Browsing by Subject "Mutual Coupling"
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Item Open Access Design and applications of A Z-gradient array in magnetic resonance imaging(Bilkent University, 2019-01) Ertan, Niyazi KorayArray of gradient coils driven by independent power amplifiers can generate gradient fields with dynamically changing gradient field profiles. Nine channel prototype z-gradient coil array with a diameter of 25 cm is designed and manufactured. Previously designed gradient power amplifiers with maximum voltage of 50 V and maximum current of 20 A are used to independently drive the coils. Mutual coupling between gradient coils are investigated to maintain high time fidelity in the gradient waveform. A first-order circuit model including the mutual couplings is provided to analytically calculate the input voltages and minimum achievable rise times for a given set of gradient array currents and amplifier limitations. Mutual impedance of the system is measured which is in a good agreement with the first order circuit model inside the operating bandwidth of the amplifiers (<10kHz). An example z-gradient profile is optimized and used in Magnetic Resonance Imaging (MRI) phantom experiment as a readout gradient. After validating the proper functioning of the hardware with current measurements and MRI experiments, advantages of dynamically arrangeable field profiles generated by z-gradient array are investigated. Firstly, linear gradient in variable Volume of Interests (VOIs) with variable linearity errors are optimized with four different performance parameters such as maximization of gradient strength for unit amplifier current limits, maximization of slew rate for unit amplifier voltage limits, minimization of current norm and peak vector B-field for a unit gradient strength. Decreasing the size of the gradient VOI and allowing more linearity error increases all performance parameters more than five times among the sweep ranges. The advantage of dynamic field optimization is demonstrated in Diffusion Weighted Imaging (DWI). Maximization of gradient fields only inside the slice volume rather than entire coil volume results in 4 times higher gradient strength which decreases the diffusion encoding gradient duration 3 times and halves the echo time. Increased signal to noise ratio (SNR) of the diffusion weighted images results in better estimate for the apparent diffusion coe cient (ADC) values inside the phantom. Secondly, gradient array is also capable of generating nonlinear gradient field distributions. In addition to many applications of nonlinear gradients in MRI, two novel applications based on nonlinear gradients are proposed. In the first application, nonlinear gradients are used to encode multiple slice locations to the same frequency. Excitation of multiple slices is achieved with a single band radio frequency (RF) pulse in contrast to multi-band RF pulses with higher specific absorption rate (SAR) and peak power. Two different field design method is presented and both of them are analyzed in terms of slice thickness error, center location variation, gradient strength per unit norm current, power dissipation. Two and three slices are excited with a single band RF pulse in phantom experiments. In the second application, a single channel nonlinear gradient field is simultaneously used with linear gradients during spoke excitation to mitigate the B+ 1 inhomogeneity. Excitation k-space with increased dimension are introduced for simultaneous use of linear and nonlinear gradients by including independent k-space variables for nonlinear gradient channel. Simulations are performed for 1D, 2D, RF power limited and RF power unlimited cases to demonstrate the enhanced B+ 1 homogeneity for simultaneous use of linear and nonlinear gradients compared to using only linear or only nonlinear gradients. Proposed method results in 2.3 times more decrease in the excitation inhomogeneity compared to using only linear gradients in MRI experiments.Item Open Access Efficient analysis of input impedance and mutual coupling of microstrip antennas mounted on large coated cylinders(IEEE, 2003-04) Ertürk, V. B.; Rojas, R. G.An efficient and accurate hybrid method, based on the combination of the method of moments (MoM) with a special Green’s function in the space domain is presented to analyze antennas and array elements conformal to electrically large material coated circular cylinders. The efficiency and accuracy of the method depend strongly on the computation of the Green’s function, which is the kernel of the integral equation that is solved via MoM for the unknown equivalent currents representing only the antenna elements. Three types of space-domain Green’s function representations are used, each accurate and computationally efficient in a given region of space. Consequently, a computationally optimized analysis tool for conformal microstrip antennas is obtained. Input impedance of various microstrip antennas and mutual coupling between two identical antennas are calculated and compared with published results to assess the accuracy of this hybrid method.Item Open Access Mutual coupling reduction in microstrip antennas using defected ground structures(Bilkent University, 2012) Yayan, S. MelikşahMutual coupling between microstrip antenna elements (through space and surface waves) has a significant role in the performance merits of the microstrip antenna arrays. In many applications, low mutual coupling levels are desired such as bistatic radar systems where isolation is essential in order not to have any interference between the transmitter and receiver antennas. Furthermore, presence of mutual coupling among the antenna elements can affect the sidelobe levels, beam position and frequency bandwidth of arrays. Mutual coupling among the array elements usually occurs as a result of surface waves and space waves. Mutual coupling through the space waves are very strong if the array elements are very close to each other. However, they die out quickly as the separation between the array elements become larger. On the other hand, although the mutual coupling due to the surface waves are weaker than that of space waves when the array elements are close to each other, they remain as the only coupling mechanism when they are far away from each other, in particular for arrays of microstrip antennas. In this thesis, the main goal is to reduce the mutual coupling between the microstrip antennas resulting from the surface waves by using a defected ground structure (DGS). The DGS is formed by etching either a dumbbell shape or a slotted complementary split ring resonator (SCSRR) to the part of the ground plane that remains between the microstrip antennas along their E-plane direction. It has been observed that although a considerable reduction in the mutual coupling can be achieved, the radiation patterns of the antennas are deteriorated due to a significant increase in the backlobe radiation. Hence, a reflector and a cavity combination is used to decrease the backlobe radiation to a certain level. Finally, to test the DGS in an array environment, the performance merits of a 2×2 microstrip antenna array is investigated in the presence of a dumbbell DGS, where each microstrip is backed with a cavity and a reflector. Based on both the simulations and the measurements, it has been concluded that despite the achieved mutual coupling reduction between the microstrip antennas in the array environment, the far-zone radiation patterns related merits have not been improved.