SU‐FF‐I‐24: The Influence of Bowtie Filtration On Cone‐Beam CT Image Quality

Purpose: The large variation of x‐ray fluence at the detector across the imaged field‐of‐view in cone‐beam CT causes the loss of skin‐line and reduces CT number accuracy and image uniformity. We report the performance of a bowtie filter (BTF) as a compensator for improved uniformity, skin‐line and CT number accuracy. Method: Image enhancement is performed using a BT filter with a 20cm collimator. The BTF was inserted 30cm from the x‐ray source on an Elekta Synergy XVI. This filter modulates the 2D x‐ray fluence making it non‐uniform across the field‐of‐view. This compensates for the limited attenuation near the skin, resulting in enhancement of skin‐line and uniformity. Two phantoms (1) CatPhan and 2) CatPhan with irregular acrylic annulus (Cat‐Irreg) were scanned on an Elekta CBCT system. The reconstructed images with and without a BTF were analyzed. The images were transformed into polar coordinates to allow quanitfication of radial lag artifacts (radar artifact) for the Cat‐Irreg phantom. Results: The use of the bowtie filter demonstrated a considerable improvement in CT accuracy in the skin‐line region. The uniformity increases 30% for Cat‐Irreg phantom, and skin‐line edges for both phantoms are visible. CT number accuracy with the BT filter improved by 2% for the CatPhan phantom while no improvement was evident for the Cat‐Irreg phantom. The CT♯ linearity for the catphan phantom with the BT filter improved by 8%. In addition, a 45% reduction in the “radar artifact” was observed in the Cat‐Irreg phantom images acquired with the BT filter. Conclusion: The implemented BTF shows improvement in image quality including uniformity and skin‐line reconstruction. This compensator is static and makes many compromises for anatomical imaging site, patient size, and imaged field‐of‐view. The ideal compensator would optimize the fluence profile to account for numerous properties of the patient and imaging system. Research sponsored by Elekta.