Image Quality from High-Resolution CT of the Lung: Comparison of Axial Scans and of Sections Reconstructed from Volumetric Data Acquired Using MDCT

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1 Studler et al. High- Resolution CT and MDCT of the Lung Chest Imaging Original Research Ueli Studler 1 Thomas Gluecker Georg Bongartz Jakob Roth Wolfgang Steinbrich Studler U, Gluecker T, Bongartz G, Roth J, Steinbrich W Received June 27, 4; accepted after revision October 22, 4. 1 All authors: Department of Radiology, University Hospital Basel, Petersgraben 4, Basel, Switzerland. Address correspondence to U. Studler (studleru@tiscali.ch). AJR 5; 185: X/5/ American Roentgen Ray Society Image Quality from High-Resolution CT of the Lung: Comparison of Axial Scans and of Sections Reconstructed from Volumetric Data Acquired Using MDCT OBJECTIVE. Our objective was to compare the image quality of reconstructed thin sections obtained from a 16-MDCT scanner with that of axial high-resolution CT scans of the same patient. SUBJECTS AND METHODS. Fifty consecutive patients referred for CT of the chest underwent 16-MDCT and, subsequently, axial high-resolution CT. The volumetric raw data from the MDCT scans were reconstructed into slices 2-mm thick using a high-spatial-frequency reconstruction algorithm. Two blinded reviewers independently rated the images from both methods for subjective image-quality criteria. The results were tested for statistical significance using Wilcoxon s signed rank test, and p values of less than.5 were considered significant. The effective dose for axial high-resolution CT and volumetric MDCT was calculated. RESULTS. Motion artifacts were significantly more common on high-resolution CT scans than on reconstructed thin-section images (p <.1). The two methods differed significantly in lung attenuation (p =.8), mainly because of the presence of ground-glass opacities. The assessment of ground-glass attenuation was superior on axial high-resolution CT. The effective doses were 3.8 msv for MDCT and.9 msv for high-resolution CT. CONCLUSION. Reconstructed high-resolution images generated from a single MDCT data acquisition are of comparable quality to images obtained using conventional axial highresolution CT. However, contiguous MDCT is not recommended for diseases showing predominantly ground-glass patterns, because axial high-resolution CT delineates ground-glass attenuation significantly better. hin-section CT has become a T proven imaging technique for evaluation of patients with diffuse infiltrative lung disease [1, 2]. High-resolution CT depicts both fine normal lung parenchyma and morphologic alterations produced by various diseases [3, 4]. The technical landmarks of high-resolution CT that are applied to improve resolution are thin collimation and image reconstruction with a high-spatial-frequency algorithm [5]. High-resolution CT usually is performed in a noncontiguous manner, with images obtained at 1- to 2-cm intervals. However, many focal or diffuse chest diseases require examination of the entire thorax, without gaps. For example, in both sarcoidosis and lung cancer, mediastinal lymph nodes and the lymphatic system of the lung may be affected. In other diseases, such as silicosis, random nodules may be missed on high-resolution CT because of the limited volume of lung that is imaged. Thus, acquisition of thin-section images and simultaneous coverage of the entire chest using thick-section CT would be desirable [6]. The advantage of the newly introduced 4 (or more)-mdct over single-detector CT consists in a high-volume coverage speed without a loss of diagnostic image quality. This advantage has been shown for different section thicknesses, including high-resolution CT [7, 8]. A reduction of threefold or even more in scanning time of the chest means, for the patient, a bearable breath-hold duration and, for the radiologist, fewer breathing artifacts. Furthermore, the same raw data set obtained from MDCT using thin sections can be reconstructed to both volumetric thick-section images and thin-section highly resolved images using a high-spatialfrequency reconstruction algorithm. This technical advantage of MDCT may allow delivery of all required information about the chest in a single CT scan, with the further benefit of sparing the patient the radiation dose of an additional high-resolution CT 62 AJR:185, September 5

