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(Chest. 2002;121:1651-1660.)
© 2002 American College of Chest Physicians

Recent Advances in Central Airway Imaging*

Phillip M. Boiselle, MD, FCCP and Armin Ernst, MD, FCCP

* From the Departments of Radiology (Dr. Boiselle) and Interventional Pulmonology (Dr. Ernst), Division of Pulmonary and Critical Care Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA.

Correspondence to: Armin Ernst, MD, FCCP, Director, Interventional Pulmonology, Pulmonary and Critical Care Division, Beth Israel Deaconess Medical Center, Harvard Medical School, One Deaconess Rd, Boston, MA 02115; e-mail: aernst{at}caregroup.harvard.edu


   Abstract
TOP
 Abstract
Introduction
 Helical CT Scanning
 MRI
 Future Directions: Where Do...
 References
 
The purpose of this article is to familiarize chest physicians with recent advances in airway imaging, with an emphasis on the emerging role of two-dimensional reformatted and three-dimensional CT reconstructed images in the assessment of central airway disorders.

Key Words: airway disease • bronchoscopy • CT • tracheal stenosis • virtual bronchoscopy


   Introduction
TOP
 Abstract
 Introduction
Helical CT Scanning
 MRI
 Future Directions: Where Do...
 References
 
In the past decade, there has been a revolution in noninvasive imaging of the central airways. It is now possible to obtain high-quality CT images of the entire central airways in < 10 s.1 2 3 4 5 The CT data can subsequently be reconstructed into elegant two-dimensional (2-D) reformations and three-dimensional (3-D) images, including internal virtual endoscopic renderings that closely simulate images from conventional bronchoscopy (Fig 1 ).6 7 8 9 10 11 12 13 14 Noninvasive cross-sectional airway imaging also has progressed from a static technique to one that provides a dynamic assessment.5 15 16



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  		Figure 1.. 3-D reconstruction images of airway in patient with non-small cell lung cancer. Top: a 3-D external rendering of the airway shows the complete obstruction of the bronchus intermedius and a subtle narrowing of the left mainstem bronchus. Middle: conventional (left) and virtual (right) bronchoscopic images show an obstruction of the bronchus intermedius from an internal perspective. Bottom: conventional (left) and virtual (right) bronchoscopic images show a narrowing of the left mainstem bronchus. Surface irregularity on a virtual image corresponds to mucosal disease on conventional bronchoscopic image.

 

   Helical CT Scanning
TOP
 Abstract
 Introduction
 Helical CT Scanning
MRI
 Future Directions: Where Do...
 References
 
The clinical introduction of helical CT scanning in 1991 dramatically improved the quality of CT images of the airway and other thoracic structures.17 18 In comparison with conventional CT scanning, which uses a long scan time and acquires a series of individual axial slices with repeated breath holds, helical CT scanning has a short scan time and acquires a single, volumetric data set of the entire thorax during one breath hold (Fig 2 ).17 Importantly, helical CT scanning reduced respiratory and cardiac motion, markedly improved the quality of 2-D and 3-D reformation images, and eliminated respiratory misregistration (a term that refers to the inadvertent failure to image portions of the lung due to differences in respiratory excursion on repeated breath holds).10 18 More recently, multidetector helical CT scanners have been developed (Fig 3 ). Such scanners, which employ four data-acquisition systems that are connected to multidetector arrays, increase the speed of scanning by a factor of 4. Compared to single-detector helical CT scanning, multidetector CT scanning further decreases the number of motion artifacts and creates even higher quality 2-D and 3-D reformation images.1 2 3 4 19 20 21 22 Rapid advances in technology will soon allow for the acquisition of more than four channels of data simultaneously. Indeed, multidetector CT scanners will be able to simultaneously detect eight or more channels by the time of this publication.



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  		Figure 2.. Single-detector helical CT scan. A continuously rotating x-ray tube and detector obtains a single volumetric data set of the thorax as the patient is continuously transported through the scanner.

 


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  		Figure 3.. Multidetector helical CT scan. With four active detectors, a single gantry rotation results in four simultaneous slice acquisitions rather than one. With other imaging factors being equal, this reduces the scanning time by a factor of 4 compared to single-detector CT scanning.

