Magnetic resonance imaging (MRI) has rapidly become an important tool in the investigation of patients with hepatobiliary disease, particularly for the characterization and staging of liver lesions seen on other imaging tests. MRI also has a role as a noninvasive means of imaging the biliary tree. (See "Magnetic resonance cholangiopancreatography".)
MRI uses a strong magnetic field to align rotating hydrogen protons within the tissue being imaged. During realignment of the protons, energy is released and sampled at different time intervals. The measured signal intensity from this energy depends upon the degree and rate of realignment within a very specific time period, which in turn depends upon the water and fat content of the different tissues. These signals are then converted into gray scale cross-sectional images that can be depicted in multiple planes or in three dimensions .
The T1 and T2 relaxation times are important parameters determining image and lesion contrast with reference to normal liver parenchyma.
- The T1 relaxation time (and the resulting T1-weighted image) refers to the time required for protons to fully realign within an external magnetic field following exposure to a radio wave pulse of specific strength and duration
- The T2 relaxation time (and the resulting T2-weighted image) describes the rate at which protons are put out of phase with respect to adjacent protons
These two measurements affect the signal intensities of tissues being imaged and are therefore crucial for creation of the final images. As an example, water has low signal intensity (dark) on T1-weighted images but high intensity (bright) on T2-weighted images, with the reverse being true for fat. Each tissue in the body has characteristic T1- and T2-weighted signal intensities. In particular, abnormal tissue such as tumor will differ from adjacent normal tissue because of a variety of factors including tissue water content, different contrast enhancement properties, and vascularity .