2 High-Resolution CT and MDCT of the Lung examination. The purpose of this study was to assess whether quality differed significantly between images obtained using reconstructed volumetric data and images obtained using axial high-resolution CT. Subjects and Methods Patients Fifty consecutive patients referred for chest CT because of known or suspected lung disease were prospectively included (23 men and 27 women; age range, years; mean, 56 years). Chest CT was part of a comprehensive clinical evaluation, and the decision to perform additional high-resolution CT was based solely on the specific clinical question. Exposure of the patient to radiation from the additional high-resolution CT examination was not considered to require approval from our institutional review board because all CT examinations were performed using the standard protocols established by our department and no CT examination was performed for study purposes only. Informed consent to review the data was obtained from the patients. The patients underwent chest CT for the following known or suspected abnormalities: idiopathic pulmonary fibrosis (n = 4), sarcoidosis (n =5), bronchiolitis obliterans (n = 4), lymphangitic carcinomatosis (n = 5), tuberculosis (n = 3), lung involvement in collagen vascular disease (n =3), various pulmonary complications after stem cell transplantation (n = 9), bronchiectasis (n =6), emphysema (n = 8), chronic eosinophilic pneumonia A Fig year-old man with severe emphysema. Images are displayed at window level of -5 H and width of 1,4 H. A, Image obtained with axial high-resolution CT (1 mm) reveals relevant motion artifacts such as doubling of left major fissure (arrow) and double images of pulmonary vessels and bronchi (arrowheads). B, Reconstructed (2-mm) high-resolution image from volumetric MDCT at corresponding level shows no motion artifacts. Detailed evaluation of image for presence of bronchiectasis and bronchial wall thickening (arrowheads), septal lines (plain arrow), and vessels (tailed arrow) is possible. Extension of emphysema is seen (asterisks). (n = 1), idiopathic pulmonary hemorrhage (n =1), and cryptogenic organizing pneumonia (n =1). None of the patients with known interstitial lung disease had been evaluated by simultaneous highresolution CT and MDCT solely for the purpose of assessing disease activity or of monitoring treatment. All patients presented with unexplained symptoms such as acute dyspnea (n = 15), fever (n = 5), and weight loss (n = 3); thus, when the underlying interstitial lung disease had been taken into account, the specific question that led to the use of both high-resolution CT and MDCT in these patients was whether pulmonary embolism, pneumonia, or lung cancer was present. MDCT Protocols All scans were obtained using 16-MDCT (Somatom Sensation, Siemens Medical Systems) with the patient supine. First, MDCT covering the whole lung was performed in a caudal-to-cranial direction during one breath-hold, resulting in a volumetric data set. Second, the same patient underwent noncontiguous axial high-resolution MDCT at 1-cm intervals. The settings used to obtain the volumetric data were 12 kv, mas (CARE Dose, Siemens), a section collimation of mm, a table feed of 3 mm per rotation, and a rotation time of.5 sec per 36 tube rotation, resulting in a beam pitch of 1.25 according to the definition of Silverman et al. [9]. From the raw data, thick contiguous sections of 3-mm collimation were reconstructed using a soft-tissue kernel and a lung kernel for routine diagnostic practice. In addition, a series of noncontiguous images consisting of the thinnest possible collimation, 2 mm, was reconstructed at 1-mm intervals using a high-spatial-frequency reconstruction algorithm (B 8s) for this study. For axial high-resolution CT, a collimation of 2 1 mm was used. A collimation of 1 1 mm was not possible because of the geometric configuration of the detector array on this scanner. The other settings for axial high-resolution CT were 12 kv, 12 mas, and a rotation time of.75 sec. Images were reconstructed with a high-spatial-frequency algorithm (B 8f). Axial high-resolution CT was not performed during a single breath-hold for each image. Instead, depending on the patient s ability to hold the breath, we scanned during two or three breathholds, between which the patient was allowed to breathe. The scanning time for both methods was recorded by the scanner. Image Analysis All images were transferred to a Magic View 1 workstation (Siemens) for Windows (Microsoft) and were displayed on the monitor for interpretation. The scans were displayed at lung window settings (level, 5 H; width, 1,4 H) and at soft-tissue window settings (level, 4 H; width, 35 H) for evaluation of the pleura. Images were analyzed independently by two experienced, boardcertified radiologists. The two reviewers were unaware of the clinical information, patient data, and acquisition technique. To avoid recall biasing, the B AJR:185, September 5 63

