Non‐Invasive determination of cerebral blood flow changes by ¹⁹f NMR spectroscopy

The build‐up and clearance of halothane in rat brain have been measured non‐invasively by 19 F NMR spectroscopy using a surface coil placed on the intact scalp. When the halothane supply (3% in O 2 /N 2 O, 33/66%) was turned off, the 19 F signal decreased exponentially to approximately 50% of the initial value, with a time constant, in normal rats, of 8.6 ± 0.7 min (mean ± SEM, n = 16), followed by a decay slower by at least one order of magnitude. The time constant of the rapid decay (r), which was found to be specific for brain, was reduced in hypoxic/hypercapnic (5% O 2 /5% CO 2 ) rats to 2.9 ± 0.2 min ( p = 0.001, n = 4), in rats infused with physostigmine (20 m̈/kg/min i.v.) to 5.7 ± 0.3 min ( p = 0.005, n = 6) and increased in rats injected with pentothal (40 mg/kg i.p.) to 10.7 ± 1.6 min ( p = 0.2, n = 5). Based on the theory of exchange of inert gas at the lungs and tissues developed by Kety, the rapid exponential decay of the 19 F signal was used to calculate relative cerebral blood flow (CBF). Assuming the cortical CBF in a normal rat to be about 130 mL min –1 100 g –1 , the following CBF values (means ± SEM) were obtained: controls 130 ± 10, hypoxia/hypercapnia 390 ± 59, hypercapnia 220 ± 25, physostigmine 195 ± 26, pentothal 105 ± 23 mL min –1 100 g –1 . These values are in good agreement with published values obtained with established methods. The rate of halothane elimination in the lungs does not significantly influence the relative CBF values, since hypercapnia (pCO 2 = 77–87) had the same effect on r in both mechanically ventilated and spontaneously breathing rats. It can be concluded that it is possible, using 19 F NMR spectroscopy, to determine relative tissue perfusion rates accurately and non‐invasively, averaged over time and volume, in live animals by measuring the exponential disappearance of a 19 F labelled inert gas, such as halothane.