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1 Department of Radiology, Osaka University Graduate School of Medicine, 2-2
Yamadaoka, Suita, Osaka, 565-0871, Japan.
2 Department of Radiology, Osaka University Hospital, Osaka, 565-0871,
Japan.
3 Department of Radiology, Vancouver Hospital and Health Sciences Centre and
University of British Columbia, 855 W. 12th Ave., Vancouver, B. C. V5Z 1M9,
Canada.
Received January 2, 2002;
accepted after revision March 29, 2002.
Address correspondence to 0. Honda.
Abstract
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MATERIALS AND METHODS. Axial multidetector CT (MDCT) scans were obtained through the entire lung in 10 normal autopsy lung specimens using an MDCT scanner. Four protocols were used: 0.5-mm collimation with a 0.5-mm reconstruction interval; 0.5-mm collimation with a 0.3-mm reconstruction interval; 1-mm collimation with a 0.5-mm reconstruction interval; and 2-mm collimation with a 1-mm reconstruction interval. Multiplanar reconstruction images with 0.5-mm slice thickness were obtained from the four types of data sets. Direct coronal thin-section CT of the same 10 autopsy lung specimens was performed using 0.5-mm scan collimation, a 0.3-mm reconstruction interval, a 25.6-cm field of view, and a 512 x 512 matrix. Two independent observers compared the image quality of each of the four coronal multiplanar reconstruction sets with that of direct coronal thin-section CT scans. The observers analyzed visualization of anatomic features and artifacts.
RESULTS. The total image quality of the multiplanar reconstructions obtained from 0.5-mm collimation data with or without 0.3-mm overlapping reconstruction was equal to that of direct coronal thin-section CT scans in all 20 interpretations. The image quality of multiplanar reconstruction images from 0.5-mm collimation data either with or without overlapping reconstruction was superior to multiplanar reconstruction images obtained from 1- or 2-mm collimation scans (p < 0.01, Fisher's exact test). Stairstep artifacts in multiplanar reconstructions using 0.5-mm collimation without overlapping reconstruction were equal to those with overlapping reconstruction and were fewer than those on 1- or 2-mm collimation (p < 0.01, Mann-Whitney U test).
CONCLUSION. The image quality of coronal multiplanar reconstructions from isotropic voxel data obtained using 0.5-mm collimation, with or without overlapping reconstruction, is similar to that of direct coronal thin-section CT scans.
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Optimal resolution on thin-section CT is achieved using the highest matrix size (512 x 512) and the lowest field of view that will include the entire region of interest. CT performed using 1-mm collimation and a 512x512 matrix requires a field of view of 51.2 cm to obtain isotropic voxel data sets. The introduction of MDCT scanners with the ability to obtain 0.5-mm-thick sections allows the reduction of the field of view to 25.6 cm.
To our knowledge, no study has evaluated the image quality of multiplanar reconstruction lung images obtained from isotropic voxel data sets. The purpose of this study was to compare the image quality of coronal multiplanar reconstructions of MDCT scans performed through normal autopsy lung specimens with that of direct coronal thin-section MDCT scans.
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Axial continuous MDCT scans were obtained on an MDCT scanner (Aquillion-VZ; Toshiba, Tokyo, Japan) using the following protocols: 0.5-mm collimation with a 0.5-mm reconstruction interval; 0.5-mm collimation with a 0.3-mm reconstruction interval; 1-mm collimation with a 0.5-mm reconstruction interval; and 2-mm collimation with a 1-mm reconstruction interval. The scanned distances were all 25.6 cm. All scans were obtained using a 6:1 pitch, 0.5 sec per rotation, 120 kV, 100 mAs, a 25.6-cm field of view, a 512x512 matrix, and a high-frequency reconstruction algorithm. As the control for image quality, direct coronal thin-section CT scans of the same 10 autopsied lungs were obtained on the same MDCT scanner. Direct coronal thin-section CT was performed using 0.5-mm scan collimation, a 6:1 pitch, a 0.3-mm reconstruction interval, 0.5 sec per rotation, 120 kV, 100 mAs, a 25.6-cm field of view, a 512 x 512 matrix, and a high-frequency reconstruction algorithm.
All images were transferred to a commercially available workstation (Advantage Windows 3.0; General Electric Medical Systems, Milwaukee, WI), and coronal multiplanar reconstruction images with 0.5-mm slice thickness and 0.5-mm slice intervals were obtained from the four axial MDCT data sets. All coronal multiplanar reconstruction images were reconstructed to fit the direct coronal thin-section CT images on the workstation. All four sets of coronal multiplanar reconstructions and the direct coronal thin-section CT scans were photographed separately on hard copies using a window width of 1200 H and a window level of—700 H.
After the CT examination, transverse sections were obtained from the same lungs every 10 mm and were stained with H and E and elastica—van Gieson stains for histopathologic assessment. Macroscopic and microscopic assessment of the lungs showed no parenchymal abnormalities.
Image quality of each coronal multiplanar reconstruction image was evaluated as being equal to or inferior to that of the direct coronal thin-section CT scan by two independent pulmonary radiologists who paid special attention to visualization of the following six normal structures: the interlobular septa, the central bronchovascular bundles coursing vertically, the central bronchovascular bundles coursing horizontally, the peripheral bronchovascular bundles, the interlobar fissures, and the pulmonary parenchyma. In addition, the total image quality of each coronal multiplanar reconstruction image was subjectively evaluated as being equal to or inferior to that of direct coronal thin-section CT scan. Stairstep artifacts of each multiplanar reconstruction image were graded as 1, none; 2, mild; or 3, severe, compared with the direct coronal thin-section CT scan. We required that at least 4 weeks elapse between each set of interpretations.
