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1
Department of Radiology, Osaka University Medical School, 2-2 Yamadaoka,
Suita, Osaka, 565-0871, Japan.
2
General Electric Yokogawa Medical Systems, 4-7-127 Asahigaoka, Hino, Tokyo,
191-0065, Japan.
Received December 28, 2000;
accepted after revision March 22, 2001.
Address correspondence to O. Honda.
Abstract
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SUBJECTS AND METHODS. Multidetector HRCT with six parameters and single-detector HRCT were performed on cadaveric lungs. The image quality and diagnostic efficacy of multidetector HRCT were evaluated in comparison with those of single-detector HRCT. A phantom was scanned, and image artifact and noise were investigated.
RESULTS. The image quality of multidetector HRCT with axial 1.25 mm x 4i (four images per gantry rotation) mode was equal to that of single-detector HRCT. The image quality of multidetector HRCT with other modes was worse than that on single-detector HRCT. The diagnostic efficacy of multidetector HRCT with high-quality mode (pitch, 3:1) and axial mode was equal to that of single-detector HRCT. The diagnostic efficacy on multidetector HRCT with high-speed mode (pitch, 6:1) was worse than that on single-detector HRCT. In the phantom study, images made in high-speed mode had strong artifacts. Noise in the axial mode was milder than that in high-speed mode but more severe than that in high-quality mode.
CONCLUSION. The image quality of axial HRCT with multidetector CT is equal to that on single-detector HRCT. Axial HRCT with multidetector CT is appropriate for evaluating subtle lung abnormalities, but high-speed mode is unsuitable. Using the high-quality mode degrades image quality but is still worthwhile.
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Originally, single-detector CT was used to obtain HRCT images. Recently, multidetector CT has been developed and brought into daily clinical practice. Multidetector CT is a promising tool for the evaluation of lung parenchyma because the fundamental advantages of this technique include shorter acquisition times and retrospective creation of both thinner and thicker sections from the same raw data [4]. Acquisition time of HRCT with multidetector CT is so short that whole-lung HRCT can be performed in one breath-hold. To our knowledge, however, the suitability of the image quality of multidetector HRCT for evaluating various pulmonary diseases has not been investigated. The aim of the our study was to compare the images of single-detector HRCT with those of multidetector HRCT that were scanned by various parameters and to evaluate the image quality and diagnostic efficacy of multidetector HRCT. In addition, the influence of noise artifact was investigated by a phantom.
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Eleven cadaveric lungs had various CT findings, including areas with ground-glass attenuation (n = 7), areas of air-space consolidation (n = 1), emphysema (n = 1), faint centrilobular nodules (n = 3), small non-lobular nodules (n = 1), large nodules (n = 1), inter-lobular septal thickening (n = 2), intralobular reticular opacities (n = 2), and honeycombing (n = 2). After CT, transverse microscopic sections were performed from these 11 lungs and stained with H and E and elastica-van Gieson for histopathologic diagnosis. Pathologic diagnoses of these 11 lungs included the the usual interstitial pneumonia (n = 2), nonspecific interstitial pneumonia (n = 1), diffuse alveolar damage (n = 1), diffuse panbronchiolitis (n = 1), pulmonary tuberculosis (n = 1), pulmonary emphysema (n = 1), pulmonary hemorrhage (n = 1), cardiogenic edema (n = 1), pulmonary metastases (n = 1), and pulmonary lymphangitic carcinomatosis (n = 1).
Two reviewers independently compared each HRCT image on multidetector CT with matching HRCT images on single-detector CT, placing them side by side without the knowledge of the histopathologic diagnoses and image-acquisition parameters of multidetector CT. They independently graded the diagnostic efficacy of HRCT on multidetector CT as sufficient for diagnosis (level 1) or insufficient (level 2) and the image quality of HRCT on multidetector CT as equal to (level 1) or inferior to (level 2) that of HRCT on single-detector CT. The interobserver variation between the two reviewers was analyzed by using kappa statistics.
