The goal of this paper is to study Cone‐beam CT scanning along a helix of variable pitch. First the rationale and applications in medical imaging of variable pitch CT reconstruction are explained. Then formulas for the minimum detection window are derived. The main part of the paper proves a necessary and sufficient condition for the existence and uniqueness of PI‐lines inside this variable pitch helix. These results are necessary steps toward an exact reconstruction algorithm for helix scanning of variable pitch, generalizing Katsevich's formula on constant pitch exact reconstruction. It is shown through an example that, when the derivative of the pitch function is not convex, or when the pitch function passes a inflection point and begins to slow down, PI‐lines may be not unique near the rim of the helix cylinder. The conclusion is that the restriction on the pitch function is weaker, if the object is placed well within the helix cylinder and far from its rim, in order to preserve the uniqueness of PI‐lines. If the object is near the rim, the restriction condition on the allowable pitch functions becomes stronger.