Purpose: A multi-purpose modular phantom (Corgi) was developed for cone-beam CT (CBCT) dosimetry and image quality assessment with software for automated analyses. We report the design and application of the Corgi phantom to the broad array of CBCT systems in clinical operation.
Methods: The phantom modules (3 cm thick acrylic slabs) can be arranged to suit various CBCT geometries. The phantom diameter (13.5 cm) is within the field-of-view (FOV) of most clinical CBCT systems and can be expanded to 16 and 32 cm for whole body CT applications. Central and peripheral holes for placement of a thimble ionization chamber allow measurement of air kerma simultaneous with image quality assessment. Modules are designed for the measurement of uniformity, HU linearity, contrast, noise, noise-power spectrum (NPS), line-spread, edge-spread, modulation transfer functions (MTFs), and cone-beam artifacts. Software enables automated module recognition with 'one-click' analysis of the above metrics, producing a CT structured report. The phantom was demonstrated on four CBCT systems varying widely in performance and clinical application: breast, orthopaedic, intraoperative C-arm, and image-guided radiotherapy (IGRT) systems.
Results: Dose measurements performed as a function of diameter, kV, and longitudinal FOV established lookup tables relating the dose measured in the (light, convenient) 13.5 cm diameter configuration to 16 and 32 cm CTDI(vol) metrics. The versatility of the phantom was demonstrated in QA/QC tests performed on CBCT systems, including breast, orthopedic, image-guided surgery and IGRT systems. The automated analyses of the image data produced graphical and textual reports of CBCT performance at known doses, and provide the potential for a standardized CT report.
Conclusion: The Corgi provides a basis for dosimetry and image quality evaluation suitable to a broad variety of CBCT systems. Automated analysis software promotes rigorous metrology (e.g., MTF, NPS) for mainstream application and may enable emerging standards for CBCT accreditation testing.