Active phased array techniques for high-field MRI

This paper describes recent developments in high field magnetic resonance imaging (MRI) concerning the application of active phased array techniques. The pulse amplitudes and phases of currents in antenna (Coil) arrays are adjusted in order to improve the homogeneity of the magnetic flux density distribution inside the inhomogeneous patient's body. This can be realized either by using the antenna elements combined with a Butler matrix which is excited by phase/amplitude steered power amplifiers or by connecting the antenna elements directly to the power amplifiers. An improved system includes a variable power combining stage for maximum utilization of installed transmit power.