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At least 1,567 records · Page 87Linked to original sources
PubMed2004

Volumetric assessment of glioma removal by intraoperative high-field magnetic resonance imaging.

OBJECTIVE: To investigate the contribution of high-field intraoperative magnetic resonance imaging (iMRI) for further reduction of tumor volume in glioma surgery. METHODS: From April 2002 to June 2003, 182 neurosurgical procedures were performed with a 1.5-T magnetic resonance system. Among patients who underwent these procedures, 47 patients with gliomas (14 with World Health Organization Grade I or II glioma, and 33 with World Health Organization Grade III or IV glioma) who underwent craniotomy were investigated retrospectively. Completeness of tumor resection and volumetric analysis were assessed with intraoperative imaging data. RESULTS: Surgical procedures were influenced by iMRI in 36.2% of operations, and surgery was continued to remove residual tumor. Additional further resection significantly reduced the percentage of final tumor volume compared with first iMRI scan (6.9% +/- 10.3% versus 21.4% +/- 13.8%; P < 0.001). Percentages of final tumor volume also were significantly reduced in both low-grade (10.3% +/- 11.5% versus 25.8% +/- 16.3%; P < 0.05) and high-grade gliomas (5.4% +/- 9.9% versus 19.5% +/- 13.0%; P < 0.001). Complete resection was achieved finally in 36.2% of all patients (low-grade, 57.1%; high-grade, 27.3%). Among the 17 patients in whom complete tumor resection was achieved, 7 complete resections (41.2%) were attributable to further tumor removal after iMRI. We did not encounter unexpected events attributable to high-field iMRI, and standard neurosurgical equipment could be used safely. CONCLUSION: Despite extended resections, introduction of high-field iMRI in conjunction with functional navigation did not translate into an increased risk of postoperative deficits. The use of high-field iMRI increased radicality in glioma surgery without additional morbidity.

Adolescent↗
PubMed2006

Recovering stimuli from memory: a statistical method for linking discrimination and reproduction responses.

Investigations of sensory memory have often relied on discrimination processes: participants judge whether a reference stimulus is identical to a remembered stimulus. Recently, investigators have used response modes in which participants directly reproduce stimuli. Findings using reproduction tasks have suggested new ways of thinking about biases that pervade psychophysics. To allow an assessment of whether results obtained using reproduction responses persevere for discrimination tasks, this paper presents a statistical model that links the two approaches. The model uses the pattern of discrimination judgements to estimate parameters that represent the means and standard deviations that would have occurred under a reproduction task. An analysis of stimuli that have been previously studied using reproduction responses illustrates the method. Simulations investigate bias and efficiency, large-sample confidence intervals and the performance of likelihood-ratio tests of the adequacy of special submodels.

Bias↗
PubMed2004

[Multisegmental image fusion of the spine].

Fusion of medical images is a technique that permits the correlation of homologous anatomical structures in different imaging modalities on the basis of a spatial transformation of the data sets. CT and MRI of the spine provide complementary information of possible relevance for diagnostic and therapeutic decisions. Methods enabling a multisegmental CT-MRI fusion of the spine were developed. These solve the problem of altered spatial relationships of the individual anatomical structures due to differing patient positioning in successive data acquisitions. Routine clinical CT and MRI data of a thoracic section of the spine were obtained and transferred to a PC-workstation. Following segmentation of the CT-data, landmarks for each individual vertebra were defined in the CT and MRI data. For each individual vertebra the algorithm we developed then carried out a rigid registration of the CT information to the MR data. The fused data sets were presented as colour-coded images or on the basis of dynamic variation of transparency. To assess registration precision, fiducial registration errors (FRE) and target registration errors (TRE) were calculated. The algorithm permitted multi-segmental image fusion of the spine. The average time required for defining the landmarks was 22 seconds per landmark for CT, and 34 seconds per landmark for MR. The average FRE was 1.53 mm. The TRE for the vertebrae was less than 2 mm. The colour-coded images were particularly suitable for assessing the contours of the anatomical structures, whereas dynamic variation of the transparency of overlapping CT images enabled a better overall assessment of the spatial relationship of the anatomical structures. The algorithm permits precise multi-segmental fusion of CT and MR of the spine, which was not possible using current fusion-algorithms due to variations in the spatial orientation of the anatomical structures caused by different positioning of the axial skeleton in successive examinations.

