Why is T2 contrast in GRE imaging referred to as T2* contrast?

T2 contrast in Gradient Echo (GRE) imaging is referred to as T2* contrast primarily because it accounts for the effects of magnetic field inhomogeneities on the signal decay. In GRE sequences, there is no refocusing pulse used, which means that while T2 relaxation (the decay of transverse magnetization) contributes to the signal loss, additional dephasing occurs due to variations in the magnetic field. These field inhomogeneities can arise from imperfections in the magnet or from the presence of materials with different magnetic susceptibilities, leading to a faster decay of the signal than what would be observed with true T2 relaxation alone.

The term T2* effectively indicates that the observed decay is a combination of true T2 decay and additional effects from the magnetic field inhomogeneities. Thus, the use of T2* reflects a more complex interaction between the signal and the magnetic environment than T2 alone would suggest.

Other options would not correctly describe why T2 contrast in GRE imaging is termed T2* contrast. The absence of a refocusing pulse in GRE sequences eliminates the focus on those interactions, and the relationship between T2* and the imaging technique is critical to understanding image contrast rather than being unrelated. While T2 imaging