Till recently the way to check for ingest coins was X-Ray, not only do these introduce a further degree of human error. They also expose people to Harmful Ionising Radiation. Metal Detectors use non Ionising Radiation (no harmful effects) & are reported to be 99.8 percent accurate in detection!
You may be interested in these active petitions
The link to the field testing of Metal detectors of 17 year old and under!
The Radiological Society Of North America's clinical error findings!