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At least 19 recordsLinked to original sources
PubMed1990

Pulsed Doppler accuracy assessment due to frequency-dependent attenuation and Rayleigh scattering error sources.

All engineering measurements are subject to inaccurate and imprecise estimates, including the estimate of blood flow velocity. An assessment of specific error sources can minimize such uncertainties. Frequency-dependent attenuation and Rayleigh scattering are significant error sources for pulsed Doppler ultrasound because the transmitted ultrasonic signal has a finite width spectrum. The former causes a frequency downshift and the latter a frequency upshift, both of which are independent of the actual Doppler frequency shift. This communication evaluates these error sources through computer stimulation and compares the computed error to experimental data.

Blood Flow Velocity↗
PubMed1982

Error sources affecting thermocouple thermometry in RF electromagnetic fields.

Thermocouple thermometry errors in radiofrequency (typically 13, 56 MHZ) electromagnetic fields such as are encountered in hyperthermia are described. RF currents capacitatively or inductively coupled into the thermocouple-detector circuit produce errors which are a combination of interference, i.e., 'pick-up' error, and genuine rf induced temperature changes at the junction of the thermocouple. The former can be eliminated by adequate filtering and shielding; the latter is due to (a) junction current heating in which the generally unequal resistances of the thermocouple wires cause a net current flow from the higher to the lower resistance wire across the junction, (b) heating in the surrounding resistive material (tissue in hyperthermia), and (c) eddy current heating of the thermocouple wires in the oscillating magnetic field. Low frequency theories are used to estimate these errors under given operating conditions and relevant experiments demonstrating these effects and precautions necessary to minimize the errors are described. It is shown that at 13.56 MHz and voltage levels below 100 V rms these errors do not exceed 0.1 degrees C if the precautions are observed and thermocouples with adequate insulation (e.g., Bailey IT-18) are used. Results of this study are being currently used in our clinical work with good success.

Diathermy↗
PubMedDate not supplied

Structured noise in computed tomography: effects of periodic error sources.

The artifact in computed tomography (CT) images due to cyclic projection errors, such as errors due to periodic fluctuations in x-ray intensity, is derived and verified by computer simulation. Depending upon the relative phase of the error between projections, the artifact is shown to be described by a Bessel function (or functions) of the radial argument which is sinusoidally modulated as a function of angle. Because of the nature of Bessel functions, the artifact is essentially zero up to some minimum radius. When the effects of sampling are neglected, a single fundamental artifact is shown to occur. In fourth-generation scanners, the fundamental artifact (neglecting sampling) will occur at a detector fan angle of about about 39 degrees (depending on the ratio of the axis-to-detector, source-to-axis distances). The radius of appearance of this fundamental artifact is independent of the frequency of the periodic error signal and will only be visible in fourth-generation CT scanners with detector fan angles greater than about 39 degrees. The effects of sampling are derived and illustrated by simulation for first-, third-, and fourth-generation CT-scanner geometries. It is shown that the effect of sampling is to cause an infinite number of such artifacts to be superimposed in the final image. The radius of appearance of all but the fundamental artifact are shown to be dependent on the frequency of the periodic signal. It is shown that by judicious choice of the sampling parameters relative to the frequency of the periodic error, the artifact can be effectively eliminated.

Technology, Radiologic↗
PubMed1985

Simulating refractive errors: source and observer methods.

There are two principal methods of simulating refractive errors. Either the retinal image can be defocused by an optical system, usually a positive lens, placed in front of an observer's eye (observer method), or the source of the retinal image can be defocused as it is projected onto a screen or photograph (source method). There are significant differences between the two methods, differences that make it difficult to compare results. However, the source method, which is the more artificial, seems to be superior for a number of reasons. The results of these two methods can be compared using a common or interchangeable parameter for specifying the level of defocus. A convenient parameter is the size of the defocused image of a point, measured either in image space (linear or angular diameter on the retina) or in object space (angular diameter of the blur disc projected back into object space), with the angular diameter measured from the respective nodal point of the eye. Methods of measuring the angular blur-disc diameter for both methods are discussed and the validity of the formula omega = D delta L, is investigated, where omega is the angular diameter of the blur disc, D is the observer's pupil diameter, and delta L is the dioptric defocus.

Humans↗
PubMed1989

[Error sources in recording three-dimensional mandibular movements. An in vitro study using the V-type stereognathograph].

