What interpolation methods can I use to get the tightest fit for these curves? I am working with MRI images of the brain that have certain areas marked by hand like  and .

I am trying to come up with an interpolating function that will let me describe such curves so that I can use machine learning techniques to automatically generate such markings on unmarked images like  and .

I've considered using cubic spline interpolations to approximate the curve, but as far as I know, I would have to split the curve into multiple parts to interpolate it.

Am I on the right track or are there other (better?) ways to do this?
If you have a time series of images, I recommend the CONDENSATION algorithm for the interpolation of curves (see also the wikipedia page). There are a number of implementation available, notably in opencv.