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At least 1,099 records · Page 61Linked to original sources
PubMed2005

In vivo validation of a new technique that compensates for soft tissue artefact in the upper-arm: preliminary results.

BACKGROUND: Soft tissue artefact is the dominant error source for upper extremity motion analyses that use skin-mounted markers, especially in humeral axial rotation. METHODS: A new in vivo technique is presented that is based on the definition of a humerus bone-embedded frame almost "artefact free" but influenced by the elbow orientation in the measurement of the humeral axial rotation, and on an algorithm designed to solve this kinematic coupling. The technique was validated in vivo in a study of six healthy subjects who performed five arm-movement tasks. For each task the similarity between a gold standard pattern and the axial rotation pattern before and after the application of the compensation algorithm was evaluated in terms of explained variance, gain, phase and offset. In addition the root mean square error between the patterns was used as a global similarity estimator. FINDINGS: After the application, for four out of five tasks, patterns were highly correlated, in phase, with almost equal gain and limited offset; the root mean square error decreased from the original 9 degrees to 3 degrees . INTERPRETATION: The proposed technique appears to help compensate for the soft tissue artefact affecting axial rotation. A further development is also proposed to make the technique effective also for the pure prono-supination task.

Adult↗
PubMed2006

Effective wavelength calibration for moiré fringe projection.

The fringe patterns seen when using moiré instruments are similar to the patterns seen in traditional interferometry but differ in the spacing between consecutive fringes. In traditional interferometry, the spacing is constant and related to the wavelength of the source. In moiré fringe projection, the spacing (the effective wavelength) may not be constant over the field of view and the spacing depends on the system geometry. In these cases, using a constant effective wavelength over the field of view causes inaccurate surface height measurements. We examine the calibration process of the moiré fringe projection measurement, which takes this varying wavelength into account to produce a pixel-by-pixel wavelength map. The wavelength calibration procedure is to move the object in the out-of-plane direction a known distance until every pixel intensity value goes through at least one cycle. A sinusoidal function is then fit to the data to extract the effective wavelength pixel by pixel, yielding an effective wavelength map. A calibrated step height was used to validate the effective wavelength map with results within 1% of the nominal value of the step height. The error sources that contributed to the uncertainty in determining the height of the artifact are also investigated.

Journal Article↗
PubMed2004

Sources of registration error with image guidance systems during endoscopic anterior cranial base surgery.

OBJECTIVES: The goal of this study was to evaluate the accuracy of the registration process and to identify potential sources of error during anterior cranial base surgery. STUDY DESIGN AND SETTING: The registration accuracy of image guidance and the location of excluded fiducials were recorded prospectively from 50 endoscopic, anterior cranial base procedures in an academic university setting. RESULTS: The mean error of initial registration was 2.8 mm (range, 1.4 to 7.1 mm). Following the exclusion of fiducials the mean error of registration was 1.6 mm (range, 0.6 to 3.7 mm). There was a significant improvement in the mean error rate from initial to final registration following the exclusion of fiducials (P < 0.0001). Posterior fiducials were excluded most often and anterior fiducials were excluded the least. Registration accuracy was similar for CT and MRI (P = 0.64). CONCLUSIONS: The accuracy of the Stryker Image Guidance System is enhanced by the exclusion of individual fiducials with high registration errors. SIGNIFICANCE: The exclusion of fiducials with high registration errors increases the accuracy of image guidance in anterior cranial base surgery.

Aged↗
PubMed1990

Uncertainties in dosimetric data and beam calibration.

