Magnetic Resonance Imaging (MRI) is a medical imaging method used in the internal inspection of a patient with no risks of harm to the patient.  Just like computed tomography (CT), MRI constructs images of the internal structure being observed.  Unlike CT, however, MRI does not use X-rays to create the images. It employs strong magnetic fields to produce magnetization alignment changes in the nuclei of hydrogen atoms in the water of the body, which are detected and translated into an MR image on the monitor.   MRI is also referred to as Magnetic Resonance Tomography (MRT).

The first MR image was shown to the public in 1973, although it was not until 1977 that MRI was first used on human studies.  In January, 1974, the cross-sectional image of a living thing (a mouse) was published for the first time.

Magnetic resonance imaging operates on the principle that the protons in the nuclei of hydrogen atoms in the body's water molecules change their magnetization alignment when subjected to a magnetic field.  When a patient is put inside an MRI machine, a strong magnetic field causes all these protons to align with the direction of this magnetic field.  The magnetically-aligned protons are then subjected to a short pulse of radio-frequency electromagnetic field that causes them to change their magnetic alignment again. 

When the pulse is gone, the protons return to their original alignment with the first magnetic field, but not before releasing energy that can be picked up as small signals by coils positioned near the patient.  These signals, which vary from one type of tissue to another,  are collected, processed and used to reconstruct an image of the internal structure where the signals came from.

This cycle of subjecting protons to controlled magnetic fields and pulsating RF excitation is repeated many, many times under different parameters during the MRI scan to achieve the necessary contrast between the different tissues. MR images possess much better contrast between different soft tissues than CT.

    

Figure 1.  A photo of a MRI machine (left) and a MR image (right).

     

MRI can be used to image virtually any part of the body, but it is especially useful for analyzing conditions related to the brain, muscles, joints, heart, and blood vessels.  It is also useful in observing tumors and cancer since diseased tissue exhibits a different MRI contrast compared to normal tissue.

See Also:  Computed Tomography (CT);  More Industry Articles