Eigenmode analysis for understanding phased array coils and their limits

What are the fundamental limits to phased array SNR gain and how do non-sample related loss mechanisms affect these SNR limits? In this article we use eigenmode analysis to understand the SNR behavior of phased array coils that would manifest with changes in design parameters, such as frequency, the number of receive channels and the amount of non-sample losses, such as conductor resistance or preamplifier noise. Eigenmode analysis easily explains why a 30% change in array element conductor resistance results in only a 6% drop in isocenter SNR, as might be seen with single channel coils such as the birdcage, but a surprising 25% drop in peripheral SNR should be expected. As the number of receivers available on MRI scanners is increased, arrays of smaller sized elements can be used to produce additional eigenmodes of higher-order spatial SNR distribution. The SNR gain associated with these new volume arrays will be more and more pushed to the periphery, such that for typical head coil filling factors, the SNR gain may be outside the sample. (c) 2006 Wiley Periodicals, Inc.