The one-line answer
MRI is one of the safest scans in medicine, and almost every patient who thinks they cannot be scanned actually can. The safety questionnaire is not about excluding you, it is about spotting the small number of implants, fragments and situations where the magnet needs a specific protocol, a different sequence, or a preparatory check first.
This guide is written from the current 2026 UK guidance used by NHS and private radiology departments. It covers what the magnet actually does, which implants matter and which do not, the truth about tattoos and IUDs, what to disclose about retained metal fragments, and how MRI is handled in pregnancy. If you want to book an MRI scan and are unsure whether something disqualifies you, this is the piece to read first.
The two things a magnetic field actually does
An MRI scanner is a very strong magnet, typically 1.5 Tesla or 3 Tesla, which is roughly 30,000 to 60,000 times the strength of the Earth's magnetic field. Everything on the safety form comes down to two physical effects.
The first is that the magnet attracts ferromagnetic metal. Iron, some steels and some older surgical alloys are pulled towards the scanner with real force. A loose ferromagnetic object in the room becomes a projectile, and a ferromagnetic implant inside the body can twist or migrate. Titanium, most modern stainless steel, cobalt-chrome and the plastics used in most modern implants are not ferromagnetic and do not do this.
The second is that the rapidly switching magnetic field can induce small electrical currents in loops of wire. That matters for pacemaker leads, cochlear implant wiring, deep-brain stimulator electrodes, and, in a very small number of cases, for large tattoos containing iron-oxide pigment. Induced currents can warm tissue slightly, disrupt device programming or, in the wrong configuration, damage the device itself.
Every question on the MRI safety form is trying to catch one of those two effects. That is the whole model.
Pacemakers and cardiac devices
For a long time, a pacemaker was an absolute contraindication to MRI. That has not been true for well over a decade. Almost every pacemaker and implantable cardioverter defibrillator (ICD) implanted in the UK since around 2011 is MR-conditional, meaning the manufacturer has tested the device to a defined protocol and the scan is safe under that protocol. In practice this means most people with a modern pacemaker can have an MRI, including at 1.5T and increasingly at 3T.
What still needs a specialist review: pacemakers implanted before roughly 2008, abandoned leads left in place after a device change, epicardial leads sitting directly on the heart, and any pacemaker-dependent patient. In those cases the scan is usually done in a centre with cardiac physiologist support, with the device reprogrammed before and after the scan, and often with real-time ECG and pulse-oximetry monitoring in the scanner.
The correct answer, if you have any cardiac implanted device, is never to guess. Bring the device card. The department will read the model number and check the manufacturer's MR conditions before booking. Do not assume the answer is no.
Cochlear implants and neurostimulators
Many cochlear implants are now MR-conditional, but the conditions vary sharply by manufacturer and generation. Some models can be scanned at 1.5T with the external processor removed. Others require a head bandage. Some earlier models still require surgical removal of the internal magnet before a head scan, then re-implantation afterwards. This is one area where the specific model absolutely dictates what can be done, and the department will contact the implanting audiology team if needed.
Deep-brain stimulators, spinal cord stimulators, vagal nerve stimulators and sacral nerve stimulators are similar. Modern devices are almost always MR-conditional with a defined protocol, and older devices may need programming changes or, occasionally, exclusion. If you have any implanted neurostimulator, bring the device card and give the department a week's notice so they can check the conditions in advance.
Joint replacements, plates, screws, aneurysm clips
The overwhelming majority of orthopaedic implants used in the UK in the last two decades are made of titanium, cobalt-chrome or non-ferromagnetic stainless steel. That means hip replacements, knee replacements, shoulder replacements, spinal rods and pedicle screws, plates and screws for fracture fixation, and dental implants are all MR-safe. You can be scanned immediately after most modern joint replacements. Some centres wait six weeks after major spinal or cardiac surgery, more to allow tissues to settle than for any real magnetic concern.
The two orthopaedic exceptions worth knowing. Some cerebral aneurysm clips implanted before around 1995 were made of ferromagnetic stainless steel and remain a genuine contraindication. All aneurysm clips implanted in the UK since then are titanium or non-ferromagnetic MP35N, and are MR-safe. The department will always ask for the operative note before scanning anyone with an aneurysm clip. Second, retained shrapnel or old bullet fragments in or near the spine, spinal cord, eye or major vessels need imaging clearance before MRI: the concern is not the fragment moving in most soft-tissue locations, but the tiny risk of movement in critical structures.
Tattoos, retained metal fragments, IUDs, dental work
The four areas patients ask about most, and where the honest answer is often reassuring.
Tattoos. Some tattoo inks, particularly older black inks and homemade inks, contain iron oxide as a pigment. A large tattoo containing iron oxide can occasionally warm slightly or tingle during a long scan. Actual burns are very rare. Tell the radiographer, and if you have a large or dark tattoo in the area being imaged they can place a cool pack over it during the scan. Newer inks used in UK studios are almost universally non-ferromagnetic. Permanent make-up follows the same rule.
Retained metal fragments. This is the one area on the form where honesty really matters. If you have ever worked as a welder, metal grinder, machinist, or been injured by shrapnel, and you did not wear eye protection at the time, there is a small chance a metal fragment could be embedded in the orbit of the eye. In the magnet, a ferromagnetic fragment behind the eye can move and cause blindness. Any UK department will offer an orbit X-ray before your first MRI if this is your history. It is a five-minute check that rules out a rare but real problem.
IUDs. Both copper coils and hormonal coils (Mirena, Kyleena, Jaydess) are MR-safe at 1.5T and 3T. The coil does not move, does not warm meaningfully, and does not need to be removed. This is one of the most common worries and almost always unfounded.
Dental work. Modern titanium dental implants, crowns, bridges and fixed orthodontic braces are MR-safe. They can cause a small local image artefact if you are having a head or jaw scan, which the radiologist reads around. Removable retainers should be taken out before the scan. Very old amalgam fillings are non-ferromagnetic and cause no issue.
Pregnancy and MRI
MRI uses no ionising radiation. That is the single most important fact for pregnant patients: unlike CT or X-ray, an MRI scan does not expose the developing baby to radiation. UK guidance regards MRI as safe in pregnancy at any stage when there is a clinical indication, and it is routinely used in pregnancy for suspected appendicitis, placental problems, and detailed fetal imaging.
