How to measure tip position error and angle error between two needles in CT image?

Manually specify a line markup that is aligned with needle takes less than one minute (one click near the tip in the 3D view, then click on the dropped point and adjust in slice views; then repeat this for another point along the needle shaft, as far as possible from the tip).

You could automate the process quite easily, as the needle has very high voxel value, a unique shape, and it is not connected to any other object. For example a fully automatic process could be the following: do thresholding, split islands to segments, then compute segment statistics; the needle will have relatively small volume and very elongated shape - there will be no other segments similar to it.However, since manual definition of the needle is so fast and simple, it might not worth the effort - only if you need to process thousands of cases or you use this intraoperatively and one minute is too long or you don’t have anyone in the operating room that could do the manual marking.