Neither wound exploration nor radiologic studies are sensitive enough alone to rule out a WFB so the two should be used in complement.
WFBs that are radiopaque and should be visible on plain film x rays with soft tissue technique are:

•    Metal
•    Glass
•    Teeth
•    Pencil tips
•    Gravel

Be aware that these WFBs may still be missed radiographically if in bony areas such as the foot or hand or if very small.
WFBs that are non-radiopaque will not typically be visible on plain films regardless of size.  These include:

•    Wood
•    Plastic
•    Vegetative matter
•    Clothing

If an imaging study is needed in these scenarios, ultrasound is usually the best initial choice and should detect all WFB materials.