Enhancement of the signal-to-noise ratio in (H2O)-O-15 bolus PET activation images: A combined cold-bolus, switched protocol

To increase the signal-to-noise ratio (S/N) of (H2O)-O-15 bolus PET activat ion images, we designed and tested a data acquisition protocol that alters the relative distribution of tracer in the uptake and washout phases of the input function. This protocol enhances the S/N gains obtained with convent ional switched protocols by combining task switching and the use of a large bolus of blood free of tracer (cold bolus). The cold bolus is formed by se questering blood in the lower limbs with a double cuff before tracer inject ion. Methods: The effect of a combined cold-bolus, switched protocol on the signal from activation images was first simulated using a compartmental mo del of the uptake of (H2O)-O-15 into the brain. Then, the effectiveness of the protocol was investigated in 4 healthy volunteers performing a language task. Each volunteer underwent scanning 12 times: 3 activation/baseline an d 3 baseline/activation scans using the conventional switched protocol and 3 activation/baseline and 3 baseline/activation scans using the combined co ld-bolus, switched protocol. The S/N changes introduced when using the cold bolus were analyzed by comparing, across protocols, the magnitude and stat istical significance of the activation foci associated with the execution o f the language task identified in the averaged subtracted images, and by co mparing image noise levels. Results: In the simulated datasets, the combine d protocol yielded a substantial increase in the activation signals for sca n durations greater than 60 s, in comparison with equivalent signals yielde d by the switched protocol alone. In the PET experiments, activation foci o btained using the combined protocol had significantly higher t statistic va lues than did equivalent foci detected using the conventional switched prot ocol (mean improvement, 36%). Analysis of the S/N in the averaged subtracte d images revealed that the improvements in statistical significance of the activation foci were caused by increases in the signal magnitudes and not b y decreases in overall image noise. Conclusion: We designed a data acquisit ion protocol for (H2O)-O-15 bolus PET activation studies that combines the use of a tracer-free bolus with a switched protocol. Simulated and experime ntal data suggest that this combined protocol enhances the S/N gains obtain ed with a conventional switched protocol. Implementation of the combined pr otocol in (H2O)-O-15 bolus activation studies was easy.