In addition, a phantom for high-contrast spatial resolution, including acrylic spheres with 2- to 10- mm diameters (Helical Phantom type HT-200; Kyoto Kagaku, Kyoto, Japan), was scanned with the same parameters as those of the examinations of cadaveric lungs. In the examination, noise and artifact were evaluated. Noise values were calculated by measuring the standard deviation (SD) values in a circular region of interest (844 mm2) defined by an electric cursor on the scanner workstation [6]. The regions of interest were placed in eight homogeneous parts of the phantom and remained in the same locations of the phantom for each scan parameter. The obtained data were statistically analyzed with an unpaired t test. The existence of artifacts on HRCT images with each parameter was visually decided by the consensus of two reviewers.
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= 0.71) and fair agreement for
diagnostic ability ( = 0.38). Therefore, the analysis was performed by
combining the decisions of the two reviewers. The result of image quality is summarized in Figure 1. The image quality of multidetector HRCT by using axial 1.25 mm x 4i mode was equal to that of single-detector HRCT in all cases (100%) (Fig. 1). The image quality of multidetector HRCT with other modes was inferior to that of single-detector HRCT, particularly the helical high-speed mode at 2.5 mm. The results of diagnostic efficacy are summarized in Figure 2. The diagnostic efficacy of multidetector HRCT compared with any helical 1.25-mm high-quality mode, helical 2.5-mm high-quality mode, axial 1.25 mm x 4i mode, and axial 2.5 mm x 2i mode was equal to that of single-detector HRCT (100%) (Fig. 2). In one case (5%) with helical 1.25-mm high-speed mode and four cases (18%) with helical 2.5-mm high-speed mode, the diagnostic efficacy of multidetector HRCT was slightly worse than that of single-detector HRCT. On multidetector HRCT with helical 1.25-mm high-speed mode, intralobular reticular opacities were blurred in one case. The blurring of intralobular reticular opacities in two cases, obscurity of interlobular septal lines in one case, and vagueness of faint centrilobular nodules in one case were reasons that multidetector HRCT with helical 2.5-mm high-speed mode was determined as insufficient for diagnosis (Fig. 3A,3B,3C,3D,3E,3F,3G).
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The SD values of eight regions of interest on multidetector HRCT of a phantom are described in Figure 4. There was no significant difference between the SD values of multidetector HRCT with the axial 1.25 mm x 4i mode and those of helical CT with the 1.25-mm high-quality mode. However, there were significant differences between the SD values of the axial mode and those of helical high-quality mode (p < 0.01) at 2.5-mm collimation, between the axial mode and helical high-speed mode (p < 0.05) at 1.25-mm and 2.5-mm collimation, and between the helical high-quality mode and helical high-speed mode (p < 0.01) at 1.25-mm and 2.5-mm collimation. There were also significant differences between the SD values of 1.25-mm collimation and those of 2.5-mm collimation in each scan mode on multidetector CT (p < 0.001). The prominent artifacts were radiated from spheres on multidetector HRCT with helical 1.25-mm high-speed mode and helical 2.5-mm high-speed mode (Fig. 5A,5B,5C,5D). These artifacts were more conspicuous on multidetector HRCT with high-speed mode than on high-quality mode (Fig. 5A,5B,5C,5D).
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Multidetector CT is equipped with a multiplerow detector array. The physical performance of a dual-slice CT scanner, introduced previously, has been investigated by Liang and Kruger [10]. For a given table speed and individual detector collimation, the dual-slice scan results in better longitudinal resolution compared with a single-slice scan if the scan is obtained with nonoverlapping slices (pitch > 2). This improvement is because the dual-slice scan obtains twice the number of nonoverlapped projections for the same length. This increase reduces the degradation of the slice profile because of more densely arranged projections (in the longitudinal direction) for the interpolation. In the dual-slice scanner, the workable scan rate is extended up to a pitch of 4, which compares with a pitch of 2 for the single-slice scanner. In our study, multidetector CT is equipped with 16 detector arrays in the longitudinal direction, and four-slice CT images can be obtained per rotation of the X-ray-beam collimation. Therefore, multidetector CT has the capability of rapidly scanning a large longitudinal (z-axis) volume with high z-axis resolution [4]. McCollough and Zink [11] reported that the multislice helical CT reconstruction algorithm produced images with section-sensitivity profiles comparable to a pitch of 1.0-1.5 for single-slice systems at three to six times the table speed.
