My sister runs track and was complaining about her ankle paining so my parents took her to the hospital. They took a film of her ankle but couldn't find anything wrong.
We went home, but it didn't get better. She went to the hospital again and they took another film, but used something called sonography this time. They found the ankle effusion that time around.
The doctor said that they couldn't see the effusion from the first film because it was too small. Sometimes they need to use a more sensitive film to find it. So just because it wasn't seen in a regular film doesn't always mean that it's not there.