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Explanation: In fact that's the correct terminology, as you could see in the first reference below (only appears the term "radioopaco" as an adjetive in the fith sentece of the paragraph titled "Conservar estructuras").
I think that you are asking as well for the term BALLOON, the accepted term in the medical circles is BALÓN (check the reference below that will give you a context, the given context is for heart operations though the term is generally translated as BALÓN for other medical specialities in the context of surgery and catherer).
I would translate the sentece as: "Dos marcadores radioopacos localizados por debajo del balón..."
I hope this will help you.