Fast nuclear magnetic resonance imaging with transient-state magnetization and a cylindrical sampling scheme.

摘要

The Dissertation reports the recent development and application of the Rotating Ultra-Fast Imaging Sequence (RUFIS), a two-dimensional nuclear magnetic resonance imaging (MRI) method which incorporates free-induction-decay (FID) acquisition, transient-state nuclear magnetization, and a polar sampling scheme.; In MRI using FIDs, the initial portion of the signal is usually blanked or distorted due to the transmission of the radio-frequency pulse. A novel MRI pulse sequence for acquiring a stimulated spin echo (SSE) is devised and implemented; the SSE accurately recovers the initial portion of the FIDs. A simple numerical method of signal recovery is formulated which can merge the FID and the data from the SSE for higher extrapolation accuracy. The application of the signal recovery method is demonstrated using RUFIS. The result shows the importance of experimentally finding the correct time origin of the FID signal.; For MRI methods using transient-state magnetization, the image can be distorted by the spin-lattice relaxation. The spin-lattice relaxation during a pulse train and a sequence of multiple pulse trains are calculated for an arbitrary initial longitudinal magnetization. Together with experimental examination, it is found that when the magnetization is in the transient state, the effect of the spin-lattice relaxation should not be ignored and can be removed by signal averaging in an eliminative fashion.; Application of RUFIS to three-dimensional imaging is implemented by adding a Cartesian dimension in the preparation sequence ahead of RUFIS; that is, the entire sampling scheme is cylindrical. Consequently, the cosine, the sine, or the Fourier transformation will be used for the image reconstruction, depending on the selected Cartesian phase encoding scheme. Experiments on pipe flow and curve flow show that the present cylindrical method is efficient in flow velocity mapping.; A novel fast data acquisition method for spin-lattice-relaxation map (T1-map) is introduced. The method obtains multiple sample points for curve fitting in one batch rather than one sample point at a time, and the curve to fit is a decay exponential function instead of a recovery one in conventional methods. The present method is demonstrated by using RUFIS.