MODULE 3. Further laboratory and imaging examinations related to breast cancer diagnostics
3.9. Safety issues - including contraindications
A safe working environment in MRI is based on a thorough orientation for the health care staff. This comprises the risks of MRI, the methods and procedures of the workplace, emergency situations, patient history and contraindications, and safety instructions given by the manufacturer of the MRI device (Alanko et al 2015; STUK 2015).
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Significant safety issues
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- The radiographer must have knowledge about how the magnetic field affects the human body and different materials, in order to position the patient safely. If an electric loop is formed, e.g. by crossing one’s ankles, the patient can have burns. Burns may also result if the patient touches conductive materials during the scan. The bore of the MR system can contain such materials (Carlton & Adler 2013).
- The contrast agent used in MRI is gadolinium. Pure gadolinium is toxic, but it can be used as a chelated compound.
- When using a contrast agent, the patient's GFR must be checked to ensure sufficient kidney function.
- Contrast agents can cause allergic reactions and even anaphylaxis. The radiographer must interview the patient for previous allergic reactions (Lammentausta 2014; Haapio 2014).
Spesific Absorption Rate (SAR)
Specific absorption rate (SAR) is a measure of the rate at which the human body absorbs energy of a radio frequency magnetic field. In MRI examinations, the SAR has been given limits: in common use the SAR should stay under 2 W/kg over the whole body, under 3 W/kg in the head area and up to 20 W/kg locally in the limbs. The SAR refers indirectly to the rise of temperature in tissues, but there are many variables like air conditioning and drugs that affect the extent of tissue heating (Alanko et al 2015; Nyberg & Jokela 2006).
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Contraindications
Screen the patient thoroughly for any foreign objects, such as ferromagnetic stents, neurostimulators, shunts, coils, aneurysm clips, implants and pacemakers. Also, the compatibility of any medical devices (e.g. intravenous drip) must be ensured before entering the scanning room. The static magnetic field can interfere with the functioning of an implant, especially older pacemakers, or cause movement of ferromagnetic objects. The pacemaker is not a definite contraindication, as there are more and more MRI compatible pacemakers. The radiologist makes the final decision about whether the examination will be carried out or not. If a metal object is in the area of interest it can cause artefacts in the image, making the examination useless (Alanko et al 2015; Saunavaara 2014; Mann et al 2015).
Other possible contraindications include claustrophobia and first trimester pregnancy.
Other possible contraindications include claustrophobia and first trimester pregnancy.
MRI is a safe procedure in general, but there are some patients for whom it is not recommended. To make sure the procedure is safe, it’s important to find out if the patient has any contraindications before proceeding (Alanko et al 2015; Saunavaara 2014; Mann et al 2015).
The MRI-device produces an extremely strong magnetic field which can affect implants or metallic foreign bodies that a patient has. Interactions between the magnetic field and ferromagnetic objects can cause heating or torque. Electromagnetic interactions can interfere with the function of the pacemaker (Alanko et al 2015; Saunavaara 2014; Mann et al 2015).
Sometimes having an implant means that the patient cannot undergo the procedure, for example, cardiac pacemakers, metallic shrapnel in the eye and some types of catheters are absolute contraindications. At other times the patient can undergo the procedure, for example, sterilisation clips or a joint replacement do not necessarily prevent the procedure. An external insulin pump and a sensor can and should be removed before the procedure.
It all depends on the type of the device and the strength of the magnetic field. If a patient has any type of an implant, the compatibility between that implant and the MRI procedure needs to be checked beforehand (Alanko et al 2015; Saunavaara 2014).
Even if the implant does not pose a danger to the patient, it could cause artefacts in the image. An artefact is a distortion in the image that can prevent a lesion from being seen. In the worst case, the whole procedure could be useless (Carlton & Adler 2013).
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References
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- Carlton RR, Adler AM. Radiographic imaging concepts and principles. China, 2013.
- Haapio VM. Magneettivarjoaineiden aiheuttama nefrogeenisen systeemisen fibroosin riski. Käypä hoito. Suomalainen Lääkäriseura Duodecim, 2013 (http://www.kaypahoito.fi/web/kh/suositukset/suositus?id=nak08078)
- Lammentausta E. MRI-tehosteaineet – onko gadoliniumin käyttökään enää turvallista? Suomen Radiologiyhdistys. Sädeturvapäivät 2014. 68-69.
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- Nyberg H, Jokela K. Sähkömagneettiset kentät. Hämeenlinna: Karisto oy:n kirjapaino, 2006.
- Saunavaara J. Vierasesineet – Mitä saa kuvata, mitä ei? Sädeturvapäivät, 2014 (http://www.sadeturvapaivat.fi/index.php?id=688&cat_ids=x93x#cat93)
- STUK. Magneettitutkimus. http://www.stuk.fi/aiheet/sateily-terveydenhuollossa/magneettitutkimus, 2016.
- Tunninen V, Ryymin P, Kauppinen T. Magneettikuvauksen riskit ja vasta-aiheet. http://www.fimea.fi/documents/160140/753095/16686_5_2008TABU.pdf, 2008.