In T2 weighted imaging, why do fluids appear bright compared to fat?

In T2 weighted imaging, fluids appear bright compared to fat primarily because fluids maintain transverse magnetization longer. The T2 relaxation time is the time it takes for the transverse magnetization to decay, and different tissues exhibit different T2 times.

Fluids, such as cerebrospinal fluid or edema, have a relatively long T2 time, allowing them to sustain their signal longer than fat. This prolonged maintenance of transverse magnetization means that when imaging sequences are applied, fluids generate a stronger echo, appearing brighter on the resulting images. In contrast, fat has a shorter T2 time, leading to faster decay of the transverse magnetization and resulting in a weaker signal, which in turn appears darker in T2 weighted images.

This characteristic is fundamental in differentiating between various types of tissues in MRI, enhancing the visualization of pathological conditions where fluid accumulates.