Measurement depth of planar surface coil and adjustment of excitation angle at the NMR measurement

The measurement technique using a planar surface coil has advantages of acquiring a local Nuclear Magnetic Resonance (NMR) signal in short time from near the coil. The detection area of the coil depends on the inside diameter of the coil and the number of spin excitations. In this paper, the detection area and measurement depth of the coil in the case of measuring Free Induction decay (FID) signal and spin-echo signal were analyzed theoretically. In acquiring FID signal, the measurement depth of the coil is 0.38 time the inside diameter of the coil. On the other hand, in acquiring spin echo signal, the measurement depth decreases by 0.32 time the inside diameter of the coil. In order to verify the theoretical result, the surface coil with an inside diameter of 0.6 mm was used, and the ratio of the intensity of FID signal and spin-echo signal was compared. The theoretical result agrees with the experimental result. Furthermore, the method to adjust the excitation angle of magnetization vector on the surface of a sample to 90 degress is described in this paper.