Foreign body on X-ray means that the radiologist is seeing something that does not belong in the body. Foreign body is something that comes from the outside and ends up in the body. There are many ways for a foreign body or object to end up in the body.
This may be something left from a procedure or surgery, either purposely or by accident. Somebody can inhale or swallow a foreign body. An example would be a child swallowing a coin. Foreign bodies can be introduced into a wound sustained from a trauma. Sometimes foreign bodies are inserted into body cavities.
Some foreign bodies will be seen on X-ray but there are some that will not. Metal, glass and stone will be visible on X-ray while something like wood will not. Wood, most commonly in the form of splinters can be detected with ultrasound.
I see foreign bodies most commonly around wounds in the hands and feet. These must be removed before infection or injury to nearby structures occurs. Often, an X-ray will be obtained to detect foreign bodies around wounds.
Inhaled foreign bodies most commonly settle in the right main bronchus or airway to the right lung. They can cause the lung to lose air and collapse. Many devices are purposely left in the body after surgery or procedures. An example would be a heart valve or catheter to deliver chemotherapy.
Most common swallowed foreign body is a coin in my experience. These are easily seen on X-rays. Sharp swallowed bodies need to be removed as soon as possible to prevent injury. Foreign bodies placed into cavities such as the rectum can result in perforation or tear. This will require emergent surgery.
Many foreign bodies are therefore easily detected on X-ray, especially if they are made of metal, stone or glass. Those foreign bodies or devices that are placed purposely often have a characteristic appearance and are recognized by the radiologist. Foreign bodies left after surgery by accident such as sponges can also be easily identified.
If there is high suspicion for foreign body but the X-ray is negative, than ultrasound or a CT scan be obtained to look further. Since some foreign bodies can be life threatening, this becomes important. Some foreign bodies can not be easily detected by imaging at all.