Janice needs to have a gastro-intestinal study. So she is given 375 mg of radiocontrast dye containing Barium-142. The half-life of Ba-142 is 10.6 minutes and its decay is followed by -ray. After one half -life, of Ba-142 remains . After 31.8 minutes , of Ba-142 remains . After half-lives, 11.7 mg of Ba - 142 remains . After minutes , 0.366 mg of Ba - 142 remains. Finally, after approximately 3.5 hours - half-lives , only 0.000358 mg remains . The dye is no longer visible to the Xray and Janice's study is over .