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

The activity of CYP1A2 in newborns and infants is lower than in adults, which may result in slower metabolism of ropivacaine. This might necessitate a reduction in the dosage or concentration of ropivacaine to prevent drug accumulation and toxicity. Additionally, since the nervous system in children is still developing, excessively high concentrations of ropivacaine could have adverse effects. Therefore, the lowest effective concentration of ropivacaine should be chosen to minimize the risk of adverse reactions. Generally, the dosage of ropivacaine should be determined based on the patient's age, weight, and the type of surgery. However, there are currently no definitive dosage recommendations, and further research is needed to determine these guidelines.

The goal of this clinical trial is to exploring the minimum effective concentration and volume of ropivacaine for sacral plexus anesthesia. Using MRI to investigate the distribution of different doses of ropivacaine after caudal anesthesia, and employing infrared imaging technology to detect the blockade level after caudal anesthesia with varying doses.

Participants will receive 0.5ml/Kg, 1ml/Kg and 1.5ml/Kg ropivacaine, then undergo magnetic resonance imaging (MRI) examinations to detect the distribution and optic nerve sheath diameter (ONSD) before and after caudal injection. Also, the blockade level will be detected 15min and 30 min after caudal anesthesia

Inclusion Criteria:

BMI ≤ 30 kg/m²; lower limbs surgery; lower abdomen surgery; perineal region surgery; Normal liver and kidney function; Normal coagulation function; Hemoglobin > 70 g/L.

Exclusion Criteria:

Abnormal coagulation function; Allergy to local anesthetics; Abnormal sacral anatomy; Puncture site infection;