Gelsenkirchen, Germany
Gelsenkirchen, Germany

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Kozlov M.,MR Computer GmbH | Kozlov M.,Max Planck Institute for Human Cognitive and Brain Sciences | Schaefers G.,MR Computer GmbH | Schaefers G.,Institute for Safety Technology GmbH
Proceedings of the 12th IASTED International Conference on Biomedical Engineering, BioMed 2016 | Year: 2016

For insulated stainless steel wires of 1.5 mm in diameter with 0.5 mm insulation thickness, we evaluated the dependence of 64 MHz RF-induced power deposited at a hotspot (p) on: 1) lead length, 2) lead insulating electrical properties, 3) lead surrounding medium. Lead transfer functions (TF) were obtained with 3-D electromagnetic simulations. TF and p depended significantly on electrical properties of the insulation and lead surrounding medium. Increased insulator conductivity resulted in decreased p in most investigated cases. It is impossible to define one test surrounding medium that results in worst case power deposition for all cases: a) different lead lengths; b) different electrical properties of the lead insulator; and c) different tissues surrounding the lead in human body.


Acikel V.,Bilkent University | Acikel V.,National Magnetic Resonance Research Center | Uslubas A.,MR Computer GmbH | Atalar E.,Bilkent University | Atalar E.,National Magnetic Resonance Research Center
Medical Physics | Year: 2015

Purpose: The authors purpose is to model the case of an implantable pulse generator (IPG) and the electrode of an active implantable medical device using lumped circuit elements in order to analyze their effect on radio frequency induced tissue heating problem during a magnetic resonance imaging (MRI) examination. Methods: In this study, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the modified transmission line method (MoTLiM) and the method of moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments were used. The induced currents on the leads with the IPG case or electrode connections were solved using the proposed models and the MoTLiM. These results were compared with the MoM simulations. In addition, an electrode was connected to a lead via an inductor. The dissipated power on the electrode was calculated using the MoTLiM by changing the inductance and the results were compared with the specific absorption rate results that were obtained using MoM. Then, MRI experiments were conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead was connected to the case and placed inside a uniform phantom. During a MRI scan, the temperature rise at the lead was measured by changing the lead length. The power at the lead tip for the same scenario was also calculated using the IPG case model and MoTLiM. Then, an electrode was connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan, the temperature rise at the electrode was measured by changing the inductance and compared with the dissipated power on the electrode resistance. Results: The induced currents on leads with the IPG case or electrode connection were solved for using the combination of the MoTLiM and the proposed lumped circuit models. These results were compared with those from the MoM simulations. The mean square error was less than 9%. During the MRI experiments, when the IPG case was introduced, the resonance lengths were calculated to have an error less than 13%. Also the change in tip temperature rise at resonance lengths was predicted with less than 4% error. For the electrode experiments, the value of the matching impedance was predicted with an error less than 1%. Conclusions: Electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The concept of matching of the electrode to the lead is clarified using the defined electrode impedance and the lead Thevenin impedance. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. With these models, understanding the tissue heating due to the implants becomes easier. Also, these models are beneficial for implant safety testers and designers. Using these models, worst case conditions can be determined and the corresponding implant test experiments can be planned. © 2015 American Association of Physicists in Medicine.


Grant
Agency: European Commission | Branch: FP7 | Program: MC-ITN | Phase: FP7-PEOPLE-ITN-2008 | Award Amount: 3.71M | Year: 2009

The scope of the present consortium is to provide technology and training for the integration of ultrasound and biophotonics based imaging with magnetic resonance imaging (MRI), Computed Tomography (CT) and Positron Emission Tomography (PET) to define the specs of an Integrated Interventional Imaging Operating System (III OS) aimed at minimal invasive treatment of common life-threatening disorders, e.g., cancer, cardiovascular disease and structural heart defects. Effective therapy of these conditions will require a range of safe surgical and interventional devices used with the necessary visualization and tracking under real-time image guidance. The consortium includes a critical mass of industrial and university research institute partners with high expertise in design, development, and manufacture of these devices and instruments. To ensure medical the safety and economical usability of the system and to allow an optimal integration into the future hospital workflow, 6 university hospitals will contribute their clinical and administrative expertise to the consortium in the fields of Interventional Radiology/Cardiology, Anaesthesia, Oncology, General and Cardiovascular Surgery and preclinical Image guided procedures. The consortium of the IIIOS research and training process includes two Biomedical Technology Societies: DGBMT and SMIT&MEDIS Foundation in Rumania providing expert networking and conference organization. The is involved in the consortium and will play a key role in the exchange of knowledge and expertise to the new member states of the EU through hosting conferences such SMIT 2009 in Sinaia (www.smit2009.com).


