I've worked in and around medical imaging for most of my career. In that time I've mostly worked around MRI applications and as a result had a number of MRI brain scans as a volunteer. You can do this with MRI quite safely as there are no known mechanisms by which the magnetic fields and radio-frequency pulses used in the machines can harm human tissue. In technical terms, MRI doesn't use ionizing radiation. Other common medical imaging techniques like X-ray, CT and PET do use ionizing radiation and there is a risk-benefit aspect to their use. Most people are very wary of "radiation" and tend to regard any exposure as far riskier than it actually is. This is probably due to associations with the nuclear arms race and accidents at places like Windscale, Three-Mile Island, Chernobyl and Fukushima - which isn't to say these weren't dangerous and highly undesirable events.
