British Journal of Radiology (2004) 77, S98-S105
© 2004 British Institute of Radiology
doi: 10.1259/bjr/27596725
High resolution volume imaging of airways and lung parenchyma with multislice CT
W K Chooi, FRCR and
S K Morcos, FRCS, FRCR
Department of Diagnostic Imaging, Northern General Hospital, Sheffield Teaching Hospitals NHS Trust, Herries Road, Sheffield S5 7AU, UK
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Abstract
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The value of multislice CT (MSCT) in imaging the peripheral airways and lung parenchyma has not been widely investigated. In this article the authors' experience in the use of MSCT (4-slice scanner) in imaging patients with suspected parenchymal lung disease or airways abnormalities will be presented. The technique described should be modified with the more modern 8-slice or 16-slice scanners. The whole thorax is scanned contiguously using 4 x 1 mm collimation from the lung bases up to apices in end-inspiration while the patient is in the prone position. Collimation of 2 x 0.5 mm is used at 8–10 levels evenly spaced in expiratory scans and also in the breathless patient who is scanned during gentle breathing. High resolution images of the lungs (1 mm slice thickness) are reconstructed in the following planes: axial (10 mm apart from apices to bases), coronal (six evenly spaced through the chest) and sagittal (four images evenly spaced through each lung). Paddlewheel reconstruction is used if further assessment of the airways is required, and three-dimensional imaging is used mainly for assessment of the trachea and major bronchi. Contiguous axial images (10 mm slice thickness) of the whole lung and mediastinum are also produced and referred to as a screenogram. Axial images of 1 mm slice thickness are produced with expiratory scans and for breathless patients. All the images are produced independently by the radiographic staff and are provided as hard copies (20 frames/film) for reporting. However, if facilities are adequate, direct reporting from the workstation is more effective in reviewing large number of images. The technique is effective in assessment of infiltrative lung disease, emphysema, bronchiectasis and central airways. The screenogram offers comprehensive evaluation of the lung and mediastinum, but the radiation dose associated with high resolution volume imaging remains a source of concern.
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Introduction
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Multislice computed tomography (MSCT) revolutionized CT technology when it was introduced in 1998. It offers unparalleled speed of acquisition, spatial resolution and anatomical coverage [1–6]. Multiplanar and three-dimensional (3D) images of excellent quality can be produced that can be of great benefit in many clinical situations [5–7].
The value of MSCT in imaging the peripheral airways and lung parenchyma has not been widely documented and there are few reports in the literature [8–13]. There is also no consensus on how to make the best use of this new technology in different clinical settings [1, 2, 6]. Since MSCT is capable of producing an extensive number of axial and multiplanar reformatted images, there is a need for practical imaging protocols that can be implemented by the radiographic staff to reduce the demand on an overcommitted radiologist's time. The reconstructed images should be made available for interpretation either on the workstation or as hard copies [6]. Further image reconstruction can be performed interactively by the radiologist at the workstation [6]. Although it is possible to view large number of images at the workstation using cine display, free access to this facility may be limited. Reporting of hard copies may be performed but requires efficient imaging protocols that maximize the diagnostic yield without production of an excessive number of images, and it has the advantage of allowing easy communication of detected abnormalities to the referring physicians. However, picture archiving and communication systems (PACS) provide the ideal solution for reviewing the large number of images produced by MSCT.
In this review the authors' experience in assessing patients with suspected diffuse parenchymal lung diseases or abnormalities of the airways using MSCT will be presented.
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Multislice CT technique
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Scanning protocol (Table 1
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The whole thorax is scanned contiguously with a MSCT scanner using 4 x 1 mm collimation from the lung bases up to apices in end-inspiration with the patient in the prone position. The described technique should be modified with the more modern 8-slice or 16-slice scanners.
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Table 1. High resolution volume imaging with multislice CT (4-slice scanner): technical parameters. (The described technique should be modified with 8-slice or 16-slice scanners)
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Expiratory scans are performed at five evenly spaced levels using 2 x 0.5 mm collimation if there is clinical suspicion of airways disease or emphysema.
