Z-gradyan dizisinin tasarımı ve manyetik rezonans görüntülemedeki uygulamaları

Array of gradient coils driven by independent power amplifiers can generate gradient fields with dynamically changing gradient field profiles. Nine channel z-gradient coil array together with the driver system are designed and manufactured. A first order circuit model including the mutual coupling is proposed and validated in bench-top and magnetic resonance imaging (MRI) experiments. Firstly, linear gradient in variable Volume of Interests (VOIs) with variable linearity errors are optimized for four different performance parameters such as maximization of gradient strength, maximization of slew rate, minimization of current norm and peak vector B-field. Smaller VOI and/or more linearity error improves performance parameters more than five times among the sweep ranges. This advantage is demonstrated in Diffusion Weighted Imaging (DWI). Maximization of gradient fields inside the slice rather than entire coil volume results in increased signal to noise ratio (SNR) and better estimate for the apparent diffusion coefficient (ADC). Secondly, two novel applications based on nonlinear gradients are proposed. 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 without increase in specific absorption rate (SAR) and peak power. Moreover, a single channel nonlinear gradient field is simultaneously used with linear gradients during spoke excitation to mitigate the B1+ inhomogeneity. Simulations and MRI experiments validates that simultaneous use of linear and nonlinear gradients provides better B1+ homogeneity compared to using only linear gradients or only nonlinear gradients.