Four-dimensional volume-of-interest reconstruction for cone-beam computerized tomography based image-guided radiation therapy of the lung

In image-guided radiation therapy of moving lung lesions, four-dimensional cone-beam CT (4D-CBCT) can be used to produce time-resolved images for tracking the target throughout the breathing cycle. The requirements in 4DCBCT are short scan time and image quality sufficient to localize the target. Short scans are desirable but result in image-distorting streak artifacts in 4D-CBCT reconstruction, which may affect image-guidance. Motion-averaged (also called conventional or 3D) CBCT reconstruction does not suffer from streak artifacts, but lacks the temporal resolution to depict the tumor breathing motion. We define a new composite four-dimensional volume-of-interest (4D-VOI) reconstruction which combines the features of pure 4D and motion-averaged reconstruction image sets. A 4D reconstruction is performed inside of a VOI which contains the moving tumor, and the higher quality motionaveraged reconstruction is performed outside of the VOI. The three image sets (motion-averaged 3D, 4D, and 4DVOI) are compared. The 3D reconstruction has very few streak artifacts but lacks the temporal resolution to depict moving structures. On the other hand, the full-4D reconstruction without VOI processing is severely distorted by streak artifacts. The 4D-VOI reconstruction has good temporal resolution in the volume of interest and low streak artifact in most of the image.