Radial probe endobronchial ultrasound (rEBUS) is a small ultrasound probe placed through the working channel of a bronchoscope used to locate peripheral lung abnormalities, such as pulmonary nodules. The probe provides a circumferential 360-degree view of its surrounding structures ( Fig. 3.1 ).
Normal lung is filled with air, which is highly reflective of ultrasound waves ( Fig. 3.2 ). When the radial probe is placed within or adjacent to a solid lesion in the periphery of the lung, a hyperechoic image with a clear border will be seen. If the radial probe is placed within the lesion, a concentric ultrasound view will be obtained ( Fig. 3.3 ). If the radial probe is placed adjacent to the lesion, an eccentric ultrasound view will be obtained ( Fig. 3.4 ). Radial EBUS allows for real-time localization of the target lesion. The rEBUS probe must be removed from the working channel prior to sampling. It is important to recognize that rEBUS does not provide the bronchoscopist with a road map to the target nodule; rather, the bronchoscopist will have to charter a path based on their own review of the chest computed tomography (CT) or use a guidance system such as navigational bronchoscopy. Radial EBUS is used once the bronchoscopist feels that the tip of the scope or guide sheath is close to the lesion and can confirm in real time whether the nodule has been reached based on an ultrasound image.
The American College of Chest Physicians Lung Cancer Guidelines recommend rEBUS as an adjunct imaging modality for patients suspected of having lung cancer, who have a peripheral lung nodule, and a tissue diagnosis is required due to uncertainty of diagnosis or poor surgical candidacy.
Patient selection is dependent on the location and size of the lesion, the experience of the bronchoscopist, and weighing the risk of complications versus the probability of a diagnosis. The presence of a bronchus sign, a visible airway on the CT scan leading to the peripheral lesion, should favor the use of rEBUS because of the higher probability of finding and diagnosing the lesion ( Fig. 3.5 ). Lesions located adjacent to the chest wall should be considered for CT-guided percutaneous needle aspiration, though the decision to proceed with rEBUS is dependent on the comfort level of the bronchoscopist. Operator inexperience and inability to tolerate procedural sedation are contraindications to rEBUS.
Radial endobronchial ultrasound probe
Probe driving unit
Universal ultrasound processor
Sampling instrument (forceps, brush, needle)
Guide sheath kit (optional)
Guiding double-hinged curette (optional).
Sedation nurse or anesthesia team.
The procedure can be done in an endoscopy suite or operating room. Anesthesia can be with either moderate/conscious sedation, monitored anesthesia care, or general anesthesia. The bronchoscope can be inserted into the airway with a nasal or oral approach. The use of an endotracheal tube or laryngeal airway mask is optional. These practices are variable among institutions.
Radial EBUS Without Guide Sheath
Prior to starting the bronchoscopy, the operator will need to determine where the lesion is located. Lesion localization can be performed with navigational assistance or by CT-anatomic correlation using coronal, sagittal, and axial planes. If navigation is being used for localization, the rEBUS probe can be used to confirm in real time the lesion location prior to sampling. When using CT imaging as a reference for lesion localization, the majority of nodules can be identified with rEBUS. A retrospective review of 467 peripheral nodule bronchoscopy cases found that when using CT imaging for lesion localization, 96% of nodules were identified with rEBUS; however, this level of expertise requires ample practice.
The bronchoscope is inserted into the target lobe and segment. The proceduralist will need to be in eyesight of their endoscopic, radial ultrasound, and fluoroscopic views (fluoroscopy is optional but recommended). Using the endoscopic and fluoroscopic views, the radial probe is inserted sequentially into the preplanned target airways until a lesion is identified with radial ultrasound. The bronchoscopist should aim to obtain a concentric ultrasound image because of the increased diagnostic yield compared with an eccentric ultrasound image. If an eccentric image is obtained, the bronchoscopist should attempt to reposition the radial probe in nearby airways seeking to obtain a concentric ultrasound view. Unfortunately a concentric ultrasound view is not always achievable, and an eccentric ultrasound image may occur in nearly half of patients. When satisfied with the rEBUS image, the bronchoscope is positioned a few centimeters proximal to the lesion if possible. The closer the bronchoscope is to the lesion, the more likely the sampling instruments will follow the same path as the radial probe.
Before removing the radial probe to sample the lesion, a static or “still frame” of the fluoroscopic image may be captured and projected adjacent to the “live” fluoroscopy. The still frame of the radial probe localizing the lesion can be used as a template for sampling the lesion. The radial probe is then removed and a sampling instrument is placed into the working channel. Using “live” fluoroscopy, the sampling instrument is advanced to the lesion in a similar manner as the radial probe, aiming to replicate the still frame. When sampling, the goal is to place the sampling tool (with live fluoroscopy) in the same location as the radial probe (still frame).
rEBUS can also be done without fluoroscopy. Once the lesion is located, the radial probe is removed and the length can be measured from the working channel insertion site. A sampling tool is then placed into the working channel and samples are taken at the same length of the radial probe.
rEBUS can be done with any size bronchoscope, though thinner bronchoscopes improve access deeper into the periphery of the lung. Ultrathin bronchoscopes with a 3-mm outer diameter are also an option and have been shown to increase diagnostic yield.
Radial EBUS Plus Guide Sheath
If the bronchoscope cannot be placed in close proximity to the peripheral lesion, a guide sheath can be used. A guide sheath is an optional tool that can be utilized with rEBUS. The guide sheath is a plastic catheter with a distal radio-opaque marker placed through the working channel of the bronchoscope. The radial probe and sampling instruments can then pass independently through the guide sheath. Only one instrument can be placed in the guide sheath at a time. Once the target lesion is identified with rEBUS, the radial probe is withdrawn and the guide sheath is kept in place just proximal to the lesion. Fluoroscopy is optional and can be used to guide sampling. Transbronchial needle aspiration, forceps biopsy, and brushings can be placed through the guide sheath. Two guide sheath sizes are available, with the smaller size being compatible with a working channel of 2 mm.
Radial EBUS Plus Guide Sheath and Guiding Curette
If there is difficulty locating a peripheral lesion, a guiding curette can be used to enter airways not directly accessible with the radial probe or bronchoscope. The guiding curette is placed through the guide sheath and can be flexed, extended, or rotated to access more acutely angled airways. This is most easily performed using fluoroscopic guidance. Once the curette has accessed the airway, the guide sheath is advanced over the curette into the airway. The curette is then withdrawn and the radial probe is inserted into the guide sheath to examine the newly accessed airway ( Fig. 3.6 ).