x-ray machine前面用a还是an（x-ray设备操作需要什么证件）

前面用an，例如：

1.For the procedure itself, you stand in front of an X-ray machine specially designed for mammography. 

2.The invention relates to an x-ray machine which comprises an x-ray tube.

3.Haney said in 1988, an infant in a car seat went through an X-ray machine at the Los Angeles airport.