Fast-Scan NMR Imaging

Abstract

This paper describes the Fast Recovery (FR) method for fast-scan Nuclear Magnetic Resonance imaging. The FR method uses a sequence of four radio frequency pulses-alternating selective 90° nutation pulses and nonselective 180° pulses. One free induction decay (FID) signal and one echo signal are detected and averaged to compute a 2-D image. In the modified FR method, extra 180° pulses are applied between 90° pulses to cause refocusing and the resultant spin echo signals are averaged to improve the signal to noise ratio. For the FR and modified FR sequences, the macroscopic magnetization is restored to equilibrium quickly and exactly; scan time can consequently be less than that for conventional pulse sequences, such as used in the saturation recovery method, without any penalty in signal to noise ratio.
This paper derives expressions for the signal to noise ratio, scan time ratio and contrast noise ratio, compares the FR and modified FR methods with the saturation recovery method and presents experimental results for human body images.
In theory and practice, the signal to noise ratio for the FR method is larger than that for the modified FR method. For a given signal to noise ratio the scan time is between one half and one fourth that for the saturation recovery method. The optimum repetition period, T_r, is 0.07∼0.25s for the FR method, and 0.1∼0.5s for the modified FR method. Contrast noise ratio is low for high speed imaging, T_r=0.07∼0.25s, but, high contrast noise ratio image is obtained for T_r>0.5s.