3 Studler et al. Reviewer 1: Distribution of HRCT s Reviewer 1: Distribution of MDCT s 4 Reviewer 2: Distribution of HRCT s 4 Fig. 2 Histograms show distribution of scores for 1 categories by reviewer and method. HRCT = high-resolution CT. 4 interpretation sessions for high-resolution CT and MDCT images were separated by at least 4 weeks. The reviewers were asked to evaluate the image quality of the normal lung structures subjectively by using the following scale: 1, excellent image quality with sharp demarcation of the structures; 2, slight blurring of the structures, with unrestricted image evaluation possible; 3, moderate blurring of the interface structures, with slightly restricted assessment; 4, severe blurring and poorly defined structures, with uncertainty about the evaluation; and 5, severely reduced image quality making reliable interpretation impossible. The assessment of normal lung structures was divided into four categories: interlobular septa; pleural and subpleural regions; centrilobular region, consisting of the centrilobular artery; and large bronchi and vessels. In a next step, the reviewers looked for the following abnormal conditions: reticular patterns, small nodules, areas of decreased lung attenuation (bulla, emphysema, or cysts), areas of increased lung attenuation (ground-glass attenuation, consolidation, or mosaic perfusion), and bronchiectasis. Both anatomic and pathologic conditions were rated by a scale ranging from 1 (best) to 5 (worst). In addition, a score of was used for absence of abnormal findings. Subsequently, the images were evaluated for evidence of artifacts, including streak artifacts and respiratory or cardiac motion artifacts (i.e., doubling of fissures, vessels, or bronchi and blurring of the contour of the heart, mediastinum, or diaphragm). The motion artifacts were not graded using a scale. Instead, the reviewers were told to record the number of nondiagnostic images (Fig. 1). Finally, the two reviewers were asked to assess the overall image quality of the entire study using the same 5-point scale. In summary, using 11 categories for each scan, both reviewers had to rate the image quality of 5 scans obtained by each method. Radiation Dose To compare the radiation exposure, the effective doses for the volumetric and axial techniques were calculated using commercially available software (WinDose, Scanditronix-Wellhoefer) that was derived from Monte Carlo calculations [1]. Statistical Analysis Statistical analyses were performed using SPSS software (release 1.1.3, SPSS, Inc.). The image quality scores from corresponding categories were summed for each reviewer, resulting in a total score for each CT technique. Wilcoxon s signed rank test was used to test pairwise for significant differences between these total scores, and p values of less than.5 were considered to indicate a statistically significant difference. Weighted kappa was used to assess the rating agreement between the two reviewers for corresponding categories of each method. Results CT Protocols and Radiation Doses The mean scanning time for volumetric MDCT was 6 sec, depending on the scan length. The net mean scanning time for axial Reviewer 2: Distribution of MDCT s 4 high-resolution CT was 24 sec. As already mentioned, this scanning time was composed of two or three packages, during which the patients had to hold their breath for 12 or 8 sec, respectively. The average breath-hold time for all axial high-resolution CT scans was 9.7 sec. The mean effective dose was 3.8 msv (3.6 msv for men and 3.9 msv for women) for volumetric MDCT and.9 msv (for both men and women) for the axial method. Image and Statistical Analyses The histograms in Figure 2 show the distribution of scores for each reviewer and method for 1 categories. The category artifact was ignored because this data type was not ordinal and could take on a continuous range of values. The shape of the histograms shows a skewed distribution of scores. Pairwise comparison of scores for normal lung structures showed no significant difference between the two methods. A good image quality for these four categories was expressed by a median score of 2. for both axial high-resolution CT and volumetric MDCT. The scores for the category increased lung attenuation were significantly higher for reconstructed images from volumetric MDCT than for the corresponding axial high-resolution CT scans (p =.82). This difference in image quality was mainly due to blurring from ground-glass attenuation on the reconstructed images of some patients 64 AJR:185, September 5