Adult↗
PubMed2004

A Matlab function to estimate choice model parameters from paired-comparison data.

Tversky (1972) has proposed a family of models for paired-comparison data that generalize the Bradley-Terry-Luce (BTL) model and can, therefore, apply to a diversity of situations in which the BTL model is doomed to fail. In this article, we present a Matlab function that makes it easy to specify any of these general models (EBA, Pretree, or BTL) and to estimate their parameters. The program eliminates the time-consuming task of constructing the likelihood function by hand for every single model. The usage of the program is illustrated by several examples. Features of the algorithm are outlined. The purpose of this article is to facilitate the use of probabilistic choice models in the analysis of data resulting from paired comparisons.

Algorithms↗
PubMedDate not supplied

Graphic representation of data resulting from measurement comparison trials in cataract and refractive surgery.

BACKGROUND AND OBJECTIVES: The evaluation of new diagnostic measurement devices allows intraindividual comparison with an established standard method. However, reports in journal articles often omit the adequate incorporation of the intraindividual design into the graphic representation. PATIENTS AND METHODS: This article illustrates the drawbacks and the possible erroneous conclusions caused by this misleading practice in terms of recent method comparison data resulting from axial length measurement in 220 consecutive patients by both applanation ultrasound and partial coherence interferometry. RESULTS: Graphic representation of such method comparison data should be based on boxplots for intraindividual differences or on Bland-Altman plots. Otherwise, severe deviations between the measurement devices could be erroneously ignored and false-positive conclusions on the concordance of the instruments could result. CONCLUSION: Graphic representation of method comparison data should sensitively incorporate the underlying study design for intraindividual comparison.

Cataract↗
PubMed1998

[Data processing computer system for evaluation of health effects due to occupational diseases diagnosed in Poland].

General consequences of occupational diseases for both employees and the country's economy have been known for many decades. Nevertheless there were no legal instruments and financial means to carry out studies leading to a comprehensive evaluation of health effects induced by occupational diseases diagnosed in Poland. Only recently, the legal basis has been provided by the revision of the regulations on occupational diseases (Official Journal of the Ministry of Health and Social Welfare no 9, heading 51, 1989), and the financial means allocated according to the Governmental Strategic Programme (SPR-1) for the years 1995-1998. The objectives of the study and the results obtained have been presented in another publication. Here the authors concentrated on some aspects of data collection and their analysis. An essential element in the establishment of data base on health effects of occupational diseases was its integration, namely the number of diagnosed occupational disease had to correspond with the entity in the Register of Occupational Diseases. Such an approach helped to avoid including in the base all metrical data, as information on effects of occupational diseases was collected exclusively for persons with detected and diagnosed occupational disease, which had to be previously placed in the Register of Occupational Diseases. The correctness of data input and the data analysis were carried out outside the Paradox system. The analysis of histograms of characteristics under study and the explanation of departing values were employed to evaluate the freedom from bias, and the statistical package for social sciences (SPSS) was used to analyse the data.

Computer Systems↗
PubMed2005

Cost-effective G-CSF therapy strategies for cyclical neutropenia: mathematical modelling based hypotheses.

Using computer simulations of a mathematical model for the regulation of stem cell and neutrophil production in dogs, we have studied the efficacy of four different treatment protocols for cyclical neutropenia involving granulocyte colony stimulating factor (G-CSF). The first treatment scheme is based on the bifurcation analysis of the mathematical model and proposes a daily, phase-dependent, protocol. The second involves alternate day administration of G-CSF. The third triggers G-CSF administration whenever neutrophil levels fall below a predetermined level, and the fourth one follows a random administration protocol. The computer simulations predict that clinically desirable results can be achieved with the three last methods, using far less G-CSF than would be needed with the standard daily treatment. If the results of this modelling are borne out clinically, they will entail a considerable financial savings for patients.

Animals↗
PubMedDate not supplied

A FORTRAN program for multivariate survival analysis on the personal computer.

