Methods to evaluate the performance of kilovoltage cone-beam computed tomography in the three-dimensional reconstruction space

Cone-beam computed tomography (CBCT) scanners for image-guided radiotherapy are in clinical use today, but there has been no consensus on uniform acceptance to verify the CBCT image quality yet. The present work proposed new methods to fully evaluate the performance of CBCT in its three-dimensional (3D) reconstruction space. Compared to the traditional methods with only two-dimensional axial slice evaluated, the new methods were designed to evaluate the CBCT image quality not only in the axial slices but also along three dimensions and three sections. Image parameters were evaluated by scanning the corresponding phantom. The new methods demonstrated the high spatial resolution, the image uniformity, the high CT numbers linearity, geometric accuracy and relatively poor performance in low contrast resolution, compared to the results of plan CT. Additionally, the spatial resolution, the CT number linearity and the image uniformity proved to be more superior along the z direction and in the cross-section. Furthermore, the spatial resolution and the CT value linearity decreased gradually when deviating from the center of rotation axis and accelerates the reduction towards the periphery area, suggesting that locating the target within the range of 4 cm around the isocenter during radiotherapy ensure the optimal spatial resolution and the CT value linearity.   Results indicate that the proposed three-dimensional image quality evaluation holds the potential to complement existing standards to provide sufficient information for the CBCT image performance and thereby guiding the clinical applications of CBCT.