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Threshold selection in virtual bronchoscopy: phantom study and clinical implications

Purpose: To investigate the effect of the threshold selection on the apparent diameter of a virtual bronchus and the virtual wall morphology and to examine the parameters that can affect the selection of the appropriate threshold for virtual bronchoscopy with single slice computed tomography. Material and Methods: A bronchial phantom containing plastic tubes simulating the airways of the tracheobronchial tree was constructed. The diameters of the virtual images of these tubes were measured using various thresholds for two different materials surrounding the tubes: air and water. These measurements were correlated with the density in HU of the walls. Furthermore, data from 20 virtual bronchoscopies in patients were retrospectively analyzed in the same way. Results: The diameter of a virtual bronchus is strongly affected by the selected threshold. The appropriate threshold for accurate diameter representation depends on the density of the bronchial wall. Conclusion: Our results suggest that in clinical practice a single threshold value cannot be used for imaging all segments of the tracheobronchial tree. While a value of −520 is appropriate for the trachea and lobar segments, values down to −720 could be needed on the level of segmental and subsegmental bronchi. At these levels, a threshold value about 65 HU more negative than the value where the artificial holes appear on the virtual bronchial walls could be used.