4 High-Resolution CT and MDCT of the Lung (Fig. 3). Among the other assessed alterations of lung parenchyma, including reticular changes, small nodules, increased lung attenuation, and bronchiectasis, no significant differences were found (Fig. 4). Overall image quality was rated slightly better for volumetric MDCT than for axial high-resolution CT. However, this difference was not statistically significant. The comparison of the number of motion artifacts achieved statistical significance (p=.4). Both reviewers noted more artifacts on images acquired using axial high-resolution CT (Fig. 1). The average interobserver agreement was.6 ±.16 (range,.39.82) for axial highresolution CT and.59 ±.15 (range,.36.8) for the volumetric method. Discussion Without a doubt, MDCT represents a true revolution in the assessment of chest diseases. The simultaneous acquisition of multiple slices and the use of a higher tube-rotation speed allow acquisition times to be reduced by twofold or more without compromising image quality [8]. One important benefit of the faster scanning time is the ability to examine uncooperative patients or critically sick patients who cannot reliably suspend breathing. Furthermore, with thin sections at a low pitch, near-isotropic resolution can be obtained, allowing multiplanar reconstructions [11]. Although this advancement has had substantial implications for vascular thoracic imaging [12], fewer studies have evaluated the potential advantage of A MDCT for normal and altered lung parenchyma [6, 13]. We performed the examinations with a CT unit from the latest generation of MDCT scanners, consisting of 16 detectors with an adaptive array configuration. The use of a 1.5-mm collimation in combination with a pitch of 1.25 resulted in a mean scanning time of 6 sec for the entire thorax, a duration for which most patients even those severely dyspneic could hold the breath. This contribution to the patient s comfort is reflected in a statistically significant reduction of motion artifacts on images obtained using MDCT, compared with images obtained using axial high-resolution CT (Fig. 1). We did not classify the artifacts by their various origins; instead, we considered the number of artifacts reducing image quality. This approach seems reasonable because prevention of cardiac motion artifacts using ECG-triggered thin-section CT has been shown not to improve diagnostic accuracy [14, 15]. Hence, the diagnostic relevance of heart movement artifacts might be questioned. Our findings contrast with the previous observations of Kelly and colleagues [13] that volumetric MDCT causes more motion artifacts than does axial high-resolution CT. Most of the patients in that study were scanned with 4-MDCT using a gantry rotation time of.8 sec and a slice width of 1.25 mm. The equipment used in that study had to result in a scanning time of up to 2 sec compared with 6 sec in this study to cover the chest, although no information about scan duration, pitch, or B Fig year-old man with recurrent large cell lymphoma (asterisk) in right middle lobe after stem cell transplantation. Images are displayed at window level of -5 H and width of 1,4 H. A, Image obtained with axial high-resolution CT clearly depicts, in right lower lobe, ground-glass attenuation (arrows) that represents viral infection in this patient undergoing immunosuppressive treatment. B, Corresponding image obtained from volumetric MDCT shows slight blurring of ground-glass attenuation (arrows). Minor cardiac motion artifacts with doubling of pulmonary vessel adjacent to heart (arrowhead) were not considered to compromise image evaluation. table feed was provided. Their technique used a single breath-hold for each image obtained with axial high-resolution CT, whereas we acquired high-resolution CT scans using two or three breath-holds with respiration allowed in between, resulting in a longer breath-hold period (average breath-hold time, 9.7 sec, compared with 6 sec using MDCT) and, consequently, in more motion artifacts. Using our high-resolution CT protocol, the gantry rotation time was.25 sec longer (.75 sec) than the time needed for a 36 tube rotation using the MDCT protocol (.5 sec). However, this slight time difference has little effect on substantial artifacts; if it has any effect, that effect may be on cardiac motion artifacts. In contrast to Kelly et al., we applied a 1-cm spacing interval, resulting in a doubling of the number of images to be interpreted and a more pronounced effect on the statistical analysis. Altogether, the longer scanning time (by three to four times) used for 4-MDCT in the work of Kelly et al. is the most important factor to consider when evaluating the discrepancy between our results and theirs. Schoepf et al. [6] compared the quality of thin-section images obtained from a singledetector scanner to that of reconstructed images obtained from an MDCT scanner and found no significant difference. However, the MDCT scanner they used required a mean duration of 27 sec to cover the entire lung, a situation hardly comparable to that of our study regarding artifacts. Furthermore, because the two techniques were compared in two different patient populations, those results could AJR:185, September 5 65

