Browsing by Author "Ertan, Koray"
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Item Open Access Detection of MR signal during RF excitation using a transmit array system(ISMRM, 2012-05) Özen, Ali Çağlar; Ertan, Koray; Atalar, ErginMost of conventional MRI applications are based on a time interleaved approach for RF excitation and signal detection. The idea of concurrent RF excitation and reception was implemented in a few recent studies using sideband excitation [1, 2] and swift [3] techniques. In sideband excitation technique, the necessary decoupling between MR signal and RF pulse is achieved by filtering out the excitation frequency band. In the continuous swift technique, partial decoupling is achieved using mechanical adjustment of receiver and transmitter coils. The remainder of signal was separated using a series of signal processing technique. According to our simulations (MATLAB: Mathworks, Natick, MA), voltage induced on a receiver coil due to RF excitation pulse from the transmit coils must be reduced by at least 80dB to make this voltage comparable to the voltage induced from the spins. Note that more decoupling is needed to bring coupling to the noise level depending on the required SNR. In this study, magnetic field decoupling of 75dB is achieved between transmitter and receiver. This method of transmit/receive isolation enables detection of MR signal during RF excitation which is significant for imaging species with ultra short T2 values, and measurement of spin properties during excitation.Item Open Access Driving mutually coupled gradient array coils in magnetic resonance imaging(International Society for Magnetic Resonance in Medicine, 2019) Ertan, KorayPurpose In contrast to conventional linear gradients, gradient coil arrays with arbitrary spatial dependency might experience strong mutual coupling. Although conventional gradient power amplifiers with feedback loop might compensate the effect of coupling, required voltages for the compensation are generally unknown and has to be considered beforehand to ensure that amplifier voltage limits are not exceeded. A first‐order circuit model is proposed to be used as a feedforward model which enables analytical formulas of required voltages to drive the mutually coupled gradient coil arrays. Theory and Methods 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. Previously designed 9‐channel Z‐gradient coil array and home‐built gradient amplifiers (50 V and 20 A) are used in the experiments. Three sets of currents optimized for linear Z‐gradient, second‐order Z2, and third‐order Z3 fields are used in the bench‐top experiments. The current weightings for the linear Z‐gradient are also used as the readout gradient in the 3T MRI experiments. Results Current measurements for the example magnetic field profiles with minimum rise times are demonstrated for the simultaneous use of 9‐channel gradient coils and amplifiers. MRI experiments verify that a linear Z‐gradient field with a desired time waveform can be generated using a mutually coupled array coils. Conclusion Bench‐top and MRI experiments demonstrate the feasibility of the proposed circuit model and analytical formulas to drive the mutually coupled gradient coils.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.