Why do fluids have long T1 times compared to fat based tissues?

Fluids have long T1 times compared to fat-based tissues primarily due to their molecular structure and interactions. In the case of fluids, the molecules are often free to move more rapidly compared to fat tissues, which contain long-chain fatty acids that are more densely packed and have limited mobility.

The long T1 times in fluids are influenced significantly by their ability to exchange energy with surrounding molecules in a manner that returns to equilibrium more slowly. This is a manifestation of the relaxation processes that occur after the removal of the radiofrequency pulse in MRI. Fluids, due to their lower viscosity and weaker intermolecular interactions, allow for a slower absorption of energy and longer recovery time to align back to the magnetic field.

In contrast, fat-based tissues exhibit a more efficient energy transfer and quicker relaxation due to their higher packing density and interactions among the fatty molecules, leading to shorter T1 times. Therefore, the long T1 times in fluids are a reflection of their distinct physical properties, making the chosen answer relevant in explaining the behavior of these tissues in an MRI context.