5 Studler et al. A B Fig year-old man with sarcoidosis. Images are displayed at window level of -5 H and width of 1,4 H. A, Image obtained with axial high-resolution CT shows decreased regional attenuation due to mosaic perfusion (open arrows) and irregular septal thickening (solid arrows). Diffuse small nodules are visible (arrowheads). B, Reconstructed image from MDCT reveals no significant difference in image quality. Focal areas of decreased attenuation (open arrows), septal thickening (solid arrow), and diffuse nodules (arrowheads) are clearly depicted. have been biased. Our study setting permitted both methods to be performed on the same patients, excluding this kind of bias. A significant difference in increased lung attenuation between the two methods indicated that high-resolution CT better depicted the presence of ground-glass attenuation (Fig. 3). We performed MDCT with a collimation of 1.5 mm, for which the thinnest possible image reconstruction is 2 mm. Another option would have been to use the thinner,.75-mm collimation with the same pitch. This protocol offers the advantage of 1-mmthick slice reconstructions and, therefore, may reduce blurring and improve image detail without a significant increase in radiation dose. However, the use of a narrower collimation halves the table speed and increases the tube rotation time, resulting in longer scans and, consequently, more artifacts. In addition, the huge amount of data generated by such protocols becomes increasingly difficult to manage. We believe that the parameters used, including a collimation of 1.5 mm, were appropriate for the imaging quality required, and although image resolution with this protocol was significantly reduced because of ground-glass opacities, neither reviewer considered the overall image quality restricted. The average interobserver variability as measured by weighted kappa was moderate to good for both methods. The two lowest values for either method were in the category artifacts. The values were less than.4, indicating poor agreement. This was attributed to the fact that one reviewer used more rigorous criteria to characterize images containing motion artifacts as nondiagnostic than the other reviewer did. However, kappa values from both methods exceeded chance levels. The calculated effective doses 3.8 and.9 msv for volumetric MDCT and axial high-resolution CT, respectively were similar to those found in other studies [6, 13]. The higher effective dose received from axial high-resolution CT in this study than in the study of Kelley et al. [13] (.9 vs.58 msv) was due to the different spacing intervals in our study and theirs (1 and 2 mm, respectively). A potential limitation of our study was that it was not designed to assess the diagnostic accuracy of images obtained from the two methods. Although all the important high-resolution CT criteria for normal and pathologic lung parenchyma structures were included in the rating, our results are based on subjective image interpretation. An additional limitation of the study involves the different collimations and reconstruction thicknesses used between the two methods. The MDCT collimation of 1.5 mm and the resulting reconstruction slice thickness of 2 mm constitute a compromise between a short examination time and the slice width. The different slice thicknesses soon may be fully compensated for when the faster 32- or 64-MDCT scanners are implemented into routine clinical practice. In summary, the current study has revealed that reconstructed thin-section images of good quality can be obtained using a standard chest MDCT protocol. However, reconstructed thin sections obtained from MDCT cannot be recommended for diseases showing predominantly ground-glass patterns. References 1. Nishimura K, Izumi T, Kitaichi M, Nagai S, Itoh H. The diagnostic accuracy of high-resolution computed tomography in diffuse infiltrative lung disease. Chest 1993; 14: Remy-Jardin M, Remy J, Deffontaines C, Duhamel A. Assessment of diffuse infiltrative lung disease: comparison of conventional CT and high-resolution CT. Radiology 1991; 181: Itoh H, Murata K, Konishi J, Nishimura K, Kitaichi M, Izumi T. Diffuse lung disease: pathologic basis for the high-resolution computed tomography findings. J Thorac Imaging 1993; 8: Kang EY, Miller RR, Muller NL. Bronchiectasis: comparison of preoperative thin-section CT and pathologic findings in resected specimens. Radiology 1995; 195: Murata K, Khan A, Rojas KA, Herman PG. Optimization of computed tomography technique to demonstrate the fine structure of the lung. Invest Radiol 1988; 23: Schoepf UJ, Bruening RD, Hong C, et al. Multislice helical CT of focal and diffuse lung disease: comprehensive diagnosis with reconstruction of contiguous and high-resolution CT sections from a single thin-collimation scan. AJR 1; 177: Honda O, Johkoh T, Tomiyama N, et al. High-res- 66 AJR:185, September 5

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