Browsing by Subject "Radio frequency identification systems."

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Finite element method based simulation, desing, and resonant mode analysis of radio frequency birdcage coils used in magnetic resonance imaging
(2012) Gürler, Necip
Radio Frequency (RF) birdcage coils are widely used in Magnetic Resonance Imaging (MRI) since they can generate very homogeneous RF magnetic field inside the coil and have high signal-to-noise ratio (SNR). In practice, designing a birdcage coil is a time-consuming and difficult task. Calculating the capacitance value, which is necessary for the coil to resonate at the desired frequency, is the starting point of the design process. Additionally, it is also important to know the complete resonance frequency spectrum (or resonant modes) of the birdcage coil that helps the coil designers to be sure that working mode is far away from the other modes and so that tuning and matching procedures of the coil can be done without interfering with the other modes. For this purpose, several studies have been presented in the literature to calculate the capacitance value and the resonant modes of the birdcage coil. Among these studies, lumped circuit element model is the most used technique in capacitance and resonant modes calculations. However, this method heavily depends on the inductance calculations which are made under quasi-static assumptions. As a consequence of this assumption, error in the calculations increases as the frequency increases to a point at which the wavelengths are comparable with the coil dimensions. Additionally, modeling the birdcage coil in a 3D simulation environment and making electromagnetic analysis in the volume of interest is also important in terms of observing the electromagnetic field distributions inside the coil. In this thesis, we have proposed three different Finite Element Method (FEM) based simulation methods which are performed using the developed low-pass and high-pass birdcage coil models in COMSOL Multiphysics. One of these methods is the FEM based optimization method in which magnitude of the port impedance or variance of H+ is used as the objective function and the capacitance value is used as the control variable. This is a new method proposed for calculating the capacitance value of the birdcage coils. The other method is the eigenfrequency analysis which is used to determine not only the resonant modes of the birdcage coil but also the electromagnetic fields distributions inside the coil at these resonant modes. To the best of our knowledge, FEM based eigenfrequency analysis of a birdcage coil is also a new study in the field of MRI. The last method is the frequency domain analysis which is used solve for the electromagnetic fields of a birdcage coil for the specified frequency (or frequencies). One can also use this method to estimate Specific Absorption Rate (SAR) at any object inside the coil. To make these three simulation methods easily and according to the user-specified parameters, we have developed two software tools using MATLAB which have also graphical user interface (GUI). In order to compare the results of the proposed methods and the results of the methods that use lumped circuit element model with the experimental results, we have constructed two handmade birdcage coils and made measurements for different capacitance values. Then, we have compared the measured resonant modes with the calculated resonant modes; used capacitance values with the calculated capacitance values. For the worst case (in which the frequency is the highest), proposed FEM based eigenfrequency analysis method calculates the resonant modes with a maximum of 10% error; proposed FEM based optimization method calculates the necessary capacitance values with 20-25% error. Methods which use lumped circuit element model, on the other hand, calculate the resonant modes and capacitance values with 50-55% error for the worst case.
Open Access
Imitation of radiofrequency ablation with fiber delivered laser system for magnetic resonance guided treatment of atrial fibrillation
(2010) Kerse, M. Can
Atrial Fibrillation (AF) is among the most common cardiac arrhythmias with a high risk of mortality and morbidity. As a cure several minimally invasive catheter approaches are performed under imaging guidance. These treatments imitate linear and transmural cuts and sutures along the atrial walls similar to the widely accepted surgical Cox Maze procedure to block undesired currents. Catheter delivery of RF energy to the cardiac chamber is widely used and approved as safe and successful. The operation is commonly performed under X-Ray which is deprived of soft tissue contrast. Besides, combination of the image with ECG (electrocardiogram) data makes the operation technically difficult and time consuming. Due to the long exposure times, X-Ray burns may be seen on the patient. MR images can be taken during RF ablation with proper matching and tuning circuits, however, during the operation RF and ECG catheters may cause artifacts in the image for some orientations. On the other hand, fiber delivery of laser energy has no significant MR compatibility issues and can be used under MR guidance. Nevertheless, MR guided laser ablation is not in clinical practice as a minimally invasive technique for curing AF possibly because of the risk of perforating the myocardial wall. Excess light intensity at the end of the fiber tip causes rapid changes in the temperature gradients which may cause charring. This is an undesired effect and especially in cardiac ablations, light intensity should be diffused. There are several diffusing tip designs to emit light in cylindrical symmetry, but, due to their orientation with respect to the cardiac chamber, common RF delivery methods cannot be applied directly. In this thesis, we propose a novel multiple fiber laser energy delivery with catheter approach and a system that imitates the scars created with RF probes under MR guidance. The system closely imitates the ablation pattern of RF delivery and therefore is expected to have quick adaptation by physicians. As a proof of principle, we used 3 fibers oriented in different directions and obtained real time MR thermometry maps of the ex-vivo and in-vitro ablation zones during laser delivery. In addition, various light diffusion methods are considered for single fiber power delivery. We believe the combination of these methods will be the solution for the MR compatible RF laser ablation system.
Open Access
A method of decoupling of radio frequency coils in magnetic resonance imaging : application to MRI with ultra short echo time concurrent excitation and acquisition
(2013) Özen, Ali Çağlar
In this thesis, it was both experimentally and theoretically shown that decoupling of transmit and receive coils can be achieved by using a transmit array system such that individual currents induced from transmit coils will cancel each other resulting in a significantly reduced coupling. A novel method for decoupling of radio frequency (RF) coils was developed and implemented in a transmit array system with multiple transmit coil elements driven by RF current sources of different amplitude and phase. It was shown that this method for decoupling provides isolation over 70dB between transmit and receive coils. Decoupling procedure was described and its performance was analyzed in terms of obtained isolation. It was shown that MR signal can be detected during RF excitation with the achieved amount of decoupling. NMR spectroscopy and MRI with concurrent excitation and acquisition (CEA) was implemented. As an alternative to existing CEA methods, this method reduces dynamic range requirements so that CEA sequences can be applied in standard MRI scanners with minimal hardware modification. It was also demonstrated that this method can be used to implement ultra-short echo time (UTE) imaging with shorter acquisition delay. For CEA approach, acquired raw data was formulated as convolution of the free induction decay (FID) signal and the input B1 field. First proof of concept images were reconstructed from nonuniformly sampled k space data using both UTE and CEA sequences. UTE and CEA were shown to be feasible to implement using the same custom made decoupling setup in a clinical 3T MRI scanner. Significance of imaging of samples with ultra short T2* values was discussed.
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.