It has been well recognized that respiratory motion diminishes the accuracy of radiotherapy in the processes of imaging (1
), treatment planning (5
) and radiation delivery (6
). If respiratory motion is not taken into account, it leads to artifacts in the images. Many investigators reported significant distortions of moving organs in CT images (1
), which can result in a systematic error in subsequent treatment processes, including target definition and dosimetry. Such artifacts can be mitigated by using methods that account for respiratory motion, including respiratory gating, breath-hold, and four-dimensional (4D) CT. With 4D-CT images, one can assess 3D tumor motion and directly incorporate that information into treatment planning. Since the first developments using single slice helical scan (10
), 4D-CT scans have been acquired using different methods (12
) and clinically implemented (16
) by many investigators. Two types of approaches have been commonly used to retrospectively sort 4D-CT images: phase-based sorting (10
) and displacement-based sorting (16
Even with 4D-CT imaging, the current acquisition and reconstruction methods can still lead to artifacts (16
). For example, Pan et al.
) showed discontinuities in the diaphragm and heart, which were due to an inaccurate determination of respiratory phases. The current procedures generate a 4D-CT data set by assembling CT slices that have the nearest phase or displacement to the target one for all positions. There are often mismatches in the phase or displacement between adjacent couch positions, which could manifest as artifacts in the images. Several authors have investigated the approaches to reduce the artifacts in 4D-CT images. Rietzel and Chen (23
), Mutaf et al.
) and Pan et al.
) improved determination of respiratory phases originally assigned by the Varian Real-time Position Management (RPM) system (Varian Medical Systems, Palo Alto, CA) system using in-house software, which then led to 4D-CT images with fewer artifacts. Studies by several investigators (16
) have shown that displacement-based sorting performed better than phase-based sorting; however, this method could result in insufficient data at some couch positions if there is no corresponding displacement in the respiratory cycle (20
). More recently, Schreibmann et al.
) and Ehrhardt et al.
) have developed another approach, which deduces a 3D data set at an arbitrary respiratory phase by interpolating the original CT images using deformable models. Ehrhardt et al.
) compared the 4D-CT image generated using their method with the original one sorted based on tidal volume, and showed reduction of artifacts. Even with these approaches, residual artifacts still remain. Artifacts in 4D-CT images can affect the delineation of target volume and the shape of beam aperture, and subsequently manifest as systematic errors. It is important to understand the characteristics of artifacts and the limitations of the current procedures for future developments of strategies to improve 4D-CT. However, to date there has been insufficient literature on that.
The purpose of this study was to quantify the type, frequency and magnitude of artifacts in 4D thoracic or abdominal CT images acquired using the multislice cine method. We also performed statistical analyses to evaluate the relationships between patient- or breathing pattern-related parameters and the occurrence as well as magnitude of artifacts.