 
Although axial CT images are sufficient for evaluating most airway abnormalities (Fig 4 ),10 23 there are inherent limitations of axial images for assessing the airways, including the following: (1) limited ability to detect subtle airway stenosis; (2) underestimation of the craniocaudad extent of disease; (3) difficulty displaying the complex 3-D relationships of the airway to adjacent mediastinal structures; (4) inadequate representation of airways oriented obliquely to the axial plane; and (5) difficulty assessing the interfaces and surfaces of airways that lie parallel to the axial plane.7 8 10 24 25 Another relative limitation of axial CT scanning is the generation of a large number of images for review, especially with multidetector scanners, which may generate data sets containing hundreds of images.25



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  		Figure 4.. Adenoid cystic carcinoma of the trachea. Axial CT image (left) shows an excellent correlation with the gross pathology specimen (right) of a lobulated tracheal mass.

 
The creation of 2-D and 3-D images reformatted from the original axial CT data set can help to overcome these limitations (Fig 5 ). Such images also offer other potential advantages, including the following: improved diagnostic confidence of interpretation; enhanced communication among radiologists, clinicians, and patients; and improved preprocedural planning.13 24 By reducing a large axial CT data set into a few images, CT reconstructions also offer clinical physicians the potential advantage of efficiency of review, which is increasingly important in this era of increasing clinical demands in daily practice.



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  		Figure 5.. Improved detection of focal tracheal stenosis using 3-D external rendering of CT data. Left, a: 3-D rendered image of the trachea shows a focal stenosis (arrows) that was not readily visible on a series of contiguous axial images. Top right, b, and bottom right, c: conventional bronchoscopic images confirm the presence of focal stenosis of the trachea and show the normal appearance of the tracheal cartilagenous rings just below the stenosis.

 
It is important to recognize that 2-D and 3-D images do not actually provide or create new information from the original axial CT data set. Rather, they offer a complementary way of viewing the same data. The axial images are the "gold standard" of images and remain an important point of reference for optimal interpretation. They also aid in the recognition of artifacts from motion or retained secretions. Thus, it is imperative that all axial images be reviewed by the radiologist interpreting the study.

In order to optimize the 2-D and 3-D reformatted images, the use of narrow (ie, 2.5 to 3.0 mm) collimation and overlapping reconstruction intervals (approximately 50%) is recommended.6 13 14 24 Because these technical parameters differ from those usually employed for standard helical CT examinations of the chest, it is usually necessary to prospectively plan for such reconstructions before the patient undergoes scanning or to obtain additional sequences through the area of suspected abnormality after the initial scan. However, an important advantage of the new multidetector CT scanners is their ability to retrospectively create images with thinner collimation than those prospectively selected.1 2 3 4 Thus, with multidetector CT scanning, high-quality reconstructed images can be obtained routinely without the need for prospective planning or additional imaging of the patient.

Although the use of IV contrast agents is not necessary for imaging the airway, its use is recommended when there is a suspected paratracheal abnormality such as enlarged lymph nodes or a thyroid mass. With regard to the phase of respiration for imaging, CT scanning of the airway is routinely obtained at end-inspiration during a breath hold. Additional imaging sequences obtained during dynamic breathing or at end-expiration can be helpful in the assessment of tracheomalacia.5 10 15 24 Such additional sequences can be performed with a "low-dose" technique in order to decrease radiation exposure.26

The radiation exposure for a dedicated CT examination of the central airways can be kept to a level comparable to the dose used in a routine CT scan of the thorax by limiting the area of acquisition to the area of interest rather than by scanning the entire thorax. For patients in whom a CT scan of the entire thorax is desired, an important advantage of multidetector CT scanning is the ability to recreate retrospectively slices of narrower collimation without additional imaging of the patient. Thus, with multidetector CT scanning, additional airway reconstructions can be performed from a CT data set of the thorax without additional radiation exposure to the patient.

Reconstruction methods that are applicable to airway imaging include 3-D internal and external rendering techniques and 2-D multiplanar methods. In the following sections, we define these techniques and describe their evolving roles in the assessment of a variety of airway disorders.

