Factors Affecting the Appearance

A myriad of factors affect the appearance of the final MRI image, including tissue composition, machine parameters such as the timing with which the radiofrequency (RF) pulse is sent in and when the machine "listens" for the returning MRI signal, the use of intravenous contrast, and whether the structure being imaged is static or dynamic (i.e. flowing blood).

A complete discussion of this is beyond the goals of this introductory module. It is not necessary to memorize the physics or the appearance of certain tissues on MRI, however you should be familiar with the concepts and realize that the same structures might look different on different types of MRI images.

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.