Recent studies indicate that the calibration of therapeutic beams is one of the main sources of uncertainty in the mean absorbed dose to the target volume in radiotherapy. Interaction coefficients and data used through the different steps in the calibration are pointed out as the main contribution to this uncertainty. Procedures used to select dosimetric data, that is, input parameters used in the specification of the quality of the beam, cause another contribution. In this paper the actual status of the data used for the dosimetry of photon and electron beams is introduced first. Uncertainties along the dosimetric chain are analyzed according to the procedure and data used in recent publications. Uncertainties in stopping-power ratios, considered the main contribution, are discussed in detail starting from the basic electron stopping-power data. Overall uncertainties in the presently available set of stopping-power ratios are analyzed. Recent developments in the dosimetry of electron beams, related to the effect of energy and angular spread and electron and photon contamination, are discussed in connection with the procedure to select stopping-power ratios for clinical dosimetry. Uncertainties along the dosimetric chain are evaluated in terms of the present knowledge of error sources.

Calibration↗
PubMed1999

Effect of transit times on quantification of cerebral blood flow by the FAIR T(1)-difference approach.

The effect of finite transit times for the tagging bolus is known to be a significant error source for perfusion quantification using the flow-sensitive alternating inversion recovery (FAIR) technique. It is shown that, in the presence of transit times, both the slice-selective (SS) and nonselective (NS) inversion recovery experiments actually consist of an NS period followed by an SS period. This mixed process can be described using a newly defined time constant called the "switching time," which separates the two periods. Calculations predict that finite transit times always lead to decreased flow values in the signal-intensity-difference approach, but that the measured flows in the T(1)-difference approach may be decreased or increased. This theory well explains our recent experimental flow results on cat brain as a function of predelay. The results show the signal-intensity-difference method is superior over the T(1)-difference approach in terms of convenience and ease of quantification. Magn Reson Med 42:890-894, 1999.

Animals↗
PubMed2000

Flip angle considerations in (3)helium-MRI.

3Helium-MRI ((3)He-MRI) can be used for analysis of lung function, e. g. dynamic imaging of ventilation and gas diffusion within the lung, assessment of intrapulmonary oxygen concentrations and their time course. During imaging, the irreversible signal loss due to depolarizing radio frequency excitations can be described using the flip angle (FA) alpha. This parameter has to be quantified in order to account for it during quantitative assessment of the (3)helium signal intensity and its temporal development. This technical report reviews two different methods to determine alpha. Limitations and possible error sources of each method are discussed.

Computer Simulation↗
PubMed1992

Small integrating meter for assessing long-term exposure to magnetic fields.

A small, lightweight meter has been developed for magnetic-field measurements, particularly those needed for exposure-assessment purposes. This meter, known as the AMEX-3D, continuously measures all three axes of magnetic-flux density and electronically combines the data into a single estimate of cumulative exposure to the root-mean-square (rms) resultant flux density. The AMEX-3D weighs about 120 g, measures 2.7 cm x 5.1 cm x 10.2 cm, and is battery powered. Two panel-mounted jacks are provided for measuring battery voltage and for reading cumulative exposure data from the unit. The instrument has, within 3 dB, a flat response to magnetic flux densities at all frequencies in its 30-1,000 Hz bandwidth. A detailed analysis of error sources in the AMEX-3D leads to an estimate of +/- 20% as the accuracy of the instrument over its dynamic range, which extends from 0.02 to 15 microT. The AMEX-3D was tested in the field by asking electric-utility distribution linemen to wear AMEX-3D and EMDEX meters simultaneously while working. Agreement between the two measures of exposure was excellent.

Electromagnetic Fields↗
PubMed1991

Precision of DNA flow cytometry in inter-institutional analyses.

A Bladder Cancer Flow Cytometry Network study has been carried out to further identify and quantify sources of inter- and intra-laboratory variability. Replicate samples containing four mixtures of peripheral blood lymphocytes and aneuploid cell lines were distributed together with reference standards to six laboratories. The samples were stained for DNA using propidium iodide, with each laboratory using its own staining protocol. Two of each of the four sample types and a reference standard were analyzed by each laboratory on 3 separate days to obtain cellular DNA distributions. DNA index (DI) and hyperdiploid fraction (HDF) were calculated for each histogram using an automated technique. The results showed significant inter- and intra-laboratory differences. Results were evaluated by a two-way analysis of variance to estimate components of the overall variation attributable to individual sources. Error variation was found to be the major component of random variation. Specimen means were also compared for each laboratory. No significant differences were noted in mean DI for similar specimens; however, agreement in HDF between similar specimens was lacking in most laboratories. Prediction intervals were computed to estimate the range of values expected for a single specimen based on the analysis of the previous six. Prediction intervals for DI were quite good while those for HDF were troublesome due to wide variation. The results of these studies indicate that intra- and inter-laboratory variability are high enough that results for a single sample may not be sufficiently precise to allow comparison to results obtained in other laboratories.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed2002

