A Green's function approach to local rf heating in interventional MRI

Yeung, Christopher; Atalar, Ergin

doi:10.1118/1.1367860

Cited by 76 publications

(73 citation statements)

References 20 publications

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“…As perfusion increases, the value of the safety index (°C/(W/kg)) decreases, indicating a safer condition. Perfusion values range from 1.4 ml/100 g/min for bone to 54 ml/100 g/min for brain (10). Parameters: tissue thermal conductivity ϭ 0.4 W/m°C, tissue electrical conductivity ϭ 0.5 S/m, and relative permittivity of insulation ϭ 3.3.…”

Section: Discussionmentioning

confidence: 99%

“…In each case, the safety index for the wire is shown in comparison to the wire-free safety index. The wire-free safety index is the SAR-to-temperature scaling factor for the case of uniform local heating (10). Since the safety index is a relationship between in vivo temperature and applied SAR, a safety index exists for the wire-free case that is a function of perfusion.…”

Section: Experimental Verificationmentioning

confidence: 99%

“…The second system is modeled with the bioheat transfer equation. We have previously shown that for the case of local RF heating in deep tissues, the bioheat equation can be solved by convolving the SAR distribution with the Green's function of the bioheat equation (10). This approach, which can be thought of as a weighted spatial averaging, gives a physiological model-based prediction of thermal heating (10) that is more relevant than a simple arithmetic average over an arbitrary fixed volume as prescribed by current regulatory guidelines (11,12).…”

Section: Modelingmentioning

confidence: 99%

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RF safety of wires in interventional MRI: using a safety index

Yeung

Susil

Atalar

2001 Conference Proceedings of the 23rd Annual International Conference of the IEEE Engineering in Medicine and Biology Society

111

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With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wireshaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value, assuming a straight wire, a homogeneous RF coil, and a wire that does not extend out of the tissue. The local specific absorption rate (SAR) amplification from induced currents on insulated and bare wires was calculated using the method of moments. This SAR gain was combined with a semianalytic solution to the bioheat transfer equation to generate a safety index. The safety index (°C/(W/kg)) is a measure of the in vivo temperature change that can occur with the wire in place, normalized to the SAR of the pulse sequence. This index can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. For the case of a straight resonant wire in a tissue with very low perfusion, only about 100 mW/kg/°C spatial peak SAR may be used at 1.5 T. But for ≤10-cm wires with an insulation thickness ≥30% of the wire radius that are placed in well-perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. Further model development to include the influence of inhomogeneous RF, curved wires, and wires that extend out of the sample are required to generate safety indices that are applicable to common clinical situations. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase. Magn Reson Med 47: 187-193, 2002.

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Section: Discussionmentioning

confidence: 99%

Section: Experimental Verificationmentioning

confidence: 99%

Section: Modelingmentioning

confidence: 99%

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RF safety of wires in interventional MRI: using a safety index

Yeung

Susil

Atalar

2001 Conference Proceedings of the 23rd Annual International Conference of the IEEE Engineering in Medicine and Biology Society

111

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“…The MR safety issue is particularly critical in endovascular iMRI due to the potential hazards associated with the use of long conductors in the MR environment, since they could act as linear RF antennas and lead to dangerous local temperature increases. Several solutions have been proposed, ranging from spatially distributing the temperature increase along the cable [90] to developing theoretical models of the local tissue heating as a function of the specific absorption rate (SAR) [91]. The ultimate solution, however, has not been found yet.…”

Section: Mri-guided Vascular Applicationsmentioning

confidence: 99%

Interventional and intraoperative MR: review and update of techniques and clinical experience

et al. 2004

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The concept of interventional magnetic resonance imaging (MRI) is based on the integration of diagnostic and therapeutic procedures, favored by the combination of the excellent morphological and functional imaging characteristics of MRI. The spectrum of MRI-assisted interventions ranges from biopsies and intraoperative guidance to thermal ablation modalities and vascular interventions. The most relevant recently published experimental and clinical results are discussed. In the future, interventional MRI is expected to play an important role in interventional radiology, minimal invasive therapy and guidance of surgical procedures. However, the associated high costs require a careful evaluation of its potentials in order to ensure cost-effective medical care.

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“…13 In this work, the value of C was not calculated, and therefore, the above equation was considered as a proportionality.…”

Section: C1 Formulation Of the Rise In Tip Temperature At The Leamentioning

confidence: 99%

Modeling of electrodes and implantable pulse generator cases for the analysis of implant tip heating under MR imaging

Açıkel

Uslubas²,

Atalar

2015

Medical Physics

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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 det...

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A Green's function approach to local rf heating in interventional MRI

Cited by 76 publications

References 20 publications

RF safety of wires in interventional MRI: using a safety index

RF safety of wires in interventional MRI: using a safety index

Interventional and intraoperative MR: review and update of techniques and clinical experience

Modeling of electrodes and implantable pulse generator cases for the analysis of implant tip heating under MR imaging

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