The metal objects may have become lodged in the body as shrapnel, for instance, during the war or have been deliberately implanted by surgical means (cardiac pacemakers, artificial joints, prosthetic heart valves).
The risk is the magnetic field
The risk posed by MR imaging is due partly to the static magnetic field and partly to the effects of the rapidly changing magnetic fields and high frequency exposure.
The risk in the static magnetic field arises by virtue of the fact that the implants can be displaced and cause damage, for instance, to nerve structures or blood vessels.
A decisive factor it the composition of the alloys used for the implant.
Doctors hold conflicting views on how exposure to high frequencies and rapidly changing magnetic fields acting on metal implants can cause parts of the body to heat up.
Decisive is the conductivity of the metal
Furthermore, metal implants such as cardiac pacemakers can act as additional antennae and couple with the high frequency coil of the MR tomograph.
Turbulences then occur in the implant, possibly leading to heat generation. A decisive factor is the conductivity of the metal. The heat that is produced is conducted to the surrounding tissue so that, in a worst-case scenario, the tissue surrounding the implant may sustain burns.
Uncertainty among doctors and patients
The theoretical risk to patients with implants as outlined above during MR tomography has in many cases made doctors and patients wary, with the result that the MR scan was not performed.
No untoward effects on mechanical heart valves
The same applies to patients following heart valve replacement surgery who now have implanted mechanical heart valves and metallic wires around their breastbone. Extensive experiments have shown that patients do not suffer any untoward effects through warming of the prosthetic heart valve or through magnetic forces.
Several scientific publications concerning the effects of magnetic fields and high frequency radio waves on various mechanical heart valves (see table) conclude that the mechanical replacement heart valves named are unproblematic and safe for the heart valve patients investigated.
In the meantime large numbers of patients throughout the world who have MR-compatible replacement heart valves have received MR scans. There are no reports to date of any serious complications.
One thing to note, however!
A doctor sending a patient for an MR scan and the MR doctor performing the scan must make sure that the implanted prosthetic heart valve is properly seated before the MR scan commences.
The magnetic force attempting to deflect the mechanical heart valve is very small compared to the forces that can be generated when the heart muscle contracts, but even the small force created by the magnetic field may the “last straw” that displaces a valve hanging on by only a single thread. This is merely a theoretical consideration.
Use echo cardiography as an exploratory scan
Patients with very poor haemodynamics (factors affecting the flow of blood within a vessel, e.g. hypertension) who are suspected of having severe valvular dysfunction should be sent for an EC scan before an MR scan is attempted.
Be careful with heart valve prostheses that were manufactured before 1969
Patients with mechanical heart valves not tested for MR-compatibility (manufactured before 1969) should not be sent for an MR scan. Care with MR scans should also be exercized in freshly operated patients whose blood vessels were clipped without the use of titanium/tantalum alloys. In addition, all patients with unspecific metal fragments in their body close to critical organs should not undergo MR tomography for safety reasons.
What is magnetic resonance tomography?
Magnetic resonance (MR) tomography is a computer-assisted imaging technique. A strong magnetic field acts on the body and electromagnetic waves emitted from the body following exposure to an outside source are recorded. These waves are converted into very accurate images and so allow the doctor to detect any early signs of tumours, etc.
Dr. med. Heinrich Koertke, Heart- and Diabetes-Center, North-Rhine Westphalia, Georgstr. 11, 32545 Bad Oeynhausen/Germany (2005)
