  Purpose: To develop bone material analogues that can be used in construction
of phantoms for simultaneous PET/MRI systems.
  Methods: Plaster was used as the basis for the bone material analogues tested
in this study. It was mixed with varying concentrations of an iodinated CT
contrast, a gadolinium-based MR contrast agent, and copper sulfate to modulate
the attenuation properties and MRI properties (T1 and T2*). Attenuation was
measured with CT and 68Ge transmission scans, and MRI properties were measured
with quantitative ultrashort echo time pulse sequences. A proof-of-concept
skull was created by plaster casting.
  Results: Undoped plaster has a 511 keV attenuation coefficient (~0.14 cm-1)
similar to cortical bone (0.10-0.15 cm-1), but slightly longer T1 (~500 ms) and
T2* (~1.2 ms) MR parameters compared to bone (T1 ~ 300 ms, T2* ~ 0.4 ms).
Doping with the iodinated agent resulted in increased attenuation with minimal
perturbation to the MR parameters. Doping with a gadolinium chelate greatly
reduced T1 and T2*, resulting in extremely short T1 values when the target T2*
values were reached, while the attenuation coefficient was unchanged. Doping
with copper sulfate was more selective for T2* shortening and achieved
comparable T1 and T2* values to bone (after 1 week of drying), while the
attenuation coefficient was unchanged.
  Conclusions: Plaster doped with copper sulfate is a promising bone material
analogue for a PET/MRI phantom, mimicking the MR properties (T1 and T2*) and
511 keV attenuation coefficient of human cortical bone.