In this paper a FORTRAN program is presented for multivariate survival or life table regression analysis in a competing risks' situation. The relevant failure rate (for example, a particular disease or mortality rate) is modelled as a log-linear function of a vector of (possibly time-dependent) explanatory variables. The explanatory variables may also include the variable time itself, which is useful for parameterizing piecewise exponential time-to-failure distributions in a Gompertz-like or Weibull-like way as a more efficient alternative to Cox's proportional hazards model. Maximum likelihood estimates of the coefficients of the log-linear relationship are obtained from the iterative Newton-Raphson method. The program runs on a personal computer under DOS; running time is quite acceptable, even for large samples.

Actuarial Analysis↗
PubMedDate not supplied

Computational gestalts and perception thresholds.

In 1923, Max Wertheimer proposed a research programme and method in visual perception. He conjectured the existence of a small set of geometric grouping laws governing the perceptual synthesis of phenomenal objects, or "gestalt" from the atomic retina input. In this paper, we review this set of geometric grouping laws, using the works of Metzger, Kanizsa and their schools. In continuation, we explain why the Gestalt theory research programme can be translated into a Computer Vision programme. This translation is not straightforward, since Gestalt theory never addressed two fundamental matters: image sampling and image information measurements. Using these advances, we shall show that gestalt grouping laws can be translated into quantitative laws allowing the automatic computation of gestalts in digital images. From the psychophysical viewpoint, a main issue is raised: the computer vision gestalt detection methods deliver predictable perception thresholds. Thus, we are set in a position where we can build artificial images and check whether some kind of agreement can be found between the computationally predicted thresholds and the psychophysical ones. We describe and discuss two preliminary sets of experiments, where we compared the gestalt detection performance of several subjects with the predictable detection curve. In our opinion, the results of this experimental comparison support the idea of a much more systematic interaction between computational predictions in Computer Vision and psychophysical experiments.

Animals↗
PubMedDate not supplied

Two quality control procedures on radiotherapy beam calibration and treatment planning system implementation.

New challenging dosimetric approaches, such as narrow beams and 3D algorithms, are being used in radiotherapy. In this paper two quality control (QC) procedures are reported. The first one concerns the QC of the dosimetry of small x-ray beams, generally carried out by using silicon detectors. The comparison of dose values obtained by a silicon diode, a diamond detector, and radiochromic films shows that for x-ray beams of high energy, the silicon diode can give an overestimation of the output factors in phantom, up to 4%. This is due to the higher than unit density silicon diode and the surrounding envelope that restore the lateral electron equilibrium. About the 3D algorithms for breast treatment planning, a quality control test has been adopted to verify the accuracy of the computed dosimetry when "loss of scatter" occurs. The results show a sensible agreement (within 1.5%) between computed and experimental data.

Humans↗
PubMed2000

Detection of compound spatial patterns: further evidence for different channel interactions.

It is shown that contrast interrelation functions for small compound gratings and for small compound edge-type patterns have different shapes: the former are lozenges, the latter ellipses in the normalised contrast space. These findings can be described by a simple p-norm model, comprising one channel which is most sensitive to the first pattern, another channel which is most sensitive to the second pattern and a non-linear summation of the channel outputs. Since the value of the summation exponent p is constant within but varies between pattern classes, an interpretation of the model parameters is limited to pattern class. Channel models based on the p-norm are discussed, taking into account the interpretation of the first derivative of the contrast interrelation function and the summation exponent, p.

Computer Simulation↗
PubMed1991

Conditional power for arbitrary survival curves to decide whether to extend a clinical trial.

This article describes a computer simulation methodology to calculate conditional power for comparing two arbitrary survival curves as an aid in deciding whether or not to extend a clinical trial. The method is a modification of that by Halpern and Brown, which computes unconditional power for comparing arbitrary survival curves at the beginning of a study. The advantage of this method is that it permits conditional power calculations for comparisons of distribution-free survival curves. Power is computed for two commonly used statistical tests, the log-rank test and Gehan's generalization of the Wilcoxon test, and for a modified Kolmogorov-Smirnov test, which is particularly sensitive to crossing survival curves. This method for estimating conditional power should be useful in the situation in which a decision must be made regarding the benefit of continuing a clinical trial. The application of this method is shown using VA Cooperative Study No. 90, "Prognosis and Outcome Following Heart Valve Replacement."

Aortic Valve↗