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.
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