Browsing by Subject "Magnetic resonance imaging--Safety measures."

Now showing 1 - 4 of 4

Open Access
Analysis of current induction on thin conductors inside the body during MRI scan
(2014) Açıkel, Volkan
The aim of this thesis is to develop a method to analyze currents on thin conductor structures inside the body during Magnetic Resonance Imaging (MRI) scan based on Modified Transmission Line Method (MoTLiM). In this thesis, first, Active Implantable Medical Devices (AIMDs) are modeled and the tissue heating problem, which is a result of coupling between AIMD and incident Radio Frequency (RF) fields, is examined. Then, usage of MoTLiM to analyze the currents on the guidewires is shown by solving currents on guidewire when a toroidal transmit receive coil is used with guidewire. At first, a method to measure MoTLiM parameters of leads using a network analyzer is shown. Then, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the Modi- fied Transmission Line Method (MoTLiM) and the Methods 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 are used. The induced currents on the leads with the IPG case or electrode connections are solved using the proposed models and MoTLiM. These results are compared with the MoM simulations. In addition, an electrode is connected to a lead via an inductor. The dissipated power on the electrode is calculated using MoTLiM by changing the inductance and the results are compared with the specific absorption rate results that are obtained using MoM. Then, MRI experiments are conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead is connected to the case and placed inside a uniform phantom. During a MRI scan the temperature rise at the lead is measured by changing the lead length. The power at the lead tip for the same scenario is also calculated using the IPG case model and MoTLiM. Then an electrode is connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan the temperature rise at the electrode is measured by changing the inductance and compared with the dissipated power on the electrode resistance. Second, based on the similarity between currents on guidewires and transmission lines, currents on the catheter are solved with MoTLiM. Current distributions on an insulated guidewire are solved and B1 distribution along the catheter is calculated. Effect of stripping the tip on the tip visibility is analyzed. It is shown that there is an increase in the B1 at the insulation and bare guidewire boundary. Then, a characteristic impedance is defined for the guidewires and impedance seen at the point where guidewire is inserted into the body is calculated. It is shown with EM simulations that if the impedance converges to the characteristic impedance of the guidewire, tip visibility of the guidewire is lost. At last, a new method to measure electrical properties of a phantom material is proposed. This method is used for validation of the coaxial transmission line measurement (CTLM) fixture, which is designed for measurement of electrical properties of viscous phantom materials at MRI frequencies, and which is previously presented by our group. The new method depends on the phenomena of the lead tip heating inside a phantom during MRI scan. Electrical properties of a phantom are influential on the relationship between tip temperature increase and the lead length. MoTLiM is used and the relationship between the lead length and the tip temperature increase is formulated as a function of conductivity and permittivity of the phantom. By changing the lead length, the tip temperature increase is measured and the MoTLiM formulation is fitted to these data to find the electrical properties of the phantom. Afterwards the electrical properties of the phantom are measured with the CTLM fixture and the results that are obtained with both methods are compared for an error analysis. To sum, electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. An analytical analysis of guidewire with toroidal transceiver is shown. This analysis is helpful for better usage and improvements of toroidal transceiver. Also, MoTLiM analysis can be extended to other MRI guidewire antennas.
Open Access
Analysis of the electromagnetic field inside the gradient coils and investigation of the nerve and cardiac stimulation risk for the patients during MRI
(2008) Abacı, Esra
During magnetic resonance imaging (MRI), the static magnetic field, the radio frequency field and the gradient fields are utilized. In the literature, there are several studies with the aim of understanding the bioeffects of these fields. Due to the time varying gradient fields, an electric field is induced in the body which may cause nerve and cardiac stimulation. In order to investigate this risk, researchers have been working on solving the induced electric field both analytically and by using computational methods. The field distribution inside the body has also been investigated. It is vital to verify the risk of MRI to patients with an implant. In order to improve the understandability of the field pattern, the field should be expressed as simple as possible. In this thesis, the simplified expressions of induced electric field are derived inside and outside of the cylindrical, homogenous volume, which is taken as the human body model. For this derivation, low frequency based assumptions are used and gradient field is assumed to be perfectly uniform. The obtained results satisfy the expected conditions for the electric and magnetic fields. The field patterns obtained with these simplified expressions are compared with the former studies and the maximum electric field values obtained for a different gradient field and slew rates are used to investigate the stimulation risk. Moreover, by using these electric field values, the worst position for an implant lead and the length of the lead is determined. We believe that with these simplified expressions, the understandability of the field distribution is enhanced and to comment on the risk of MRI to a patient with an implant becomes easier.
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 RF heating of active implantable medical devices during MRI using safety index
(2007) Irak, Halise
Magnetic Resonance Imaging (MRI) is known as a safe imaging modality that can be hazardous for patients with active implantable medical devices, such as a pacemakers or deep brain stimulators. The primary reason for that is the radio frequency (RF) heating at the tips of the implant leads. In the past, this problem has been analyzed with phantom, animal and human experiments. The amount of temperature rise at the lead tip of these implants, however, has not been theoretically analyzed. In this thesis, a simple approximate formula for the safety index of implants, which is the temperature increase at the implant lead tip per unit deposited power in the tissue without the implant in place, was derived. For that purpose, an analytical quadrature birdcage coil model was developed and the longitudinal incident electric field distribution inside the body was formularized as follows: ER H z () R = −ω µ − in which ω is the angular frequency, µ is the magnetic permeability of the tissue, H- is the left hand rotating component of the RF magnetic field and R is the radial distance from the center of the body. This formula was examined by simulations and phantom experiments. The analytical, simulation and experimental results of that model are in good agreement.Then, depending on the quadrature birdcage coil model safety index (SI) formula for active implants with short leads was derived as shown below: 2 max 2 peak 1 ( ) 2 j t b T SI Rl Ae f Dv SAR c R θ α ∆ == + where ∆Tmax is the maximum temperature increase in the tissue, SARpeak is the maximum deposited power in the body when there is no implant in the body, α is the diffusivity of the tissue, ct isthe heat capacity of the tissue, Rb is radius of the body, R is the radial distance from the center of the body, l is the length of the implant lead, A is the area of the curvature of the lead, θ is the angle that curvature of the implant makes with the radial axis, and f(Dv) is the perfusion correction factor, which is function of the diameter of the electrode and perfusion. The safety index formula was tested by simulations. Simulation results showed that the theoretical safety index formula approximates and identifies the RF heating problem of active implants with short leads accurately. The safety index formula derived in this thesis is valid for only short wires. However, the formulation for long wires is currently under investigation. Despite the fact that the results obtained for short leads can not be generalized for the safety of patients with active implants, it is believed that this study is the first step towards safety of these patients. Using safety index as a measure of safety is very beneficial to ensure the safety of patients with active implants. Because, it uses the MR scanner-estimated deposited power that does not take the existence of the implant in the patient body into account. This formulation is the first study illustrating the advantage of the safety index metric for RF heating studies of active implants.