Visualization of a target positions using the 4 dimensional cone-beam CT image reconstruction with the extracorporeal infrared monitor

To determine the accurate radiation dose for a specific small volume of tissue, confirmation of the temporal location of a target position is very important. The temporal location of the target can help improve the accuracy of target localization. A kilovoltage cone-beam computed tomography (CBCT) system mounted on a linear accelerator can verify the target on each treatment day, but this system requires a long data acquisition time; therefore, reconstructed images are affected by motion. In this study, we implemented a method to reconstruct target motions obtained by a four-dimensional CBCT using sorted projection data according to the phase and displacement of the extracorporeal infrared monitor signal. We evaluated the effect of reconstruction methods on image quality using a moving phantom and respiratory signals of an actual patient. We modified the relationship between the motion cycle and positions of the marker and projection data and evaluated the effect on image quality. Moreover, the effect of target motions in patients on the quality of reconstructed images was investigated. The results of the experiments showed that phase binning and displacement binning reduced the blurring of the reconstructed image. The quality of the reconstructed images was significantly affected by the amplitude as well as the inhalation and exhalation slopes of the marker signal. The missing number of projections in the displacement binning method was caused by non-linearity of breathing pattern. This method was based on projection binning, in which the signal of the infrared monitor served as the surrogate of tumor motion. Therefore, further major efforts are needed to ensure the accuracy of the infrared monitor signal.