Biomedical subjects

Bernd Müller-Bierl

Publications and source records attributed to Bernd Müller-Bierl.

4 recordsLinked to original sources

PubMed2006

[Field perturbations by magnetic dipoles as a model for magnetically labelled cells in MRI].

UNLABELLED: Due to the creation of intense local magnetic fields, iron oxide nanoparticles are used as a contrast agent to produce signal loss in Magnetic Resonance Imaging (MRI) in regions where labelled cells have migrated. OBJECTIVE: To study effects of the intracellular distribution of magnetic moments on the extracellular magnetic field by means of numerical simulations. METHODS: Various geometries of intracellular particle distributions were scrutinized and the extracellular field distortions were computed. The total magnetic moment of a labelled cell was assigned to various magnetic subcompartments. The implications on the intravoxel frequency distribution and the static MR signal decay were assessed. RESULTS: The extracellular field perturbation was affected by the intracellular particle distribution only in close proximity to the labelled cell. With increasing distance from the labelled cell, the effects of the intracellular particle distribution were less pronounced. The intravoxel frequency distribution induced by a single labelled cell was non-lorentzian. CONCLUSION: The magnetic fields created by an iron loaded cell are sensitive to the intracellular distribution of nanoparticles only in close proximity to the cell. Far from the cell the field perturbation cannot be distinguished from the magnetic dipole field produced by a magnetic sphere with the same total magnetic moment.

Humans↗

PubMed2005

Magnetic susceptibility effects on the accuracy of MR temperature monitoring by the proton resonance frequency method.

PURPOSE: To evaluate the error of MR temperature assessment based on the temperature-dependent Larmor frequency shift of water protons, which can result from susceptibility effects caused by the radiofrequency (RF) applicator. MATERIALS AND METHODS: Local frequency shifts due to RF applicator displacements were simulated numerically by means of a three-dimensional elementary dipole model. Experimental examinations using a water tank phantom equipped with a high-precision screw thread were applied to examine temperature and movement effects for five commercially available, MR-compatible RF applicators. Measurements were performed at 1.5 Tesla. RESULTS: For single-needle electrodes perpendicular to the external field, a distortion of 0.1 ppm and 0.2 ppm was recorded at a distance of 17.5 mm and 12.5 mm, respectively, to the needle shaft. Cluster applicators and umbrella-shaped applicators caused distortions of 0.1 ppm up to distances of 36 mm. Sinusoidal dependence on applicator orientation was found with the highest values for perpendicular orientation and the lowest values for orientation parallel to the magnetic field. With a single electrode oriented perpendicular to the field at a distance of 1.5 cm and 2.0 cm, a needle displacement of 5 mm led to an error in temperature measurement of 16.3 degrees C and 7.5 degrees C, respectively. CONCLUSION: In MR temperature measurement, displacement of the RF applicator by patient movement or breathing leads to significant errors that have to be taken into account when PRF temperature maps are used to monitor tumor ablation in the presence of paramagnetic applicators.

Catheter Ablation↗

PubMed2005

Compensation of magnetic field distortions from paramagnetic instruments by added diamagnetic material: measurements and numerical simulations.

In minimally invasive procedures guided by magnetic resonance (MR) imaging instruments usually are made of titanium or titanium alloys (e.g., nitinol), because other more MR-compatible materials often cannot provide sufficient mechanical properties. Artifacts depending on susceptibility arise in MR images due to incorrect spatial encoding and intravoxel dephasing and thereby hamper the surgeon's view onto the region of interest. To overcome the artifact problem, compensation of the paramagnetic properties by diamagnetic coating or filling of the instruments has been proposed in the literature. We used a numerical modeling procedure to estimate the effect of compensation. Modeling of the perturbation of the static magnetic field close to the instruments reflects the underlying problem and is much faster and cost efficient than manufacturing prototypes and measuring artifact behavior of these prototypes in the MR scanner. A numerical model based on the decomposition of the susceptibility distribution in elementary dipoles was developed by us. The program code was written object oriented to allow for both maximum computational speed and minimum random access memory. We used System International units throughout the modeling for the magnetic field, allowing absolute quantification of the magnetic field disturbance. The field outside a simulated needlelike instrument, modeled by a paramagnetic cylinder (out of titan, chi =181.1) of length 8.0 mm and of diameter 1.0 mm, coated with a diamagnetic layer (out of bismuth, chi=-165.0) of thickness 0, 0.1, 0.2, 0.3, and 0.4 mm, was found to be best compensated if the cross-sectional area of the cylinder, multiplied by the absolute susceptibility value of the cylinder material, is equal to the cross-sectional area of the coating, multiplied by the absolute susceptibility value of the coating material. At the extremity of the coated cylinder an uncompensated field distortion was found to remain. We studied various tip shapes and geometries using our computational model: Suitable diamagnetic coating or filling of paramagnetic instruments clearly reduced tip artifacts and diminished the dependency of artifact size on orientation of the instrument with respect to B0 in the numerical studies. We verified the results of the simulations by measuring coated and uncoated titanium wires in a 1.5 T MR scanner.

Computer Simulation↗

PubMed2004

Numerical modeling of needle tip artifacts in MR gradient echo imaging.

Exact determination of needle tip position is obsolete for interventional procedures under control of magnetic resonance imaging (MRI). Exact needle tip navigation is complicated by the paramagnetism of microsurgical instruments: Local magnetic field inhomogeneities are induced resulting in position encoding artifacts and in signal voids in the surrounding of instruments and especially near their tips. The artifacts generated by the susceptibility of the material are not only dependent on the material properties themselves and on the applied MRI sequences and parameters, but also on the geometric shape of the instruments and on the orientation to the static magnetic field in the MR unit. A numerical model based on superposition of induced elementary dipole fields was developed for studying the field distortions near paramagnetic needle tips. The model was validated by comparison with experimental data using field mapping MRI techniques. Comparison between experimental data and numerical simulations revealed good correspondence for the induced field inhomogeneities. Further systematic numerical studies of the field distribution were performed for variable types of concentric and asymmetric tip shapes, for different ratios between tip length and needle diameter, and for different orientations of the needle axis in the external static magnetic field. Based on the computed local inhomogeneities of the magnetic field in the surroundings of the needle tips, signal voids in usual gradient echo images were simulated for a prediction of the artifacts. The practically relevant spatial relation between those artifacts and the hidden tip of the needle was calculated for the different tip shapes and orientations in the external field. As needle tip determination is crucial in interventional procedures, e.g., in taking biopsies, the present model can help to instruct the physician prior to surgical interventions in better estimating the needle tip position for different orientations and needle tip shapes as they appear in interventional procedures. As manufacturing prototypes with subsequent measurements of artifacts in MRI are a costly procedure the presented model may also help to optimize shapes of needle tips and of other parts of MR-compatible instruments and implants with low expense prior to production if some shape parameters can be chosen freely.

Artifacts↗