The difference in image quality of each multiplanar reconstruction image was analyzed using Fisher's exact probability test. The scores of stairstep artifacts of each multiplanar reconstruction were evaluated using the Mann-Whitney U test. The interobserver variation of the image quality between two reviewers was evaluated using kappa statistics. The interobserver agreement was classified as follows: poor, 0-0.20; fair, 0.21-0.40; moderate, 0.41-0.60; good, 0.61-0.80; and excellent, 0.81-1. The interobserver variation for noise and stairstep artifacts was analyzed using Spearman's correlation coefficient.
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= 0.76) for the evaluation of
image quality and stairstep artifacts (Spearman's correlation coefficient,
r > 0.7; p < 0.01). Therefore, the statistical
analysis of the comparison of image quality among the various sets of
multiplanar reconstruction images was performed by combining the results of
the two observers. The image quality results are summarized in Table 1. The total image quality of multiplanar reconstructions obtained from the 0.5-mm collimation data sets with or without 0.3-mm overlapping reconstruction was equal to that of the direct coronal thin-section CT scans in all 20 interpretations (10 lungs, two observers). The image quality of the other multiplanar reconstructions from 0.5-mm collimation data sets with or without 0.3-mm overlapping reconstructions was also equal to that of direct coronal thin-section CT scans except in one case of interlobular septa. The total image quality of multiplanar reconstruction images from 0.5-mm collimation data sets either with or without overlapping reconstruction was superior to that of multiplanar reconstructions obtained from 1-and 2-mm collimation data sets (Fisher's exact probability test, p < 0.01) (Fig. 1A,1B,1C,1D,1E).
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Stairstep artifacts in multiplanar reconstruction images are summarized in Table 2. The scores of stairstep artifacts in multiplanar reconstructions obtained from 0.5-mm collimation data sets without overlapping reconstruction were equal to those obtained from 0.5-mm collimation data sets with overlapping reconstruction. The scores of stairstep artifacts in multiplanar reconstructions obtained from 0.5-mm collimation data sets with overlapping reconstruction were superior to those from 1- and 2-mm collimation data sets (Mann-Whitney U test, p < 0.01).
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In this study, we used 0.5-mm collimation, a 25.6-cm field of view, and a 512 x 512 matrix. Pixel size in the x—y plane was calculated as follows: 256-mm field of view / 512 pixels = 0.5 mm. Therefore, isotopic imaging has been theoretically achieved. The results of the study showed similar image quality for coronal multiplanar images reconstructed from axial scans obtained using 0.5-mm collimation and for direct coronal CT scans. Furthermore, these multiplanar reconstructions had no stairstep artifacts. These findings confirm the hypothesis that isotropic imaging was achieved.
Caldemeyer et al. [8] reported that the diagnostic efficacy of coronal multiplanar reconstruction images with 0.5-mm collimation on dual-detector helical CT was comparable to or superior to that of direct coronal CT scans with 1-mm collimation on temporal bone. Venema et al. [9] also reported that coronal multiplanar reconstructions from axial dual-detector helical CT obtained with 0.5-mm collimation could replace direct coronal sequential CT scans of petrosal bone. In their study, overlapping reconstruction was used. However, our results suggest that overlapping reconstruction is not necessary for coronal multiplanar reconstructions from isotropic voxel data sets for lung parenchyma.
Our study has several limitations. First, because we used autopsy lung specimens, it was not possible to evaluate the influence of motion artifacts. However, ethical concerns exist about performing repeated CT in any given patient. Furthermore, it is not possible to obtain direct coronal images of human lungs using commercially available CT scanners. Second, the study was limited to normal lungs. It was reported that MDCT high-resolution CT images of various modes were compared with single-detector high-resolution images in the axial mode, and high-resolution MDCT images of a 6:1 pitch were unsuitable for detecting subtle lung abnormalities on axial MDCT images of 1.25- and 2.5-mm collimation [10]. In our study, we used a pitch of 6:1 and 0.5-mm collimation. We chose 0.5-mm collimation to obtain the multiplanar reconstruction images from isotropic voxels, and a 6:1 pitch to cover a large area in one breath-hold. If axial high-resolution CT is used for direct coronal high-resolution CT instead of a high-resolution MDCT pitch of 6:1, multiplanar reconstruction images from MDCT data of a 6:1 pitch would be inferior to direct coronal high-resolution axial CT scans. Further study is needed with scanning of abnormal lungs, especially lungs with subtle abnormalities. If more rapid scanning could be achieved, it would be possible to obtain superior multiplanar reconstruction images of the whole lung from isotropic voxel data. The third potential limitation is a possible bias because the observers knew which of the images were obtained using the direct coronal technique, which may have biased them to make a superior evaluation of the multiplanar reconstruction images with 0.5-mm collimation. However, comparing the images with direct coronal images has merit to test for the inferiority of multiplanar reconstruction images as well. As for the multiplanar reconstructions, the observers did not know how the images were scanned and reconstructed; therefore, their findings—that the image quality of coronal multiplanar reconstruction images from isotropic voxel data sets without overlapping reconstruction is almost identical to that with overlapping reconstruction—are reliable.
In conclusion, the results of our study indicate that the image quality of coronal multiplanar reconstructions obtained from isotropic voxel data sets is similar to the image quality of direct coronal thin-section CT scans. The image quality of coronal multiplanar reconstructions from isotropic voxel data sets without overlapping reconstruction is almost identical to that of sets with overlapping reconstruction.
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