Grey values (GV) to HU units conversion asked a similar question since your post.
Additionally, I’ve found literature that states that calibrating CBCT values based on correlation with Multi-slice CT (MSCT) will not provide accurate density measurements (Pauwels et al., 2013).
I’m assuming that MSCT and MDCT are similar enough, in contrast to CBCT, that the challenge between MSCT and CBCT is translatable to MDCT and CBCT.
Reference:
Pauwels, R., Nackaerts, O., Bellaiche, N., Stamatakis, H., Tsiklakis, K., Walker, A., Bosmans, H., Bogaerts, R., Jacobs, R., Horner, K., & The SEDENTEXCT Project Consortium. (2013). Variability of dental cone beam CT grey values for density estimations. British Journal of Radiology, 86(1021), 20120135. https://doi.org/10.1259/bjr.20120135