Limitations Of Virtual Bronchoscopy

In order to create a virtual bronchoscopy study, images need to be obtained on a prospective basis: Therefore, the clinician and radiologist have to be in direct communication for an appropriate study to be performed. This is critical since the collimation necessary may be thinner than what is routinely obtained in the radiology department. For example, 5- to 10-mm CT scan slices may be routinely obtained rather than the 3- to 5-mm slices required for VB. Without the appropriate slice thickness and reconstruction, the program may not be able to reconstruct the data into a satisfactory three-dimensional image. If images of 7- to 10-mm slice thickness were reconstructed retrospectively, they would be suboptimal due to stairstep artifacts.

A protocol for VB must be established in order to select the appropriate slice thickness and reconstruction algorithm. The reconstruction of images for VB formatting has to be performed while the original helical CT raw data is still available on the CT scanner's computer. Raw data requires a substantial amount of memory on a CT scanner's computers and unless the data is moved to a separate storage unit (e.g., disk or disk drive) it is discarded as new incoming scans of other patients are performed throughout the day.

Not all patients are candidates for VB imaging. Virtual bronchoscopy requires continuous scanning of the trachea and proximal airways during a single breath hold; in other words, a patient is required to hold his or her breath for up to 30 sec. With the multidetector CT scanners, this time can be shorted substantially and, unless a patient is in respiratory distress, a reasonable scan should be obtainable, especially if prescanning hyperventilation is applied.

Although VB has advantages over actual endoscopy in terms of simultaneously imaging extraluminal and endoluminal anatomy, it is not capable of evaluating the mucosa of the airways and small lesions such as ulcers or plaques. In addition, mucus in the airways may mimic an endobronchial lesion. In this sense, VB should be considered an imaging guide and bronchoscopy should still be performed on any patient with a suspected airway lesion.

Virtual bronchoscopy appears to be useful for imaging pediatric airways for disorders such as vascular rings, tracheomalacia, bronchomalacia, or congenital tracheoesophageal fistula [19]. However, respiratory motion is a significant limitation to its use with small children who are unable to hold their breaths on command [20]. Infants and the very young are unable to perform the breath holding necessary to avoid artifacts such as stairstepping and resultant pseudonarrowing of the airway. In addition, radiation dosage is a concern in this patient group. Although a single helical CT scan is standard for imaging a child with suspected or known thoracic disease, thin collimation inspiratory and expiratory CT images of the entire airway to diagnose disorders such as bronchomalacia and tracheomalacia exposes the patient to excess radiation. Even though multidetector CT scanners decrease scan time, a child still has to be able to comply with breath holding instructions, even if it is for less than 10 sec.

VI. THE FUTURE OF VIRTUAL BRONCHOSCOPY

As discussed above, the power of VB lies in its three-dimensional imaging of the airway with simultaneous imaging of the mediastinum. Three-dimensional VB creates images that a physician without formal image training can comfortably understand. It is noninvasive and can be readily reconstructed from clinically obtained CT scans. To date studies have shown that VB is useful as a preprocedure guide to bronchoscopy by providing excellent anatomy of the airway and demonstrating the presence of stenosis [21,22]. Preliminary studies show that VB can add to the confidence of the bronchoscopist performing transbronchial needle aspiration biopsy by mapping out mediasti-nal masses and locating lymph nodes.

Conventional CT scanning has significant limitations in detecting abnormal lymph nodes [23]. Contrast enhanced helical CT has provided finer resolution from thinner sliced and reconstructed images and as a result can better distinguish enhancing vascular structures from the hilar and mediastinal lymph nodes [24]. The juxtaposed hilar and paratracheal nodes are the most difficult lymph nodes to distinguish from one another. However, distinction of nodes

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