SSFP signal with finite RF pulses

Abstract

The theoretical description of steady state free precession (SSFP) sequences is generally well accepted and unquestioned, although it is based on instantaneously acting radiofrequency (RF) pulses. In practice, however, all excitation processes are finite, thereby questioning the overall validity of the common SSFP signal description for use with finite RF pulses. In this paper, finite RF pulse effects on balanced SSFP signal formation are analyzed as a function of the RF time, the pulse repetition time, the flip angle (alpha) and relaxation times (T(1,2)). The observed signal modulations from finite RF pulses (compared to infinitesimal ones) can range from only a few percent (for RF time/pulse repetition time << 1, alpha << 90 degrees, T(2)/T(1) approximately 1) to over 10% (for RF time/pulse repetition time << 1, alpha approximately 90 degrees, T(2)/T(1) << 1) and may even exceed 100% in the limit of RF time/pulse repetition time --> 1 (for alpha approximately 90 degrees, T(2)/T(1) << 1). As a result, a revision of SSFP signal theory is indicated not only for reasons of completeness but also seems advisable, e.g., for all quantitative SSFP methods. A simple modification for the common balanced SSFP equation is derived that provides an accurate framework for SSFP signal description over a wide variety of practical and physiologic parameters.