In our study, the images of multidetector HRCT with helical high-speed mode (pitch, 6:1) showed blurring of intralobular reticular opacities, obscurity of interlobular septal lines, or vagueness of faint centrilobular nodules, which could lead to misdiagnosis. Increased table speed may increase the volume-averaging artifact and may result in indistinctness of subtle pulmonary abnormalities. Although the images on multidetector helical CT have higher z-axis resolution than those on single-detector helical HRCT, the full width at half-maximum of multidetector helical CT with high-speed mode with a pitch of 6:1 is equal to that of single helical CT with a pitch of 2:1 [4]. Increasing helical pitch leads to a decrease in the amount of photons, which makes the images unclear. McCollough and Zink [11] tested in the axial mode, helical high-quality mode, and helical high-speed mode with a multislice CT scanner and reported that helical artifacts and geometric distortion were more pronounced in some high-speed helical modes. Therefore, in evaluating the subtle parenchymal abnormalities such as faint nodules or thin linear opacities, axial HRCT or multidetector HRCT with helical high-quality mode (pitch, 3:1) should be more useful than helical high-speed mode (pitch, 6:1). However, dense centrilobular nodules, nodules, bronchiectasis, traction bronchiectasis, ground-glass attenuation, consolidation, emphysema, and honeycombing can be depicted even on multidetector HRCT with helical high-speed mode. Multidetector HRCT with helical high-speed mode can be used for evaluating these findings because multidetector HRCT with helical high-speed mode sufficiently illustrate abnormalities such as dense centrilobular nodules, nodules, bronchiectasis, traction bronchiectasis, ground-glass attenuation, consolidation, emphysema, and honeycombing.
Noise is also one of the important factors for image quality [7,9]. In our phantom study, the artifact was more conspicuous on helical 2.5-mm high-speed mode than on the helical 1.25-mm high-speed mode, but noise was milder on helical 2.5-mm high-speed mode than on helical 1.25-mm high-speed mode. In addition, noise of the axial 4i mode was greater than that of the helical high-quality mode with the multidetector HRCT images on 2.5-mm collimation in our phantom study, although the image quality of axial 4i mode was superior to that of helical high-quality mode in our study with cadaveric lungs. Therefore, the poor quality of the helical mode, especially of the helical high-speed mode, is mainly due to artifact.
There are several limitations in our study. First, this study included a small number of cases. Second, because we used lung specimens, there was no evaluation of the influence of motion artifact on image quality. Severe motion artifacts blur the detail of pulmonary parenchyma, and subtle motion artifacts can cause false images of pulmonary parenchyma. Principally, motion artifacts are caused by respiratory and cardiac motion. It is reported that the double fissure occurs most commonly at the base of the left lung, suggesting that cardiac motion is the cause [12]. Tarver et al. [13] also reported that a double image of the pulmonary vessels, which is caused by respiratory and cardiac motion, is similar to bronchiectasis. Therefore, it is necessary to investigate how motion artifacts of multidetector HRCT influence the image quality. If motion artifacts were investigated, image quality of multidetector HRCT might be better than that of single-detector HRCT. Third, multidetector CT scanners from different manufacturers have various kinds of detector configuration [11]. Thus, the results of our study may not apply to other manufacturers' scanners. Finally, the image quality and diagnostic efficacy of various kinds of multidetector HRCT were evaluated subjectively. However, the object of this study was to evaluate the image quality and diagnostic ability of various kinds of multidetector HRCT by comparing them with those of single-detector HRCT. The object was achieved, we believe, considering the support of the phantom.
In conclusion, the image quality of axial HRCT with multidetector CT is equal to that with conventional single-detector CT. The image quality of helical HRCT with multidetector CT is worse than that of axial HRCT with single-detector CT because of the blurring of faint parenchymal abnormalities. However, diagnostic efficacy is equal except for the high-speed helical mode. Axial and high-quality helical HRCT with multidetector CT is satisfactory for evaluating subtle abnormalities of the lung.
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= mean value.
