Endobronchial Ultrasound

History and Historical Perspective

Ultrasound has long been used to visualize intrathoracic structures, but prior to the introduction of endoscopic ultrasound, the bronchoscopist’s view was limited to those structures seen within the airways or with uoroscopy. In 1980, the use of ultrasound to visualize intrathoracic structures was frst described in the feld of gastroenterology for staging esophageal and gastric malignancies [1]. It wasn’t until the early 1990s when ultrasound technology was introduced within the airway. Small ultrasound probes, also known as “miniprobes,” were the frst version of radial probe ultrasound endobronchial ultrasound (RP-EBUS) and became the frst endobronchial ultrasound to visualize structures surrounding the airway wall [2]. In 2002, Herth et al. published the frst report of RP-EBUS-guided transbronchial lung biopsy (RP-EBUS-TBBx) for solitary pulmonary nodules and peripheral lesions [3]. At this point, the endobronchial ultrasound could provide confrmation that the target was reached but the biopsy was still performed without imaging guidance. This led to the development of the convex probe EBUS (CP-EBUS) in 2002 which provided both target confrmation and real-time

A. A. Goizueta · G. A. Eapen (*)

Department of Pulmonary Medicine, The University of Texas MD Anderson Cancer Center,

Houston, TX, USA

e-mail: geapen@mdanderson.org

guidance during the biopsy [4]. The wide acceptance of CP-EBUS-guided transbronchial needle aspiration (CP-EBUS-TBNA) as a safe and effective tool for mediastinal and hilar lymphadenopathy resulted in a rapid decline in the use of mediastinoscopy from 21.6 to 10.0% from 2006 to 2010 [5, 6]. While RP-EBUS continues to be a tool used for the confrmation of peripherally located lesions, its use has been limited by the fact it does not provide real-time biopsy imaging.

The Basics of Endobronchial

Ultrasound

Ultrasound is an imaging modality that utilizes the mechanical properties of high-frequency sound waves passing through different densities of tissue to produce images of internal body structures. The ultrasound waves are created by applying an electrical current to crystals with unique piezoelectric properties within the ultrasound probe, resulting in the vibration of the crystals to generate sound waves. The sound waves then penetrate the tissue and are re ected as an echo onto the crystals causing them to vibrate and generate electrical current that is analyzed by the ultrasound machine and reconstructed into an image. Sound waves are measured by their wavelength and frequency. Wavelength is the distance sound travels in one cycle and fre-

quency refers to the number of cycles of rarefaction in a sound wave per second. Wavelength is an important factor in the axial resolution of the ultrasound image and is inversely proportional to frequency. This means that the shorter the wavelength, the higher the frequency and resolution, but less depth of penetration, making higher frequency probes better for visualizing superfcial structures and lower frequency probes better for deeper structures. The production of an ultrasound image is also dependent on the density of tissue penetrated and propagation speed which is collectively known as acoustic impedance. If two materials have no difference in acoustic impedance, an echo will not be produced (e.g., pleural effusion, simple cysts) and when there is a large difference, the echo is completely re ected resulting in total acoustic shadowing (e.g., bone). The direction from where the sound wave is re ected, and the time taken to reach and return from the tissue gives information on the location and distance of the target from the probe, respectively. These properties of ultrasound provide the ability to accurately and clearly visualize structures not visible to the naked eye.

Radial Probe Endobronchial

Ultrasound

Description oftheEquipment and Technique

Equipment

The RP-EBUS is a catheter-based probe that is introduced through the working channel of the bronchoscope into the airways to provide a 360° view of the adjacent structures (Fig. 23.1). The outer diameter of the radial probes currently available are 1.4 mm and 1.7 mm which are compatible with the working channel of most bronchoscopes. The most commonly used frequency of RP-EBUS is 20 MHz which can provide a depth of penetration up to 5 cm and a resolution of less than 1 mm [2]. The high-resolution probe can easily differentiate structures with different echogenicities within millimeters of each other. In order to use the RP-EBUS, additional equip-

Fig. 23.1  RP-EBUS (UM-S20-17S, Olympus, Tokyo, Japan) advanced through the working channel of a exible bronchoscope (Reprinted with permission from Olympus)

ment is required including a bronchoscope, light source, video processor, ultrasound processor, and a monitor (Fig. 23.2). The radial probe is connected to an ultrasound processor with the ability to process the image and project it on to a monitor with outstanding image quality. There are different ultrasound processors on the market which are compact and have capabilities to process both RP-EBUS and CP-EBUS ultrasound images. Although the RP-EBUS cannot provide Doppler imaging, like CP-EBUS, the images can be captured, and the size of the target can be measured.

Technique

Once the bronchoscope has been advanced into the appropriate location and is determined to be near the target, a scanning technique is used to locate the target by advancing the radial probe outside of the bronchoscope toward where the lesion is suspected. Normal aerated lung tissue will typically appear as a “snowstorm like” whitish image due to the difference in impedance (Fig. 23.3). If the ultrasound probe is not in direct contact with the wall, the image will not be visible due to the lack of ultrasound penetrance and complete re ection of the ultrasound waves. Solid tumors can usually be differentiated from normal lung by a homogeneously gray area with a well-demarcated white echogenic border that commonly lacks a discrete air bronchogram [7] (Fig. 23.4). Necrotic areas will appear darker and

Fig. 23.2  Endobronchial ultrasound system that includes a radial probe endobronchial ultrasound, bronchoscope, light source, video processor, ultrasound processor, and a monitor (Reprinted with permission from Olympus)

Monitor

Bronchoscope

Video processor

Light source

Ultrasound

Processor

Suction pump

blood vessels will appear as black well demarcated circular or tubular structures. Benign lesions and atelectasis will usually have an inhomogeneous pattern with air bronchograms and white spots caused by varying structures within the lung. The RP-EBUS lacks the capability of Doppler ultrasonography requiring full evaluation of the length and presence of pulsatility to confdently differentiate between necrotic tissue and blood vessels.

When using RP-EBUS to biopsy a lesion within the lung, uoroscopy is commonly used to

assist in directing not only the bronchoscope but the radial probe and the biopsy tool of choice toward the target lesion. This combination of tools has been proven to increase diagnostic yield compared to uoroscopy alone. The use of forceps to perform TBBx remains one of the most frequently used tools to biopsy peripheral lesions allowing for histological and immunohistological examination. In addition to the standard forceps biopsies, brushings, TBNA, and cryobiopsy can be performed. In certain situations, the use of a guide sheath to extend the working channel of the