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At least 1,315 records · Page 73Linked to original sources
PubMed1996

Temperature mapping using water proton chemical shift obtained with 3D-MRSI: feasibility in vivo.

This article discusses the applicability to a living animal of the temperature mapping method using the water proton chemical shift obtained with three-dimensional magnetic resonance spectroscopic imaging (3D-MRSI). There are several sources of error in obtaining the spectra with 3D-MRSI: signal noise, limitation in the frequency resolution due to the finite signal length, intravoxel inhomogeneity in the static magnetic field, and variation in the magnetic field due to the eddy current magnetic field. A spectral estimation method called phase deduction complex Lorentzian fitting (PD-CLF) was proposed. Numerical simulations demonstrated that this method reduces the error in the chemical shift to one third of that obtained with the simple frequency subtraction method that uses zero-padded first Fourier transformation (FFT). The temperature images obtained using 3D-MRSI with PD-CLF clearly visualized the changes and distribution of temperature in an anesthetized rat.

Animals↗
PubMed1988

Summary attributable risk estimation from unmatched case-control data.

We propose an alternative method to obtain summary estimators, variances and confidence intervals for attributable risk measures. This method combines weighted exposure prevalences for cases and controls across strata formed by the cross-classification of relevant covariates to form estimates of attributable risk among the exposed and attributable risk in the target population. The major benefit of this approach over those previously proposed in the literature is that it operates on data summed across strata rather than on estimation of statistics within each stratum. This alternative method for attributable risk measures utilizes the Mantel-Haenszel estimate of an average odds ratio, and can be implemented using the matrix procedure in SAS. This method is appropriate even when the within-stratum sample sizes are too small for other methods to be valid. Simulation results indicate that this method is superior to others with respect to bias and coverage probability for confidence intervals.

Alcohol Drinking↗
PubMed2001

Time averaging and fitting of nonlinear metabolic changes: the issue of the time index choice applied to 31P MRS investigation of muscle energetics.

We present an exact analytical method dedicated to fitting time-dependent exponential-like changes in MR spectra. As an illustration, this method has been applied to fitting metabolic changes recorded by 31P MRS in human skeletal muscle occurring during a rest-exercise-recovery protocol. When recording metabolic changes with the accumulative method, the time averaging of the MR signals implies the choice of a time index for fitting any changes in the features of the associated MR spectra. A critical examination of the different ways (constant, linear, and exponential) of choosing the time index is reported. By numerical analysis, we have calculated the errors generated by the three methods and we have compared their sensitivity to noise. In the case of skeletal muscle, both constant and linear methods introduce large and uncontrolled errors for the whole set of metabolic parameters derived from [PCr] changes. In contrast, the exponential method affords a reliable estimation of critical parameters in muscle bioenergetics in both normal and pathological situations. This method is very easy to implement and provides an exact analytical solution to fitting changes in MR spectra recorded by the accumulative method.

Computer Simulation↗
PubMed2002

Computational models of MRI characteristics of focal cortical dysplasia improve lesion detection.

In many patients, focal cortical dysplasia (FCD) is characterized by minor structural changes that may go unrecognized by standard radiological analysis. We previously demonstrated that visual analysis of a composite map based on three simple models of MRI features of FCD increased the sensitivity of FCD lesion detection, compared to visual analysis of conventional MRI. Here we report on the use of improved methods for characterizing FCD which improve contrast in the composite maps: a Laplacian-based metric for measuring cortical thickness, a convolutional kernel to model blurring of the GM-WM interface, and an operator to measure hyperintense T1 signal. To validate these methods, we processed the MRIs of 14 FCD patients with our original set of image processing operators and an improved set of image processing operators. Comparison of the composite maps associated with the two sets of operators revealed that contrast between lesional tissue and nonlesional cortex was significantly increased in the composite maps associated with the set of improved operators. Increasing this contrast is an important step toward the goal of automated FCD lesion detection.

Brain Mapping↗
PubMed1992

Computer-assisted 3D-reconstruction and statistics of the limbic system. 2. Spatial statistics of the hippocampal formation, the fornix, and the mammillary bodies.

A statistical method is described to show the distribution of neuroanatomical structures within a Cartesian coordinate system from any given number of examinations. The algorithm is based on polygons derived from the outlines of neuroanatomical structures in parallel canthomeatal-orientated cutting planes. These polygons are transformed in virtual voxels, rotated into the bicommissural coordinate system, and projected onto the three main planes of this coordinate system. Areas with the same probability for the structures examined are given in these planes. As an example this method is applied to the hippocampal formation and the results attained are shown.

