If you want to more accurately simulate bone, Data Spectrum sells a fillable spine that is a replacement for the Teflon one. A few years ago, a solution was proposed that was very similar to cranial bone, as far as attenuation characteristics are concerned. It consists of:100 gr of K2HPO4 (dipotassium hydrogen phosphate) dissolved in 67gm of H2O.Reference: De Dreuille, O., V. Strijckmans, et al. (1997). “Bone equivalent liquid solution to assess accuracy of transmission measurements in SPECT and PET.” IEEE Transactions on Nuclear Science 44(3): 1186-1190.We have found that it is best to use a magnetic stirrer and to heat the water/salt mixture to facilitate dissolving the salt. Once dissolved, the salt will remain in solution at room temperature.It was shown in the reference above that the attenuation properties of this solution is nearly equivalent to skeletal cranium bone over a range of 50 to 600 keV. This was also tested at the site of one of DSC’s customers and it worked very well from 140 keV up to 511 keV in their situation.The density of the solution is 1.68, while the density (according to the above reference.) of bone is 1.61. By weight, the composition of the solution is: 26%(K), 10%(P), 56%(O), 8%(H), while bone is: 17.6%(Ca), 8.1%(P), 43.5%(O), 5%(H), 21.2%(C), 4%(N). K (Z = 39) is used in place of Ca (Z = 40).