use and risk of ovarian cancer: a meta-analysis.

AIM: Ovarian cancer remains the most fatal gynaecological malignancy. Several observational studies have examined paracetamol as a potential chemopreventive agent. The nonconclusive nature of the epidemiological evidence prompted us to conduct a detailed meta-analysis of the studies published on the subject in peer-reviewed literature. METHODS: A comprehensive search for articles published up to 2004 was performed, reviews of each study were conducted and data were abstracted. Prior to meta-analysis, the studies were evaluated for publication bias and heterogeneity. Pooled relative risk estimates (RR) and 95% confidence intervals (CIs) were calculated using the random and the fixed-effects models. RESULTS: Eight studies (four case-control and four cohort studies), published between 1998 and 2004, were included. We found no evidence of publication bias or heterogeneity among the studies. The analysis revealed an inverse association between paracetamol use and ovarian cancer risk. This association was marginally significant assuming a random-effects model (RR=0.84, 95% CI 0.70, 1.00), but not statistically significant assuming a fixed-effects model (RR=0.90, 95% CI 0.80, 1.01). When the analysis was stratified into subgroups according to study design, the association was inverse in both case-control and cohort studies, but only in the former was it statistically significant. The sensitivity analysis strengthened our confidence in the validity of this association. Furthermore, our results provided evidence for a dose effect; 'regular use' was associated with a statistically significant 30% reduction in the risk of developing ovarian cancer compared with non-use (RR=0.70, 95% CI 0.51, 0.95). CONCLUSIONS: Our meta-analysis supports a protective association between paracetamol use and ovarian cancer, and provides evidence for a dose effect. However, the question of paracetamol's potential association with ovarian cancer deserves further verification, since proof of chemoprevention would represent a major public health advance.

Acetaminophen↗
PubMed1997

Hin-mediated inversion on positively supercoiled DNA.

Hin recombinase requires negatively supercoiled DNA for an efficient inversion. We have generated positively supercoiled plasmid DNA using reverse gyrase from Sulfolobus shibatae and subjected it to the Hin-mediated inversion reaction. Both Hin and Fis showed the same DNA binding activity regardless of the superhelical handedness of the substrate plasmid. However, inversion activity on positively supercoiled DNA was less than 1% of negatively supercoiled DNA. Assays designed to probe steps in inversion, showed that on positively supercoiled DNA, Hin was able to cleave the recombination sites with the same efficiency shown on negatively supercoiled DNA but was not able to exchange the cleaved DNA. Based on the theoretical differences between positive and negative supercoiling, our data may suggest that unwinding of the double helix at recombination sites is needed after DNA cleavage for strand exchange to occur.

Binding Sites↗
PubMed2002

Acid-base and surface energy characterization of grafted polyethylene using inverse gas chromatography.

For a specific design of interfaces, i.e. in composites and blends, it is essential to know the surface thermodynamics of the components. Polyethylene grafted with maleic anhydride and maleic anhydride-styrene mixtures, respectively, was the component of interest of our investigations. Inverse gas chromatography (IGC) at infinite dilution was shown to be an appropriate method to evaluate the dispersive and acid-base surface characteristics although there is an influence of bulk absorption and morphology when performing IGC above the glass transition temperature of the polymer.

Calorimetry, Differential Scanning↗
PubMed2003

Identification of material constants for piezoelectric transformers by three-dimensional, finite-element method and a design-sensitivity method.

In this paper, an inversion scheme for piezoelectric constants of piezoelectric transformers is proposed. The impedance of piezoelectric transducers is calculated using a three-dimensional finite element method. The validity of this is confirmed experimentally. The effects of material coefficients on piezoelectric transformers are investigated numerically. Six material coefficient variables for piezoelectric transformers were selected, and a design sensitivity method was adopted as an inversion scheme. The validity of the proposed method was confirmed by step-up ratio calculations. The proposed method is applied to the analysis of a sample piezoelectric transformer, and its resonance characteristics are obtained by numerically combined equivalent circuit method.

