Find a study and sign up.

Look for research groups in your local institutions and send an email to a couple grad students asking about studies they or their colleagues are running in the near future. A researcher may give you the scan if you ask nicely and say it's for personal interest, plus you'll be compensated for your time /and/ simultaneously help with the advancement science :)

To expand on Deniz' answer, Klarismo (YC S15) does a full body MRI, including a high resolution scan of your head for $249, available in SF and LA at the moment. We extract your brain, segment out gray and white matter and measure your brain volume. In addition we segment, measure and visualize visceral fat, subcutaneous fat and major muscle groups.

You can: https://klarismo.com/ (YC S15)