31P MR spectra of intact perfused cells, acquired in our MR-compatible perfusion system, showing spectra from (a) MCF-12A human mammary epithelial cells and (b) MDA-MB-231 human breast cancer cells. Breast cancer cells exhibit high PC levels, whereas mammary epithelial cells exhibit low PC and high GPC levels. Following peaks are assigned in the 31P MR spectra: GPC, glycerophosphocholine; GPE, glycerophosphoethanolamine; NDP, nucleoside diphosphate; NTP, nucleoside triphosphate; PC, phosphocholine; PCr, phosphocreatine; Pi, inorganic phosphate; PME, phosphomonoesters; PDE, phosphodiesters.