Depending on the context could be "röntgenläpivalaisu" (but "läpivalaisu" is used more often), or some additional word, like "kuva" or "laite", may have to be added.

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Note added at 59 mins (2005-12-07 17:09:56 GMT)
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OK, now I realise you have a whole sentence there: "Monissa kirurgisissa toimenpiteissä tarvitaan läpivalaisua."