Development of phantom materials with independently adjustable CT- and MR-contrast at 0.35, 1.5 and 3 T

Quality assurance in magnetic resonance (MR)-guided radiotherapy lacks anthropomorphic phantoms that represent tissue-equivalent imaging contrast in both computed tomography (CT) and MRimaging. In this study, we developed phantom materials with individually adjustable CT value as well as T-1- and T-2-relaxation times inMRimaging 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 CTvalue was varied by adding potassium chloride (KCl). By multiple linear regression, equations for the determination of agarose, Ni-DTPA andKCl concentrations for given T-1, T-2 andCT valueswere derived and employed to produce nine specific soft tissue samples. Experimental T-1, T-2 andCT 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 phantommaterialswith independently adjustableCT 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 andCT value. Dual-energy CT allows accurate prediction of the carbon ion range in these materials.