3-D Reconstruction Methods
There are two basic types of 3-D reconstruction images of the airway, involving internal and external rendering.6 7 8 10 13 24 Both types involve postprocessing of CT scan data at a 3-D workstation. Internal rendering, also referred to as virtual bronchoscopy, provides an intraluminal perspective.10 24 27 28 29 30 31 32 33 34 35 36 37 This method combines helical CT data and virtual reality computing techniques to allow the viewer to navigate through the airways in a fashion similar to that of conventional bronchoscopy (Fig 6 ). Potential applications of this emerging technology include evaluating airway stenoses, guiding transbronchial needle aspiration (TBNA) procedures, screening for endobronchial neoplasms, and assessing tracheomalacia.



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  		Figure 6.. Nodular thickening of the tracheal wall due to sarcoidosis. Virtual (left) and conventional (right) bronchoscopic images of the distal trachea approaching the carina show diffuse nodularity of the tracheal surface as well as anterior narrowing from extrinsic compression. The latter was shown to be due to enlarged mediastinal lymph nodes on axial CT images.

 
Airway Stenosis: Virtual bronchoscopy may play a complementary role to bronchoscopy in the assessment of patients with high-grade airway stenoses, particularly with regard to assessing the patency of the airways beyond the site of a stenosis.14 37 For example, in a study by Fleiter et al37 that assessed virtual and conventional bronchoscopy in 20 cancer patients with airway stenoses, high-grade stenoses were viewed equally well with both techniques, but virtual bronchoscopy offered the advantage of viewing the airway beyond the site of stenosis in 5 of 20 patients (25%) in whom the bronchoscope could not pass through the lesion. A limitation of such studies, however, is the absence of a "gold standard" for comparison with virtual bronchoscopic findings when bronchoscopy cannot pass beyond a stenosis. Further research in this area with animal models is necessary.

TBNA of Mediastinal Lymph Nodes: Guidance by TBNA of mediastinal and hilar nodes was one of the earliest potential roles identified for virtual bronchoscopy, which can be used to provide a "road map" to guide the bronchoscopist performing this procedure (Fig 7 ).12 35 38 This technique has been shown to improve the yield and to reduce the time of TBNA. For example, McAdams et al38 assessed the role of virtual bronchoscopy in guiding TBNA in 17 patients and found that it improved the yield of this procedure, with an overall sensitivity of 88% on a per-node basis, and reduced both the amount of preprocedure preparation and the procedural time.



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  		Figure 7.. Virtual bronchoscopic guidance of TBNA. Left, a: external 3-D rendering of the trachea and lymph nodes (n). Top right, b: a virtual bronchoscopic image at the level of the carina shows a normal airway lumen, but airway walls preclude the visualization of the adjacent lymph nodes. Bottom right, c: same image rendered with airway walls made transparent in order to show the relationship of the lymph nodes (n) to the airway. Reprinted with permission of Boiselle et al.27

 
Despite these promising results, virtual bronchoscopy is being supplanted by another new technology, CT fluoroscopy, which offers the advantage of real-time, direct guidance during the procedure.39 40 41 In a study by Goldberg et al,39 TBNA with CT fluoroscopic guidance was performed in 12 consecutive patients who previously had undergone nondiagnostic TBNA without imaging guidance. A tissue diagnosis was established in all patients. The identification of needle placement outside of the intended target site in roughly 40% of needle passes highlights the importance of real-time guidance of TBNA. In such cases, CT fluoroscopy guided successful repositioning of the needle into the intended lymph node site. Although CT fluoroscopic guidance is superior to virtual bronchoscopic guidance, the latter method should still be considered in practice settings where CT fluoroscopy is not available.

Lung Cancer Screening: Virtual bronchoscopy is theoretically an appealing complementary tool for lung cancer screening in conjunction with low-dose helical CT scanning of the lungs, a technique that has a high sensitivity for peripheral adenocarcinomas but identifies few central squamous cell lesions.42 43 With multidetector CT scanning, both techniques can be created from the same CT data set. At present, however, virtual bronchoscopy is not yet ready to assume a role as a primary screening modality for patients with early lung cancer.43

Summers et al34 assessed the accuracy of computer-assisted detection of polypoid lesions in the airway using virtual bronchoscopy and reported a high sensitivity (> 90%) for lesions that were > 5 mm in diameter. However, this technique was limited by a high false-positive rate due to the difficulty in differentiating retained secretions from true airway lesions. Additional limitations of virtual bronchoscopy for lung cancer screening include the following: a limited ability to detect and define small lesions that are typical of early endobronchial neoplasm; difficulty in distinguishing mucosal from submucosal disease; and the limited general experience with this technique for screening purposes.24 43 Future advances will hopefully overcome many of these obstacles.