Hippocampus↗
PubMed2003

[Dynamic in vitro measurement of pressure and movement with the LCS prosthetic system].

This study measured the femoral inlay pressure and the movement of mobile bearing inlays after implantation of the LCS knee prosthetic system (DePuy,Warsaw, USA). The prostheses were implanted in seven fresh frozen knee specimens. This system offered three different types of inlays [classic (CL), A/P glide (APG), and rotating platform (ROT)] with the same type of femoral component. A pressure-sensitive film (Tekscan,Boston, USA) was attached on the surface of the inlay.T he movement of the inlay was measured by an ultrasonic positioning system (Zebris, Isny,Germany). The specimens were fixed in a hydraulic knee apparatus simulating an isokinetic extension movement from 120 degrees flexion to full extension. The hydraulic cylinder that simulated the musculus quadriceps femoris extended up to 1200 N. The pressure measurement showed a peak pressure of 4.4+/-1.5 MPa (CL),3.6+/-1.0 MPa (APG),and 9.8+/-1.3 MPa (ROT). The mobile bearing inlays moved 1.4+/-2.7 mm (CL) and 2.9+/-5.8 mm (APG) from posterior to anterior during extension of the knee. During the extension the inlays rotated 0.8+/-2.6 degrees (CL), 0.9+/-5.2 degrees (APG), and 3.2+/-6.9 degrees (ROT) internally.

Biomechanical Phenomena↗
PubMed2000

Assessment of the geometry of human finger phalanges using quantitative ultrasound in vivo.

Quantitative Ultrasound (QUS) methods have been shown to be useful in the assessment of bone status. Nevertheless, ultrasound transmission depends on a variety of skeletal parameters, and a detailed understanding of ultrasound propagation through bone is important for the accurate interpretation of QUS results. In this study we wanted to elucidate the pathways of an ultrasound wave through finger phalanges and determine correlations between geometric and QUS parameters. Phalanges of a subject group were measured using QUS and magnetic resonance imaging (MRI). MRI was used for the derivation of the geometric parameters. Similar assessments were performed on cylindrical tubes and with a simulation program. New parameters related to speed of sound (SOS) and amplitude of the wave (A2P) were calculated. Strong correlations between QUS parameters and morphologic cross-sectional areas were observed in vivo and in phantoms. Similar correlations could be found in the calculations using the simulation software. Cross-sectional cortical area, medullary canal area and relative cortical area could be calculated from the QUS parameters (subjects: R2 = 0.71 for cortical area, R2 = 0.45 for medullary canal area and R2 = 0.61 for relative cortical area; phantoms: R2 = 0.98 for cortical area, R2 = 0.78 for medullary canal area and R2 = 0.77 for relative cortical area). In vivo, phantom and simulation results consistently showed that SOS was correlated with cortical area but not with medullary canal area while the opposite was found for A2P. Pathways of the ultrasound wave through solid cortical bone and the medullary canal could be identified and the propagation of the wave could be depicted. These results help to interpret QUS findings and provide information that may be helpful in improving the performance of QUS.

Adult↗
PubMed2005

(1)H MR spectroscopy of the brain in multiple sclerosis subtypes with analysis of the metabolite concentrations in gray and white matter: initial findings.

Many MR spectroscopy (MRS) studies of multiple sclerosis (MS) have focussed on metabolism in normal-appearing white matter (NAWM) and in white matter lesions (WML). In this study, eight patients suffering from primary or secondary progressive MS (PPMS/SPMS) and seven patients with relapsing/remitting MS (RRMS) were examined by (1)H-MRS to assess metabolite levels in gray matter (GM) as well. (1)H-MRS chemical-shift imaging of a cerebral volume of interest of 8x8x2 cm(3) above the lateral ventricles revealed differences between the metabolite concentrations in the three groups varying from almost significant [NAWM, choline (cho); P=0.0547] to highly significant [GM, N-acetylaspartate (NAA); P=0.0003]. In PPMS/SPMS patients, the decreases in choline, creatine (Cr) and N-acetylaspartate compared with six healthy controls were significant in GM and to a lesser extent, in NAWM. No significant differences in metabolite concentrations were found between RRMS and controls. In WML, all metabolites were reduced compared with white matter in controls (Cho: P=0.0020; Cr and NAA: P<0.0001, both). In conclusion, the concentrations of Cho, Cr and NAA are reduced in PPMS/SPMS patients, especially in GM and in WML. Despite contrary observations in previous studies, increases in the concentrations of Cr and/or Cho were not observed.

Adult↗
PubMed2004

[Clinical trial for differentiation between corticoid-induced osteoporosis and periarticular demineralization via digital radiogrammetry in patients suffering from rheumatoid arthritis].

