1Attix FH. Introduction to radiological physics and radiation dosimetry [ M] . New York, NY: Wile, 1986:20 -34.
2Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations: a guide to medi- cal management [ M ] .2nded. Madison, Wis: Medical Physics Publishing, 1997.
3Johns HE, Cunningham JR. The physics of radiology [ M ] .4th ed. Sprinfield, Ill: Thomas, 1983: 336-338.
