Transmit Surface Coil Array using RF Current Sources

INTRODUCTION: The transmit SENSE technique can reduce imaging time by shortening the trajectory in k-space which a multi-dimensional RF pulse follows for spatial selective excitation [1, 2]. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity compensate for the missing parts of the excitation k-space. To implement Transmit SENSE, independent transmit channels, each driven by a specific RF waveform, are required. However independent transmit channel operation is difficult to achieve in conventional arrays due to coupling between elements. S11 measurements of a 50 ohm tuned coil at 200.237 MHz without (Fig. 1-a) and with (Fig. 1-b) the other coil which is also tuned at 50 ohm are shown in Fig. 1. Note that the coil is detuned and there is a significant mode split demonstrating that the two coils are strongly coupled. This mode split makes it very difficult to control transmit channels independently when conventional 50 ohm coils are used in a transmit phased array. A novel approach to minimizing the effects of mutual inductance between coils elements is to use RF current sources driving non-resonant structures. This is an effective method for achieving independent control of the current on each element in a transmit array. Experiments show that a coil driven by the RF current sources 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. It makes independent current control on each array element of transmit phased array possible and gives the ability to create desired field patterns [3]. In this paper we demonstrate that the RF current source driving a non-resonant coil can be used for Transmit SENSE by showing the ability to produce the independent transmit field pattern in the presence of other coil.