Journal Article↗
PubMedDate not supplied

Survey of foot deformities among 166 geriatric inpatients.

A clinical survey on foot deformities among 166 geriatric inpatients was carried out. Direct measurements on hallux valgus, great toe movements, great toe deformities, lesser toe deformities, arch of the foot, callosities, ankle movements, inversion, and eversion were done by using the goniometer. A specially designed goniometer was created to measure inversion and eversion. The Harris Mat was used to study foot prints in correlation with the various deformities. About 50% of geriatric patients were found to have foot deformities of various types. This compared well with available data in literature. The target population was also checked for their symptomatology which, amazingly, was found to be really insignificant compared with the deformities present. This finding was found to be very much unlike data for Caucasian individuals with foot deformities, 50% of whom had symptoms related to pain and footwear. This major difference in the clinical presentation encourages the authors to carry on their study and, perhaps, direct cross-cultural studies in the future.

Aged↗
PubMed2004

Effective blood signal suppression using double inversion-recovery and slice reordering for multislice fast spin-echo MRI and its application in simultaneous proton density and T2 weighted imaging.

PURPOSE: To design a multislice double inversion-recovery fast spin-echo (FSE) sequence, with k-space reordered by inversion time at slice position (KRISP) technique, to produce black-blood vessel wall magnetic resonance imaging (MRI). MATERIALS AND METHODS: In this sequence, central k-space sampling for each slice is required at inversion time (TI) of the blood signal. To fill the entire k-space, the peripheral lines are obtained less or greater the TI and using a rotating slice order. Blood flow signal suppression was first evaluated using a phantom. Simulation studies were used to investigate FSE image quality. The final sequence was then applied to the rabbit abdominal aorta MRI at 4.7 T. RESULTS: In the flow phantom study, artifacts from slow-flowing water were substantially reduced by the KRISP technique; residual water spins were dephased by the strong phase-encoding gradient required for peripheral k-space. These dephased spins flowed into the slice plane where the center of k-space was being acquired at the TI of the flowing water signal. Multislice black-blood MR images were successfully obtained in the rabbit abdomen using the sequence with the k-trajectory optimized by the simulation study. CONCLUSION: The KRISP technique was effective both in multislice double inversion-recovery FSE and in blood signal suppression.

Abdomen↗
PubMed2006

Resampling: an optimization method for inverse planning in robotic radiosurgery.

By design, the range of beam directions in conventional radiosurgery are constrained to an isocentric array. However, the recent introduction of robotic radiosurgery dramatically increases the flexibility of targeting, and as a consequence, beams need be neither coplanar nor isocentric. Such a nonisocentric design permits a large number of distinct beam directions to be used in one single treatment. These major technical differences provide an opportunity to improve upon the well-established principles for treatment planning used with GammaKnife or LINAC radiosurgery. With this objective in mind, our group has developed over the past decade an inverse planning tool for robotic radiosurgery. This system first computes a set of beam directions, and then during an optimization step, weights each individual beam. Optimization begins with a feasibility query, the answer to which is derived through linear programming. This approach offers the advantage of completeness and avoids local optima. Final beam selection is based on heuristics. In this report we present and evaluate a new strategy for utilizing the advantages of linear programming to improve beam selection. Starting from an initial solution, a heuristically determined set of beams is added to the optimization problem, while beams with zero weight are removed. This process is repeated to sample a set of beams much larger compared with typical optimization. Experimental results indicate that the planning approach efficiently finds acceptable plans and that resampling can further improve its efficiency.

Computer Simulation↗
PubMed1994

De novo design and creation of a stable artificial protein.

Protein de novo design has been performed, as an exercise of the inverse folding problem. A beta/alpha-barrel protein was designed and synthesized using the Escherichia coli expression system for the structural characterization. A tertiary model with a two-fold symmetry was built, based upon the geometrical parameters extracted from X-ray crystal structures of several beta/alpha-barrel proteins. Amino acid frequencies at each position on the alpha- and beta-structures were investigated, and an amino acid sequence with 201 residues was designed. The associated gene was chemically synthesized and the fusion protein with human growth hormone was expressed in Escherichia coli. The purified protein after being cleaved and refolded was found to be stable and globular with the large amount of secondary structures. However, it has similar characteristics to the molten globules of natural proteins, with loose packing of side-chains. The approach for the tight packing is discussed.

