Cell growth and volume cell fractions as a function of time. Cell growth was determined by the increase in the ratio of β-NTP signal (normalized to the external standard, 3-APP) over time (■, left side scale). The cell volume fractions derived from the 1H spectrum in presence of Gd-DTPA (○, right side scale), are defined as the ratio of the unshifted signal to shifted signals The cell volume fractions derived from the 31P spectrum of DMMP (●, right side scale) are defined as the ratio of the upfield signal (extracellular) to downfield signals (intracellular).