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要旨:
This paper describes the signal intensity in rapid FLASH NMR imaging as a function of the repetition time, the NMR relaxation times, the flip angle, and the shape of the tailored RF pulses used for slice selection. In the absence or after elimination of signal contributions from transverse coherences the theoretical treatment may be confined to a steady state of the longitudinal magnetization. It turns out that deviations from a rectangular excitation profile due to imperfect pulse shapes strongly alter both the dynamic approach to steady-state conditions and the resulting saturation behavior as expected from theoretical expressions. As a consequence the signal-to-noise and image contrast become dependent on the actual slice profile. In T1 images calculated from series of FLASH images with different flip angles or repetition times qualitative relations between tissues with different T1 values are borne out correctly, whereas the accuracy of T1 relaxation times may not be satisfactory. No restrictions are expected for 3D imaging using a spatially homogeneous RF excitation. Experiments have been carried out on phantoms and human volunteers using a Bruker 2.35 T 40 cm NMR system.
