Why precautions should be taken while using MRI machines

Krishna: Magnetic resonance imaging (MRI) is one of the non-invasive imaging techniques that have superior soft tissue contrasts and potential physiological and functional applications.
MRI uses a very powerful magnetic field, rapidly changing magnetic fields, radio waves, and a computer to obtain detailed images. However, MRI is not without risks.

MRI machine (Image Source: Shutterstock)

There are three major magnetic fields in an MRI scanner that have potential safety risks:

1. The strong static magnetic field (B0) of MRI scanners can attract and accelerate ferromagnetic objects toward the center of the machine and turn them into dangerous projectiles. This magnetic field can also displace implants or affect the function of devices such as pacemakers and pumps.
2. The radiofrequency (RF) field that is created by RF-coils can potentially cause tissue heating, especially in the presence of implants. Even non-ferromagnetic implants can cause heating due to eddy currents that propagate in metals exposed to oscillating magnetic fields(1,2)
3. The Time-varying fast-switching gradient magnetic field function is a spatial encoding of the MRI signal and can stimulate muscles or peripheral nerves and induce implant heating. They also produce noise in the MRI scanner space, which can reach levels of 100 dB or more and damage the hearing system. Using hearing protection such as Headphones and earplugs are essential for patients and anybody present in the room during an MRI examination.[3]

These magnetic fields in MRI scanners can cause five dangerous interactions in patients with metallic foreign bodies: projectile effect, twisting, burning, artifacts, and device malfunction (interference with a pacemaker).[4] Therefore, all patients need to thoroughly be screened individually for foreign bodies before undergoing an MRI scan.
Radiologists, referring physicians and MR technologists, need to be able to assess MRI safety, patients' condition, and compatibility of medical devices to keep patients safe. Therefore, they check for these things (5):

The cardiac implantable electronic device (CIED) such as pacemakers, implantable cardioverter defibrillators (ICDs) and cardiac resynchronization therapy (CRT) devices: Patients with CIED are at risk for inappropriate device therapy, device heating/movement, and arrhythmia during MRI. These patients must be scheduled in a CIED blocked slot or scheduled with electrophysiology nurse or technician support. But nowadays MRI conditional cardiac implantable electronic devices are widely available.
Metallic intraocular foreign bodies: The patient should be asked if he/she has ever welded without eye protection or had any facial injury with metal; if yes, an orbit x-ray must be taken and reviewed by the radiologist for approval before the MRI.
Implantable neurostimulation systems
Cochlear implants/ear implant: BAHA cochlear implant type can be scanned on a 1.5-tesla scanner only after the patient removes the battery. Cochlear implant wrapping scheduling must take place before the patient's MRI appointment.
Drug infusion pumps (insulin delivery, analgesic drugs, or chemotherapy pumps): If possible, the patient has to remove the device.
Catheters with metallic components (Swan-Ganz catheter), metallic fragments such as bullets, shotgun pellets, and metal shrapnel, cerebral artery aneurysm clips, magnetic dental implants, tissue expander, artificial limb, hearing aid, and piercing.

Patients will also be asked about these things (5):

Coronary and peripheral artery stents
Programmable shunts: patients must understand that they have to reprogram their shunt with their provider after the scan.
Airway stents or tracheostomy: If the tracheostomy is plastic, continue with an MRI scan. If it is unknown or unsafe, have to swap out for a plastic trach before the patient enters the MRI scanner.
Intrauterine device (IUD): Make and model merit investigation. Unknown IUDs are acceptable with 1.5 teslas only.
Ocular prosthesis
Stapes implants
Surgical clips or wire sutures
Penile prosthesis
Joint replacement or prosthesis
Inferior vena cava (IVC) filter: If make and model of filter unknown, the patient can be scanned on a 1.5 tesla after six weeks of being implanted.
Harrington rods: These patients are scannable on 1.5-tesla scanners.
Medication patch: The patches require removal before the procedure.
Tattoos: If the tattoo is in the area of interest and is less than six weeks old, reschedule the patient. Ice packs or padding should be used against any tattoo that is in contact with the bore of the scanner or the MRI coil. Patients must be encouraged to squeeze the communication ball if they feel any warmth around their tattoos.
Colonoscopy procedure in the last eight weeks: Obtain confirmation whether any endoscopy clips or pill cam were used or placed during the procedure with the patient or through the patient's medical records. If existence is confirmed or unknown, there is a need for a consultation with the radiology or referring physician.

