Magnetic resonance imagining (MRI) is a technique that uses a strong magnetic field that is not harmful (see module MRI Technique). MRI is based on influencing protons (hydrogen atoms) with a strong magnetic field. MRI provides an image with good contrast between soft tissues, making it possible to characterize soft tissue pathologies. In addition, contrast (non-iodinated) can be given to obtain further information; this is particularly used for the detection and characterization of tumors, metastases, infections, and the imaging of vessels/vascular pathologies (= MR angiography). See e.g. figure 12.
With MRI you can view organs in multiple planes and axis and unlike CT, it does not use ionizing radiation. Air contains few protons and therefore emits no signal. Air appears as black on MRIs (for example think of the lungs). CT or conventional investigations are therefore more suitable for imaging the lungs.
A disadvantage of MRI is that it takes on average between 20 to 45 minutes to acquire the images. In this time, it is only possible to image a small area of the body (e.g. only the head or focused on an upper abdominal organ). Further, it is expensive and less available than CT scans. MRI is not/less suitable for patients with claustrophobia, patients that cannot lay still, patients with certain foreign bodies (e.g. certain pacemakers, vascular clips in the brain or metal splinters in the eyes). Heavily obese patients do not fit in the narrow tunnel. Implants such as prostheses are not contraindicated, however they can give strong artifacts locally due to the disruption of the magnetic field.
Examples of MRI indications:
Figure 11 Overview of common MRI indications. HNP = Herniated nucleus pulposus MRCP = Magnetic Resonance Cholangiopancreatography, IBD = Inflammatory Bowel Disease.
Figure 12. Abdominal MRI of a patient with Crohn’s disease in a coronal section. There is wall thickening of the terminal ileum, which shows (on the T1 sequence after administration of gadolinium) reduced diffusion; compatible with terminal ileitis. See module MRI Technique for more information on the various MRI series/sequences.