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Biomedical subjects

N Meier

Publications and source records attributed to N Meier.

At least 37 records · Page 2Linked to original sources
PubMed1997

[Telemedicine. Possibilities and perspectives].

The discussion about the implementation of telemedicine and teleresourcing and its consequences has been under way for some time now. The rate at which telemedical applications are being developed, leaves little time for consideration of the economic, scientific and social aspects. There is a need for integration of all the existing fragments into one coherent telemedical concept. Different aspects of telemedical concepts are discussed.

Computer Communication Networks↗
PubMed1997

Unraveling the role of structure and density in determining vertebral bone strength.

The strength of bone is determined not only by bone density but also by structure. Therefore, quantification of the structure in radiographs by texture parameters may result in a better prediction of fracture risk. Since in radiographs density and structure are strongly correlated, the predictive power of texture parameters should be corrected for the influence of BMD to determine the additional information conveyed by these parameters. In this study, we evaluated the predictive power of various texture parameters based on the Grey-Level Dependence Method and the Morphological Gradient Method. This study was performed on 67 vertebrae obtained from 20 male and 12 female human cadaver thoracolumbar spines. BMD and area of the vertebral body were determined from QCT images and texture parameters were derived from direct magnification (DIMA) radiographs. The fracture force, measured under conditions simulating the in vivo situation, was corrected with the area of the vertebra to yield the fracture stress (FS). Results of the study indicate that BMD correlates significantly with FS r = 0.82 (P < 0. 001, n = 24) and r = 0.94 (P < 0.001, n = 43) for female and male vertebrae, respectively. Correlation coefficients of the investigated texture parameters were as high as 0.80 (P < 0.001) and 0.67 (P < 0.001) for the female and male vertebrae, respectively. Multiple regression analysis showed that in female vertebrae, the addition of one texture parameter to BMD results in a better prediction of strength. The multiple correlation coefficient was 0. 87 (P < 0.001) in this case. In male vertebrae, BMD was the best predictor of fracture stress. These results suggest that texture parameters, as measured in magnification radiographs, can predict bone strength. Whereas in all cases BMD is the best single predictor of bone strength, for women texture parameters contain useful additional information.

Bone Density↗
PubMed1997

Texture analysis of direct magnification radiographs of vertebral specimens: correlation with bone mineral density and biomechanical properties.

RATIONALE AND OBJECTIVES: The authors used direct magnification radiographs, combines with texture analysis, to investigate the trabecular structure of human vertebral specimens and compared these techniques with measurement of bone mineral density (BMD) by using quantitative computed tomography to predict bone strength. METHODS: Direct magnification radiographs and BMD measurements were obtained from 38 motion segments from the thoracolumbar spines of 11 female human cadavers. Maximum compressive strength (MCS) was determined with a materials testing machine. Morphologic parameters, digital skeletons, and fractal dimension were obtained from the radiographs in three different regions of interest. RESULTS: Correlations between BMD and MCS were statistically significant (r = .81, P < .01). With morphologic parameters, correlation coefficients of up to .64 (P < .01) were obtained. Use of multivariate regression analysis with one morphologic parameter (the width of the black pixels, or thicknessB) in addition to BMD improved correlations versus MCS (P < .01). CONCLUSION: In an experimental setting, BMD showed statistically significant correlation with bone strength, whereas the structural parameters demonstrated only modest correlations. BMD together with one of these measures (thicknessB), however, showed the highest correlation.

Adult↗
PubMed1996

[Radiation exposure in low dose computerized tomography of the thorax].

Computed tomography accounts for a high percentage of the populations exposure to radiation. Every possible means of reducing patient exposure must be considered. Exposure reduction will be most appropriate for organs with large absorption differences, since the accompanying decrease in signal-to-noise ratio will have a smaller effect. It has been shown in the literature that high-contrast details in the lung can be depicted, even with the lowest exposure rates. Therefore, experimental data are presented by means of thermoluminescence dosimetry and calculations using conversion factors to determine the technical parameters to be adopted for achieving the effective equivalent dose close to that of a conventional chest X-ray in two projections. With helical CT (pitch 2, 50 mAs, 120 kV, 5-mm slice thickness) it was possible to reduce the equivalent dose from 9.6 mSv to 0.6 mSv, which is comparable with the exposure in a conventional chest X-ray, measured to be 0.2 mSv.

Equipment Design↗
PubMed1996

Artificial spine fractures: detection with helical and conventional CT.