The breathless patient is scanned in the supine position during gentle breathing at 8–10 evenly spaced levels using 2 x 0.5 mm collimation.
Imaging protocol (Table 1
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High resolution imaging of the lungs
The lungs are imaged using 1 mm slice thickness reconstructed at a window level of –600 Hounsfield units (HU) and a window width of 1600 HU, utilizing a high spatial frequency algorithm. Reconstructions are made in the following planes: axial (10 mm apart from apices to bases), coronal (six evenly spaced through the chest) and sagittal (four images evenly spaced through each lung).
Paddlewheel reconstruction
In cases with suspected bronchiectasis, paddlewheel image reconstruction is performed to ensure imaging of the bronchi along their long axis [13]. This technique was initially described for optimal visualization of the pulmonary vasculature [14]. Reconstruction is carried out at 1° intervals in a 180° arc centred on the distal trachea. Two sets of paddlewheel reconstructions are performed: one set rotated from the axial to coronal plane and back to the axial plane, and the other from sagittal to coronal to sagittal plane. These create 180 images (1 mm slice thickness) for each set, which are reviewed at the workstation by the radiologist. Optimal images displaying the bronchi along their longitudinal axis are selected for hard copying.
Screenogram
In addition to the high resolution images, contiguous axial images of 10 mm slice thickness of the whole lung (level –600 HU, width 1600 HU, smooth kernel) and mediastinum (level 40 HU, width 400 HU) are produced (Table 1).
Imaging the breathless patient and expiratory scans
Axial images of 1 mm slice thickness of the lungs are produced for expiratory scans and for breathless patients. Mediastinal axial images (level 40 HU, width 400 HU) are also produced in the breathless patient with a history of asbestos exposure in order to demonstrate calcified pleural plaques or if mediastinal lymphadenopathy is suspected.
Hard copying and reporting
The images are produced independently by the radiographic staff and provided as hard copies (X-ray films (30 cm x 40 cm) with 20 frames/film) for reporting. Paddle wheel imaging is used only when further information is required. On average, 4–6 films are produced per examination.
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Discussion
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Traditional sequential high resolution computed tomography (HRCT) of the thorax has been the mainstay for assessment both of airways and parenchymal lung disease [11, 15, 16]. Development of helical and MSCT has improved the speed of the examination and has allowed imaging free of breathing artefacts. The use of MSCT in imaging airways and parenchymal lung diseases will be discussed below.
Imaging the airways
Peripheral airways
The role of helical CT in imaging diseases of the airways was recently reviewed by Grenier et al [16]. Sequential HRCT is accepted as a reliable method for imaging bronchiectasis but it has some limitations. The gaps between image slices could result in focal areas of bronchiectasis being overlooked within the "skip" regions. In addition, depiction of the bronchi along the long axis is limited in axial imaging [16]. MSCT can overcome these difficulties with its ability to scan the whole thorax in a single breath-hold and to perform multiplanar image reconstruction (MPR). The latter is particularly effective in imaging the airways, since a good percentage of the bronchi can be depicted along their long axis, allowing better assessment of abnormalities such as bronchiectasis or bronchial stricture (Figure 1) [16, 17]. While the longitudinal extent of abnormalities of the airways are better demonstrated with MPR, the transverse extent of the disease and its relationship to adjacent structures are better shown on axial imaging [17]. The use of cine mode at the workstation to view contiguous thin slice images as a "stack" is also helpful to follow the bronchi throughout their divergent pathway from the hilum [16].
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Figure 1. (a) Collapsed bronchiectatic left lower lobe is clearly depicted with paddlewheel image reconstruction. Normal left lower lobe bronchus (arrow) is demonstrated, excluding a proximal obstructive lesion. (b) High resolution volume imaging of a patient with bronchiectasis of the left lower lobe. The dilated bronchi are clearly demonstrated in a coronal image of the left lung.
