INORGANIC-PHOSPHATE COMPARTMENTATION IN THE NORMAL ISOLATED CANINE BRAIN

In vivo P-31 magnetic resonance spectra of 16 isolated dog brains were studied by using a 9.4-T wide-bore superconducting magnet. The observ ed P(i) peak had an irregular shape, which implied that it represented more than one single homogeneous pool of P(i). To evaluate our abilit y to discriminate between single and multiple peaks and determine peak areas, we designed studies of simulated P-31(i) spectra with the sign al-to-noise (S/N) ratios ranging from infinity to 4.4 with reference t o the simulated P(i) peak. For the analysis we used computer programs with a linear prediction algorithm (NMR-Fit) and a Marquardt-Levenberg nonlinear curve-fit algorithm (Peak-Fit). When the simulated data had very high S/N levels, both methods located the peak centers precisely ; however, the Marquardt-Levenberg algorithm (M-L algorithm) was the m ore reliable at low S/N levels. The linear prediction method was poor at determining peak areas; at comparable S/N levels, the M-L algorithm determined all peak areas relatively accurately. Application of the M -L algorithm to the individual experimental in vivo dog brain data res olved the P(i) peak into seven or more separate components. A composit e spectrum obtained by averaging all spectral data from six of the bra ins with normal O2 utilization was fitted using the M-L algorithm. The results suggested that there were eight significant peaks with the fo llowing chemical shifts: 4.07, 4.29, 4.45, 4.62, 4.75, 4.84, 4.99, and 5.17 parts per million (ppm). Although linear prediction demonstrated the presence of only three peaks, all corresponded to values obtained using the M-L algorithm. The peak indicating a compartment at 5.17 pp m (pH 7.34) was assigned to venous pH on the basis of direct simultane ous electrode-based measurements. On the basis of earlier electrode st udies of brain compartmental pH, the peaks at 4.99 ppm (pH 7.16) and 4 .84 ppm (pH 7.04) were thought to represent interstitial fluid and the astrocyte cytoplasm, respectively.