PURPOSE: To investigate a new bone densitometric technology based on digital radiogrammetry (DXR) with respect to its ability to measure severity-dependent variations of bone mineral density (BMD) in patients with rheumatoid arthritis and to differentiate between corticoid-induced and periarticular bone mineral density loss. PATIENTS AND METHODS: A total of 153 randomly selected patients suffering from verified rheumatoid arthritis underwent digitally performed plain radiographs of the non-dominant hand and also measurements of dual-energy X-ray absorptiometry (DXA) regarding total femur and lumbar spine in 102 patients and peripheral quantitative computed tomography (pQCT) regarding the distal radius in 51 patients. Using DXR the radiographs of the non-dominant hand were analyzed for cortical bone mineral density calculation. The severity was classified in the DXA group using the Ratingen score. Furthermore, both study populations were divided into patients with and without corticoid therapy. RESULTS: Correlations between BMD determined by DXR and by DXA (R=0.44 for lumbar spine and R=0.61 for total femur) versus pQCT (0.46<R<0.59) were all significant. An appropriate association was confirmed between pQCT and DXA (R=0.61 for total femur and 0.73 for LWS). In the subgroup of patients with corticoid therapy (mean dose: 5 mg/d for a period of more than 6 month), our data showed-similar to the collective of all patients-significant correlations (0.34<R<0.59) between DXR and the other methods. In contrast to pQCT (0.37<R<0.59) the study revealed a poor association between DXR- and DXA-parameters in the subgroup of patients without corticoid therapy; only the correlation between DXA-BMD of total femur and DXR-BMD achieved a significant level (R=0.38, p<0.05). The mean value of BMD measured by DXR decreased severity dependently from 0.59 g/cm(2) (Stage 1) to 0.46 g/cm(2) (Stage 5). Similar results were verified for the metacarpal index (DXR). The relative decrease of BMD between the highest and lowest score was 21% (p<0.05). Otherwise the reduction of bone mineral density using DXA revealed no significant results. CONCLUSION: The DXR-based BMD calculation can distinguish severity and progress of disease-related periarticular demineralization in contrast to those of DXA. In this context, DXA primarily measures the systemic (corticoid-induced) osteoporosis and pQCT partially estimates disease-related bone mineral density loss, whereas DXR can predominantly analyze and quantify the periarticular demineralization, which often shows a manifestation at an early stage of rheumatoid arthritis. Therefore DXR seems to be a diagnostic tool in the course of rheumatoid arthritis.

Absorptiometry, Photon↗
PubMed1988

Dynamical dosage regimen calculations in linear pharmacokinetics.

The objective of this analysis of a linear compartment system is to compute drug input functions that are optimal in producing nontoxic pharmacological responses of maximal therapeutic efficacy. Pharmacokinetics should underlie the rational use of drugs and when a therapeutic range is known, the achievement of safe and effective target concentrations may be assured by a dosage regimen computed for a given administration schedule. The method developed herein is based on linearity and superimposition principles applicable to the class of systems considered. This method requires estimated values of model individual parameters and computes optimum dosage regimens in an iterative scheme, corresponding to a real time dynamical context. An interactive computer program has been developed to perform dosage regimen calculations.

Amikacin↗
PubMed1987

Power spectral analysis of normal and pathological brainstem auditory evoked potentials.

The brainstem auditory evoked potential (BAEP) recording has become a powerful investigational tool in neurological diagnosis. The BAEPs of patients have different latencies and morphologies when compared to those of normals. In this paper the power spectra (PS) of BAEPs of 21 normals, 17 patients with multiple sclerosis (MS) and 12 patients with head injury (HI) computed by Blackman-Tukey (BT) and Maximum Entropy (ME) methods are examined for their frequency composition. Three major peaks appear at approximately 170 Hz, 520 Hz and 950 Hz in PS of normal BAEPs. The average power contained in the frequency bands spread around these frequency bands for BAEPs of patients differed significantly (P less than 0.05) from those of normal BAEPs. The peaks observed in ME spectra were found to match those computed using BT method. The model order for representing both normal and patient BAEPs is greater than 40 and data compression afforded by modelling the BAEPs is of the order of 5:1.

Adolescent↗
PubMed1982

An automated microprocessor-controlled data collection device for use with the Technicon AutoAnalyzer system.

