Factors affecting the appearance  
 

TISSUE COMPOSITION

As discussed in "how images are obtained", hydrogen atoms provide the basis for MRI imaging of the body. Tissues which are abundant in hydrogen atoms, such as water and fat, are responsible for producing most of the MR signal. Likewise, tissues which are devoid of hydrogen atoms, such as cortical bone, do not give off much signal.

Tissues which emit a strong MRI signal will appear white on MRI images and are termed "high signal intensity" and those which emit a weak MRI signal appear blacker and are termed "low signal intensity".

On the right is a sagittal MRI of the cervical spine. Note the cerebrospinal fluid which has a very high water content, has a high signal intensity on this image, appearing white. In contrast, the cortical bone appears black, "low signal intensity" due to its low water content.