Array Coil Types and Design Principles

Introduction Before we embark on the topic of array coils designs, the reader is referred to a historic paper by Hoult [1] that provides a detailed overview and discussion on the NMR receiver. Most of today’s RF receive coil designs are based upon the phased array coil technology proposed by Roemer et al. in 1990 [2], and its application to volume imaging was discussed by Hayes et al. in 1991 [3]. (For the details of original and general phased-array radar technology, the reader may be referred to [4, 5].) The reader is also encouraged to read a detailed review of array coil theory and application by Wright and Wald [6]. It is well known that a small surface coil yields higher SNR at the distances closer to the surfaces of the coil when compared with that of a volume coil such as birdcage coil [7]. On the other hand, a small surface coil BB1-sensitive region is (much) smaller than that of a volume coil. What is remarkable in the phased array coil approach is that the high SNR associated with a small surface coil can be achieved and maintained over an extended FOV, by using a set of RF coils, which may be comparable to that of a volume coil. This is enabled by the development of various array coil designs for imaging applications of interest and the techniques to decouple the mutual inductances and thus the crosstalk among the array coil elements. Adjacent coil elements are often partially overlapped to cancel the mutual inductance between the elements. Next nearest and other coil elements can be mutually decoupled by the use of low input impedance (typically less than 5 Ohm) preamplifier decoupling circuit (other decoupling techniques and methods are described briefly later in this paper). For detailed discussions of low noise amplifiers, the reader may refer to [8]. What the low input impedance preamplifier effectively does in conjunction with a coil matching/decoupling circuit is to eliminate the current flow in the coil element loop, thereby eliminating the magnetic fields induced in the neighboring coil elements. In designing array coils, baluns and management of cable routing are also important factors to consider, and they are discussed elsewhere in this educational course.