Since steady-state free precession (SSFP) was first described by Carr in 1958, several groups have analyzed the behavior of the magnetization and the resulting image contrasts. Major interests have been, however, the two components of SSFP signals : namely, the signals obtained just after and before the RF pulses which are known as FISP and CE-FAST, respectively. In practice, simultaneous acquisition of above two signal has become more popular since each of these signals usually exhibits different contrasts. Furthermore, by positioning the two signals exactly to the same time, and inhomogeneity map can also ne measured form the interference pattern of the two signals.
In the steady-state free precession with an applied linear gradient, the transverse magnetizations are periodically distributed. Fourier analysis of this periodic distribution leads to the understanding of many interesting phenomena in SSFP. It is found that there are many other higher order echoes in SSFP in addition to the previously known echoes.
In this thesis, a generalized equation of the transverse magnetization of SSFP are derived and the echo time independent term of the transverse magnetization are expanded with Fourier series. By this Fourier expansion, the higher order echoes including the two previously known FISP and CE-FAST are understood and explained. The theory of these higher order echoes is studied in detail, and computer simulations and experiments are preformed to confirm the validity of this analysis by using KAIS 2.0T superconducting NMR system.
For the portential application of the higher order echoes, a motion insensitive imaging technique is developed. Even if SSFP imaging technique have very high signal-to-noise ratio for the tissues with long T2, it has a disadvantage of their great sensitivity to motion. The signals from moving spins are strongly sttenuated and motion artifacts are observed because a steady state cannot be attained for those spins that move along a magnetic field gradient. To avoid this sensitivity to motion, i.e., to attain a steady state for moving spins, the time integral of the magnetic field gradients must vanish over a repetition time. This condition, however, causes interference pattern to appear in the image because all the higher order echoes are coincided. This superimposed higher order echoes are separated by using RF phase cycling technique, and motion insensitive images are obtained. In the appendix, a correction technique of the total magnetic inhomogeneity effects including the localized object induced inhomogeneities such as chemical shift and susceptibility is included and its experimental results are given.
