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Biomedical subjects

K Rohr

Publications and source records attributed to K Rohr.

16 recordsLinked to original sources
PubMed2002

Coupling of fluid and elastic models for biomechanical simulations of brain deformations using FEM.

In order to improve the accuracy of image-guided neurosurgery, different biomechanical models have been developed to correct preoperative images with respect to intraoperative changes like brain shift or tumor resection. All existing biomechanical models simulate different anatomical structures by using either appropriate boundary conditions or by spatially varying material parameter values, while assuming the same physical model for all anatomical structures. In general, this leads to physically implausible results, especially in the case of adjacent elastic and fluid structures. Therefore, we propose a new approach which allows to couple different physical models. In our case, we simulate rigid, elastic and fluid regions by using the appropriate physical description for each material, namely either the Navier equation or the Stokes equation. To solve the resulting differential equations, we derive a linear matrix system for each region by applying the finite element method (FEM). Thereafter, the linear matrix systems are linked together, ending up with one overall linear matrix system. Our new approach has been tested and compared to a purely linear elastic model using synthetic as well as tomographic images. It turns out from our experiments, that the integrated treatment of rigid, elastic and fluid regions improves the physical plausibility of the predicted deformation results as compared to a purely linear elastic model.

Biomechanical Phenomena↗
PubMed2001

Landmark-based elastic registration using approximating thin-plate splines.

We consider elastic image registration based on a set of corresponding anatomical point landmarks and approximating thin-plate splines. This approach is an extension of the original interpolating thin-plate spline approach and allows to take into account landmark localization errors. The extension is important for clinical applications since landmark extraction is always prone to error. Our approach is based on a minimizing functional and can cope with isotropic as well as anisotropic landmark errors. In particular, in the latter case it is possible to include different types of landmarks, e.g., unique point landmarks as well as arbitrary edge points. Also, the scheme is general with respect to the image dimension and the order of smoothness of the underlying functional. Optimal affine transformations as well as interpolating thin-plate splines are special cases of this scheme. To localize landmarks we use a semi-automatic approach which is based on three-dimensional (3-D) differential operators. Experimental results are presented for two-dimensional as well as 3-D tomographic images of the human brain.

Algorithms↗
PubMed1999

Biomechanical modeling of the human head for physically based, nonrigid image registration.

The accuracy of image-guided neurosurgery generally suffers from brain deformations due to intraoperative changes. These deformations cause significant changes of the anatomical geometry (organ shape and spatial interorgan relations), thus making intraoperative navigation based on preoperative images error prone. In order to improve the navigation accuracy, we developed a biomechanical model of the human head based on the finite element method, which can be employed for the correction of preoperative images to cope with the deformations occurring during surgical interventions. At the current stage of development, the two-dimensional (2-D) implementation of the model comprises two different materials, though the theory holds for the three-dimensional (3-D) case and is capable of dealing with an arbitrary number of different materials. For the correction of a preoperative image, a set of homologous landmarks must be specified which determine correspondences. These correspondences can be easily integrated into the model and are maintained throughout the computation of the deformation of the preoperative image. The necessary material parameter values have been determined through a comprehensive literature study. Our approach has been tested for the case of synthetic images and yields physically plausible deformation results. Additionally, we carried out registration experiments with a preoperative MR image of the human head and a corresponding postoperative image simulating an intraoperative image. We found that our approach yields good prediction results, even in the case when correspondences are given in a relatively small area of the image only.

Biomechanical Phenomena↗
PubMed1996

[Does the physician need religion?].

The enormous progress in natural and physical sciences leads medicine to limits, namely limits of ethics, credibility and economy. The term 'religio' does not mean religion in the traditional sense, but stands for the norms guiding the physician's actions. Simultaneously, 'religio' searches for clues in fairy tales and myths that guide the physician, on his delicate way between feasibility and responsibility.

Ethics, Medical↗
PubMed1995

Determination of ochratoxin A in bile of swine by high-performance liquid chromatography.

