Figure 8. Nature of precessing blood proton spins.

Hydrogen-based molecules in blood maintain coherent spin phases in static fluid (as shown in subfigure A), but are distributed chaotically in turbulent fluid and this disrupts the alignment of proton spins from their order (as illustrated by subfigure B). Due to the incoherent or random precession of spins, the net magnetization is weak and gives a low MR signal intensity. Therefore regions of blood at different levels of turbulence appear with contrasting pixel intensities in the cardiac MR image. The groups of incoherently precessing proton spins move along with the global blood flow within the cardiac chamber, and presence of intensity flow, which coincide with the blood flow, can be observed from time-dependent sequences of MR images.