Exploring the Minimum Effective Concentration and Volume of Ropivacaine for Sacral Plexus Anesthesia

The goal of this clinical trial is to explore the minimum effective concentration and volume of ropivacaine for sacral plexus anesthesia. The main questions it aims to answer are: * The distribution of different volumes of ropivacaine after caudal anesthesia * The blockade level after caudal anesthesia with varying doses * The minimum effective concentration of ropivacaine among pediatric patients of different age groups Researchers will inject different doses or concentrations of ropivacaine into the caudal canal to observe the drug's spread range, the level of blockade, and the duration of its effect. Participants will: * Undergo magnetic resonance imaging (MRI) examinations before and after caudal anesthesia * Will be detected the blockade level after caudal anesthesia