Tracheomalacia: CT scan images obtained at end-inspiration and during dynamic breathing provide an accurate, noninvasive method for assessing tracheomalacia.5 15 24 In a study of patients with tracheobronchomalacia who underwent dynamic multidetector CT imaging and bronchoscopy, Gilkeson et al5 reported a strong correlation between dynamic CT scan findings and bronchoscopic results. In this study, virtual bronchoscopy images often were preferred over axial images by clinicians and occasionally obviated the need for further bronchoscopic evaluation in patients who were poor candidates for this procedure.

External 3-D rendering of the airways, or CT tracheobronchography, depicts the external surface of the airway and its relationship to adjacent structures. This method improves the detection of subtle airway stenoses and aids in the assessment of complex airway abnormalities.7 8

Airway Stenosis: Although slight variations in the size and shape of the airway can be difficult to appreciate in a series of contiguous axial reconstructed images, such subtle stenoses are usually readily visible on external 3-D rendered images (Fig 5) .7 8 24 These images also depict the craniocaudad extent of airway stenoses with a higher level of accuracy than axial images.7 8 24

Remy-Jardin et al8 performed CT scanning including axial and 3-D external rendered images in 47 patients with benign tracheobronchial stenoses and found that the 3-D images provided important supplemental information in one third of cases by enabling a more precise evaluation of the shape, length, and/or degree of the airway stenoses. In several cases, 3-D rendering enabled a confident recognition of mild stenoses that were not clearly depicted on axial images.

Similarly, Kauczor et al14 assessed the accuracy of 3-D rendering in 36 patients with airway stenoses and reported agreement between the results of CT scanning and bronchoscopy in the detection of stenoses involving the trachea, mainstem bronchi, and proximal segmental bronchi. In this study, CT findings helped to guide laser ablation, to determine the size of stents and applicators for transbronchial irradiation, and to assess airway patency in the postprocedural follow-up period.

Complex Airway Abnormalities: The mental and visual information processing required for the spatial integration of data collected from a large stack of contiguous axial CT sections is particularly challenging in the setting of complex airway abnormalities.8 24 In such cases, 3-D external rendering can help to clarify complex anatomic relationships.

Remy-Jardin et al8 assessed the role of 3-D external rendering of the airway in 15 patients with a variety of complex tracheobronchial deformities and found that 3-D images provided relevant supplemental information in over half of cases and corrected interpretive errors on axial images in about 10% of cases.

2-D Reformation Methods
2-D reformation methods include multiplanar reconstructions (MPR) and multiplanar volume reconstructions (MPVR).10 13 24 25 44 MPR images are single-voxel-thick sections that may be displayed in the coronal and sagittal planes, orthogonal to a point of reference, or in a curved fashion along the axis of the airway. MPVR images comprise a thick slab of adjacent thin slices and represent a block of contiguous MPR images. MPVR images, thus, combine the spatial resolution of thin slices with the anatomic display of thick slices.

2-D images are the easiest reformations to generate and can be interactively performed in real-time at the CT scanner console. Unlike 3-D reconstructions, they do not require the transfer of data to a separate workstation. Such workstations are, however, becoming increasingly available in a variety of practice settings. 2-D reformation images are helpful in selected cases of airway stenosis and in the evaluation of tracheomalacia.

Airway Stenosis: 2-D reformation images performed along the axis of the airway offer the advantage of quickly displaying the regional extent of a stenosis on a single image (Fig 8 , left, a, and top middle, b).10 24 44 A review of such images can aid preprocedural planning prior to stent placement or surgery. These images are also helpful for assessing stent complications, including migration, fracture (Fig 8 , left, a, middle bottom, c, and right, d), and the development of granulomatous reactions.45



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  		Figure 8.. 2-D reformation of the airway for the assessment of stent complications. Left, a: 2-D sagittal reformation image of the trachea shows the focal disruption of the posterior wall of a tracheal stent (solid arrow). Also note the presence of stenosis above the level of the stent (open arrows). Top middle, b: a conventional bronchoscopic image shows free stent fragments (arrows). Bottom middle, c: a conventional bronchoscopic image demonstrates tracheal stenosis. Right, d: an axial CT image shows lateral disruption of the stent (arrow). The craniocaudad extent of both the stent fracture and the stenosis both were seen to better detail on the 2-D image than on the serial axial images.

