Reconstructions, protocols aid virtual bronchoscopy

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Advances in CT scanner technology, particularly the deployment of 64-slice systems, have seen greater interest in the virtual bronchoscopy (VB), however, quality images of the lung and airways using this technique require careful attention to scanning protocols and reconstruction techniques, according to a recent article published in the American Journal of Roentgenology.

“It is clear that VB can be a useful adjunct to conventional axial CT in the evaluation of patients with suspected airway abnormalities,” the authors wrote.

Researchers from the Johns Hopkins Medical Institutions in Baltimore use VB to detect and grade benign and malignant airway stenosis; bronchogenic carcinoma, endoluminal lesions; anatomic variants; foreign body aspiration; imaging guidance for biopsy; and miscellaneous applications such as tracheoesophageal fistula stent planning and follow-up, burn injuries and trauma.

The clinicians wrote that IV contrast may or may not be needed, depending on the study. If IV contrast is indicated, they typically inject 100-120 mL of nonionic contrast medium at a rate of 3 mL/s through a peripheral angiocatheter. They noted that scan timing can be tailored to highlight the arteries (25 seconds after injection) or veins (40 seconds after injection), depending on the clinical scenario.

“In this setting, CT of the airway can be combined with CT angiography to show the relations between the vessels and the trachea,” they wrote.

When conducting a dedicated airway study, the group employs a 64-slice CT scanner at 0.6-mm collimation setting to generate 0.75-mm slices reconstructed every 0.5 mm for 3D review. The other settings the team uses are 200 effective mAs at 120 kVp with a rotation time of 0.5 seconds. They observed that scanning takes only a few seconds, which allows even infants to undergo the procedure without sedation.

“CT of the airways usually is performed during inspiration,” they wrote. “In certain clinical applications, however, such as tracheobronchomalacia, both inspiration and expiration acquisitions should be performed to detect airway narrowing and collapse. Similarly, expiratory phase imaging can be valuable for detecting airway stent failure.”

When the image acquisition is completed, the data is transferred to a dedicated 3D workstation (Leonardo, Siemens Medical Solutions). Clinicians review the data with a combination of volume rendering, maximum intensity projection and multiplanar reformation (MPR).

“Volume rendering is especially helpful because the degree of transparency can be controlled and, when slab clip plane editing is used, can display the airways in great detail,” they wrote. “Color assignments can be used to highlight stents. Volume visualization of the lungs can be performed with varying degrees of transparency to highlight differences in lung aeration.”

The group has found that the precision and accuracy of their radiologic findings can be improved when axial images, MPRs and endoluminal views are viewed simultaneously on a workstation, in a display manner similar to that for virtual colonoscopy.

They cautioned that in the performance of surface or volume rendering of the airway, it is essential to select appropriate thresholds because the threshold affects the diameter of the airways, and that inappropriate thresholds may cause artifacts.

“It is often necessary to use different threshold values when visualizing the central airway and the distal airway,” the authors noted.