The instrument-induced measurement errors in V-type stereognatography have been analyzed. An optical bench was used as a measurement apparatus. In measuring not too small movement distances, the linearity errors remained within acceptable limits. The variations in linearity due to calibration errors were almost negligible. In practical applications, the stereognatography proved to be insensitive to external factors such as type of illumination, room temperature and voltage fluctuations. Factors affecting the effectiveness of the projection error correction system, such as tilting of the filter, different sites of measurement of the six optoelectronic canals, and mandibular protrusion during mediotrusive movements, might be reduced by slight modifications of the measurement system.

Cephalometry↗
PubMed1990

Polyadenylated RNAs as error sources in ribosomal RNA turnover analyses.

An approach to ribosomal RNA turnover studies in which cytoplasmic RNA was extracted and subsequently fractionated to isolate ribosomal RNA is reported. The presumption that the pool of 28S and 18S RNAs represented ribosomal RNA, exclusively, proved false and led to erroneous results of ribosomal RNA turnover. Polyadenylated RNAs exhibited a heterogeneous size distribution and, although constituting only 3% (w/w) of the cytoplasmic RNA extract, accounted for fully 10% of radioactivity of the presumptive ribosomal RNA pool. Profiles from the radioactivity data suggested that the discrepant results were due to these polyadenylated RNAs. An additional analytical procedure, an oligo (dT) cellulose column chromatography of the RNA extract prior to the sucrose density gradient fractionation step, performed as described in this paper, proved an effective remedy for this error.

Animals↗
PubMed1987

Sources of error in registering suicide.

Sources of error theoretically possible in the registration of suicide are discussed. Using cause-of-death statistics for each of the Nordic countries, the relative influence of the various sources of error is evaluated. Two types of sources of error are identified, one is when cause-of-death is registered as drowning and the other is when it is registered as "unknown". Results have shown however, that the low probability of suicide in these cases, and consequently small numbers involved, could hardly influence differences in rates between the Nordic countries. The differences found in suicide statistics therefore reflect true differences in the frequency of suicide between these countries.

Accidents↗
PubMed1979

Ultrastructure of skin biopsy specimens in lysosomal storage diseases: common sources of error in diagnosis.

Common sources of error in the diagnosis of lysosomal storage diseases by ultrastructural examination of skin specimens have been identified in a series of biopsies from 72 patients. Four principal factors have emerged as leading pitfalls and sources of error in diagnosis. First, the skin biopsy technique itself may lead to alterations of normal skin ultrastructure. Second, artifacts may be produced during fixation and preparation of tissue for electron microscopy. Third, cellular organelles and structures normally present in human skin may be mistakenly interpreted as pathological. Fourth, the use of cultured skin fibroblasts for ultrastructural identification of storage material is often accompanied by artifacts induced in tissue culture and is not recommended. Recognition of these common problems may aid interpretation of the fine structure of skin abnormalities. Furthermore, when skin biopsy specimens are used as the primary source of diagnostic material, correlation of both skin ultrastructure and assay for specific lysosomal enzymes in cultured dermal fibroblasts will facilitate diagnostic accuracy.

Biopsy↗
PubMed1976

Bone density measured by photon scattering. II. Inherent sources of error.

The origins of inherent sources of error in results of measurements of bone density by a photon scattering technique are described and their effects are predicted theoretically. The predictions are confirmed experimentally. The prime source of error is multiple scattering. A procedure has been developed to correct the observed densities for effects of multiple scattering.

Bone Diseases↗
PubMed1988

Interobserver variability. A source of error in obstetric ultrasound.

Several sources of error encountered in obstetrical ultrasound examination have been analyzed from a computerized ultrasound database. The variability in measuring fetal landmarks and visualization of fetal structures were found to be significantly different among three registered sonographers who examined 1,410 consecutive second and third trimester patients. The rate of successful measurement of biparietal diameter (BPD) and average abdominal diameter (AAD), as well as successful visualization of fetal stomach and/or kidneys were statistically different among the three sonographers. Only the fetal femur length was consistently measured by all three sonographers. The bias in terms of actual millimeters measured was significantly different for both BPD and AAD. The bias related to the BPD was in the magnitude of 1 mm, while the bias related to the AAD was almost 3 mm. The clinical significance of these findings and the value of the computer to perform periodic checks to assure quality control in a busy ultrasound service are discussed.

Diagnostic Errors↗