 
Tracheomalacia: In patients with tracheomalacia, paired end-inspiratory and end-expiratory sagittal 2-D images along the axis of the trachea are helpful for displaying the craniocaudad extent of excessive tracheal collapse during expiration (Fig 9 ). These images are complementary to the standard axial images, which allow for precise measurements of cross-sectional area changes in different phases of respiration.



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  		Figure 9.. 2-D reconstruction images of the trachea in a patient with tracheomalacia. Left: an end-inspiratory, sagittal, 2-D reconstruction image of the trachea is normal. Right: an end-expiratory image shows excessive collapse of a long segment of the intrathoracic trachea that is consistent with tracheomalacia, which was confirmed at bronchoscopy.

 
Current Clinical Indications for Advanced Airway Imaging Methods
Presently, axial helical CT images should still be considered the "gold standard" for noninvasive imaging of the airways. However, there are several clinical indications for the supplemental use of advanced reconstruction techniques including the diagnostic assessment of suspected central airway stenosis, tracheomalacia, and complex airway abnormalities. Once a diagnosis has been established, CT reconstructions may also be helpful for planning procedures such as stent placement and laser or radiation therapy. In the postprocedural setting, reconstruction images may prove to be useful for confirming airway patency and for assessing for stent complications. Large-scale prospective studies are necessary, however, to clarify the precise contribution of advanced CT imaging to the diagnosis and management of patients with central airway disease and to further define its complementary role to conventional bronchoscopy.


   MRI
TOP
 Abstract
 Introduction
 Helical CT Scanning
 MRI
Future Directions: Where Do...
 References
 
Although CT scanning generally is the preferred method of imaging the airway, there are two major instances in which MRI should be considered. First, because MRI does not involve ionizing radiation, it is the preferred modality for assessing paratracheal abnormalities in children (Fig 10 ).46 Indeed, this technique is ideal for assessing paratracheal "rings and slings" that compress the airway. Second, because MRI does not require the use of an iodinated contrast agent, it is the preferred modality for imaging paratracheal masses in patients with contraindications to iodinated contrast media. Gadolinium, an MRI contrast agent, can be administered safely to such patients.



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  		Figure 10.. Extrinsic tracheal compression in a male infant due to a double aortic arch. A coronal T-1-weighted MRI shows narrowing of the trachea (T) due to extrinsic compression by the right (R) and the left (L) aortic arches. Reprinted with permission of Boiselle.47

 
In addition to static imaging, MRI also can be performed dynamically.16 A preliminary investigation has shown a potential role for dynamic MRI in the evaluation of tracheomalacia by imaging the airway during a coughing maneuver. Future studies are necessary to compare the relative accuracies of dynamic MRI and multidetector helical CT scanning in the assessment of tracheomalacia.


   Future Directions: Where Do We Go From Here?
TOP
 Abstract
 Introduction
 Helical CT Scanning
 MRI
 Future Directions: Where Do...
References
 
In the near future, multidetector CT scanners and computerized reconstruction methods will become even faster and more widely accessible.25 Additionally, viewing station advances will provide an ability to rapidly navigate between a variety of display methods, including 3-D internal and external renderings, in order to meet the needs of specific airway abnormalities and individual viewer preferences. With such advances, it is likely that 2-D reformation and 3-D reconstruction will eventually become a primary mode of viewing central airway studies. Future advances also will allow for interactive, real-time virtual reality guidance of airway procedures such as bronchoscopy and surgery.24 Before these concepts become reality, however, further improvements in automated processing, processing speed, and user interface capabilities are necessary.25


   Footnotes
 
Abbreviations: 2-D = two-dimensional; 3-D = three-dimensional; MPR = multiplanar reconstruction; MPVR = multi-planar volume reconstruction; TBNA = transbronchial needle aspiration

Received for publication June 13, 2001. Accepted for publication September 25, 2001.


   References
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 Abstract
 Introduction
 Helical CT Scanning
 MRI
 Future Directions: Where Do...
 References
 

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