Accuracy evaluation of a 3D ultrasound-based neuronavigation system.

We have investigated the 3D navigation accuracy of a frameless ultrasound-based neuronavigation system (SonoWand) for surgical planning and intraoperative image guidance. In addition, we present a detailed description and review of the error sources associated with surgical neuronavigation based on preoperative MRI data and intraoperative ultrasound. A phantom with 27 precisely defined points was scanned with ultrasound by various translation and tilt movements of the ultrasound probe (180 3D scans in total), and the 27 image points in each volume were located using an automatic detection algorithm. These locations were compared to the physically measured locations of the same 27 points. The accuracy of the neuronavigation system and the effect of varying acquisition conditions were found through a thorough statistical analysis of the differences between the two point sets. The accuracy was found to be 1.40 +/- 0.45 mm (arithmetic mean) for the ultrasound-based neuronavigation system in our laboratory setting. Improper probe calibration was the major contributor to this figure. Based on our extensive data set and thorough evaluation, the accuracy found in the laboratory setting is expected to be close to the overall clinical accuracy for ultrasound-based neuronavigation. Our analysis indicates that the overall clinical accuracy may be as low as 2 mm when using intraoperative imaging to compensate for brain shift.

Humans↗
PubMed2004

Estimating the arterial input function using two reference tissues in dynamic contrast-enhanced MRI studies: fundamental concepts and simulations.

In dynamic contrast-enhanced MRI (DCE-MRI) studies, an accurate knowledge of the arterial contrast agent concentration as a function of time is crucial for the estimation of kinetic parameters. In this work, a novel method for estimating the arterial input function (AIF) based on the contrast agent concentration-vs.-time curves in two different reference tissues is described. It is assumed that the AIFs of the two tissues have the same shape, and that simple models with two or more compartments, and unknown kinetic parameters, can describe their tracer concentration-vs.-time curves. Based on the principle of self-consistency, one can relate the two tracer concentration-vs.-time curves to estimate their common underlining AIF, together with the kinetic parameters of the two tissues. In practice, the measured concentration-vs.-time curves have noise, and the AIFs of the two tissues are not exactly the same due to different dispersion effects. These factors will produce errors in the AIF estimate. Simulation studies show that despite the two error sources, the double-reference-tissue method provides reliable estimates of the AIF.

Arteries↗
PubMed1998

Measurement of R'2 in the presence of multiple spectral components using reference spectrum deconvolution.

A method is described for measuring R'2, the RF reversible contribution to the effective transverse relaxation rate in yellow trabecular marrow, as a means to evaluate trabecular bone structure and density. The method exploits the similarity in spectral composition of the marrow and fat in subcutaneous tissue. Under these conditions the gradient echo envelope of the marrow signal can be regarded as a convolution of a function describing the bone marrow intravoxel line broadening (R'2) with a function expressing chemical shift modulation, which is obtained from the echo envelope of the subcutaneous fat signal in a reference region. Simple division of each of a series of echoes by the reference signal is shown to afford a smooth decay which can be fitted to a model to extract R'2. The method has been evaluated in the upper femur of test subjects and a strong correlation of the thus derived R'2 values with those obtained by the GESFIDE technique is demonstrated. The close correspondence in spectral composition of proximal femur marrow and subcutaneous fat is further illustrated by means of localized spectroscopy. The major potential error source is global inhomogeneity in the reference region which can lead to an underestimation of the demodulation-derived R'2.