Patients who are unable to be still or obey breathing instructions in the scanner need special attention. Some patients in pain might move during the procedure, which degrades the quality of the images, restricts the interpretation, and decreases the accuracy of the report. Some MRI sequences need to be obtained while patients hold their breath and lie motionless.
Patients with high body mass index (BMI) might have difficulty fitting into the narrow bore of the MRI machine. MRI scanners with more substantial space can scan these patients (5).

MRI contrast agents are gadolinium chelates with different stability, viscosity, and osmolality. Gadolinium is a relatively very safe contrast; however, it rarely might cause allergic reactions in patients. Following patients need to be evaluated carefully before injection of gadolinium for MRI procedure (5):

Patients on dialysis
Patients with a history of renal disease (renal transplant, single kidney, renal cancer)
Patients who had received a dose of contrast in the last 24 hours
Patients who had a previous allergic or anaphylactic reaction to gadolinium
Patients who have risk factors for nephrogenic systemic fibrosis (NSF)

Patients with impaired renal function are at risk of NSF associated with gadolinium chelate. Patients with known or at risk of renal impairment need to require evaluation regarding their renal function before an MRI scan.

Patients with diabetes mellitus or hypertension who are receiving treatment with medications; calculate these patients' estimated glomerular filtration rate, and if less than 35 mL/min/1.73 m2, there is a need to consult a radiologist for further instructions.
Patients with eGFR below 30 mL/min/1.73 m2.
Patients who are pregnant: The administration of contrast during pregnancy is predicated on a risk-benefit assessment.

MRI image ( Source: Wikipedia)

On the whole MRI's are relatively safe if you take care of these things.

The strong magnetic field of an MRI scanner can affect medical implants that contain metal or magnets.

Some contrast agents might cause some problems in some people.

Some people are afraid of the sounds coming out of the machine. Technicians have to reassure them.

I read this story reported by news papers some time back:

A few days back it was reported in the media here that an illiterate relative of a patient was asked to carry an oxygen cylinder along with the patient into an MRI chamber by an illiterate ward boy. This resulted in the death of the relative as the iron oxygen cylinder he was carrying was sucked into the MRI machine with such speed that hit him on the head so severely that he was killed instantly. A scientific literate would have definitely avoided such a mishap!

Despite all these precautions sometimes mishaps like these do happen.

It is not about living tissue. It is about metallic/ magnetic implants and devices that can cause some problems. They get heated and can burn the living tissue. People without any metallic implants can safely go for MRI scans.

Footnotes:
1. Shellock FG, Crues JV. Corneal temperature changes induced by high-field-strength MR imaging with a head coil. Radiology. 1988 Jun;167(3):809-11. [PubMed]
2. Shellock FG, Crues JV. Temperature changes caused by MR imaging of the brain with a head coil. AJNR Am J Neuroradiol. 1988 Mar-Apr;9(2):287-91. [PMC free article] [PubMed]
3. Sammet S. Magnetic resonance safety. Abdom Radiol (NY). 2016 Mar;41(3):444-51. [PMC free article] [PubMed]
4. Stecco A, Saponaro A, Carriero A. Patient safety issues in magnetic resonance imaging: state of the art. Radiol Med. 2007 Jun;112(4):491-508. [PubMed]
5.https://www.ncbi.nlm.nih.gov/books/NBK551669/#:~:text=The%20strong%....