PURPOSE: To compare the accuracy of detection of artificial spine fractures with helical computed tomography (CT) versus conventional CT. MATERIALS AND METHODS: Twenty-six motion segments from 15 human cadaver thoracolumbar spines were imaged with helical and conventional CT both before and after fractures were artificially induced. The vertebrae were scanned with different collimations, reconstruction indexes, and exposure doses. A total of 900 images were analyzed by four radiologists, and a receiver operating characteristic (ROC) analysis was performed. RESULTS: ROC analysis showed a large area under the curve for conventional CT (0.913) than for helical CT (0.844) when 3-mm collimation was used. The ability to detect fractures with helical CT increased when collimation was decreased and exposure dose was increased. CONCLUSION: Conventional CT allows more accurate detection of artificial spine fractures than does helical CT. Helical CT requires thinner collimation for fracture detection comparable with that of conventional CT.

Cadaver↗
PubMed1995

[In vitro validation of intravascular ultrasound, computerized and magnetic resonance tomography in diagnosis of atherosclerotic vascular segments in comparison with direct magnification radiography].

The depiction of atherosclerotic vessel abnormalities is a prerequisite for percutaneous interventional therapy and long-term observations of peripheral artery disease. The aim of this in-vitro study was to determine the potentials and limitations of 12.5 and 20 MHz intravascular ultrasound, computed tomography and magnetic resonance (MR) imaging in comparison to direct magnification radiography for the localization and quantification of peripheral vessel wall calcifications. Forty-three postmortem, human iliac segments were examined by intravascular ultrasound (12.5 and 20 MHz), computed tomography and magnetic resonance tomography (gradient echo-and spin echo-technique). For comparative analysis, each segment was divided into eight sectors of 45 degrees each; using all five methods, the presence of calcified wall areas was examined in each sector, and luminal area (42 segments) and plaque area (32 isolated plaques) were quantitatively estimated. In the sonograms, the circumferential extension of the boundary between intima and media was measured. 122 of 344 sectors showed regional vessel wall calcifications. Sensitivity of 20 MHz intravascular ultrasound was 73% versus 59% with the 12.5 probe, specificity was 97% with 20 MHz, 96% with 12.5 MHz. Sensitivity of both 12.5 and 20 MHz intravascular ultrasound was higher with increased thickness of the calcified structures. 20 MHz ultrasound identified the intima-media boundary averaging 146.8 degrees of the vessel circumference; the corresponding value of 131.8 degree with 12.5 MHz did not differ significantly. Computed tomography detected calcifications with a sensitivity of 88%, specificity was 88%. With MR imaging, sensitivity of the gradient echo-technique was 94% versus a sensitivity of 86% with spin echo-technique. Quantification of luminal and plaque areas showed that luminal area was precisely estimated only by 20 MHz ultrasound (no significant difference to direct magnification radiography), whereas all other techniques showed significant overestimation. Plaque areas were markedly overestimated by computed tomography and MR imaging, too. In an in vitro set-up, intravascular ultrasound, MR tomography and computed tomography do not allow an authentic depiction of peripheral vessel wall architecture. Limited resolution, subintimal shadowing and and distortion are the main limitations of these new techniques so that details of regional vessel wall calcifications cannot be presented thoroughly. Relevant over-estimation of luminal and plaque areas must be considered.

Arteriosclerosis↗
PubMed1995

[Exposure of personnel in interventional radiology].

The surface dose at five typical positions around the patient couch was measured during transjugular intrahepatic porto-systemic stent shunt (TIPSS) interventions for retrospective calculation of staff dose. Additionally, the surface dose was measured on three members of staff (two radiologists and one technician) by placing thermoluminescence (TL) dosimeters directly on the eyebrows, thyroid, sternum, gonads, hands, knees, and feet. The measured surface dose values and the TL dosimeter measurements are used for estimation of effective dose per examination received by personnel. The effective dose for the radiologists is calculated to 25-35 microSv. The maximum number of examinations allowed per year is calculated on the basis of a given yearly dose limit, by looking at organ dose and effective body dose.

Female↗
PubMed1992

[Evaluation of different methods of control of film processing].

In addition to the official control method (DIN 6868), alternative test methods have been developed in the last few years: the "visuelle Methode" by Eder and the "Stuttgarter Modell." The aim of both methods is mainly to increase practicability and economy. The DIN method, however, demonstrates clear benefits, precision, practicability and economy, and it is the only method that allows a trend analysis at small intervals, which is the basis for possible preventive action.

