According to Sud, patients face injuries such as asphyxiation and cervical spine injuries should they not follow the directions.

A man who entered an M.R.I. room during a scan in Westbury, N.Y., on Wednesday was pulled into the machine by his chain necklace and was hospitalized in critical condition, the authorities said.

The necklace was described as a “large metallic chain,” according to a statement posted online by the Nassau County Police Department.