Calculation of in vivo concentrations requires knowledge of the B1 field. A common solution to this problem has been to use field maps measured in phantoms, but this becomes increasingly difficult at high field. The size of the effect of material and B0 field strength determining B1 in the liver using phosphorus (31P) phantoms was investigated at 1.5, 3, and 7T using CST simulations. The effect of concentration differences at 7T was demonstrated using 15 and 30mM phosphate phantoms. At 1.5T, using phosphate phantoms with concentrations between 5-40mM give an error of less than 3%. This increases to 10% at 3T, and 20-114% at 7T.