Analysis of Artifacts in Four-dimensional CT Images of 50 Abdominal and Thoracic Radiotherapy Patients

Abstract

It has been recognized that four-dimensional (4D) CT is useful to account for respiratory motion in the treatment planning. Even with 4D-CT imaging, the current acquisition methods can still lead to artifacts. The purpose of this study was to quantify the type, frequency and magnitude of artifacts in 4D-CT images acquired using a multislice cine method. Fifty patients, who underwent 4D-CT scanning and radiotherapy for thoracic or abdominal cancers, were included in this study. All the 4D-CT scans were performed on the GE multislice PET/CT scanner with the Varian RPM system in cine mode. The GE Advantage 4D software was used to create 4D-CT data sets. The artifacts were then visually and quantitatively analyzed by reviewing the images at different positions and respiratory phases. We determined the four types of artifacts: blurring, duplicate structure, overlapping structure, and incomplete structure. All the artifacts observed in this study were classified into one of these four types. The magnitude of each artifact was evaluated by measuring the distance in the superior-inferior direction between the edge of the artifact and the “true” edge of the organ. We further performed statistical analyses to evaluate the relationships between patient- or breathing pattern-related parameters and the occurrence as well as magnitude of artifacts. It was found that 46 of 50 patients (92%) had at least one artifact with a mean magnitude of 11.6 mm (range, 4.4 - 56.0 mm) in the diaphragm or heart. We also observed at least one artifact in 6 of 20 lung or mediastinal tumors (30%). Statistical analysis revealed that there were significant differences between several breathing pattern-related parameters, including abdominal displacement (p < 0.0001), respiratory period (p < 0.0001) and respiratory irregularity (p = 0.0236), for the subgroups of patients with and without artifacts. The magnitude of an artifact in the diaphragm was found to be significantly correlated with the abdominal displacement difference between two adjacent couch positions (R = 0.3383, p = 0.0039). This study has identified that the frequency and magnitude of artifacts in 4D-CT is alarmingly high (over 90%). This high value is of particular concern as the true number of artifacts is probably higher than that given by the current method. Significant improvement is needed in 4D-CT imaging, through either breathing training, improved acquisition methods during scanning, or post-scan image processing.