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In a recent report we have demonstrated that MPR employing the technique described in this report (1 x 4 mm collimation) had no important impact on assessment of bronchiolitis but increased the accuracy of diagnosing bronchiectasis and improved interobserver agreement [13]. However, another study found that scanning the whole thorax employing 4 x 2.5 mm collimation and the use of axial imaging only (3 mm slice thickness at 10 mm intervals) without MPR is effective for screening of bronchiectasis [12]. This approach allowed a 20% reduction in radiation dose compared with the use of 4 x 1 mm collimation for scanning the lungs. The diagnostic accuracy of 3 mm reconstructed axial images in detection and characterization of bronchiectasis was comparable with that of 1 mm axial images [12].
Central airways
The usefulness of helical single-slice and MSCT in assessing the central airways and trachea has been investigated in several reports [17–22]. Coronal, sagittal and 3D images were found to be extremely helpful in assessing major airways before and after thoracic surgery or endoscopic procedures such as laser ablation, dilatation and stenting [17–22]. However, 3D imaging can be time consuming and should always be combined with MPR including axial images for evaluation of surrounding structures and optimal spatial resolution [17–22]. The 3D images of the airways can be displayed as a solid cast of the air passages or using virtual bronchography [23] (Figure 2).
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Figure 2. (a) Benign stricture of the trachea (arrow) complicating endotracheal intubation is shown clearly in a three-dimensional (3D) image (solid cast). (b) Stricture of the distal part of the right main bronchus and the origin of the bronchus intermedius (arrow) post lung transplantation is shown clearly with 3D imaging (solid cast). (c–e) Displacement of the trachea to the right (arrow) by a large goitre is shown in a 3D image of the trachea (virtual bronchography). (c). The impression of the aorta (X) on the left lung is also shown. Large goitre of the right lobe of the thyroid gland (asterisk) is shown in coronal (d) and sagittal (e) imaging.
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The data acquired by MSCT can also be used for virtual bronchoscopy, but the clinical usefulness of the technique has not been conclusively proven [19–22]. In addition, it cannot replace bronchoscopy, which allows both direct visualization of the airway lumen and biopsy sampling [20]. The technique is also not accurate in assessing mild stenosis, submucosal infiltration and superficial spreading tumours [16]. It is important to emphasise that virtual bronchoscopy, if performed, should always be combined with axial and MPR images for accurate assessment of the airways and adjacent structures [16, 20].
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Parenchymal lung diseases
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The ability of MSCT to produce high quality MPR images of the lung parenchyma has been demonstrated in an experimental study using normal autopsy lung specimens [11]. However, the value of MPR over axial imaging in assessing diffuse parenchymal lung disease has not been extensively evaluated [8–10]. Schoepf et al [8] reported that MSCT can produce excellent quality high resolution axial images of the lungs following volume acquisition using 4 x 1 mm collimation in patients with diffuse lung disease. The quality of the images was comparable with that produced with the standard single-slice high resolution CT technique. Arakawa et al [9] compared coronal reformatted images (1.9 mm slice thickness at 5 mm intervals) produced by MSCT following scanning the lungs using 4 x 1 mm collimation with axial (1 mm slice thickness at 10 mm intervals) HRCT images produced by the single-slice standard CT technique. They found that coronal images were comparable with axial HRCT in the assessment of diffuse lung disease and revealed additional information regarding lesion conspicuity and distribution in 22% of cases. In another study, Remy-Jardin et al [11] concluded that coronal imaging alone could be used to accurately assess infiltrative lung disease. The vertical predominance of lung changes was more precisely assessed by this approach compared with axial imaging alone. In addition, coronal imaging allowed a significant reduction in the number of images. In our experience, MPR in assessing diffuse infiltrative parenchymal lung disease did not add important new diagnostic information compared with axial imaging alone (Figure 3). However, sagittal and coronal images were very helpful in assessing the distribution of emphysema in patients considered for lung volume reduction surgery (Figure 4) [13].
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Figure 3. (a) Lipoid pneumonia of the lung causing diffuse shadowing in the lung fields shown in axial (a) and coronal (b) imaging, and sagittal imaging of right (c) and left (d) lung. Multiplanar reconstruction in such cases does not seem to offer important additional information compared with axial imaging.
