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Enhancement of the Signal-to-Noise Ratio in H₂¹⁵O Bolus PET Activation Images: A Combined Cold-Bolus, Switched Protocol

Department of Neurology and Neurosurgery, Neuropsychology/Cognitive Neuroscience Unit, and McConnell Brain Imaging Center, Montreal Neurological Institute, Montreal, Quebec
Department of Anaesthesiology, Royal Victoria Hospital, McGill University, Montreal, Quebec, Canada

Correspondence: For correspondence or reprints contact: Jorge J. Moreno-Cantú, PhD, PET Imaging Service (11P), VA Medical Center, 1 Veterans Dr., Minneapolis, MN 55417.

ABSTRACT

To increase the signal-to-noise ratio (S/N) of H₂¹⁵O bolus PET activation 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 conventional 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 sequestering blood in the lower limbs with a double cuff before tracer injection. Methods: The effect of a combined cold-bolus, switched protocol on the signal from activation images was first simulated using a compartmental model of the uptake of H₂¹⁵O 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 and 3 baseline/activation scans using the conventional switched protocol and 3 activation/baseline and 3 baseline/ activation scans using the combined cold-bolus, switched protocol. The S/N changes introduced when using the cold bolus were analyzed by comparing, across protocols, the magnitude and statistical significance of the activation foci associated with the execution of the language task identified in the averaged subtracted images, and by comparing image noise levels. Results: In the simulated datasets, the combined protocol yielded a substantial increase in the activation signals for scan durations greater than 60 s, in comparison with equivalent signals yielded by the switched protocol alone. In the PET experiments, activation foci obtained using the combined protocol had significantly higher t statistic values than did equivalent foci detected using the conventional switched protocol (mean improvement, 36%). Analysis of the S/N in the averaged subtracted images revealed that the improvements in statistical significance of the activation foci were caused by increases in the signal magnitudes and not by decreases in overall image noise. Conclusion: We designed a data acquisition protocol for H₂¹⁵O bolus PET activation studies that combines the use of a tracer-free bolus with a switched protocol. Simulated and experimental data suggest that this combined protocol enhances the S/N gains obtained with a conventional switched protocol. Implementation of the combined protocol in H₂¹⁵O bolus activation studies was easy.

Key Words: PET • switched protocol • activation studies • water • signal-to-noise ratio