On a T2 weighted image, edema appears bright because it has a _________ relaxation time.

On a T2 weighted image, edema appears bright because it has a long T2 relaxation time. In MRI, T2 relaxation time refers to the time it takes for the protons in a tissue to lose coherence among the magnetic moments due to interactions with surrounding tissue.

When protons in tissues such as edema are subjected to a magnetic field, they align with that field. However, those in fluids, such as the edema, retain their signal longer compared to tissues with short T2 relaxation times, like fat or compact structures. In T2 weighted imaging, longer T2 values lead to a brighter appearance on the image because the signal remains strong for a longer duration before decaying.

This characteristic is particularly useful in diagnosing conditions with increased fluid content, such as edema, which is often associated with inflammation or injury. Thus, the long T2 relaxation time associated with edema is what causes it to appear bright on a T2 weighted image.