A numerical approach to non-circular birdcage RF coil optimization: Verification with a fourth-order coil

It is demonstrated that birdcage resonators, satisfying conditions of quadr ature operation and radiofrequency field homogeneity, can be realized in pr actice on formers of non-circular cross section described by an equation of the form \x/a\(n) + \y/b\(n) = 1 where a and b are constants and n greater than or equal to 2 is an integer. Using a ladder network analogous to that of a conventional circular birdca ge, optimization algorithms were employed to determine the elemental curren t distribution on the non-circular cylindrical surfaces. A comparison of ci rcular, elliptical, symmetric and asymmetric fourth-order (n = 4) section b irdcage current distributions is presented. A short, asymmetric fourth-orde r cage was constructed and tested experimentally at 3 T and compared with a conventional circular-section head coil. (C) 1999 Wiley-Liss, Inc.