Good day to you.
My suggestion is similar to the above, however after setting the threshold I use the sphere tool to highlight the aorta (simply run it up the length), while not being to concerned about getting small sections of the spine.
Then I use islands to keep only the largest island.
Finally I do another 4 steps
1 create a new segment
2 use logical operators to copy the aorta (making sure you are not overwriting segments)
3 use Margins to grow by a small amount (1.6 mm)
4 back to logical operators to subtract your original segment from the new one - leaving you with a hollow artery.
Hopefully this is sufficient for your purposes