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Original Report |
1 Department of Radiology, Beth Israel Deaconess Medical Center and Harvard
Medical School, 330 Brookline Ave., Boston, MA 02215.
2 The University of Arizona College of Medicine, Tucson, AZ.
Received May 7, 2003;
accepted after revision September 24, 2003.
Address correspondence to P. M. Boiselle
(pboisell{at}caregroup.harvard.edu).
Abstract
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CONCLUSION. Volumetric high-resolution CT is associated with significantly greater motion artifact compared with axial noncontiguous high-resolution imaging.
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We used our hospital record system to retrospectively identify a consecutive series of patients who underwent imaging using this protocol between January 2001 and October 2002. For each patient, volumetric and axial high-resolution CT images were reviewed at three anatomic levels (upper, mid, and lower lung zones) by two observers using a PACS (picture archiving and communication system) workstation (Centricity, General Electric Medical Systems, Milwaukee, WI). The upper lung zone was defined as the level of the aortic arch; the mid lung zone, as the level of the carina; and the lower lung zone, as the level of the right inferior pulmonary vein. For each patient, the volumetric and axial high-resolution CT images obtained closest to these anatomic landmarks were analyzed for evidence of motion artifact, including double-imaged structures, such as doubling of fissures, vessels, or bronchi or doubling of the contour of the heart, mediastinum, or diaphragm; blurring of pulmonary vessels; and pulsation artifacts [5].
The degree of motion at each anatomic level was graded by consensus agreement using the following scale: 0, no motion; 1, mild motion artifact; 2, moderate motion artifact; and 3, severe motion artifact. A total motion score for each patient was calculated by summing the scores from each of the three lung zones. Thus, the total motion score could range from 0 to 9, with scores from 1 to 3 defined as mild; 4–6, moderate; and 7–9, severe.
Statistical analysis was performed using Wilcoxon's rank sum test. A p value of less than 0.05 was considered statistically significant.
The effective radiation doses for the volumetric and axial sequences on the 8-MDCT scanner were calculated using commercially available software.
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The total motion scores are as follows: For the volumetric high-resolution CT images, the median score was 3.0 (range, 1–9) and for the axial high-resolution CT images, the median score was 2.0 (range, 1–9). This difference was statistically significant (p = 0.02). At each of the three levels that were analyzed, the mean score for motion artifact was higher for volumetric high-resolution CT images than for axial high-resolution CT images. The difference in motion score between volumetric and axial high-resolution CT methods was slightly greater in the upper and middle lung zones compared with the lower lung zones (0.2 vs 0.1, respectively). The mean values at the upper, middle, and lower lung zones for the volumetric versus axial CT images were as follows: 0.9 versus 0.7, 1.0 versus 0.8, and 1.3 versus 1.2, respectively.
The effective radiation doses were 6.17 mSv for the volumetric sequence and 0.58 mSv for the axial high-resolution CT sequence. The addition of the axial sequence increased the total dose of the procedure by approximately 9%.
The estimated additional time for performing the axial sequence, including changing the position of the patient and obtaining a series of individual axial high-resolution CT images, was approximately 6 min.
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Schoepf et al. [4] found no significant difference in overall image quality when comparing high-resolution CT images obtained using volumetric MDCT with axial high-resolution CT images acquired using single-detector CT. However, the two methods of imaging were compared in two separate patient populations. Differences between the two study groups, such as the type and extent of disease, could have potentially introduced bias in that study. In contrast, in our study, both techniques were performed in the same patients, eliminating this potential bias.
Prone high-resolution CT images have been shown to be helpful for
distinguishing subpleural ground-glass opacity from dependent atelectasis on
supine images in selected populations
[6] (Fig.
1A,
1B). Our data suggest that the
addition of several axial prone high-resolution CT images to a volumetric MDCT
study also provides higher-quality high-resolution CT images with less motion
artifact. However, we did not directly compare the ability to make specific
diagnoses with these methods. In our clinical experience, we have found that
higher-quality axial high-resolution CT images are most helpful for allowing
confident exclusion of disease in patients without evidence of interstitial
lung disease and for increasing diagnostic confidence for the detection of
subtle interstitial disease. We believe that the potential advantage of the
axial high-resolution CT sequence outweighs the slight increase (
9%) in
radiation dose and the additional few minutes of time added to the
procedure.
A potential limitation of our study is that the volumetric and axial high-resolution CT images were obtained with patients in a different position, supine and prone, respectively. Although differences in positioning might have contributed to the differences in respiratory motion, we believe that the primary causative factor for this difference is the longer breath-hold required for volumetric high-resolution CT images than for individual axial high-resolution CT images. Future studies comparing volumetric and axial high-resolution CT with the patient in the same position would be helpful to confirm our findings.
The difference in respiratory motion that we observed between the two techniques was likely minimized by the caudal-to-cranial imaging direction that we used for the volumetric sequence; this factor has been shown to reduce respiratory motion, particularly in the lower lung zones. Because the middle and upper lung zones were imaged later in the breath-holding period, it is not surprising that we observed a slightly greater difference in motion artifact between volumetric and axial high-resolution CT methods in these regions than in the lower lung zones. The relatively higher level of motion artifact that was observed in the lower lung zones compared with other zones with both imaging methods reflects the fact that the lower lung zones are more susceptible to motion from the heart and diaphragm.
In summary, axial high-resolution CT images have significantly less motion artifact than volumetric high-resolution CT images. Because of the higher quality of these images and the capability to reveal the cause of dependent opacities, the addition of several prone axial high-resolution CT images may prove to be a useful routine addition to volumetric supine high-resolution CT studies.
Acknowledgments
We thank John F. Copeland for assistance with radiation dose measurements,
Michael Larson for photography, and Alexis Potemkin for administrative
assistance.
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