Browsing by Subject "Gradient array"
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Item Open Access Nonlinear droop compensation for current waveforms in MRI gradient systems(John Wiley and Sons Inc, 2022-03-28) Babaloo, Reza; Atalar, ErginPurpose: Providing accurate gradient currents is challenging due to the gradient chain nonlinearities, arising from gradient power amplifiers and power supply stages. This work introduces a new characterization approach that takes the amplifier and power supply into account, resulting in a nonlinear model that compensates for the current droop. Methods: The gradient power amplifier and power supply stage were characterized by a modified state-space averaging technique. The resulting nonlinear model was inverted and used in feedforward to control the gradient coil current. A custom-built two-channel z-gradient coil was driven by high-switching (1 MHz), low-cost amplifiers (<$200) using linear and nonlinear controllers. High-resolution (<80 ps) pulse-width-modulation signals were used to drive the amplifiers. MRI experiments were performed to validate the nonlinear controller's effectiveness. Results: The simulation results validated the functionality of the state-space averaging method in characterizing the gradient system. The performance of linear and nonlinear controllers in generating a trapezoidal current waveform was compared in simulations and experiments. The integral errors between the desired waveform and waveforms generated by linear and nonlinear controllers were 1.9% and 0.13%, respectively, confirming the capability of the nonlinear controller to compensate for the current droop. Phantom images validated the nonlinear controller's ability to correct droop-induced distortions. Conclusion: Benchtop measurements and MRI experiments demonstrated that the proposed nonlinear characterization and digitally implemented feedforward controller could drive gradient coils with droop-free current waveforms (without a feedback loop). In experiments, the nonlinear controller outperformed the linear controller by a 14-fold reduction in the integral error of a test waveform.Item Open Access Technical innovations in gradient array systems for MRI applications(Bilkent University, 2023-02) Babaloo, RezaIn Magnetic Resonance Imaging, gradient array coils have lately been employed in a variety of applications, such as field profiling. This capability of array technology can be used to minimize electric fields induced by gradient waveforms. For this purpose, a whole-body gradient array with all three gradients is being investigated. Gradient current amplitudes are optimized to produce a target magnetic field within a desired region of linearity (ROL) while minimizing induced electric fields. By reducing the diameter of ROL, generating a target gradient within a slice, and relaxing the linearity error, array coil electric fields are significantly reduced compared to a conventional coil. When a linear gradient is required in a small region, higher gradient strengths and slew rates can be achieved without exceeding peripheral nerve stimulation thresholds. Because of a high number of channels in the array design, feedback controllers significantly raise the system cost due to the expensive current sensors used for gradient current measurements. Thus, a nonlinear second-order feed-forward controller is introduced for the gradient array chain. The feed-forward controller is then modified to update the controller coefficients based on thermal behavior prediction to deal with time-varying parameters caused by temperature-dependent resistances. Gradient current measurements and MRI experiments are conducted to show the effectiveness of the proposed method. In the scope of this thesis, novel applications and hardware solutions are proposed to make array technology valuable and feasible.Item Open Access A z-gradient array for simultaneous multi-slice excitation with a single-band RF pulse(John Wiley and Sons, 2018) Ertan, Koray; Taraghinia, Soheil; Sadeghi, Alireza; Atalar, ErginPurpose: Multi-slice radiofrequency (RF) pulses have higher specific absorption rates, more peak RF power, and longer pulse durations than single-slice RF pulses. Gradient field design techniques using a z-gradient array are investigated for exciting multiple slices with a single-band RF pulse. Theory and Methods: Two different field design methods are formulated to solve for the required current values of the gradient array elements for the given slice locations. The method requirements are specified, optimization problems are formulated for the minimum current norm and an analytical solution is provided. A 9-channel z-gradient coil array driven by independent, custom-designed gradient amplifiers is used to validate the theory. Results: Performance measures such as normalized slice thickness error, gradient strength per unit norm current, power dissipation, and maximum amplitude of the magnetic field are provided for various slice locations and numbers of slices. Two and 3 slices are excited by a single-band RF pulse in simulations and phantom experiments. Conclusion: The possibility of multi-slice excitation with a single-band RF pulse using a z-gradient array is validated in simulations and phantom experiments. Magn Reson Med 80:400-412, 2018.