Magnetic resonance imaging, or MRI, is an important tool for medical diagnostics. It allows doctors to see parts of the body that X-rays can't image, and it can even provide 3D visualisations of body structures.
MRIs use powerful magnetic fields and radio waves to view inside the body. However, the intensity of these fields can cause serious problems for some patients.
The MRI's magnetic fields and radio waves can affect any magnetically susceptible metal (e.g., iron). Implants such as aneurysm clips may move during an MRI, while cardiac pacemakers or insulin pumps may malfunction. Foreign bodies, such as metal fragments in the eye, may shift and cause damage.
For these reasons, MRI technicians must conduct a careful evaluation of each patient, and may need to alter the field strength to ensure the safety of the exam.
Contrast agents increase the visibility of different types of tissue during an MRI, by making them appear lighter or darker. Injection of contrast agents normally occurs during the MRI.
Some patients have allergic reactions to contrast agents; in rare cases, these reactions can be life threatening. Although it's possible to conduct an MRI exam without contrast agents, doing so may reduce the exam's effectiveness.
Closed MRI machines may cause panic attacks in claustrophobic individuals, making it impossible for them to complete the exam. The loud noises produced by both open and closed MRIs can also cause autistic individuals to panic. Sedation can relax these patients, and allow them to proceed with the exam.
A closed MRI may be too narrow to fit grossly obese patients. In such cases, doctors must choose an alternative procedure.
MRI is not always the diagnostic tool of choice to image particular tissues. For instance, Doppler ultrasound is equal or superior to an MRI for injuries or diseases of the tendons. Ultrasounds are also preferable for the imaging of foetuses during pregnancy.