Magnetophosphenes are induced by gradient fields at high field strength. Fast spin echo sequences use a lot of RF pulses and thus far generate the most heating. Loops of wire, ECG electrodes, metallic patches, etc. have burned patients during sequence acquisition. Always use hearing protection, especially at high field strength. As magnetic field strength increases, the resonant frequency rate increases.
Shorter exposures give the same effect at higher field strengths.
Magnetic fields are invisible. MRI can cause serious injury or death if the wrong things are allowed into the scan room. Many people have metal in their bodies. Most metal implants are safe for the MR scanner. Need complete medical history of patient for MR staff to determine safety.