What effect does reducing the TE have on image contrast?

Reducing the echo time (TE) in MRI affects the amount of T2 relaxation information that is captured in the image. T2 is the time it takes for protons in tissue to lose phase coherence after a radiofrequency pulse, which corresponds to the loss of the MR signal due to interactions with surrounding molecules. When TE is shortened, the imaging process captures the signal before a significant amount of T2 decay has occurred. This results in an image where T2 contrast, or the differentiation based on T2 relaxation times, is decreased because the time allowed for T2 effects to influence the signal is minimized.

In practical terms, tissues with short T2 values will still produce a measurable signal, but the longer T2 tissues may not have sufficient time to emit a strong signal by the time the echo is captured, leading to a lower overall contrast that distinguishes between different T2 tissues. Therefore, decreasing the TE results in a lesser ability to visually differentiate tissues based on their T2 characteristics.