Absorptiometry, Photon↗
PubMed1994

Postmortem validation of the automated coronary analysis (ACA) software package.

Goal of this study was to compare the quantitative coronary arteriographic (QCA) results obtained with the Philips DCI/ACA analytical software package with those from postmortem casts in an animal experimental setting. Standard digital coronary arteriograms were obtained from 6 mongrel dogs. After the imaging procedure, the dogs were sacrificed and casts were made of the coronary trees by filling the vessels with a mixture of radio-opaque barium and silicone gel at a fixed pressure of 100 mmHg. Vessel diameters were measured from the digital arteriograms at a total of 118 selected locations with the ACA package. Thin slices were cut from the casts at these same measurement locations and the areas of the cross sections were obtained by manual tracing of the outline of each slice in an approximately 40 x magnified image. From these cross-sectional areas, cast diameters were derived using the formula for circular cross-sections. Cast diameters ranged in size from 0.69 to 3.30 mm. The systematic error between the measurements was found to be 0.058 mm; (p < 0.015) and the standard deviation of the signed difference 0.255 mm; the correlation coefficient was r = 0.91. The largest error sources are supposed to be the slight differences in the selection of identical positions in the X-ray images and on the casts, and the 'out-of-plane' magnification for a number of vessel locations. This postmortem study demonstrates that the diameters of coronary vessels can be measured from digital arteriograms with the DCI/ACA package with a high degree of accuracy and precision.

Animals↗
PubMedDate not supplied

Complex bioelectric impedance measurement system for the frequency range from 5 Hz to 1 MHz.

Analytic techniques that have been successfully employed in materials science, and to a lesser extent in the study of biologic systems, have potential for improving the application of bioelectric impedance provided that both real and imaginary impedance components can be measured with sufficient accuracy over a given frequency range. Since biologic tissue, particularly animal tissue, is typically highly conductive, phase angles are small, making accurate measurements difficult. A practical four-terminal system employing commercial lock-in amplifiers is described and error sources and corrective techniques are discussed.

Amplifiers, Electronic↗
PubMed1993

Detection of late potentials in the signal-averaged ECG combining time and frequency domain analysis.

The paper reports experience of flexible, computer-based instrumentation for high-resolution ECG, designed for laboratory research and clinical applications. It is an attempt to improve methods of detection of late potentials when identifying patients at increased risk of ventricular tachycardia (VT) and sudden cardiac death. Several factors influencing the efficiency of a high-resolution ECG system are examined. Of primary importance are signal, gain, type of filter, length and position of the time interval taken for spectral analysis, type of window used etc. Some possible error sources are analysed. Time-domain analysis is illustrated by representative XYZ-vector magnitude tracings of a non-VT and a VT patient. In an effort to further refine the diagnostic strategy, an approach to data analysis is presented, combining time-domain with frequency-domain measurements. A spectral ratio is proposed that helps recognise patients prone to VT. Two-dimensional plots of this ratio against time-domain parameters reveal clearly distinguishable normal and pathologic groups. It can be concluded that an appropriate frequency analysis can add significant diagnostic power to existing instrumentation for signal-averaged ECG to predict VT risk.

Amplifiers, Electronic↗
PubMed1986

Birth order in Australian vital statistics.

The author examines factors contributing to the poor quality of Australian vital statistics on birth order. Among these are the lack of standardization of birth registration procedures, census respondents' reluctance to provide information concerning illegitimate births, and problems associated with differences in instructions to parents registering births. Instructions for Australian birth registration forms are suggested. The author stresses that "information needs to be gathered on previous issue in respect of all births registered, not just marital births; and it needs to be gathered in respect of all children born alive to the mother, not just those of the current marriage or relationship."

Australia↗
PubMed2001

The use of sector field ICP-mass spectrometry for Rb-Sr geochronological dating.

