Magnetic imaging (MRI) is a medical imaging method for obtaining accurate cross-sectional images of the human body.
Magnetic imaging is particularly well suited for examining the central nervous system, musculoskeletal system and the abdomen. Blood vessels can also be imaged. As magnetic imaging does not use ionising radiation, it does not involve radiation exposure as radiographic imaging does.
Instead of X rays, magnetic imaging uses three different types of magnetic fields to form the image:
- strong static magnetic field
- slowly changing magnetic fields, or gradients, and
- radio-frequency magnetic field.
Magnetic imaging is a relatively new examination method; the first imaging device was deployed in Finland in 1984. In 2018, there were 150 devices in use, used for over 400,000 imaging studies. The number of MRI devices has increased by a few each year. The strength of the static magnetic field of the devices has increased as old devices of under 1.5 Tesla are mainly replaced by 3-Tesla devices. Magnetic imaging is also known as magnetic resonance imaging (MRI).
The patient is exposed to strong magnetic fields
During magnetic imaging, the patient is exposed to all three magnetic fields required to form the image, with different effects on the body:
The static magnetic field can disturb the functioning of implanted electronic devices. Therefore, magnetic imaging is not used for people with pacemakers, defibrillators or infusion pumps, for example. In addition, rapid movements in the strong static field can result in such high fields in the head that the person being imaged feels dizzy.
Slowly moving gradient fields can cause slight muscle tremor or tingling. These sensations are temporary and harmless, and in practice they occur only rarely.
The radio-frequency field transmits energy to the patient’s body, which might cause a slight increase in the temperature of tissues. This excess heat only rarely affects the normal function of the patient’s body.
All implanted metallic objects interfere with the quality of the magnetic image and may cause heating up of the surrounding tissue or start moving. Therefore, it is very important for the patient to tell the hospital staff before magnetic imaging if they have metallic implants, artificial joints, shrapnel, metal working residues, etc., in their body. In such cases, the safety of imaging has to be evaluated on a case-by-case basis.
Currently used implants are usually safe with regard to magnetic imaging. Problems are mainly caused by old implants and foreign objects in the body. The certificates supplied with implants specify whether magnetic imaging is possible with the implant.
Some tattoo inks or pigment used for permanent eyebrow and eyeliner makeup may contain metallic compounds, which heat up in magnetic imaging. Their effects must be discussed with the radiologist before the imaging. Previously performed magnetic imaging does not guarantee the safety of new imaging for the patient, as the situation may change considerably due to the strength of the magnetic field, patient’s position, object of imaging and imaging sequence.
All metal objects in clothes should be removed before imaging. The worst hazardous situations and accidents have in fact taken place when a metallic object in the imaging room has been thrown to the imaging device by the magnetic field, causing bodily or material damage. The imaging site must report all hazardous and near-miss situations to Valvira.
Also personnel may be exposed to magnetic fields
The staff is mainly only exposed to the static stray field of the MRI device as the radio-frequency field and gradient fields do not extend much outside the device. The exposure of employees to magnetic fields is comprised of short periods when the patient is being positioned for the examination. Even though fields of this magnitude do not have harmful long-term effects according to current knowledge, unnecessary exposure should be avoided. This is possible by keeping the following in mind:
- Fields outside the imaging room do not differ much from normal background fields.
- The radio-frequency magnetic field and gradients are active only during the imaging sequence.
- Sudden head movements near the imaging device, especially at its mouth, may cause dizziness.
- Being unnecessarily in the immediate vicinity of the device should be avoided.
In procedural radiology, such as when collecting biopsies, the doctor or nurse may be exposed to almost as high fields as the patient. Rapid movements in the static field may induce strong flows and fields in the head, causing dizziness and nausea.