| 摘要 |
In NMR tomography, the nuclear spins of a selected type present in a selected area of a body are selectively excited by applying first a selection gradient field and exciting thereafter the nuclear spins by a HF signal which initially effects a rotation of the nuclear spins situated in a selected plane by an angle alpha of less than 90 DEG , then a rotation of all selected nuclear spins by 90 DEG and finally again a rotation of the nuclear spins contained in the selected plane by an angle of 90 DEG - alpha . As a result thereof, the nuclear spins contained in the selected area have resumed at the end of this excitation their original orientation determined by a homogeneous magnetic field, while nuclear spins located outside this area lie within a plane extending almost vertically thereto. Thereafter, a measuring gradient field may be switched on without disturbing the orientation of the nuclear spins contained in the selected area. With the measuring gradient field switched on, the nuclear spins are now excited by a 90 DEG pulse or usual pulse sequences to produce an induction signal. The measuring processes are carried out in pairs, with the selection gradient field applied, using different excitation signals so that the nuclear spins contained in the selected area are alternately oppositely directed which means that the induction signals generated by them are phase-shifted by 180 DEG , whereas the nuclear spins situated outside the selected area are shifted in the same sense so that they supply identical signals, if any.
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