Amino Acid Sequence↗
PubMed1999

The use of a semi-customized phantom for verification of conformal plans.

We have developed a technique for inverse treatment planning of prostate therapy designed to improve the degree of conformation between the dose distribution and the target volume. We compared the inverse plan with a "standard" four-field box technique as well as a four-field technique using oblique fields ("cross technique"). We validated the dosimetry of the inverse plan using Fricke gel solution in phantom specifically designed for this purpose. The phantom is a Plexiglas tank with a cross section, which approximates the dimensions of the pelvis. Anatomical data from computed tomography (CT) images of a patient were used to simulate organs in our phantom. This allows us to calculate dose distributions with the external geometry of the phantom and internal anatomy of the patient. Dose-volume histograms (DVHs) for the three different plans were calculated. The phantom containing the Fricke gel was irradiated according to the inverse plan. Magnetic resonance (MR) images was used to determine the dose distribution delivered to the phantom. We observe, on DVHs, that the inverse plan significantly reduces the dose to the rectum and the bladder but slightly increases the inhomogeneity inside the target volume. Correlation is good between isodoses on MR images and calculated isodoses. We conclude that inverse planning software can greatly improve the conformal degree of treatment to the prostate. This technique could be applied to other complex anatomic sites at which dose to organs at risk is a limiting factor and increased dose to the target volume is indicated. Our phantom and the Fricke gel solution are convenient to carry out validation of conformal treatments.

Humans↗
PubMed1998

Protective effect of fruits and vegetables on stomach cancer in a cohort of Swedish twins.

Observational studies, primarily of a case-control design, have shown an inverse association of fruit and vegetable consumption with the risk of stomach cancer, a finding tentatively attributed to anti-oxidant vitamins. Ensuing randomized-intervention trials of these vitamins, however, have been mostly negative. Therefore, the seemingly protective effect of fruit and vegetables in case-control studies is suspected to be influenced by the information bias inherent in the retrospective assessment of exposure, particularly since pre-conceptions about the wholesome effects of these foods are common among the public. Our aim was to examine the association of fruit and vegetable intake with the risk of stomach cancer in a prospective cohort study. Fruit and vegetable consumption was assessed in 1967 in 11,546 individuals in the Swedish Twin Registry, along with a wide range of potentially confounding factors. Complete follow-up through 1992 was attained through record linkage to the National Cancer and Death Registers. The relative risk of stomach cancer was estimated in proportional hazards models, with confidence intervals (CIs) adjusted for correlated outcomes. The risk of stomach cancer was inversely related to fruit and vegetable consumption. Controlling for potentially confounding factors, the relative risk among subjects with the lowest compared to those with the highest intake was 5.5 (95% CI 1.7-18.3) with a statistically significant dose-risk trend (p < 0.05). Our results indicate that information bias is not likely to explain the discrepancy between the results of observational studies and of randomized-intervention trials.

Aged↗
PubMed2007

Validity and power in hemodynamic response modeling: a comparison study and a new approach.

One of the advantages of event-related functional MRI (fMRI) is that it permits estimation of the shape of the hemodynamic response function (HRF) elicited by cognitive events. Although studies to date have focused almost exclusively on the magnitude of evoked HRFs across different tasks, there is growing interest in testing other statistics, such as the time-to-peak and duration of activation as well. Although there are many ways to estimate such parameters, we suggest three criteria for optimal estimation: 1) the relationship between parameter estimates and neural activity must be as transparent as possible; 2) parameter estimates should be independent of one another, so that true differences among conditions in one parameter (e.g., hemodynamic response delay) are not confused for apparent differences in other parameters (e.g., magnitude); and 3) statistical power should be maximized. In this work, we introduce a new modeling technique, based on the superposition of three inverse logit functions (IL), designed to achieve these criteria. In simulations based on real fMRI data, we compare the IL model with several other popular methods, including smooth finite impulse response (FIR) models, the canonical HRF with derivatives, nonlinear fits using a canonical HRF, and a standard canonical model. The IL model achieves the best overall balance between parameter interpretability and power. The FIR model was the next-best choice, with gains in power at some cost to parameter independence. We provide software implementing the IL model.

