P-31 SPECTROSCOPIC IMAGING OF PIG BRAINS DURING CIRCULATORY ARREST

An experimental protocol was developed for the localized in-vivo study of brain energetics in large animals. A double tuned semi-cylindrical surface coil specially designed to conform to the back of a pig's sku ll was used to acquire P-31 NMR spectra from 8 cm3 volume localized in side the brain. The work reported here demonstrates the ability to obt ain localized P-31 NMR spectra of pig brains. This is important becaus e the P-31 signal due to muscle will not interfere with the spectral i nterpretation. We have improved upon the use of the standard surface r .f. coil which normally requires the removal of the scalp and skull of large animals where the brain is located deep in the skull. The semic ylindrical r.f. coil used here facilitated the study of pig brains in- vivo and under physiological conditions (scalp and skull intact) by al lowing deeper r.f. penetration into the head. This experimental protoc ol can be readily expanded to include quantifiable localized multi-vox el P-31 and H-1 NMR spectroscopy and H-1 imaging experiments in future studies of brain protection methods in the pig model. As a test of ou r MR approach, three methods of brain protection for use during cardia c surgery requiring circulatory arrest were evaluated: deep hypothermi a alone (15-degrees-C) and deep hypothermia with deep hypothermic retr ograde and antegrade cerebral perfusion. P-31 spectroscopic imaging of pig brains was used to observe the pH and high energy phosphate metab olite signals during and following circulatory arrest. We found that t he P-31 NMR spectra and the pH remained constant during the entire exp erimental protocol for the control groups (anaesthesia and cardiopulmo nary bypass alone, both n = 5) and the antegrade perfusion group (n = 6). The pH of both the retrograde perfusion and circulatory arrest gro ups was acidic at the end of the circulatory arrest period (6.4 +/- 0. 1 and 6.2 +/- 0.1, respectively). The pH returned to normal at the end of the recovery period (7.1 +/- 0.2) only for the retrograde perfusio n group (n = 6). In the circulatory arrest group (n = 6), three animal s showed no recovery at the end of the experimental protocol (pH = 6.2 +/- 0.1 and no or very little observable signal from ATP and Pi). For the remaining three animals in this group, the final pH at the end of the recovery period was 6.9 +/- 0.1 and P-31 NMR spectra showed at le ast partial recovery of high energy phosphate metabolites. The spectra l characteristics of the retrograde and circulatory arrest groups indi cated significant overall loss of phosphorus metabolites suggesting ce ll death and metabolite washout due to break-down of the blood brain b arrier after the brain experienced acidic conditions.