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1 All authors: Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02215.
Received August 11, 2003;
accepted after revision October 8, 2003.
Address correspondence to H. Hatabu
(hhatabu{at}bidmc.harvard.edu).
Abstract
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SUBJECTS AND METHODS. We studied 40 consecutive patients with known or suspected diffuse lung diseases with airway abnormalities who underwent volumetric expiratory high-resolution CT between May and July 2003. Respiratory motion artifacts were evaluated at upper, middle, and lower lung areas. Cardiac motion, beam-hardening, and other artifacts were evaluated throughout the lung fields. Detectability, clarity of borders, size, distribution, and extent of air trapping were compared on axial versus coronal end-expiratory high-resolution CT images.
RESULTS. Respiratory motion artifacts were either imperceptible or not diagnostically limiting in all patients except three (7%) with diagnostically limiting image degradation at lower lung areas. Other diagnostically limiting image degradation was caused by beam-hardening artifacts in two patients (5%) and by quantum noise in two other patients (5%). The borders of air trapping were more clearly identified on coronal images than on axial images (grade 1 [vague], nine vs three; grade 2 [partially clear], 23 vs 21; grade 3 [completely clear], eight vs 16; median, two vs two; p = 0.001). The coronal reformatted images were as informative as axial images for detecting and assessing the classification and extent of air trapping.
CONCLUSION. Coronal reformations of volumetric expiratory high-resolution CT scans were acceptable in image quality and provided additional value by affording clearer visualization of the borders of air trapping than was found in contiguous axial images.
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The recent advent in MDCT technique with 4–16 channels of simultaneous acquisition has enabled volumetric data acquisition of the entire thorax with thin collimation. This MDCT technology can be applied to acquire volumetric data of the lung at both end inspiration and end expiration. We recently developed a new volumetric expiratory high-resolution CT protocol that allows volumetric data acquisition of the lung both at end inspiration and end expiration. By reducing the X-ray tube current from 340 to 240 mA, this new protocol provides combined volumetric inspiratory and expiratory high-resolution CT with an examination time and radiation dose equivalent to the standard expiratory high-resolution CT protocol. The total estimated effective radiation dose of the volumetric expiratory high-resolution CT protocol was 11.61 mSv, which was essentially equal to the total estimated radiation dose of 11.63 mSv for the standard high-resolution CT protocol consisting of a volumetric inspiratory high-resolution CT and nonvolumetric supine expiratory high-resolution CT at three selected levels. Using a pair of inspiratory and expiratory volumetric high-resolution CT scans of the lung, one can now perform simultaneous reconstruction of both thin- and thick-collimated axial images as well as coronal and sagittal reformations, which show thoracic structures in different imaging planes.
Coronal reformations of the thorax have been studied in the detection of diaphragmatic injury [11, 12]. The usefulness of coronal reformations of the lung has been reported in the diagnostic evaluations of infiltrative lung disease [13]. Coronal reformation has been reported to be a diagnostic approach as precise as axial imaging but requiring fewer images. However, to our knowledge, no previous report has described the feasibility and usefulness of combining end-inspiratory and end-expiratory coronal reformatted images from volumetric expiratory high-resolution CT to evaluate lung parenchymal abnormalities caused by airway obstruction.
Our goal was to evaluate the quality of coronal reformatted images from volumetric expiratory high-resolution CT images and to compare coronal and axial images for their usefulness to detect and characterize air trapping.
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Volumetric high-resolution CT scans on end inspiration and end expiration were obtained with the patient in the supine position using an 8- or 4-MDCT scanner (LightSpeed, General Electric Medical Systems). The parameters for the 8-MDCT scanner were as follows: 5-mm collimation, 120 kVp, 240 mA, 0.5-sec gantry rotation time, and table speed of 60 mm per rotation. The parameters for the 4-MDCT scanner were as follows: 2.5-mm collimation, 120 kVp, 240 mA, 0.5-sec gantry rotation time, and table speed of 15 mm per rotation. For the expiratory high-resolution CT scan, all subjects were instructed to take a deep breath, exhale all the way, and hold their breath. Scanning was performed from the lung bases toward the apices. The volumetric axial images with 1.25-mm thickness and the coronal images with 1.3-mm thickness and 10-mm intervals were reconstructed with a high-spatial frequency algorithm on both end-inspiration and end-expiration scanning.
All images were displayed at the lung window setting using a PACS (picture archiving and communication system) workstation (Centricity, General Electric Medical Systems) and evaluated by the consensus review of two board-certified chest radiologists. Approval for this retrospective imaging study was granted by the institutional review board of our hospital.
Respiratory motion artifacts were evaluated on end-expiratory coronal reformatted images at three lung levels (upper, middle, and lower). The criteria for motion degradation included motion-induced streaking and blurring and doubling of fissures, bronchial walls, and vessels. Respiratory motion artifacts were graded subjectively on a 3-point scale: 1, imperceptible; 2, present but not diagnostically limiting; and 3, prominent and diagnostically limiting. Cardiac motion artifacts were graded on the basis of the degree and extent of serration of heart borders on a 4-point scale: 1, none; 2, mild, when cardiac borders were rippled; 3, moderate, when stair-stepping artifacts were observed only in posterior cardiac borders; and 4, severe, when stair-stepping artifacts were observed in cardiac borders throughout the heart. Beam-hardening and other artifacts, if present, were evaluated throughout the lung areas and were graded on a 3-point scale: 1, imperceptible; 2, present but not diagnostically limiting; and 3, prominent and diagnostically limiting.
