Comparison of the rebuilding effects of different computed tomography scanners and reconstructive settings for the five-line sign in normal interlobular fissures

Multi-slice spiral computed tomography (CT)-based thin reconstructed images facilitate the clear presentation of various structures and lesions in the lung from any angle [1–3]. Reconstructions based on the maximum intensity projection (MIP) algorithm can reveal the highest level of continuity of linear, high-density vascular structures in the lung. Therefore, this is an excellent method to differentiate vascular sections and small pulmonary nodules on a thin-section image [4, 5].

Using a 64-slice spiral CT scanner, we previously noticed that normal interlobular fissures presented as multiple parallel shadow lines in MIP-reconstructed images, while the number of shadow lines was equal to the ratio of the thickness of the MIP reconstruction layer to the thickness of the original layer. A five-line sign could be generated if the following parameters were adopted: 1.25-mm layer thickness, 1.25-mm space, and 6.25-mm MIP slab thickness [6]. The five-line sign of interlobular fissures could be used to differentiate the underdeveloped interlobular fissures and the accessory fissures from pulmonary linear fibrosis.

In this study, we examined the effect of two different CT units (16-slice or 256-slice), using 1.0-mm reconstruction layer thicknesses, 1.0-mm space, and also added a high-resolution reconstruction algorithm on the clarity rate and level of the five-line sign.

The following settings were used for both CT units: 1.0-mm reconstruction thickness, 1.0-mm space and 5.0-mm MIP slab thickness. Correspondingly, both units generated MIP reconstruction images at the axial position that revealed parallel five-line shadow lines for the normal pleura. In other words, the number of shadow lines was equal to the ratio of the thickness of the MIP-layer to the thickness of the original layer (Fig. 3). However, when the images derived from the two units were reconstructed by oblique MIP, the workspace added 1 mm by itself to reach a thickness of 6 mm before the five-line shadow lines could be displayed. As the MIP thickness was increased, the line number doubled, resulting in a gradual reduction in the density. Consequently, the surrounding structures blended into the shadow lines, and the external lines gradually disappeared (Fig. 4).

When the standard oblique algorithm was used, the rates of assignment to clarity level 3 from six locations were 70.3% for the 16-slice CT and 74.3% for the 256-slice CT, which were not significantly different (χ² = 1.199, p = 0.273). In the lower left zone, however, 256-slice CT yielded significantly superior results to 16-slice CT (60% versus 34%; χ² = 6.784, p = 0.009). If data from level 2 (barely clear) were included, the total clarity rates of the six locations increased to 100% and 97.3% for 16-slice CT and 256-slice CT, respectively.

Inter-observer agreement for the clarity of the five-line sign for the high resolution axial algorithm, the high-resolution oblique algorithm, the standard axial algorithm, and the standard oblique algorithm was represented by κ values of 0.142, 0.559, 0.679, and 0.922 for the 16-slice CT scanner and 0.125, 0.736, 0.617, and 0.827 for the 256-slice CT scanner, respectively. Thus, there was almost perfect agreement for the standard oblique algorithm for both CT scanners.

In the current work, the results obtained from this investigation indicated that the phenomenon of the five-line sign, which we identified previously using a 64-slice CT scanner [6], was present when using different types of CT scanners and under different reconstruction settings.

The phenomenon of the five-line sign or multi-line reflections, which appear within axial MIP reconstructions of interlobular fissures, may be caused by partial volume averaging in the lung [6, 8, 9] because the signs appear to be the beaded vessels exhibited by MIP-reconstructed pulmonary microvasculature, which was first proposed by Napel [8]. Evidence indicated that the beaded vessels and the multiple shadow lines of interlobular pleura belong to the same type of artificial shadows [6].

Of note, the 1.0-mm thin-slice is commonly used for three-dimensional reconstruction of lung microstructures and nodules [10–12]. We determined that the 1.0-mm thickness is the best for MIP-reconstruction of interlobular fissures, and that too thick or too thin slice thickness is not advisable [6]. In addition, the reconstruction space should be equal to the slice thickness. When the space is larger or smaller than the thickness, overlapping of multiple shadow lines or wider or narrower gaps between the lines can occur. Here, we used 1.0-mm thickness data derived from two CT scanners for oblique MIP reconstructions. In the Philips Brilliance™ Workspace Portal, the MIP reconstruction thickness must be increased to 6.0 mm before the five-line sign of interlobular fissures can be revealed.

In this study, high-resolution algorithms produced worse results than the standard-resolution algorithm with respect to presenting the five-line sign of the interlobular fissures. We reasoned that in comparison with a standard algorithm, the high-resolution method decreased the image smoothness and increased the sharpness, leading to a clearer microstructural edge in the lung parenchyma. However, this effect also increased image noise that may have overshadowed the density of the pulmonary microstructure.

The purpose of MIP reconstruction is to enhance the brightest pixels, thereby augmenting the contrast [8, 13]. When applying MIP reconstruction to study diffuse lung lesions, Remy-Jardin et al. [5] used high spatial-frequency reconstruction kernels to facilitate the reconstruction of transverse images in the lung window and noticed reduced intensity of the beaded vessels. This phenomenon was limited to the subpleural area and had poor imaging quality. This finding easily overcomes the challenge of distinguishing the string-of-beads artificial shadow of MIP-reconstructed microvasculature from the micronodules in diffuse lung lesions. Thus, the control of image noise is a key factor to determine the visibility of the five-line sign (Fig. 3b1-2). Remy-Jardin et al. [5] revealed some pulmonary micronodules that were less than 3 mm in diameter and had low density and a blurred boundary. MIP reconstruction with 8-mm thickness typically cannot reveal these microstructures, whereas 5-mm thickness is relatively ideal for displaying them. These findings indicate that increasing the MIP thickness may mask low-density structures in the lung, which is consistent with the observation in this study (Fig. 4).

