An optimal analgesic therapy is very important for postoperative recovery. In recent years, several studies showed that the prevalence of the moderate to severe pain after craniotomy ranged from 69 to 87% of patients. In a previous study, the investigators showed that the use of morphine based patient controlled analgesia prevented moderate to severe postoperative pain in patients undergoing supratentorial craniotomy. Morphine related side effects such as sedation, miosis, respiratory depression, nausea and vomiting produce a general reluctance for their use in neurosurgery. Therefore, all patients were closely observed to detect opioid related side effects in the intensive care unit for 24 hours following surgery in the previous study. The Integrated Pulmonary Index (IPI) is a new tool that calculates respiratory and hemodynamic parameters noninvasively. In the present study the investigators will use different doses of morphine based PCA and the IPI system to determine more effective and safer morphine dose for postoperative analgesia following supratentorial craniotomy.