Is Endobronchial Ultrasound Indispensable in Clinical Practice?

摘要

When computed tomography (CT) became a routine procedure for staging of lung cancer in the 1980s, we were confident that many structures that could not be assessed by regular chest radiography and conventional tomography would become visible easily and that staging would become much more accurate. The structures of special importance were the lymph nodes, the walls of the central airways, the mediastinum (especially the large vessels), the heart, the esophagus, and the diaphragm. However, despite preliminary promising experience, the results in prospective studies were disappointing. 1,2 For a better understanding one has to consider that radiographic imaging of the intrathoracic organs depends mainly on the difference in density of water (soft tissue) and air-containing tissue (lung). If there is no interface like fat or air between two adjacent structures that are composed of soft tissue (e.g., lymph nodes and the bronchial wall or a tumor and the esophagus), those structures cannot be separated by CT. This is also true for virtual bronchoscopy by three-dimensional reconstruction of CT images because it is derived from conventional CT and basically contains the same information. In addition, structures of water density like sticky secretions, mucoid impactions, or blood clots can be interpreted mistakenly as solid intraluminal structures on CT. More recent imaging procedures, like magnetic resonance imaging and positron emission tomography, have similar drawbacks in analyzing mediastinal structures. In magnetic resonance imaging, resolution is still too low for differentiation of more delicate structures. With regard to positron emission tomography, the diagnosis of structures by analyzing their metabolic status also is not very reliable and still has to be proved in prospective studies. The view of the endoscopist, however, is limited to the lumen and the internal surface of the airways. Processes within the airway wall and outside the airways can only be assessed by indirect signs. In addition, many processes also involve the parabronchial structures. 3 Especially in malignancies, this can be of decisive importance for the fate of the patient. This is why expanding the endoscopist's view beyond the airways is essential. We were therefore looking for an imaging procedure by which we could overcome these obstacles. THE DEVELOPMENT OF ENDOBRONCHIAL ULTRASOUND (EBUS) The imaging in ultrasound is different from the processes in radiographic imaging. The difference is in the resistance of different tissues to ultrasonic waves (impedance), which is more complex and only partly dependent on water content. The different impedance of soft tissues has made ultrasound an indispensable diagnostic tool in medicine. However, external mediastinal thoracic ultrasound is insufficient for imaging of the paratracheal and hilar structures because of the limitation of the acoustic window resulting from total reflection of the ultrasonic wave by interference of air-containing lung tissue. For analyzing structures that cannot be reached from the body surface, devices for endoluminal application have been developed. During diagnosis of mediastinal structures, endoesophageal ultrasound by dedicated ultrasonic endoscopes has been applied with great success and has proved to be superior to CT. 4 However, according to the anatomic position of the esophagus, contact to structures of interest is limited and the view is restricted to the lower and to the lateral and dorsal upper mediastinum. In contrast, most of the structures are within reach from the central airways, apart from the lower and dorsal mediastinum. Instruments that are used for gastrointestinal application could not be applied inside the airways because of their diameter. Prototypes of dedicated endoscopes with an integrated curvilinear electronic transducer at the tip have not been applied widely thus far. 5 Preliminary experience using miniaturized endovascular sonographic probes did not