When the standard axial algorithm was used for MIP reconstruction of the five-line sign, the low-clarity rates were 45% and 48% for the 16-slice and 256-slice CT scanners, respectively. In contrast, when the standard oblique algorithm was used, the low-clarity rates decreased to 0% and 2.7% for the 16-slice and 256-slice CT scanners, respectively, because the oblique lines rotated in areas where the oblique fissures were oriented at steep angles, such as the lower segments of the right oblique fissures and all three segments of the left oblique fissures. As a consequence, the oblique fissures and slices formed right angles that are exhibited as clear or barely clear in the most oblique multiplanar reformation images where they otherwise would have had an unclear axial MIP five-line sign, as shown in Fig. 2. In addition, for those areas where the five-line sign was clearly presented by the axial algorithm, the clarity level was not compromised by minor angle changes in the oblique plane. Under the generation of the five-line sign during the MIP reconstruction of interlobular fissures, the fissures and scanning plane must form a suitable angle, which can be easily accomplished by angle adjustment via multiplanar reformation on the sagittal plane of the workstation. The data in Table 4 show that the median of angles between the tangent lines and the fissures were − 27° to − 36° counter clockwise in most cases; the median of the right upper segments was the smallest, in most of which the five-line sign could be revealed in the axial MIP images directly (Table 3), and the median of the left lower segments was the biggest. The minor fissures were oriented at gentle angles with physical curvature (Fig. 2c), and the oblique lines had to be rotated clockwise or counter clockwise to form right angles between the oblique planes and fissures to show the five-line sign in some cases.

In most areas where the five-line sign could be discerned after adjustment of the oblique angle, the axial MIP images all contained band-like patterns with high density and clear boundaries. In comparison, in a few sites where the five-line sign was invisible after oblique angle adjustment, interlobular fissures in the axial MIP images were all manifested as a stripe-like pattern with low density and blurring boundaries.

This result was corroborated by the 2.7% of the samples derived from 256-slice CT and the standard oblique algorithm that failed to produce the five-line sign. These findings indicate that the proper density of interlobular fissures and suitable angles between the fissures and the section plane are necessary for the formation of the five-line sign. This requirement also explains why only the small blood vessels that are oblique to the MIP planes are manifested as strings of beads. Furthermore, the angle between the interlobular fissures and the scanning plane not only influenced the display rate of the five-line sign but also dictated the width of the lines [6]. Finally, in the lower right segment calculated with the high-resolution oblique algorithm, and the lower left segment calculated with the standard oblique algorithm, 256-slice CT displayed superior performance to 16-slice CT based on the rate of grading of the five-line sign as clarity level 3. This result may have been observed because imaging of the lower lobes of the lung was strongly affected by the heartbeat; therefore, 256-slice CT generated clearer images because of the faster scanning speed.

To outline the clinical usefulness of the five-line sign, we should consider that normal interlobular fissures feature regular morphology, smooth appearance, and uniform thickness. The MIP-rendered five-line sign has smooth lines and clear gaps and can, therefore, be used as the referencing object for other planar structures in the lung. Certain pulmonary pathological features, such as the pleural tail sign [14–16], represented a form of thickened septa [15]; the five-line sign revealed by MIP reconstruction suggested that it was a relatively smooth and regular plane, and it is difficult to determine using multiplanar or thin-section reconstruction. As we observed that most nodules with the pleural tail sign had an unclear or absent five-line sign, we speculated that the plane was rough and irregular; only a small number of nodules with the tail sign showed a clear five-line sign by MIP rebuilding, but these lines were thicker and denser than those of normal interlobular fissures. Simultaneously, it was noticed in our limited experiments that the tumour was at an advanced stage in most of the cases of lung cancer in which the five-line sign was unclear or absent, and relapse and metastasis usually occurred after resection [17]; whereas a small number of peripheral lung cancers with a clear five-line sign were at an earlier stage and no recurrence and metastases occurred following resection; these patients survived for more than 5 years disease-free [17]. In addition, a doubling time exceeding 800 days was observed in several tumours with a barely clear five-line sign and without experiencing being resected surgically. That implied character of indolent growth of those lung cancers with five-line sign. The cases of lung cancer mentioned involved almost completely solid nodules. Recent clinical studies reported the pleural tail sign or pleural indentation as having different clinical effects [16, 18, 19]; parts of these investigations focused upon pulmonary adenocarcinomas appearing as sub-solid nodules or ground-glass nodules [18, 19]. Other scholars have applied volume rendering reconstruction technique to predict whether the lung cancer invades the visceral or parietal pleura [20]. Anyhow, the authors of this article attempted to investigate the five-line sign of peripheral lung cancer through MIP reconstruction and to evaluate the effect of the five-line sign on staging and prognosis in peripheral lung cancer, especially when represented as a solid nodule; we consider that it would be worth carrying out further investigation on the basis of the large number of cases accumulated. This is the reason why we explored the five-line sign of the normal interlobular fissures using MIP reconstruction.

This study has limitations. The results were obtained from two different scanners only. Conventional scan parameters were used with half the subjects studied without tube current modulation. We did not obtain data on subjects scanned with both CT scanners (inter-individual and not intra-individual design).

In conclusion, the five-line sign of interlobular fissures can be revealed in MIP reconstructed images derived from different CT scanners. The standard oblique algorithm generated the best reconstruction outcome, whereas the clarity levels were not significantly different between the two CT scanners. The MIP-rendered five-line sign of normal interlobular fissures can be used as the referencing object for other planar structures in the lung.