What is the precessional frequency difference between fat and water attributed to?

The precessional frequency difference between fat and water is indeed attributed to the electron shielding effect in fat. In the context of MRI, protons in different chemical environments resonate at slightly different frequencies due to the influence of surrounding electrons. Fat molecules have fewer electronegative atoms compared to water, resulting in a different chemical shift for the protons in fat. This difference in electron density effectively shifts the magnetic field experienced by the protons, leading to a distinct precessional frequency compared to those in water.

In fat, the protons experience a different magnetic environment because the surrounding electron cloud shields them differently than in water. As a result, this shielding causes the resonant frequencies of the protons in fat to be lower compared to those in water, creating the well-known chemical shift observed in MRI images. This difference is critical for distinguishing between lipid-rich and aqueous tissues in imaging.