Comparison of a Current Source and a Voltage source in Transmit SENSE

H. Nam, S. M. Wright Electrical Engineering, Texas A&M University, College Station, TX, United States INTRODUCTION: An RF power amplifier feeds power into a RF coil through matching networks and generates a transverse magnetic B1 field. In the conventional voltage source configuration, the matching networks ensure maximum power transfer between the RF amplifier and the coil. Assuming the coil impedance is constant, the current delivered to the coil is directly proportional to the applied voltage. However, as the matching networks are designed for specific impedance transformation ratios, any change in the coil impedance changes the current delivered. In the presence of other transmit array elements, the impedance presented to the matching network becomes a function of the current amplitude and phase on each of those elements [1, 2]. Therefore, in order to be able to exercise accurate control over the amplitudes and phases of currents on each array element, it may be necessary to have an accurate knowledge of the mutual coupling, and take this into account in designing the RF pulse. This makes independent RF waveform transmission complicated. Alternatively, the use RF current sources driving non-resonant coils [3] has been shown to mitigate the effect of mutual coupling, and could prove beneficial in the design and application of RF pulses for transmit SENSE. Experiments show that a coil driven by the RF current source is much less sensitive to loading and an inter-element coupling than a conventional 50 ohm matched coil fed by a standard voltage source mode amplifier [4]. It makes independent current control on each array element of transmit coils possible thus gives the ability to create desired transmit field patterns in the presence of other coil [5]. In this paper we compare the current source with the conventional voltage amplifier and demonstrate the improved linearity of the voltage-to-current transformation. This could have significant advantage when using multiple transmit coils for Transmit SENSE.