Mapping extracellular pH in rat brain gliomas in vivo by 1H magnetic resonance spectroscopy imaging: Comparison with maps of metabolites

Abstract

The value of extracellular pH (pHe) in tumors is an important factor in prognosisand choice of therapy. We demonstrate here that pHe can be mappedin vivo in a rat brain glioma by 1H magnetic resonance spectroscopic imaging (SI) of the pH buffer (±)2-imidazole-1-yl-3-ethoxycarbonylpropionic acid (IEPA). 1H SI also allowed us to map metabolites, and, to better understand the determinants of pHe, we compared maps of pHe, metabolites, and the distribution of the contrast agent gadolinium1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraaceticacid (Gd-DOTA). C6 cells injected in caudate nuclei of four Wistar rats gave rise to gliomas of ∼10 mm in diameter. Three mmols of IEPA were injected in the right jugular vein from t = 0 to t = 60 min. From t = 50 min to t = 90 min, spin-echo 1H SI was performed with an echo time of 40 ms in a 2.5-mm slice including the glioma (nominal voxel size, 2.2μ l). IEPA resonances were detected only within the glioma and were intense enough for pHe to be calculated from the chemical shift of the H2 resonance in almost all voxels of the glioma. 1H spectroscopic images with an echo time of 136 ms were then acquired to map metabolites: lactate, choline-containing compounds (tCho), phosphocreatine/creatine, and N-acetylaspartate. Finally, T1-weighted imaging after injection of a bolus of Gd-DOTA gave a map indicative of extravasation. On average, the gradient of pHe (measured where sufficient IEPA was present) from the center to the periphery was not statistically significant. Mean pHe was calculated for each of the four gliomas, and the average was 7.084 ± 0.017 (± SE; n = 4 rats), which is acid with respect to pHe of normal tissue. After normalization of spectra to their water peak, voxel-by-voxel comparisons of peak areas showed that N-acetylaspartate, a marker of neurons, correlated negatively with IEPA (P < 0.0001) and lactate (P < 0.05), as expected of a glioma surrounded by normal tissue. tCho (which may indicate proliferation) correlated positively with pHe (P < 0.0001). Lactate correlated positively with tCho (P < 0.0001), phosphocreatine/creatine (P < 0.001), and Gd-DOTA (P < 0.0001). Although lactate is exported from cells in association with protons, within the gliomas, no evidence was observed that pHe was significantly lower where lactate concentration was higher. These results suggest that lactate is produced mainly in viable, well-perfused, tumoral tissue from which proton equivalents are rapidly cleared.