Germany↗
PubMed1992

[Sources of error in the quality assurance of x-ray equipment].

With the introduction of the German industrial protocol for quality assurance of X-ray equipment, everybody using X-ray machines fell into some sort of trap or found some of the procedures misleading. Vigorous discussions were held on every conceivable aspect of this and other protocols, and are still in progress. This paper points out some of the common traps and considers a new type dosimeter, which allows a definite separation of quality assurance of X-ray equipment and of processing and which can also be used in future for quality assurance of digital radiography equipment.

Germany↗
PubMed1990

[Serum level determination of local anesthetics using high pressure liquid chromatography].

We report a high resolution liquid chromatography method for simultaneous determination of lidocaine, mepivacaine and bupivacaine in serum using cyclicine as a standard. Drugs are extracted from serum using dichloromethane in an alkaline medium type columns in a mobile phase with 0.1 M phosphate buffer at pH 7.5, methanol and acetonitrile (33:17:50) were used for separation. Spectrophotometric measurement was performed at 207 nm. In a concentration ranging from 0.5 to 8 micrograms/ml, r values of 0.997, 0.989 and 0.998 were obtained for lidocaine, mepivacaine and bupivacaine, respectively. The coefficient of variation estimated at 2 micrograms/ml was 2, 19, 2.76 and 2.48%, respectively. A maximal error of 4.5%, 3.6% and 2.9% found for "inter day" repeated measurement at the above concentration for each drug. Thus, high sensitivity, reproducibility and relative simpleness of the method are demonstrated for its clinical use in determination of serum levels of local anesthetics.

Anesthetics, Local↗
PubMed1986

The importance of individual nucleotides for the structure and function of rRNA molecules in E. coli. A mutagenesis study.

Methods of in vitro mutagenesis were employed to determine the importance of individual nucleotides within the ribosomal RNAs for the structure and function of E. coli ribosomes. A series of defined nucleotides in the genes for the 5 S and 16 S RNA were altered by transition and transversion mutations using either oligonucleotide-directed or bisulfite-catalyzed mutation procedures. Plasmids harbouring the mutated rRNA genes were expressed and the ribosomes containing such altered RNAs were investigated for impairments in RNA-protein interaction assembly and mRNA-coded tRNA binding.

Base Sequence↗
PubMed1985

Effects of the ribosomal subunit association on the chemical modification of the 16S and 23S RNAs from Escherichia coli.

70S ribosomes and 30S and 50S ribosomal subunits from Escherichia coli were modified under non-denaturing conditions with the chemical reagent dimethylsulfate. The ribosomal 23S and 16S RNAs were isolated after the reaction and the last 200 nucleotides from the 3' ends were analyzed for differences in the chemical modification. A number of accessibility changes could be detected for 23S and 16S RNA when 70S ribosomes as opposed to the isolated subunits were modified. In addition to a number of sites which were protected from modification several guanosines showed enhanced reactivities, indicating conformational changes in the ribosomal RNA structures when 30S and 50S subunits associate to a 70S particle. Most of the accessibility changes can be localized in double-helical regions within the secondary structures of the two RNAs. The results confirm the importance of the ribosomal RNAs for ribosomal functions and help to define the RNA domains which constitute the subunit interface of E. coli ribosomes.

Binding Sites↗
PubMed1984

Binding of tRNA alters the chemical accessibility of nucleotides within the large ribosomal RNAs of E. coli ribosomes.

Functionally active 70S ribosomes were chemically modified with dimethylsulfate (DMS) in the presence and absence of bound tRNA. The ribosomal 16S RNA and 23S RNA were extracted, separated and labeled radioactively at their 3'-ends. DMS modification sites within the last 200 nucleotides from the 3'-ends were investigated on sequencing gels, after borohydride reduction and aniline catalyzed strand scission of the isolated RNA's. tRNA binding caused enhanced reactivity at 9 nucleotide positions while three sites showed decreased reactivity in the 16S RNA. The effects of bound tRNA on the modification of 23S RNA were limited. Only one enhancement was observed in the presence of bound tRNA. mRNA binding alone showed two more sites with enhanced reactivity, however. The results are consistent with the view that the sequence 1400-1500 of the 16S RNA plays an important functional role in the translating ribosome and possibly constitutes part of the tRNA binding site.

Base Sequence↗