Most 3-D magnetic particle imaging (MPI) scanners currently use permanent magnets to create the strong gradient field required for high resolution MPI. However, using permanent magnets limits the field of view (FOV) due to the large amount of energy required to move the field free point (FFP) from the center of the scanner. To address this issue, an alternative approach called "Traveling Wave MPI" is here presented. This approach employs a novel gradient system, the dynamic linear gradient array, to cover a large FOV while dynamically creating a strong magnetic gradient. The proposed design also enables the use of a so-called line-scanning mode, which simplifies the FFP trajectory to a linear path through the 3-D volume. This results in simplified mathematics, which facilitates the image reconstruction.