PET AND THE AUTORADIOGRAPHIC METHOD WITH CONTINUOUS INHALATION OF OXYGEN-15-GAS - THEORETICAL-ANALYSIS AND COMPARISON WITH CONVENTIONAL STEADY-STATE METHODS

The steady-state method using O-15 gas inhalation and positron emissio n tomography (PET) is a simple and practical way of imaging cerebral b lood flow (CBF) and oxygen metabolism. Several disadvantages do exist, however, including prolonged examination time, requirement of steady- state and a large tissue heterogeneity effect. To avoid the drawbacks of the steady-state method but to preserve its simplicity, we applied the PET/auto-radiographic method to the build-up phase during the cont inuous inhalation of O-15-gas with intermittent arterial sampling. A s imulation study was performed to determine the optimal scanning period , evaluate the delay and dispersion effect of the input function and e stimate the tissue heterogeneity effect. To assess the clinical feasib ility of the proposed technique for the study of oxygen metabolism, se quential measurements with this method and the conventional steady-sta te method were performed in eight patients. The simulation study showe d that a 5-min scan started 3 min after the commencement of O-15-gas i nhalation was optimal. With this method, the delay and dispersion effe ct on CBF was the same as that of the conventional steady-state method , but the tissue heterogeneity effect was reduced. In eight patients, CBF values calculated by this method showed time dependency and were s lightly higher than those obtained by the steady-state method. The oxy gen extraction fraction showed no significant time dependency and was well correlated with that obtained by the steady-state method. We conc lude that the proposed method is a simple and acceptable alternative t o the conventional steady-state method.