How images are obtained  
 

MRI is a technically complex imaging modality, and it is beyond the intentions of this module to discuss image acquisition in great detail. The following provides a brief summary of how MRI images are obtained and viewed.

The patient lies on a table which is moved into the bore of a large, powerful magnet (most of those in clinical use are 1.5 Tesla). Protons in the patient's tissues will naturally align themselves with this powerful external magnetic field.

Radiofrequency pulses are sent into the patient, causing disruption of the alignment of the protons (the protons absorb energy and are displaced from their aligned position). Over time, the protons will naturally realign themselves with the external magnetic field. When this occurs, they emit the absorbed energy as radiofrequency waves which are detected by the MRI scanner. These emitted radiofrequency waves are the MRI signal.

The location and distribution of the emitted MRI signal is detected and analyzed by a computer, and used to produce an image. Thus, a radiofrequency pulse is sent in, the patient emits a signal back which the machine reads and uses to create an image.

What types of tissue emit the MRI signal?

Only molecules with an odd number of protons have their own magnetic field and align themselves with the external magnetic field. It is these molecules that are responsible for emitting the MRI signal which is used to create the image.

Hydrogen atoms are the most abundant of these molecules in the human body and provide the basis for MRI imaging. The body is largely composed of water, and the hydrogen atoms in water are responsible for producing much of the MRI signal.