Kraff O.,University of Duisburg - Essen | Wrede K.H.,University of Duisburg - Essen | Schoemberg T.,University of Duisburg - Essen | Dammann P.,University of Duisburg - Essen | And 5 more authors.
Medical Physics | Year: 2013

Purpose: The increasing number of clinically oriented MRI studies at 7 T motivates the safety assessment of implants, since many 7 T research sites conservatively exclude all subjects with metallic implants, regardless of type or location. The purpose of this study was to investigate potential RF-induced heating during a 7 T MRI scan using a self-built transmitreceive RF coil in patients with implants used for refixation of the bone flap after craniotomy. Going beyond standard ASTM safety tests, a comprehensive test procedure for safety assessments at 7 T is presented which takes into account the more complex coupling of the electromagnetic field with the human body and the implant as well as polarization effects. Methods: The safety assessment consisted of three main investigations using (1) numerical simulations in simplified models, (2) electric and magnetic field measurements and validation procedures in homogeneous phantoms, and (3) analysis of exposure scenarios in a heterogeneous human body model including thermal simulations. Finally, 7 T in vivo images show the degree of image artifact around the implants. Results: The simulations showed that the field distortions remain localized within the direct vicinity of the implants. A parallel E-field polarization was found to be the most relevant component in creating local SAR deviations, resulting in a 10 increase in 10-g-averaged SAR and 53 in 1-g-averaged SAR. Using a heterogeneous human head model, the implants caused field distortions and SAR elevations in the numerical simulations which were distinctly lower than the maximum local SAR value caused by the RF coil alone. Also, the position of the maximum 10-g-averaged SAR remained unchanged by the presence of the implants. Similarly, the maximum absolute local temperature remained below 39 °C in the thermal simulations. Only minor artifacts from the implants were observed in the in vivo images that would not likely affect the diagnostic image quality in patients. Conclusions: The findings suggested no evidence for noteworthy RF-related heating in humans after craniotomy using the described implants and for the particular RF coil that was used in this study. Here, identical transmit power restrictions apply with or without the implants. For other RF coils, the maximum permissible input power should be reduced by 10 until further simulations may indicate otherwise. © 2013 American Association of Physicists in Medicine.


PubMed | MR Computer GmbH and University of Duisburg - Essen
Type: Journal Article | Journal: Magma (New York, N.Y.) | Year: 2015

Over the last decade, the number of clinical MRI studies at 7 T has increased dramatically. Since only limited information about the safety of implants/tattoos is available at 7 T, many centers either conservatively exclude all subjects with implants/tattoos or have started to perform dedicated tests for selected implants. This work presents our experience in imaging volunteers with implants/tattoos at 7 T over the last seven and a half years.1796 questionnaires were analyzed retrospectively to identify subjects with implants/tattoos imaged at 7 T. For a total of 230 subjects, the type of local transmit/receive RF coil used for examination, imaging sequences, acquisition time, and the type of implants/tattoos and their location with respect to the field of view were documented. These subjects had undergone examination after careful consideration by an internal safety panel consisting of three experts in MR safety and physics.None of the subjects reported sensations of heat or force before, during, or after the examination. None expressed any discomfort related to implants/tattoos. Artifacts were reported in 52% of subjects with dental implants; all artifacts were restricted to the mouth area and did not affect image quality in the brain parenchyma.Our initial experience at 7 T indicates that a strict rejection of subjects with tattoos and/or implants is not justified. Imaging can be conditionally performed in carefully selected subjects after collection of substantial safety information and evaluation of the detailed exposure scenario (RF coil/type and position of implant). Among the assessed subjects with tattoos, no side effects from the exposure to 7 T MRI were reported.


Kozlov M.,MR Computer GmbH | Schaefers G.,MR Computer GmbH
2015 IEEE International Conference on Microwaves, Communications, Antennas and Electronic Systems, COMCAS 2015 | Year: 2015

The implant model obtained by 3D EM and thermal co-simulation was used to estimate power deposition and temperature rise due to the presence of bare titanium rods placed in a high permittivity medium for uniform and non-uniform 64 MHz incident electric fields. No correlation between power deposition and temperature rise were observed. Worst-case temperature rise for an implant with a fixed diameter and variable length excited by a tangential electric field with constant amplitude can be estimated using the implant thermal model. For other cases further extensive investigations are required. © 2015 IEEE.