An automated microprocessor-controlled data collection system for use with a Technicon AutoAnalyzer in a research or clinical laboratory is described. The collection system can digitize an analog output from the AutoAnalyzer, perform mathematical operations on this digital data, detect and record peak-trough data, and perform these data acquisition and mathematical data processing operations in parallel. The data collection system can be interfaced to a variety of host computers for further data analysis. The accuracy of the data collection system was tested and compared to existing procedures using an analysis of bovine serum albumin protein. The correlation coefficient for the comparison of the results was 0.99998. Hence, the automated system is as accurate as previous methods, but is considerably faster, more efficient, and, in comparison to existing devices, less expensive.

Analog-Digital Conversion↗
PubMed1991

A structured visual language for a knowledge-based front-end to statistical analysis systems in biomedical research.

Within the last few years, several knowledge-based systems for statistical analysis systems have been proposed (see Refs. 1-4 for references). Most of these systems provide so-called 'natural-language' interfaces for acquisition and application of meta-data. Since graphics have been very efficient in displaying results (e.g., as scatter, QQ and residual plots), some attempts have been made (cf. Refs. 5,6) to use graphics also to display knowledge of the statistical strategy. In the present paper I will concentrate on the visualization of knowledge of the experimental design and its impacts on the design of a structured visual syntax language for acquisition and application of this knowledge in the field of biomedical research.

Artificial Intelligence↗
PubMed2006

Molecular dynamics - potential of mean force calculations as a tool for understanding ion permeation and selectivity in narrow channels.

Ion channels catalyze the permeation of charged molecules across cell membranes and are essential for many vital physiological functions, including nerve and muscle activity. To understand better the mechanisms underlying ion conduction and valence selectivity of narrow ion channels, we have employed free energy techniques to calculate the potential of mean force (PMF) for ion movement through the prototypical gramicidin A channel. Employing modern all-atom molecular dynamics (MD) force fields with umbrella sampling methods that incorporate one hundred 1-2 ns trajectories, we find that it is possible to achieve semi-quantitative agreement with experimental binding and conductance measurements. We also examine the sensitivity of the MD-PMF results to the choice of MD force field and compare PMFs for potassium, calcium and chloride ions to explore the basis for the valence selectivity of this narrow and uncharged ion channel. A large central barrier is observed for both anions and divalent ions, consistent with lack of experimental conductance. Neither anion or divalent cation is seen to be stabilized inside the channel relative to the bulk electrolyte and each leads to large disruptions to the protein and membrane structure when held deep inside the channel. Weak binding of calcium ions outside the channel corresponds to a free energy well that is too shallow to demonstrate channel blocking. Our findings emphasize the success of the MD-PMF approach and the sensitivity of ion energetics to the choice of biomolecular force field.

Barium↗
PubMed2005

Modelling in the study of interaction of Hemopump device and artificial ventilation.

The aim of this work is to evaluate in different ventricular conditions the influence of joint mechanical ventilation (MV) and Hemopump assistance. To perform this study, we used a computer simulator of human cardiovascular system where the influence of MV was introduced changing thoracic pressure to positive values. The simulation confirmed that haemodynamic variables are highly sensitive to thoracic pressure changes. On the other hand, Hemopump assistance raises, among the others, mean aortic pressure, total cardiac output (left ventricular output flow plus Hemopump flow) and coronary flow. The simulation showed that the joint action of Hemopump and positive thoracic pressure diminishes these effects.

Air Pressure↗
PubMed2006

Improved early stroke detection: wavelet-based perception enhancement of computerized tomography exams.

Nonenhanced computerized tomography (CT) exams were used to detect acute stroke by notification of hypodense area. Infarction perception improvement by data denoising and local contrast enhancement in multi-scale domain was proposed. The wavelet-based image processing method enhanced the subtlest signs of hypodensity, which were often invisible in standard CT scan review. Thus improved detection efficiency of perceptual ischemic changes was investigated. Data processing became more effective by initial segmentation of brain tissue and extraction of regions susceptible to tissue density changes. The new method was experimentally verified. Sensitivity of stroke diagnosis increased to 56.3% in comparison to 12.5% of standard CT scan preview.

Acute Disease↗
PubMed2006

Single-trial evoked potential estimation using wavelets.

In this paper we present conventional and translation-invariant (TI) wavelet-based approaches for single-trial evoked potential estimation based on intracortical recordings. We demonstrate that the wavelet-based approaches outperform several existing methods including the Wiener filter, least mean square (LMS), and recursive least squares (RLS), and that the TI wavelet-based estimates have higher SNR and lower RMSE than the conventional wavelet-based estimates. We also show that multichannel averaging significantly improves the evoked potential estimation, especially for the wavelet-based approaches. The excellent performances of the wavelet-based approaches for extracting evoked potentials are demonstrated via examples using simulated and experimental data.

Algorithms↗