Reducing echo time (TE) results in what sort of change to image contrast?

When echo time (TE) is reduced in MRI, it has a significant impact on the signal characteristics from tissues, particularly in relation to their T2 relaxation times. T2 relaxation time is the period it takes for spins of hydrogen nuclei in a tissue to lose phase coherence after being excited by a radiofrequency pulse.

By shortening TE, the MRI sequence captures the signal before substantial T2 decay occurs. This means that tissues with longer T2 times (which typically appear brighter in T2-weighted images) will not lose as much signal intensity, potentially leading to a decrease in the contrast that highlights T2 differences between tissues. Therefore, shorter TEs can result in decreased T2 contrast in the images, making it harder to differentiate tissues that rely on T2 weighting for contrast.

In contrast, increased T1 contrast typically results from longer TE values, as T1 weighting emphasizes tissue recovery characteristics rather than loss of signal from phase coherence. So, while T1 information can be enhanced with other parameters, reducing TE specifically affects T2 contrast adversely.

Ultimately, reducing TE primarily decreases T2 information in the images captured, leading to diminished contrast based on T2 relaxation characteristics.