Full title: A 44-year-old man went to the hospital after pus began oozing from his chest, where doctors discovered a knife that had been embedded in his body for eight years. According to the report, he showed no signs of chest pain, breathing problems, coughing, or fever, and was otherwise in good health.

Possibly because a standard magnetometer cannot detect reactive elements down to the level that would be affected by a giant insane field associated with an MRI?
A simple xray, or a pass-through of a single metal detector at a court house is one thing….
…but Imagine a tattoo with ferrous ink. In a simple airport magnetometer, it’s no thing. In an MRI it’s ripped out of flesh.
Basically tattooing itself can be a very wild scenario. I caught artists in Tunisia making do and had a friend shooting cats in Cuba as we provided ink.