The calculated ratio of R2/R1 for (Gd-DOTA-4AmP)44-G5 at four different concentrations and three different pH values at 7 T: top line, pH = 8.84, middle line, pH = 7.40, bottom line, pH = 6.96. The ratio R2/R1 does not vary significantly with concentration of contrast agent at the lower pH values, but there is a small and significant variation at the higher pH, which lies outside the range of values usually encountered in tumor tissues.