Browsing by Subject "Induced currents"

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Open Access
Modeling of radio frequency induced currents on lead wires during MR imaging using a modified transmission line method (MoTLiM)
(2010) Açıkel, Volkan
Magnetic resonance imaging (MRI) is widely used diagnosis technique. During MRI radio frequency (RF) fields are utilized to excite the spins. If these RF fields incidence on metallic implants, currents will be induce on the metallic parts of implants. Inside the body these induced currents on metallic implants cause heating of tissue and sometimes cause severe burning of tissue. This phenomena makes MRI hazardous for patients with metallic implants. Much work has been done to understand this phenomena. However, most of these work based on purely experimental or numerical methods. So to understand and to obtain a good intuition on this problem a lot of cases must be solved computationally or tested experimentally. In this study lumped element model of the transmission line is modified in order to model the conductive wires of implants inside the body. This model is based on the similarity between the damped oscillatory behavior of transmission line currents and induced currents on wires inside the body. A voltage source is added to model the effect of the incident electric field. Voltages and currents on a infinitesimally small portion of wire are solved. Solving currents and voltages simultaneously on the modified lumped element model lead to a non-homogeneous differential equation for the current. The solution of this differential equation gives the analytical solution for the induced current on the implant lead. To test the validity of this solution, wire under the uniform incident electric field is solved with the Modified Transmission Line Method (MoTLiM) and compared to Methods of Moment (MoM) solution. The results are also verified using phantom experiments. For experimental verification, the distorted flip angle distribution due to induced currents are measured using flip angle imaging techniques. In addition to this, the flip angle distribution around the wire is calculated using results obtained from MoTLiM. Finally these results are compared and an error analysis is carried out.
Open Access
Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method
(Wiley-Blackwell Publishing, 2011-11-23) Acikel, V.; Atalar, Ergin
Purpose: Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases. Methods: In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k t. These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out. Results: Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10 mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20 among compared cases. Conclusions: A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium.