Several hundred hospital workers began a 24-hour strike Wednesday outside Keck Medical Center of USC. X-ray technicians, certified nursing assistants and housekeeping staff are picketing over working conditions, particularly the ability of workers to have a say in how the hospital is staffed, said John Borsos, vice president of the National Union of Healthcare Workers, which represents 640 hospital employees. The National Labor Relations Board in Los Angeles is scheduled to hear workers' grievances later this month, Borsos said. Contract negotiations between the union and the medical center, formerly known as USC University Hospital, began in the fall of 2010. Officials said in a statement that the hospital pays its unionized employees some of the highest wages in the region. "It's disappointing that this is happening," said Matt McElrath, the hospital's human resources director. Meanwhile, he said, hospital operations have not been disrupted.