RF current element design for independent control of current amplitude and phase in transmit phased arrays

Both the optimization of B1 field homogeneity in high-field MRI and the implementation of the exciting new theory of transmit SENSE can be accomplished by individual and independent control of the amplitude and phase of current on the elements or rungs of a radio frequency (RF) transmit coil array. One way of achieving this is by using the concept of the active rung, which consists of one rung of a volume coil connected across the output terminals of an RF power MOSFET. The RF power MOSFET is used as a voltage-controlled current source and drives RF current through the rung. The amplitude and phase of the RF current are controlled by the amplitude and phase of the RF control voltage. In this article, we demonstrate that the active rung may be used as a current element by tuning the rung to series resonance. We then demonstrate, using field measurements, that the current induced by adjacent rungs is suppressed by greater than 15 dB in a current element relative to a resonant loop representing a single element of the well known TEM coil. Thus, the active rung configuration enables significantly greater isolation and independence between rungs than in conventional designs where each rung is fed by a voltage source. Measurements and theory demonstrate a dynamic range of independent control of the rung current amplitude of 17 dB. We conclude that the degree of suppression is dependent on the size of the MOSFET output parasitic capacitance. The active rung should find application as a building block in the construction of parallel transmit coils. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 29B: 75–83, 2006