Untangling the causes of elongation artifacts in dual flat-panel PET: limited angular sampling versus parallax error

Dual flat-panel PET systems provide easy patient access and high sensitivity when the panels are close together. Their images are however susceptible to elongation artifacts due to incomplete angular sampling and parallax error. These effects are reduced by respectively including depth-of-interaction (DOI) and time-of-flight (TOF) information. To guide system design, a framework is needed that predicts which effect dominates for a given object size and field-of-view (FOV) location. This study investigates the impact of TOF and DOI on elongation artifacts for a variety of phantoms in a dual flat-panel system, benchmarked against a cylindrical reference.

A large dual flat-panel system and cylindrical system with the same axial FOV were simulated. The TOF and DOI resolution were varied independently and iterative MLEM reconstruction was performed. Data were generated for point sources and spheres spanning 1-50 mm radius at multiple FOV positions. Point-source blur was quantified with full width at half maximum measurements, and sphere elongation using an activity-spillover metric. Findings were validated using the more clinically relevant NEMA IQ and XCAT phantoms with embedded lesions.

DOI uncertainty resulted in elongations independently of object size. In the cylindrical system, these only occurred at radial offsets, but in the dual flat-panel system, parallax-blur was also present at the center of the FOV. Elongations due to limited angular coverage in the flat-panel system grew with object size and dominated for features over 1-2 cm. The extent of these elongations increased towards the FOV edges and was substantially reduced by including TOF information. With adequate TOF and DOI performance, the flat-panel system matched cylindrical image quality.

The framework predicts the dominant artifact mechanism as a function of object size, position, and system geometry. The insights guide detector and system design for dual flat-panel PET and corresponding artifact correction methods.