I have a question regarding sample size calculation for my research study and was not able to find an answer elsewhere.

The study compares two imaging methods and should prove that one method is equivalent to the other method in terms of geometric measurements.

The question: Each patient will provide multiple available 'samples'. Let's say the study investigates the accuracy of height measurements performed on vertebral bodies of the thoracic spine on both imaging method (paired data). The hypothesis is that both methods will be equal. Each patient has 12 thoracic vertebral bodies, so in theory 12 measurements could be made per patient.

In this case, is it valid to consider each vertebral body as sample? To me, it makes no sense to consider each patient as a unique sample as every patient yields so many 'samples'.

Thank you very much for your help!