A method for the determination of low concentrations of the mycotoxin ochratoxin A (OA) in bile of pigs is described. OA was extracted with acidified chloroform and the extract was purified on a silicagel cartridge, followed by liquid-liquid partition. OA was determined by high-performance liquid chromatography with fluorescence detection. The detection limit was 0.3 ng/ml, the mean recovery was 84%. An enzyme-linked immunosorbent assay proved to be suitable for confirmation of positive results. Because of the low detection limit, biliary elimination of OA in pigs fed with low-contaminated feedstuffs can be examined by this method. This is an important requirement for the study of pharmacokinetic profiles of OA.

Animal Feed↗
PubMed1994

[Determination of true digestibility of amino acids from the small intestine of dairy cows].

True digestibility of 17 amino acids was determined with 2 lactating cows, fitted with rumen fistulae and T-shaped cannulae at the proximal duodenum and terminal ileum. The calculations were based on the following regression equation: I = a + b x D + c x PFT, with I = amino acid flow at the ileum, D = amino acid flow at the duodenum, and PFT = passage of non-protein dry matter at the ileum. As the factor b means the part of undigested amino acids, 1 - b represents the true digestibility. The calculations, which comprize the results of 16 individual experiments, showed that lysine, histidine, arginine, cysteine, leucine, isoleucine and tyrosine were absorbed in the range of 85% to 90%. Lowest absorption-rates (77-80%) were found for threonine, valine, methionine, aspartic acid and serine. The overall true digestibility of amino acids was 83% without any difference between essential and not essential amino acids.

Amino Acids↗
PubMed1993

Determination of ochratoxin A in urine and faeces of swine by high-performance liquid chromatography.

Sensitive methods for the determination of ochratoxin A in urine and faeces of swine are described. The samples were extracted with chloroform at pH < 2, and the extracts were cleaned up by a combination of solid-phase extraction and liquid-liquid partition. High-performance liquid chromatography with fluorescence detection was used for detection and determination. The detection limits were 0.3 ng/ml for urine and 1.5 ng/g for faeces. Recoveries of ochratoxin A from spiked samples of urine and faeces were 93% and 60%, respectively. Because of the low detection limit and the fast and relatively easy performance, the method for the determination of ochratoxin A in urine proved suitable for the estimation of possible contamination of live animals.

Animals↗
PubMed1981

[Dependence of rumen fatty acid production on the composition of rations].

In three experiments with two Black-and-White dairy cows the influence of soybean oil and coconut fat as well as that of rations rich in roughage and concentrated feed on the production of fatty acids were determined with the isotope dilution method. A change in the method of sampling from the rumen in the course of the investigations resulted in distinctly different absolute production quotas, which can presumably be traced back to the disproportionate mixing in of the isotope and/or different production quotas in various regions of the rumen. The relative differences between the production quotas dependent on the rations, however were approximately the same with both sampling methods, so that they make the comparison of the rations concerning rumen fermentation possible. The production of acetic acid and the total production of fatty acids (C2--C4) correlated closely both with the intake of digestible energy and the intake of digestible organic matter. There was also a highly significant correlation o that they make the comparison of the rations concerning rumen fermentation possible. The production of acetic acid and the total production of fatty acids (C2--C4) correlated closely both with the intake of digestible energy and the intake of digestible organic matter. There was also a highly significant correlation o that they make the comparison of the rations concerning rumen fermentation possible. The production of acetic acid and the total production of fatty acids (C2--C4) correlated closely both with the intake of digestible energy and the intake of digestible organic matter. There was also a highly significant correlation between the relation of acetic and propionic acid in the rumen fluid and the quotient from acetic and propionic acid produced. In contrast to this, a significant relation between the concentration of fatty acids and the production of fatty acids could not be ascertained. Soybean oil and coconut fat brought about a slightly better utilisation of the fat-free organic matter for the production of fatty acids in the rumen. This could mainly be traced back to the increased production of propionic acid. The production of acetic acid per kg fat-free organic matter was insignificantly reduced. A reduced quota of roughage in the ration as well as the use of feed fats resulted in a decrease in the production of acetic acid and an increase in the production of propionic acid. The influence of the quota of roughage, however, was bigger than that of the use of fats. When rations rich in roughage were given, the share the energy contained in the total fatty acids has in the total of the digested energy was, on an average of both animals, slightly lower in comparison to rations rich in concentrated feed. However, the reason for this is not to be found in a lower share the energy digested in the stomachs has in the total of digested energy but in a higher amount of fermentation losses with a nutrition rich in roughage.

Acetates↗