State - of - the - art airway imaging with CT : Part 2

Advances in CT technology afford the ability to create 3-dimensional (3-D) reconstructions of the airways in only a few minutes. The 2 basic types of 3-D reconstruction imaging methods are CT bronchography, which depicts the external surface of the airways and its relationship to adjacent structures, and virtual bronchoscopy, which allows the viewer to navigate the internal lumen of the airways by a means similar to conventional bronchoscopy. Although axial images are routinely used to evaluate the upper airways, multiplanar reformations in the coronal and sagittal planes also help evaluate upper airway pathology. Coronal multiplanar reformation images are useful in defining the anatomy of the larynx; sagittal images provide excellent delineation of the epiglottis, vallecula, and piriform sinuses. Axial images are the reference standard for assessing tracheal wall thickening and, therefore, may be particularly helpful in the differential diagnosis of tracheal stenosis. (J Respir Dis. 2006;27(6):266-273) In the May 2006 issue of The Journal of Respiratory Diseases, we reviewed advances in CT technology for noninvasive assessment of the airways, particularly multi-detector row CT scanning and multiplanar and 3-dimensional (3-D) reconstruction imaging. In this article, we examine reconstruction and reformation methods in relation to normal airway imaging anatomy. In a subsequent article, we will consider applications of 3-D reconstruction and reformation imaging. 3-D RECONSTRUCTION Advances in CT technology have enabled 3-D CT reconstructions of the airways to be created in just a few minutes, contributing to a more accurate assessment of various disorders of the central airways. 3-D CT reconstructions require the transfer of data to a separate workstation, which allows the interactive display of 3-D images in real time. Fortunately, such workstations are becoming increasingly commonplace in inpatient and outpatient radiology department settings. Most commercially available workstations provide a menu of options consisting of preset reconstruction algorithms, including dedicated airway techniques. The use of clip-editing planes (also known as trimming or extraction functions) precludes the need to trace regions of interest and significantly enhances the efficiency with which one can obtain 3-D images.1,2 Using clip-editing planes, a trained technologist or radiologist can complete a series of reconstructions in less than 10 minutes. There are 2 basic types of CT 3-D reconstruction imaging methods3-7: • External rendering depicts the external surface of the airways and its relationship to adjacent structures (Figure 1).5,6 This technique is also called CT bronchography because of the similarity of the images it produces to conventional bronchographic images. • Internal rendering combines helical CT data and virtual reality computing techniques, allowing the viewer to navigate the internal lumen of the airways by a means similar to conventional bronchoscopy.8-20 This method is also called virtual bronchoscopy because it produces images that closely correlate with conventional bronchoscopic images (Figures 2 and 3). 2-D MULTIPLANAR REFORMATION Two-dimensional (2-D) reformation methods include multiplanar reformations and multiplanar volume reformations.4,7 Images are easy to generate using these methods, which can be interactively performed in real time at the CT console or at a dedicated workstation. Multiplanar reformation images are sections of single-voxel thickness that may be displayed in coronal and sagittal planes or in a curved fashion along the axis of the airways. By comparison, multiplanar volume reformations comprise a thick slab of adjacent, thin slices and represent a block of contiguous images. The thickness of such blocks (or slabs) varies but usually ranges from 5 to 10 mm. Multiplanar volume reformation images therefore combine the spatial resolution of multiplanar reformation images with the anatomic display of thick slices.3 Multiplanar volume reformation images can also be reconstructed using a rotational (or "paddlewheel") method, along the axis of the carina, to enhance the continuous display of the central airways (Figure 4).21NORMAL AIRWAY ANATOMYUpper airways In the evaluation of the upper airways, a review of the standard axial landmarks of the airways is