Optimization of four-dimensional cone-beam computed tomography in image-guided radiation therapy of the lung

In image-guided radiation therapy of moving lung lesions, four-dimensional CBCT (4D-CBCT) can produce several images of the target through the breathing cycle with good temporal resolution. The requirements in 4D-CBCT are short scan time and image quality sufficient to localize the target. Short scans are desirable but result in imagedistorting streak artifacts. We have optimized 4D-CBCT by determining the minimum scan time for adequate image quality. We scanned 4 patients with long scan times (3.8 - 5.4 minutes) to produce high-quality oversampled data sets. These serve as the gold standard for image quality assessment. Various shorter scan times were simulated via removal of projection data from the long scans. The projection data were removed in such a way as to maintain an accurate number of total breaths for the various simulated scan times. The amount of global streak artifact and the tumor and bony anatomy shape is assessed for each image set. The original long scans display no major streak artifacts. The moving structures show no sign of motion blurring and have high contrast boundaries. As the scan time is reduced, streak artifacts increase. Images for 1.5 - 2 minute scan time have relatively little distortion of boundaries. At 1 minute or less, the streak artifacts severely distort boundaries and may compromise localization.