According to recent studies, patients following craniotomy suffer more than minimal pain; two-thirds of patients experienced moderate to severe pain. Postoperative pain most often occurs within 48 hours after surgery. Local infiltration of anesthesia is the most simple and effective analgesia. However, the analgesic effect only lasts for a short-time after surgery, and it cannot adequately meet the needs of postoperative analgesia after craniotomy. Several studies have shown that the mixture of dexamethasone with local anesthetics could reduce the postoperative pain scores better than local anesthetics alone. Lipid microsphere is a relatively new drug delivery system. It is an artificial lipid emulsion. Studies have shown that dexamethasone lipid microsphere, the dexamethasone palmitate emulsion (D-PAL emulsion), has stronger anti-inflammatory effect than dexamethasone. Therefore, the investigators hypothesize that the pre-emptive scalp infiltration with dexamethasone lipid microsphere plus ropivacaine could achieve superior postoperative pain-relief compared to ropivacaine alone for patients undergoing craniotomy