Brain↗
PubMed2001

Coronary artery imaging using contrast-enhanced 3D segmented EPI.

The purpose of the work was to evaluate the effectiveness of extracellular contrast media in improving MR coronary angiography using breath-hold segmented echo-planar imaging (SEPI). Two protocols were designed to optimize the inversion recovery-prepared contrast-enhanced SEPI method. In 15 healthy volunteers, significant improvements in signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and length of visualization were observed post-contrast. The method with two targeted scans to cover the left and right arteries, respectively, following separate 20-mL contrast injections, was found to yield thinner slices and longer right coronary artery (RCA) visualization than a single scan following a 40-mL contrast injection without compromising SNR and CNR. In conclusion, extracellular contrast media substantially improves the delineation of coronary arteries with SEPI. J. Magn. Reson. Imaging 2001;13:676-681.

Adult↗
PubMed2004

Diabetes mellitus and risk of prostate cancer: a meta-analysis.

AIMS/HYPOTHESIS: The association of diabetes mellitus with prostate cancer has been controversial. This study examines the strength of this association by conducting a detailed meta-analysis of the studies published in peer-reviewed literature on the subject. METHODS: A comprehensive search for articles published up to 2003 was performed, reviews of each study were conducted and data were abstracted. Prior to meta-analysis, the studies were evaluated for publication bias and heterogeneity. Pooled relative risk (RR) was calculated using the random- and the fixed-effects models. Subgroup and sensitivity analyses were also performed. RESULTS: We included 14 studies, published between 1971 and 2002, in the meta-analysis (five case-control studies, nine cohort studies). We found no evidence of publication bias ( p=0.89) or heterogeneity among the studies ( p=0.38). The association of diabetes with prostate cancer was statistically significant, both on the basis of a random-effects model (RR=0.91, 95% CI: 0.86 to 0.96), and on the basis of a fixed-effects model (RR=0.91, 95% CI: 0.88 to 0.94). When the analysis was stratified into subgroups according to study design, the association was inverse in both cohort and case-control studies, but only in the former was it statistically significant. The sensitivity analysis strengthened our confidence in the validity of this association. CONCLUSIONS/INTERPRETATION: Our meta-analysis findings provide strong evidence that people with diabetes have a significant decrease in risk of developing prostate cancer. There is biological evidence to support this association.

Adult↗
PubMed1998

MRI of meniscal bucket-handle tears.

OBJECTIVE: A meniscal bucket-handle tear is a tear with an attached fragment displaced from the meniscus of the knee joint. Low sensitivity of MRI for detection of bucket-handle tears (64% as compared with arthroscopy) has been reported previously. We report increased sensitivity for detecting bucket-handle tears with the use of coronal short tau inversion recovery (STIR) images. DESIGN AND PATIENTS: Three hundred and twenty-seven patients who had MRI of the knee between October 1994 and December 1996 and subsequently underwent arthroscopy were included in the study. We prospectively and retrospectively reviewed the MR examinations of the 30 patients with arthroscopically proven bucket-handle tears. Each of two observers examined each scan for the three traditional findings of bucket-handle tears: a double posterior cruciate ligament sign, a flipped meniscus sign and/or a fragment in the intercondylar notch. We also assessed STIR images in the coronal plane through the menisci looking for an area of increased signal within either meniscus with a displaced meniscal fragment. RESULTS: By using four criteria for diagnosis of meniscal bucket-handle tears, our overall sensitivity compared with arthroscopy was 93% (28 of 30 meniscal bucket-handle tears seen at arthroscopy were detected by MRI). The meniscal fragment was well visualized in all 28 cases on coronal STIR images. The double posterior cruciate ligament sign was seen in 8 of 30 cases, the flipped meniscus was seen in 10 of 30 cases and a fragment in the intercondylar notch was seen in 18 of 30 cases. CONCLUSION: By using four criteria for diagnosis of bucket-handle tears, overall diagnostic sensitivity of MRI compared with arthroscopy increased from the previously reported 64% to 93%. Coronal STIR images are useful for detecting small meniscal bucket-handle tears.

