If @Herbie’s solution uses an existing plugin then it may be the most straight forward approach. However, as a stimulatory thought experiment, I have come up with another potential solution. In truth my method is most likely a little computationally inefficient, as it is quite recursive.
My approach (see schematic below) would involve defining multiple adjacent ROIs (green circles) along a predefined arc/ellipse, multiple times if many circles are required, and a set distance (a) from a point of origin (X). This could all be achieved in multi-nested ‘for loops’. Within the inner-most loop you could then perform a check to see if the ROI contains a dendrite (e.g. if binarised, check for black pixels). If a dendrite is found, you can then add it to a higher-level counter ++ for that particular ellipse. However, importantly, you may want to not add to the counter if an adjacent ROI detected the dendrite, as it would indicate that this is the same dendrite (as in the example picture the blue dendrite passes through two adjacent ROIs). If two dendrites are very close to each other, then this adjacent check might fail to differentiate between the two, but you could always decrease the size of the ROIs. Also, if a dendrite comes back on itself, then my method would not work as well.
Your question didn’t indicate that you want to individually follow each dendrite and collect a separate measure of concentric cross-sections for each, so my plan doesn’t incorporate that.