Exact FBP reconstruction for dynamic pitch helical cone beam CT

We present an extension of the exact filtered back-projection (FBP) formula for wide cone helical trajectories to more general helical trajectories. In particular, we show that the original formula presented applies for any helical trajectory in which the velocity plus derivative of acceleration does not change sign during the acquisition. This condition allows for helical trajectories in which the pitch changes dynamically during the scan, and as a byproduct, permits gantry tilt without any additional interpolation steps.