Barium-133 has strong x-ray peaks around 31kEv and 81kEv, which are low enough energy that organic water-containing materials (like apples and bell peppers) are reasonably contrastive for imaging.
http://ejje.weblio.jp/content/Contrastive > 対照的な、対比する
含水有機物ならはっきりと画像化できる、ということのようだ。