Kozlov M.,MR Computer GmbH | Schaefers G.,MR Computer GmbH
2015 IEEE International Conference on Microwaves, Communications, Antennas and Electronic Systems, COMCAS 2015 | Year: 2015

We numerically investigated volume loss density and temperature rise generated due to presence of a titanium rod placed in an ASTM F2182-11a high permittivity medium which was excited by radio frequency (RF) 64 MHz plane waves. 3-D electromagnetic and thermal co-simulations were performed for continuous wave and time-varied excitations. If time variable RF excitation that reaches the limit set by the short duration specific absorption rate limit and the implant thermal time constant is comparable to or less than 10s, RF induced heating on and near an implant during magnetic resonance imaging can be up to factor of 2 higher than RF induced heating caused by continuous RF excitation reaches the limit set by whole body specific absorption rate limit defined by IEC 60601-2-33. If the thermal time constant is significantly longer than 10 s, continuous wave excitation results in a reliable assessment of the maximum temperature rise. © 2015 IEEE.


Lu Z.,University of Missouri | Camps-Raga B.,MR Computer GmBH | Islam N.E.,University of Missouri
Physics Research International | Year: 2012

The concept of a single frequency band, single high-refractive-index metamaterial has been extended and applied in the design of dual frequency band, dual high-refractive-index metamaterials in the THz regime. The structure design consists of twenty five unit cells with a surface area of 250 um by 250 um and a thickness of 5 um. Each cell has metallic structures embedded in a polyimide substrate. The return loss (S-parameter) analysis shows two strong electric responses at two frequency ranges, and the extracted constitutive parameters suggested high values of simultaneous dielectric constant and permeability at these frequencies. Results retrieved from the S-parameters also show high refractive index values. A first peak refractive index of 61.83 was observed at a resonant frequency of 0.384 THz, and another peak refractive index of 19.2 was observed at the resonant frequency 1.416 THz. Analysis show that higher refractive index at the second resonance frequency band is achievable through redesign of the structures, and modifications could lead to a single structure with multiple frequency, multiple high-refractive-index metamaterials that can be put to practical use. © 2012 Zan Lu et al.


Kozlov M.,MR Computer GmbH | Kozlov M.,Max Planck Institute for Human Cognitive and Brain Sciences | Schaefers G.,MR Computer GmbH
Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS | Year: 2015

We numerically investigated power deposition and temperature rise generated due to the presence of a titanium rod placed in a phantom, located inside a 1.5 T coil. The induced power deposition and temperature rise normalized to incident tangential electric field was found to be dependent on distance to the phantom wall. The different dependence of the integral of power deposition over a box surrounded the rod and the temperature rise on American Society for Testing and Materials (ASTM) phantom medium electrical conductivity was observed. The consequences of numerical domain simplification have been analyzed. © 2015 IEEE.


Schmitt S.,Gelsenkirchen University of Applied Sciences | Choli M.,MR Computer GmbH | Overhoff H.M.,Gelsenkirchen University of Applied Sciences
Progress in Biomedical Optics and Imaging - Proceedings of SPIE | Year: 2015

MRI-guided interventions have gained much interest. They profit from intervention synchronous data acquisition and image visualization. Due to long data acquisition durations, ergonomic limitations may occur. For a trueFISP MRI-data acquisition sequence, a time sparing sub-sampling strategy has been developed that is adapted to amagnetic needle detection. A symmetrical and contrast rich susceptibility needle artifact, i.e. an approximately rectangular gray scale profile is assumed. The 1-D-Fourier transformed of a rectangular function is a sinc-function. Its periodicity is exploited by sampling only along a few orthogonal trajectories in k-space. Because a needle moves during intervention, its tip region resembles a rectangle in a time-difference image that is reconstructed from such sub-sampled k-spaces acquired at different time stamps. In different phantom experiments, a needle was pushed forward along a reference trajectory, which was determined from a needle holders geometric parameters. In addition, the trajectory of the needle tip was estimated by the method described above. Only ca. 4 to 5 % of the entire k-space data was used for needle tip estimation. The misalignment of needle orientation and needle tip position, i.e. the differences between reference and estimated values, is small and even in its worst case less than 2 mm. The results show that the method is applicable under nearly real conditions. Next steps are addressed to the validation of the method for clinical data. © 2015 SPIE.

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