Adolescent↗
PubMed2006

Cloning of the mating type locus from Ascochyta lentis (teleomorph: Didymella lentis) and development of a multiplex PCR mating assay for Ascochyta species.

The mating type (MAT) locus of the lentil pathogen, Ascochyta lentis, was cloned and characterized using thermal asymmetric interlaced and inverse PCR with primers designed to the HMG-box of Ascochyta rabiei. A multiplex PCR assay for mating type was developed based on MAT idiomorph and flanking sequences. Primers were designed to specifically amplify MAT from several Ascochyta spp. including A. pisi, A. fabae and A. viciae-villosae in addition to A. lentis. Four hundred and fifty and 700 bp fragments were amplified from MAT1-1 and MAT1-2 isolates, respectively, and fragment size correlated perfectly with laboratory crosses using mating type tester strains. MAT-specific PCR allowed rapid scoring of mating type in crude DNA extracts from geographically diverse population samples of A. viciae-villosae from California and Washington State, USA. This co-dominant MAT-specific PCR assay will be a valuable tool for studying the population structure, biology and epidemiology of these fungi.

Amino Acid Sequence↗
PubMed2006

Comparison of absolute thresholds derived from an adaptive forced-choice procedure and from reaction probabilities and reaction times in a simple reaction time paradigm.

An understanding of the auditory system's operation requires knowledge of the mechanisms underlying thresholds. In this work we compare detection thresholds obtained with a three-interval-three-alternative forced-choice paradigm with reaction thresholds extracted from both reaction probabilities (RP) and reaction times (RT) in a simple RT paradigm from the same listeners under otherwise nearly identical experimental conditions. Detection thresholds, RP, and RT to auditory stimuli exhibited substantial variation from session to session. Most of the intersession variation in RP and RT could be accounted for by intersession variation in a listener's absolute sensitivity. The reaction thresholds extracted from RP were very similar, if not identical, to those extracted from RT. On the other hand, reaction thresholds were always higher than detection thresholds. The difference between the two thresholds can be considered as the additional amount of evidence required by each listener to react to a stimulus in an unforced design on top of that necessary for detection in the forced-choice design. This difference is inversely related to the listener's probability of producing false alarms. We found that RT, once corrected for some irreducible minimum RT, reflects the time at which a given stimulus reaches the listener's reaction threshold. This suggests that the relationships between simple RT and loudness (reported in the literature) are probably caused by a tight relationship between temporal summation at threshold and temporal summation of loudness.

Adult↗
PubMed1995

Total body irradiation with a sweeping 60Cobalt beam.

PURPOSE: This article describes the physical, technical, and dosimetric aspects of total body irradiation (TBI). METHODS AND MATERIALS: The continuous head swivel motion of a standard 60Cobalt unit has been used to obtain a sweeping beam that encompasses the entire length of the patient in TBI. A perspex beam flattener designed to remove the inverse square fall-off in beam intensity along the sweep axis provides a 90% field length of 200 cm in air at a treatment source-to-skin distance of 160 cm. The anterior-posterior parallel pair setup permits accurate placement of customized lead compensators to limit the dose to lungs. RESULTS: Measured beam profiles, dose buildup curves, and percentage depth dose for the technique are presented. With compensators in place, the variation in lung dose is shown to be within +/- 5% of the prescribed tumor dose. CONCLUSIONS: A sweeping beam TBI technique has been devised using a standard 60Cobalt unit. The technique allows the patient to be treated in supine and prone positions, which facilitates the accurate placement of lung compensators to limit the dose to lung tissue.

Cobalt Radioisotopes↗