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Exact determination of needle tip position is obsolete for interventional procedures under control of magnetic resonance imaging (MRI).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.

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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.

The ability of magnetic resonance imaging (MRI) to visualize magnetically labelled cells has attracted much attention for revealing cellular events. The present study addressed the geometry and the extension of signal voids in static signal dephasing MRI induced by aggregations of magnetically labelled cells by means of a three-dimensional numerical model. The magnetic field distortions around spherical cell aggregations were treated as equivalent to those of a magnetic dipole. Intravoxel signal dephasing and respective signal voids attributed to these field inhomogeneities were computed. Effects of cell concentration on the signal void in the plane of view were evaluated in terms of dipole magnetization. Signal void characteristics were scrutinized systematically for fundamental sequence parameters including echo time, voxel size and plane-of-view orientation. For all variables examined, significant changes in geometry as well as extension of signal voids were demonstrated. The results are of crucial importance to optimize and interpret MR images with regard to spatial accuracy as well as sensitivity to detect aggregations of labelled cells in vitro or even in vivo. It is anticipated that the dependence of the extension of signal voids on the local magnetization may be valuable for quantifying labelled cells.

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°C and 7.5°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. PERCUTANEOUS LOCAL interstitial thermotherapy for minimally invasive treatment of deep-seated tumors gained increasing importance during the last decade. Compared to conventional surgical resection, interventional treatment offers several advantages, such as reducing the patient's recovery time, the complication rate, and overall health-care costs. Several energy-deposition techniques have been developed, including radiofrequency (RF) ablation (1,2), laser interstitial tumor therapy (LITT) (3), microwave ablation (MW) (4), highintensity focused ultrasound (HIFU) (5), and cryotherapy (6). RF ablation has evolved as the most extensively used ablation modality thanks to its ease of applicator positioning, large achievable coagulation diameters, and relative cost efficacy.Computed tomography (CT) and ultrasound (US) are widely used to position the RF applicator in the tumor. However, neither CT nor US can confirm complete tumor ablation after the energy application. In CT, the neoplastic tissue and the coagulation lesion may show similar patterns, and in US the production of steam bubbles by the heating process disturbs the image acquisition. Magnetic resonance imaging (MRI) provides high soft-tissue contrast for delineation of tumor and applicator navigation, and T2-weighted imaging can be used to estimate the extent of coagulation.The completeness of tumor ablation and protection of susceptible normal tissue in the surrounding a...

Cells loaded with superparamagnetic iron oxide (SPIO) cause relatively strong magnetic field distortions, implying that field position effects of neighboring SPIO loaded cells have to be accounted for. We treated SPIO loaded cells as magnetic dipoles in a homogeneous magnetic field and computed the 3D frequency distribution and the related signal decay using a numerical approach under static dephasing conditions. The volume fraction of dipoles was kept constant for all simulations. For larger randomly distributed magnetic dipoles we found a non-Lorentzian frequency distribution and a non-monoexponential signal decay whereas, for smaller dipoles, the frequency distribution was more Lorentzian and the signal decay was well fitted monoexponentially. Moreover, based on our numerical and experimental findings, we found the gradient echo signal decay due to a single SPIO labeled cell to be non-monoexponential. The numerical approach provides deeper understanding of how the spatial distribution of SPIO loaded cells affects the MR signal decay. This fact has to be considered for the in vivo quantification of SPIO loaded cells, implying that in tissues with different spatial distributions of identical SPIO concentrations, different signal decays might be observed.

Needle tip visualization is of high importance in magnetic resonance imaging (MRI) guided interventional procedures, for example for taking biopsies from suspicious lesions in the liver or kidney. The exact position of the needle tip is often obscured by image artifacts arising from the magnetic properties of the needle. The authors investigated two special biopsy needle tip designs using diamagnetic coatings. For common interventional MR sequences, the needle tip can be identified in the MR image by several equidistant dark spots arranged along a straight line. A dotted instead of a solid line allows for an improved control of the movement of the needle, not only if the needle is tilted toward the imaging plane, but also if the needle leaves an empty canal with signal extinction, which cannot be distinguished from the needle material itself. With the proposed design the position of the needle tip can be estimated with a precision of approximately 1 mm using conventional FLASH, FISP, and TSE sequences, as used for interventional MR. Furthermore, the size of the biopsy probe can be estimated from the artifact. In using needles with a properly designed tip coating, taking biopsies under MR control is beginning to be greatly simplified. The approach to design artifacts using diamagnetic material in combination with paramagnetic material paves the way toward new instruments and implants, suitably tailored to the needs of the interventional radiologist.