Browsing by Subject "Specific absorption rate (SAR)"

Now showing 1 - 3 of 3

Open Access
FEM Based design and simulation tool for MRI birdcage coils Including eigenfrequency analysis
(2012) Gürler, Necip; İder, Yusuf Ziya
Radio Frequency (RF) birdcage coils are widely used in Magnetic Resonance Imaging (MRI). Before the actual construction of the coil, not only calculating the capacitance value, which is necessary for the coil to resonate at the desired frequency, but also modeling the birdcage coil in a 3D simulation environment and making electromagnetic analysis in the volume of interest have importance in terms of observing the resonance behavior of the coil, electromagnetic field distributions inside the coil, or specific absorption rate (SAR) distribution for any object. For this purpose, we have made two different electromagnetic analyses of low-pass and high-pass birdcage coils in COMSOL Multiphysics. These are frequency domain analysis and eigenfrequency analysis. We have also developed a software tool using MATLAB® to make these electromagnetic analyses easily and according to the userspecified parameters. Simulation results for these analyses are provided. For the experimental results, we have constructed a hand-made lowpass birdcage coil and measured the resonant modes of the coil for different capacitance values. Our eigenfrequency analysis tool calculates the resonant modes with high accuracy
Open Access
Innovative designs of RF transmit array coils and RF heating analysis of patients with implanted DBS
(2020-09) Kazemivalipour, Ehsan
A safe and efficient magnetic resonance imaging (MRI) test would rely on informed specification, design, implementation, assessment, and application of appropriately selected radiofrequency (RF) coils. Towards these goals, this dissertation comprises three contributions to novel RF transmit array (TxArray) coil design techniques and two contributions to RF heating reduction of deep brain stimulation (DBS) implants in RF transmit coils. TxArray coils with multiple transmit elements provide the additional degrees of freedom that can be used to enhance field uniformity, accelerate acquisition time, enable RF shimming while intending to mitigate specific absorption rate (SAR) hotspots, and increase power efficiency. How a TxArray coil is designed can have a significant impact on its gain from parallel transmission technology. Thus, the first contribution of this dissertation is on the eigenmode analysis of the scattering matrix for the design of TxArray coils to obtain their efficient operation modes and achieve an efficient RF shimming in terms of power consumption. The algorithm is tested for the design of four 3T TxArray coils with 8 to 32 channels, and it is shown that it can enlarge the dimension of the excitation space by up to 50% compared with the commonly used design techniques. The next contribution is to establish a fast finetuning procedure to precisely design an imperfectly manufactured TxArray coil using its corresponding equivalent circuit model. By fitting the measured scattering parameters to a lumped circuit model, all inductances/resistances of an 8-channel 3T TxArray coil are estimated. The manufactured coil is then appropriately tuned only in a single iteration. As another contribution, a theoretical coil size optimization is introduced to minimize the magnetic coupling between non-adjacent transmit channels of a TxArray coil. By calculating all self/mutual-inductances of a 12-channel 3T TxArray coil and minimizing mutual-inductances, its sizes are determined. The finite element simulations are performed to demonstrate the feasibility of this approach. One of the safety considerations of RF transmit coils is the localized SAR amplification due to the interaction of metallic implants with the coil's electric fields. Therefore, the fourth contribution is on evaluating SAR mitigation performance at tips of patient-derived realistic DBS implants using a 3T patient-adjustable transmit coil, which uses a mechanically rotating linearly polarized birdcage resonator. The reconfigurable coil system decreases the SAR on average by 83% for unilateral leads and by 59% for bilateral leads in comparison to a conventional quadrature birdcage coil. In the final study, the local SAR amplification surrounding the tips of a large cohort of DBS implants with realistic lead trajectories in a commercially available vertical open-bore 1.2T coil and a standard horizontal closed-bore 1.5T birdcage coil is presented. On average, SAR is decreased by 31-fold in the 1.2T vertical coil compared to the 1.5T horizontal coil. Overall, this dissertation proposes innovative approaches for designing TxArray coils and heating assessment and SAR reduction of DBS patients, which mainly contribute to improving the performance of RF transmit coils in terms of power efficiency and patient safety.
Open Access
Reconfigurable MRI technology for low-SAR imaging of deep brain stimulation at 3T: application in bilateral leads, fully-implanted systems, and surgically modified lead trajectories
(Elsevier, 2019) Kazemivalipour, Ehsan; Keil, B.; Vali, A.; Rajan, S.; Elahi, B.; Atalar, Ergin; Wald, L.; Rosenow, J.; Pilitsis, J.; Golestanirad, L.
Patients with deep brain stimulation devices highly benefit from postoperative MRI exams, however MRI is not readily accessible to these patients due to safety risks associated with RF heating of the implants. Recently we introduced a patient-adjustable reconfigurable coil technology that substantially reduced local SAR at tips of single isolated DBS leads during MRI at 1.5 T in 9 realistic patient models. This contribution extends our work to higher fields by demonstrating the feasibility of scaling the technology to 3T and assessing its performance in patients with bilateral leads as well as fully implanted systems. We developed patient-derived models of bilateral DBS leads and fully implanted DBS systems from postoperative CT images of 13 patients and performed finite element simulations to calculate SAR amplification at electrode contacts during MRI with a reconfigurable rotating coil at 3T. Compared to a conventional quadrature body coil, the reconfigurable coil system reduced the SAR on average by 83% for unilateral leads and by 59% for bilateral leads. A simple surgical modification in trajectory of implanted leads was demonstrated to increase the SAR reduction efficiency of the rotating coil to >90% in a patient with a fully implanted bilateral DBS system. Thermal analysis of temperature-rise around electrode contacts during typical brain exams showed a 15-fold heating reduction using the rotating coil, generating <1C temperature rise during ∼4-min imaging with high-SAR sequences where a conventional CP coil generated >10C temperature rise in the tissue for the same flip angle.