A good trick if you have a sample of the foreign body (such as a sliver of glass or whatever) but are unsure if it will show up on x-ray is to place it on or under a water phantom (a bag of saline works nicely, or a diposable cup full of water) and to image that. The water mimics the density of soft tissues, and you can (roughly) judge what level of contrast you'll get on resulting images, and if it's worth proceeding with the exam.

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