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Figure 4. The distribution of emphysema in the right lung is clearly shown in (a) coronal and (b) sagittal imaging.
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Finally, MSCT high resolution volume imaging may offer a powerful diagnostic tool to elucidate the functional consequences of the different pathological processes affecting the lung [24]. Quantification of the extent of emphysema and the extent of air trapping can be efficiently assessed by this technology [24].
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Screenogram
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Standard HRCT scanning with single-slice CT allows assessment of the lung at the scanned levels only and cannot exclude abnormalities in the lungs between these sections. MSCT offers an effective solution to this problem through a single scan. It can simultaneously produce images of different thickness from data acquired by contiguous thin slice scanning during a single breath-hold. This approach is often referred to as a Combi scan. The efficacy of the combination of thick (5 mm slice thickness) contiguous axial images and thin (1.25 mm) high resolution axial images at 10 mm intervals in imaging lung diseases using 1 mm slice thickness collimation for scanning the whole thorax has been reported by Schoepf et al [8]. The report indicated that this combination allowed a comprehensive diagnosis of intrathoracic abnormalities in patients with focal and diffuse lung diseases [8].
The term screenogram in this report is used to refer to the contiguous thick (10 mm) axial imaging, as this part of the examination is used to screen the lung and mediastinum for abnormalities. Preliminary experience indicates that the screenogram is particularly useful in detecting mediastinal lymphadenopathy in cases with suspected sarcoidosis or malignancy and in depicting pleural plaques in patients with a history of asbestos exposure. The screenogram is also valuable in the pre-operative assessment of patients undergoing volume reduction surgery as it may detect unexpected lung cancer (Figure 5). The importance of the screenogram in patients undergoing high resolution volume imaging of the lungs is currently under investigation.
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Figure 5. Unexpected peripheral lung cancer (arrowhead) demonstrated by the screenogram in a patient with emphysema under assessment for volume reduction surgery. This lesion was not demonstrated by the sequential (1 mm at 10 mm intervals) high resolution CT axial imaging of the lungs.
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Radiation dose
There is no significant increase in radiation dose using high resolution volume imaging compared with single-slice CT producing the same combination of images [8]. According to Schoepf et al [8] the radiation dose associated with MSCT was 5.55 mSv compared with 5.50 mSV for single-slice CT producing 1 mm slice thickness HRCT axial images at 10 mm intervals and contiguous axial images of the thorax at 5 mm slice thickness. However, the effective radiation dose for sequential HRCT acquisition (1 mm slice thickness, 15 mm gap) alone is only 1 mSV [13]. Therefore, despite the potential benefits of high resolution volume imaging, consideration of the increased radiation dose is necessary, particularly in examining young patients with suspected lung disease. One solution to reduce the radiation dose is to perform a volume scan of the thorax employing thicker collimation as shown by Remy-Jardin et al [12] in screening for bronchiectasis. However, the effectiveness of this approach in diagnosing infiltrative lung disease is not known. Reducing the tube current should also be considered.
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Conclusion
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In summary, MSCT allows high quality volume imaging that is effective in the assessment of the airways and lung parenchyma. It offers excellent quality multiplanar and 3D images. It also allows a simultaneous reconstruction of thick and thin slice images from a single set of raw data, permitting comprehensive assessment of the lungs and mediastinum without multiple exposures to the patients. Nevertheless, further clinical studies are required to elucidate the importance of high resolution volume imaging in the assessment of patients with suspected airways or infiltrative lung diseases. Although the technique has several advantages, including very fast scanning time allowing high throughput, there are certain disadvantages, including an increased radiation dose and a large number of images to interpret. In addition, hard copying of all the reconstructed images increases film cost. A summary of the advantages and disadvantages of the technique is presented in Table 2. The potential applications of high resolution volume imaging of the lung with MSCT are presented in Table 3.
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Footnotes
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Address correspondence to Dr S K Morcos. 
Received for publication July 10, 2003.
Revision received November 20, 2003.
Accepted for publication December 2, 2003.
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Abstract
Introduction