The axial and coronal high-resolution CT images were evaluated and scored for the presence of air trapping (1, absent, and 2, present) and confidence level (1, not confident; 2, slightly confident; 3, moderately confident; and 4, definitely confident). When air trapping was present, clarity of the borders of air trapping was scored as 1, vague; 2, partially clear; and 3, completely clear.
Size and distribution of air trapping were also classified on axial and coronal images: lobular, when composed of small areas of hypoattenuation corresponding to one or two adjacent pulmonary lobules in one or two regions per lung level; mosaic, when three or more areas of lobular air trapping were observed to alternate with areas of normally attenuated lung, usually in a multilobular distribution; subsegmental or segmental, when composed of a contiguous area of air trapping that was larger than three adjacent pulmonary lobules and was subsegmental or segmental in distribution; and lobar, when a contiguous area of air trapping was larger than a segment and was lobar in distribution. The extent of air trapping was qualitatively graded using a 5-point scale: 1, 0%; 2, 1–25%; 3, 26–50%; 4, 51–75%; and 5, 76–100%.
Statistical analysis was performed using Wilcoxon's signed rank test. A p value of less than 0.05 was considered to be significant.
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Cardiac motion artifacts were present in all cases. The grade was 2, mild, in 35% (14/40); 3, moderate, in 60% (24/40); and 4, severe, in 5% (2/40). The median score was 3.
Beam-hardening artifacts were present in 18% (7/40): Grade 1, present but not diagnostically limiting, was seen in five patients and grade 2, prominent and diagnostically limiting, in two patients. The artifacts were caused by cardiac pacemaker lead (grade 1, n = 1; grade 2, n = 2) (Fig. 1), rib (grade 1, n = 1), thoracic vertebra (grade 1, n = 1), calcified lymph node (grade 1, n = 1), and barium retained in the intrathoracic gastric pull-through (grade 1, n = 1).
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Other artifacts, including grade 2—prominent and diagnostically limiting—quantum noise (linear artifact), were seen in two patients because of their body habitus.
Air trapping was present in all cases on both axial and coronal images. The difference in confidence level was not statistically significant between axial and coronal images (grade 1, zero vs zero; grade 2, two vs zero; grade 3, three vs one; grade 4, 35 vs 39; median, four vs four; p = 0.156).
The borders of air trapping were discerned more clearly on coronal images than on axial images (grade 1, nine vs three; grade 2, 23 vs 21; grade 3, eight vs 16; median, two vs two; p = 0.001) (Figs. 2A, 2B and 3A, 3B).
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Classification of the size and distribution of air trapping did not show any statistically significant difference between axial and coronal images (grade 1, zero vs zero; grade 2, seven vs seven; grade 3, 33 vs 32; grade 4, zero vs one; median, three vs three; p = 0.5). The extent of air trapping did not show any statistically significant difference between axial and coronal images (grade 1, zero vs zero; grade 2, 28 vs 23; grade 3, nine vs 15; grade 4, three vs two; grade 5, zero vs zero; median, two vs two; p = 0.219).
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Respiratory motion artifacts were most frequent and severe in the lower lung areas but were diagnostically limiting only in the lower lung area in three cases (8%). Thus, respiratory motion artifact was not a limiting factor in the usefulness of coronal reformatted images.
Cardiac artifacts were present in all cases but were severe in only two cases (5%). The cardiac artifacts are inevitable in coronal reformatted images because of cardiac phase variations among contiguous axial images. Cardiac motion artifacts can potentially be reduced by ECG gating. Both prospective and retrospective ECG-gating techniques in combination with MDCT have been reported [14–19].
Beam-hardening artifacts, which were observed in seven patients, were diagnostically limiting in two patients with cardiac pacemakers. Because of their body habitus, two patients showed diagnostically limiting quantum noise artifacts that might have been overcome by increased tube current and kilovoltage. Coronal reformatting may be of limited value in patients with cardiac pacemakers and those with large body habitus.
The borders of the areas of air trapping were more clearly identified on end-expiratory coronal reformatted images than on end-expiratory axial images. On axial images, lung attenuation gradually increased in the anterior to posterior direction in supine patients [20]. On coronal reformatted images, lung attenuation is relatively homogeneous in each plane, which makes the areas of air trapping more conspicuous. No statistically significant difference was found in the confidence level of the detection of air trapping (p = 0.156), the size and distribution of air trapping (p = 0.5), and the extent of air trapping (p = 0.219) between coronal reformatted images and contiguous axial images. Coronal reformations were as informative as contiguous axial images in evaluating air trapping.
Only qualitative analysis was performed in assessing air trapping. Quantitative assessment may be needed for further comparison of axial and coronal images to evaluate the degree of air trapping in correlation with pulmonary function tests.
In conclusion, volumetric expiratory high-resolution CT provided coronal reformations of acceptable image quality for evaluating lung parenchymal abnormalities. End-expiratory coronal reformations were as informative as axial images in the detection, classification, and extent assessment of air trapping, and they provided additional value by affording clearer visualization of the borders of air trapping than was found in contiguous axial images.
Acknowledgments
We thank Donna Wolfe and Michael Larson for their assistance in manuscript
preparation.
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