Quantification of image quality of intra-fractional cone-beam computed tomography for arc irradiation with various imaging condition

BACKGROUND: 3-dimensional intra-cone beam computed tomography (intra-CBCT) could be a potentially powerful tool for use with arc irradiation such as volumetric modulated arc therapy. The aim of the study was to evaluate the image quality of intra-cone beam computed tomography (intra-CBCT) for arc irradiation with various imaging condition. MATERIALS AND METHODS: Two types of intra-CBCT imaging techniques were evaluated — intra-fractional CBCT with flattening filtered (FF) beam (intra-FF CBCT) and that with flattening filter free (FFF) beam (intra-FFF CBCT). For the intra-MV beams, four different field sizes (2 cm x 2 cm, 5 cm x 5 cm, 10 cm x 10 cm, and 20 cm x 20 cm) were used with dose rates of 500 MU/min and 1600 MU/min, for 6 MV FF and 6 MV FFF, respectively. For all image acquisitions, two rotation angles (full-arc and half-arc) were investigated. Thereafter, the linearity, contrast-to-noise ratio (CNR), and uniformity index (UI) of intra-CBCT image were compared with those of conventional CBCT image. RESULTS: All acquisition conditions had good linearity of the CT value (R2 > 0.99). For CNR, the change rates from conventional CBCT ranged from 0.6–33.7% for a 2 cm x 2 cm beam, whereas that for a 20 cm x 20 cm beam ranged from 62.7–82.3%. Similarly, the UI increased from 1.5% to 7.0% as the field size increased. CONCLUSION: Quality of intra-CBCT image was affected by the field size and acquisition angle. Image quality of intra-CBCT was worse than that of conventional CBCT, but it was better under a smaller field and wider correction angle and would be acceptable for clinical use.