State-of-the-art airway imaging with CT: Part 1

Advances in CT technology have revolutionized noninvasive imaging of the central airways.^1-3 It is now possible to image all of the central airways in a few seconds and to create elegant 3-dimension-al (3D) CT reconstructions of the airways in only a few minutes (Figure 1).^1-3 In addition, the speed of CT scanning has enhanced the ability to diagnose noninvasively a va-riety of functional airway abnormalities, such as tracheobroncho-malacia (Figure 2).^4-6

In this article, the first of a 3-part series, we delineate advances in airway imaging, with particular emphasis on the potential contribution of CT reconstruction methods to the evaluation of various airway disorders. In coming issues of The Journal of Respiratory Diseases, we will review reconstruction and reformation methods in the context of normal airway imaging anatomy. We will then consider clinical applications of 3D reconstruction and multiplanar reformation imaging.

HELICAL CT IMAGING

The clinical introduction of helical CT imaging in 1991 dramatically improved the quality of the CT image of the airways and other thoracic structures.⁷ Compared with conventional "stop-and-shoot" CT scanners, which acquire a series of individual axial slices using repeated breath-holds in a long scan time, helical CT scanners collect a single volumetric dataset in 1 breath-hold in a short scan time. Helical CT has eliminated respiratory misregistration, reduced respiratory and cardiac motion, and markedly improved the quality of 2-dimensional and 3D reformation images.⁷

More recently, multi-detector row CT (MDCT) scanners have been introduced.^8-11 Unlike the standard, single-detector row CT (SDCT) scanners, this type of helical scanner allows for simultaneous registration of multiple channels of information with each gantry rotation.^7-11Depending on their configuration, MDCT scanners have up to a 64-times-greater capacity for registering slices than do SDCT scanners. The scanning speed is further enhanced by fast gantry rotation times, which are less than 0.5 second with state-of-the-art scanners.

MDCT scanners can obtain thin-section images of the central airways in only a few seconds, creating an isotropic dataset with identical resolution in the axial, coronal, and sagittal planes.⁸ Compared with standard helical CT scanners, MDCT scanners provide higher spatial resolution, faster speed, greater anatomic coverage, and higher-quality multiplanar reformation and 3D reconstruction images. For these reasons, MDCT scanning is preferred over SDCT scanning for airway imaging.

AXIAL CT IMAGING

With the high spatial resolution afforded by MDCT scanners, axial CT imaging provides excellent anatomic information about the airway lumen, airway wall, and adjacent mediastinal and lung structures. Although CT imaging of the axial plane is considered the reference standard for airway imaging, it is important to be aware of its limitations: a limited ability to detect subtle airway stenoses, an underestimation of the craniocaudad extent of disease, difficulty in displaying complex 3D relationships of the airways, and inadequate representation of airways that are oriented obliquely to the axial plane.^1,2,12-17 These limitations have important implications for the assessment of certain airway disorders, such as airway stenoses and complex airway abnormalities.

Multiplanar and 3D reconstructions

These alternative CT display methods can help overcome the limitations of axial imaging by providing a more anatomically meaningful display of complex structures, such as the airways.^1,2,12-18 Multiplanar and 3D reconstruction images have been shown to enhance the detection of airway stenoses, aid the assessment of the craniocaudad extent of stenoses, and clarify complex congenital airway abnormalities (Figure 3).¹⁵

They have also been shown to improve diagnostic confidence of interpretation; enhance preprocedural planning for bronchoscopy and surgery; and improve communication among radiologists, clinicians, and patients.^1,2,12Because multiplanar and 3D images effectively reduce large CT datasets to a considerably smaller number of images, these methods also facilitate the efficient review of pertinent findings between radiologists and referring clinicians.

Although multiplanar and 3D reconstruction images do not actually create new data, they do provide an alternative method of viewing CT data that is often more visually accessible and anatomically meaningful. However, axial images provide a more comprehensive review of the entirety of the thoracic structures and serve as an important point of reference for optimal interpretation of multiplanar and 3D images. It is therefore essential that the radiologist review the traditional axial images, in addition to the alternative display images, when interpreting a CT study of the central airways.

Image quality optimization

Although multiplanar and 3D images can use data obtained with any type of helical CT scanner, MDCT scanners are preferred over SDCT scanners because of their greater speed and improved resolution. To enhance the quality of multiplanar and 3D images, the use of narrow collimation (less than 3 mm) is recommended.^1-3,13,18,19

The use of overlapping reconstruction intervals (with 50% overlap) is also suggested.However, this is less important when very narrow collimation (0.5 to 1 mm) is used²⁰ because such thin collimation results in an isotropic dataset in which spatial resolution is the same, regardless of whether images are viewed in the axial, coronal, or sagittal planes.

Ideally, airway studies should be prospectively planned and tailored to the area of interest. An advantage of MDCT scanning, however, is its ability to change slice thickness retrospectively, which allows for high-quality reconstruction images from routine CT studies. In routine cases, this feature enables patients to benefit from the creation of multiplanar and 3D reconstructions without the need for additional imaging.

Intravenous contrast is not routinely used for assessment of benign airway disorders, such as malacia and postintubation stenoses. However, it is recommended for evaluation of airway involvement by malignancy and in the setting of extrinsic airway compression by enlarged lymph nodes, vascular anomaly, or thyroid mass. Intravenous contrast is also recommended for the assessment of lobar collapse, since it helps distinguish an obstructing mass from the adjacent collapsed lung (Figure 4).

Airway imaging is routinely performed at end-inspiration during a single breath-hold; state-of-the-art scanners allow the central airways to be imaged in a few seconds.³ Such speed is of particular benefit when used in the evaluation of patients with airway disorders, many of whom cannot tolerate longer breath-holds. It also aids in the assessment of young children who may not understand breathing instructions. With fast scanners, younger children can be successfully imaged during quiet breathing, without the need for sedation. For children, it is strongly recommended that published guidelines for radiation dose reduction in the pediatric population be consulted.²¹

The quickness of MDCT scanning is also advantageous for patients with suspected tracheobronchomalacia,^4-6 which is characterized by excessive collapse of the airways during expiration. Increasingly, acquired tracheobronchomalacia has been recognized as a relatively common, but often overlooked, cause of chronic cough, dyspnea, and respiratory infections.

To assess for malacia, an additional helical sequence is performed during dynamic exhalation.²² The dynamic expiratory imaging method (imaging during a forced exhalation) has been shown to be more sensitive for detecting malacia than the traditional end-expiratory method (imaging at the end of forced expiration).²²

To reduce radiation exposure, this sequence can be performed using a low-dose (40 mAs) technique.⁵The estimated radiation dose (expressed as dose-length product) for a dual-phase CT of the central airways (including a standard-dose inspiratory phase and a low-dose expiratory phase) for a 70-kg patient at our institutions is about 500 mGy.cm. This compares with a reference standard of 650 mGy.cm for a routine chest CT examination.

For patients who are at risk for vocal cord paralysis, the quickness of scanning with MDCT allows for the dynamic assessment of the vocal cords during active phonation to evaluate whether vocal cord movement is normal. This assessment can be performed using a "cine" acquisition mode.

In general, multiplanar and 3D images transform axial data into an anatomic image that is more familiar to clinicians and easier for patients to understand.This can improve communication between clinicians and patients and can aid in surgical planning.

References:

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