Investigating the influence of baseline drifts of respiratory signals in amplitude-based gating for positron emission tomography

Respiratory gating of coincidence data acquired during positron emission tomography (PET) scans is widely considered as the standard solution to the problem of respiration-induced blurring effects of thoracic and abdominal structures. Retrospective gating methods require respiratory signals over the PET scanning time. For this purpose, usually only surrogate signals are measured, e.g. by tracking abdominal markers or measuring abdominal pressure changes. In this study, we investigate the quality of such signals for amplitude-based gating compared with respiratory signals derived from the internal motion by using a raw data-driven gating approach. Specifically, changes in baseline of the respiratory signals (slow drifts, sudden shifts) are of interest in this context. We analyzed this using 25 single-bed position cardiac [¹⁸F]FDG list mode datasets where both types of signals were available. Of these, 12 demonstrated clear differences in baselines between internal and external respiratory signals. This resulted in noticeable differences in the optimally-gated PET images and apparent metabolically active myocardial volumes as well as a reduced motion resolution as measured in end-expiration/-inspiration images for the external gating. On the other hand, no relevant differences were observed in the other 13 cases where no or few apparent baseline differences between internal and external signals were present. This demonstrates that external respiratory motion may be overlaid with motion patterns that are not well correlated to internal motion patterns, thus potentially affecting the quality of amplitude-based gating.