Better precision X ray tube, and wider analysis scope, long lifetime up to 10 years.
激发源X射线光管精度更高，范围更宽，使用寿命长达10年以上；
2
Conclusion: From 1997 to 2004, the comprehensive occurrence possibility and Nerve Tube Defects (NTDs) occurrence possibility appear a decrease trend year by year, moreover, the scope keep the same.