Quality assurance in magnetic resonance (MR)-guided radiotherapy lacks anthropomorphic phantoms that represent tissue-equivalent imaging contrast in both computed tomography (CT) and MR imaging. In this study, we developed phantom materials with individually adjustable CT value as well as
T
1
- and
T
2
-relaxation times in MR imaging at three different magnetic field strengths. Additionally, their experimental stopping power ratio (SPR) for carbon ions was compared with predictions based on single- and dual-energy CT. Ni-DTPA doped agarose gels were used for individual adjustment of
T
1
and
T
2
at
0.35
,
1.5
and 3.0 T. The CT value was varied by adding potassium chloride (KCl). By multiple linear regression, equations for the determination of agarose, Ni-DTPA and KCl concentrations for given
T
1
,
T
2
and CT values were derived and employed to produce nine specific soft tissue samples. Experimental
T
1
,
T
2
and CT values of these soft tissue samples were compared with predictions and additionally, carbon ion SPR obtained by range measurements were compared with predictions based on single- and dual-energy CT. The measured CT value,
T
1
and
T
2
of the produced soft tissue samples agreed very well with predictions based on the derived equations with mean deviations of less than
3.5
%
.
While single-energy CT overestimates the measured SPR of the soft tissue samples, the dual-energy CT-based predictions showed a mean SPR deviation of only
0.2
±
0.3
%
.
To conclude, anthropomorphic phantom materials with independently adjustable CT values as well as
T
1
and
T
2
relaxation times at three different magnetic field strengths were developed. The derived equations describe the material specific relaxation times and the CT value in dependence on agarose, Ni-DTPA and KCl concentrations as well as the chemical composition of the materials based on given
T
1
,
T
2
and CT value. Dual-energy CT allows accurate prediction of the carbon ion range in these materials.