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Technical considerations for automated low-pitch spiral 4D CT scanning protocol selection

Respiration-correlated CT (4D CT) represents the basis of radiotherapy treatment planning for thoracic and abdominal tumor patients. A common approach is low-pitch spiral 4D CT. Similar to standard spiral 3D CT, CT projection data are continuously acquired while the patient couch is moving through the gantry. To ensure sufficient projection data coverage for 4D CT reconstruction, the so-called 4D CT data sufficiency condition (DSC) has to be fulfilled: For a fixed pitch factor and gantry rotation time, the patient breathing rate must be above a certain threshold; otherwise, artifacts impair image quality. For the current Siemens 4D CT scanner generation, three 4D CT protocols can be selected manually, associated with DSC thresholds of 6, 9 and 12 breaths per minute (BPM). Due to, e.g., a limited achievable z-range during scanning with lower BPM protocols, these options are, however, often not selected in practice. As a result, a high fraction of artifact-affected 4D CT data are reported. Aiming to optimize respective 4D CT workflows and improve image quality, this study systematically investigates the influence of parameters to be considered for automated scanning protocol selection and their interrelation (e.g. severity of artifacts due to DSC violation vs. clinically required z-scan range).