(Easy) motion path trace question?

All,

Sorry if this question is dumb. I like tweening along paths in animate cc--specifically, I have some complex mathematical functions like hyperbolic spirals that I want to have little particles follow in animate. The problem I'm having is that these are not amenable to my shoddy pen-tool hand--they need to come from sreen shot jpegs.

This seems like it should be easy but hasn't been. Take a simple b+w spiral--i can get a screenshot of arbitrary thickness. Now, though, trace bmp results in a mesh of varying paths (like thousands of closed loop paths to approximate the spiral graphic rather than a single, open path curved spiral vector motion path).

Approach 2, come from Photoshop. Now, I have the same screen shot, and I do a path trace. The result is a closed path spiral (the vector path runs along the perimeter of the spiral line, so eventually the path repeats). I can use this though I'd prefer the single line open path--ill just classic tween the particles along part of the spiral. But wait! The import of the .psd file results in a box with the image inside (the closed path spiral)--the spiral is like a flattened image again...the Photoshop vector path is gone. Tracing bitmap now results in the same problem discussed above.

Can't I just have a path (pure black and white single path, like sin x) as an image file and do some sort of 1-d trace without absolutely without question HAVING to do it by hand?? Some of these could take forever, and it seems like such a stupid thing for me to have to do when the image file is crisp and exactly smooth already.