Image quality assurance in the Lung Screening Study component of the National Lung Screening Trial was considered important for optimizing the ability to detect and evaluate small pulmonary nodules and assess for annual changes, while also minimizing radiation exposure, at all screening locations throughout the screening period. A similar rationale and quality monitoring approach were used in the American College of Radiology Imaging Network component of the National Lung Screening Trial [3
]. Although investigators in other multicenter imaging trials have reported implementation of imaging quality control procedures [7
], we are unaware of any published reports on the results of such measures. Our experience illustrates the challenges of formally assessing and maintaining image quality across a large number of trial centers and provides a point of reference for judging image quality in other multicenter trials.
Overall, the severity of quality defects identified in the Lung Screening Study was low, and the occurrence of defects was sporadic. This finding may have been due in part to the front-line level of quality control provided by the uniform training and certification of CT technologists in the National Lung Screening Trial scanning protocol, assessment of image quality by the screening center radiologists at interpretation, and the regular feedback and emphasis on quality control by the quality assurance working group. However, detection and correction of several recurrent errors with potential clinical implications through centralized review of screening examinations are evidence of the importance of continual quality control monitoring in multicenter imaging trials. In the Lung Screening Study, the recognition and correction of errors in effective tube current– time settings at a screening center early in the trial likely prevented unintended radiation exposure of additional trial subjects. In addition, identification of 3-mm section thickness images from a new scanner at one screening center and excessive image noise in the lung apices on images from a new scanner at another screening center likely served to limit the number of cases in which low anatomic detail might have compromised the detection of small pulmonary nodules.
Our experience suggests that there may be increased susceptibility to image quality defects soon after initiation of a multicenter imaging trial and after the addition of new imaging equipment. Increased attention to quality assurance issues in multicenter imaging trials may be particularly important during these times. It also may be important to ensure that a centralized image quality control process is completely implemented and functioning before initiation of image acquisition in a multicenter imaging trial. If our centralized image collection and review process had been established sooner, the recurrent errors in effective tube current–time settings at one screening center found when reviews began may have been detected and rectified earlier than they were.
Other features of CT image quality with relatively higher rates of error were field of view and scan length. Although the reduction in rates of error in these parameters during the second half of the screening period further illustrates the potential positive effect of a quality assurance program, the frequency of these errors remained greater than 2% overall in most months. This finding suggests that these parameters may be particularly susceptible to technologist error in multicenter imaging trials involving CT. The persistent errors in field of view may reflect in part a reluctance of some technologists to exclude any anatomic structure despite protocol instructions to limit the field of view to the widest dimension of the lungs. The repeated scan length errors (incomplete scanning of the inferior sulci) may have been due to inadequate review of all images by technologists before the subjects left the scanner. A need to maintain rapid throughput in busy clinical settings may have contributed to this tendency. Encouraging the use of internal quality control mechanisms, such as technologist checklists, was not sufficient to maintain lower error rates. In retrospect, periodic repeated viewing of the training slide presentation by technologists with emphasis on the features of a proper field of view and scan length for the National Lung Screening Trial may have been helpful. For multicenter imaging trials in general, such technologist recertification probably would be beneficial.
Level of inspiration is another parameter influenced by CT scanner operators but was less frequently a problem than field of view and scan length. Motion usually was related to cardiac pulsation, and streak artifacts probably were a function of the low radiation dose technique. Improving image quality related to these two parameters would require addressing the limitations of the scanner technology (scan acquisition time) and acquisition parameters (tube current) used.
In few instances did two of three quality assurance radiologists believe a quality defect warranted consideration of repetition of an examination. These defects were primarily due to incomplete coverage of the lungs rather than to defects in the images obtained. This finding may explain in part why none of the recommendations to consider repetition of a study were followed. Although we did not track the outcome of repetition recommendations, it is also possible that lung nodules were detected in some of these examinations, so further diagnostic testing or short-term CT follow-up was already planned. In addition, with the time lag in review of scans obtained during the first 8 months of screening, reporting of the repetition recommendations for some subjects may have occurred close to the date of their next annual screening examination. Finally, subjects may have refused requests to undergo repeated examinations. Regardless, our experience suggests that if repetition of an examination is strongly indicated for certain quality deficiencies in a multicenter imaging trial, making it a protocol requirement may be preferable to making it an option.
Complete CT screening requires coverage of the entire lung, but it is also desirable to limit the amount of abdominal radiation exposure. Our data show that lung CT screening is reasonably effective at limiting abdominal exposure, but we found a greater tendency toward extending scanning farther into the abdomen than was necessary (4 cm or more below the lung in 16%) than toward not extending it far enough (4.3% inadequate scan length). Because no benefit of abdominal CT screening has been proved, emphasis during technical training on potential means of minimizing abdominal scanning (such as maximizing subject inspiration during both the topogram and spiral image acquisitions and using a lateral topogram) should be considered in multicenter imaging trials involving thoracic CT.
One limitation of our study was the subjective nature of the visual image quality evaluations, illustrated by the variability in defect rates recorded by the reviewers. This variability might have been reduced if the reviewers had received specific training, though the subjectivity inherent in deciding whether to state that an image quality feature is suboptimal would still exist. Nevertheless, the averages of results from four reviewers with extensive experience interpreting thoracic CT scans likely provided reasonable estimates of image quality defect rates. In addition, blinding the reviewers to the source of the screening examinations should have limited bias in error rates by reviewer among the screening centers. Bias may have remained, however, because reviewers might have tended to designate more defects than existed owing to a desire to optimize image quality or not to designate defects owing to a desire to portray the Lung Screening Study favorably. Such biases also might have been present had external reviewers been enlisted to perform the quality reviews. Another limitation was that from our observational data, we could not determine whether the defect rates and their occurrence over time would have been different without an ongoing quality assurance program.
The results of this analysis illustrate the effect of centralized image quality assurance procedures in a multicenter imaging trial. Despite effective communication of a defined imaging protocol to screening center personnel through a well-organized infrastructure and the use of local quality control measures, errors occurred. Although complete elimination of acquisition and image quality errors is unrealistic, minimizing them will help to ensure the validity of multicenter trial data and protect clinical research subjects. The approaches used and data obtained in this study may be useful in the design and planning of quality control measures in other multicenter imaging trials.