2D Segmentation possible?

Hello, a few days ago I downloaded slicer with the plan to measure the skeletal muscle index of patients. The measurement is based on a 2D segmentation. I’ve already played around with 3D segmentation, but so far I haven’t found a way to segment in 2D. Is this even possible? If so, how can I do this? Thank you for your help in advance!

Most segmentation tools work on 2D (single-slice 3D) images, but “Grow from seeds effect” and maybe a few more do not. Let us know if you have any specific questions (how to do something, explain why something behaves as it does, etc.).