Rb-Sr dating, one of the most important tools in geochronology, requires determination of the Rb/Sr concentration ratios and the 87Sr/86Sr isotope ratios in co-genetic minerals or rocks and is traditionally performed by thermal ionization mass spectrometry (TIMS). In this work we investigated whether sector field inductively coupled plasma mass spectrometry (ICP-MS), which is characterized by a high sample throughput and straight-forward sample introduction, could be used as an alternative to TIMS. To avoid spectral overlap of the ion signals of the isobaric nuclides 87Sr and 87Rb, Sr was separated from Rb by cation-exchange chromatography. A mathematical correction was applied to take into account the small amount of Rb that can be present in the Sr fraction. The isotope ratio accuracy and precision attainable with ICP-MS were evaluated by analysis of several reference materials from the US Geological Survey. The results of this evaluation show that excellent accuracy could be achieved; the internal precision (repeatability) of the isotope ratio (expressed as the relative standard deviation for 10 successive 1-min measurements) was 0.04-0.12%. An attempt was made to calculate the total or combined uncertainty on the isotope ratio results, by also taking into account other possible error sources (corrections for mass discrimination, detector dead time, blank signal and Rb fraction). Finally, the same procedure was used for dating two rock formations that were 2,500 Ma and 350 Ma old, according to age determinations previously performed by the Rb-Sr laboratory of the University of München (Germany) using TIMS. The ICP-MS results (2,520 +/- 150 Ma and 379 +/- 48 Ma) obtained for these formations compare well with the corresponding TIMS data (2,509 +/- 120 Ma and 357 +/- 25 Ma).

Journal Article↗
PubMed1998

Sample handling for determination of free platinum in blood after cisplatin exposure.

Cisplatin is a commonly used cytostatic drug that can be pharmacokinetically monitored by measurement of non-protein-bound platinum (Pt) in plasma. The present report elucidates some practical aspects on blood sample preparation with the purpose of identifying methodological error sources and simplifying sample handling. Human blood was incubated in vitro with clinically relevant doses of cisplatin and then stored at different temperatures for various periods prior to deproteinization. Pt concentrations were analyzed by inductively coupled plasma mass spectrometry, which is a very sensitive technique that is well suited for determination of free Pt in biological samples. Free Pt concentration is usually determined after ultrafiltration of plasma. An alternative approach used to study the protein-free fraction is to precipitate the proteins with ice-cold ethanol and then analyze the aliquot. We compared the Pt concentrations in protein-free plasma obtained by these two methods and the levels were very similar. Thus, ethanol deproteinization may be an attractive alternative to ultrafiltration, since it is both simple to perform and very cost-effective. Storage of nondeproteinized whole blood or plasma at 4 degrees C or 20 degrees C did not affect the free Pt fraction significantly for the first 2 h, after which a gradual decrease was noted. This suggests that saving the blood sample for approximately 1 h before processing should not have any major impact on the result. On long-term storage of frozen blood samples prior to deproteinization the freezing temperature proved to be of critical importance. At -20 degrees C there was a gradual decrease in the free Pt fraction during the study period of 14 days, whereas the free Pt concentration remained unchanged at -70 degrees C for at least 3 months. Thus, samples may be stored unprocessed at -70 degrees C for several months, whereas -20 degrees C is not sufficiently cold.

Cisplatin↗
PubMed1999

Experimental animal models of traumatic brain injury: medical and biomechanical mechanism.

The numerous traumatic brain injury models were designed to study the nature of the human brain injury. The properties of six experimental injury models were reviewed in this article. Weight-drop models with or without skull protection were compared in terms of experimental setup, possible error source, and biomechanical prospect. The modified percussion models with or without rigid cortical impact were contrasted with regard to reliability, histopathological production, and deformation. The focal contusion model by mechanical suction force represented isolated cortical injury without compression brain injury. As a class of traumatic brain injury, brain retraction damage was reviewed in this article.

Journal Article↗