  We study the four dimensional Ricci flow with the help of local invariants.
If $(M^4, g(t))$ is a solution to the Ricci flow and $x \in M^4$, we can
associate to the point $x$ a one-parameter family of curves, which lie in the
product of two projective lines. This allows us to reformulate the
Cheeger-Gromov-Hamilton Compactness Theorem in the context of these curves. We
use this result, in order to study Type I singularities in dimension four and
give a characterization of the corresponding singularity models.
