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Endobronchial Ultrasonography: An Overview

Noriaki Kurimoto

Department of Internal Medicine, Division of Medical Oncology and Respiratory Medicine, Shimane University Faculty of Medicine, Izumo, Japan

Introduction

Endobronchial ultrasonography (EBUS) is a diagnostic modality whereby a miniature ultrasonic probe is introduced into the bronchial (tracheal) lumen, providing tomographic images of the peribronchial (peritracheal) tissue. Endoscopic ultrasonography (EUS) is an established, indispensable technique for examining the gastrointestinal tract, particularly the stomach and large intestine. The applications of EUS include assessment of the depth of tumor invasion, detection of lymph node metastases, tumor staging, and fine‐needle aspiration (FNA) under EUS guidance.

In 1988, Pandian et al. [1] were the first to report the clinical use of a narrow‐gauge ultrasonic probe for intravascular ultrasonography. In 1990, EBUS was first mentioned in the study by Hürter et al. on EBUS of the lung and mediastinum. Since then, research and development in this field have been primarily conducted by Becker (Germany) and us (Japan).

Typically, radial EBUS probes are of the 20 MHz radial type. Therefore, tissue penetration of the ultrasound waves is of the order of approximately 2–3 cm; in other words, EBUS provides a tissue cross‐section image with a radius of approximately 2–3 cm centered on the trachea or bronchus.

Some important EBUS studies are:

• Hürter and Hanarath [2]. Endobronchial sonography in the diagnosis of pulmonary and mediastinal tumors (in German).
• Iizuka et al. [3]: Evaluation of airway smooth muscle contractions in vitro by high‐frequency ultrasonic imaging.
• Ono et al. [4]: Bronchoscopic ultrasonography in the diagnosis of lung cancer.
• Goldberg et al. [5]: US‐assisted bronchoscopy with use of miniature transducer‐containing catheters (delineation of central and peripheral pulmonary lesions).
• Becker [6]: EBUS – a new perspective in bronchology (tracheobronchial wall seven‐layer structure).
• Kurimoto et al. [7]: Assessment of usefulness of EBUS in determination of depth of tracheobronchial tumor invasion (tracheobronchial wall five‐layer structure).

Based on these studies, the current applications of EBUS are as follows:

• Determination of the depth of tumor invasion of the tracheal/bronchial wall (allocation of patients to localized endobronchial treatments such as photodynamic therapy).
• Identification of the location of a peripheral lung lesion during bronchoscopic examination (more accurate than fluoroscopy in determining contact between lesion and bronchus, thereby reducing abrasions, the time to determine biopsy sites, and duration of fluoroscopy).
• Qualitative diagnosis of peripheral lung lesions and differentiation between benign and malignant lesions.
• Determination of position and shape of peribronchial structures, particularly lymph nodes (at the time of transbronchial needle aspiration).
• Determination of the spatial relationship between bronchus and lesion in the short‐axial image of the bronchus (if the bronchus is situated near the center of the lesion, the lesion might have arisen from the bronchus).

Issues arising from the application of EBUS to date and the results of studies include the following:

• Standardization of how the layers in the tracheobronchial wall structure are interpreted (how many layers are seen).
• Changes in the layer structure of the tracheobronchial wall with the use of higher frequencies (e.g. 30 MHz).
• Evaluation of the qualitative diagnosis accuracy and differentiation between benign and malignant lesions from EBUS images of peripheral lung lesions.
• Evaluation of peribronchial lymph node metastases.
• Complications of EBUS‐guided transbronchial needle aspiration (TBNA).
• Technique of needle aspiration from the esophagus using the convex bronchoscope.

EBUS facilitates examining the state of the bronchial wall and extramural tissue that cannot be visualized with bronchoscopy alone. This book will present an overview of EBUS with reference to actual clinical cases.

Principles of Ultrasonography

What Is a Sound Wave?

Definition of Ultrasound

Typically, ultrasound refers to sound wavelengths >20 MHz that cannot be heard by the human ear. Because considerable variations exist in the range of frequencies audible to humans, we often define sounds in terms of their purpose. In this case, ultrasound is “sound not intended for humans to hear.”

Frequency and Wavelength

The frequency of a sound tells us whether it is high or low in pitch. The unit of frequency is hertz (Hz), defined as the number of oscillations per second. For example, a sound with a frequency of 20 MHz has 20 × 106 oscillations per second. Medical ultrasonography equipment produces sounds with a frequency range of 2–50 MHz. Wavelength is the length of a soundwave, and it varies inversely with frequency; thus, the higher the frequency, the shorter is the wavelength (Figure 1.1).

Speed of Sound

Sound travels through various materials, such as air and water (hereafter media), and the speed at which it travels through each medium is the speed of sound for that medium. The speed of sound through the human body is generally considered to be 1530 m/s, although the actual speed of passage varies for different organs and tissues.

Production of Ultrasound Images

Transmitting and Receiving Ultrasound Waves

Ultrasonic probes used in medical ultrasonography comprise a sensor that transforms electrical signals into ultrasound and vice versa. When an electrical signal is applied to an electrode of an ultrasonic transducer (also known as an oscillator/transformer), ultrasound waves are transmitted from the device surface; when ultrasound waves are received by the device surface, an electrical signal is generated (Figure 1.2).

Figure 1.1 Correlation between frequency and wavelength.

Figure 1.2 Transmitting and receiving ultrasound waves.

Propagation and Attenuation of Ultrasound Waves

Ultrasound waves produced by an ultrasonic transducer travel through a medium – called propagation. As the soundwave is propagated, the energy of its oscillations is absorbed and scattered, thereby weakening steadily; this phenomenon is called attenuation. Typically, the higher the frequency, the higher is the attenuation rate. Medical ultrasonography equipment uses high frequencies that do not propagate well through the air owing to the high attenuation ratio. Hence, a medium such as water is required between the ultrasonic transducer and the study object to allow the efficient propagation of ultrasound waves.

Reflection and Penetration

As with light, a proportion of ultrasound waves is reflected at the boundary between different media, and a proportion penetrates the boundary; the ultrasonic processor uses these reflections to construct images.

The ultrasonic transducer emits ultrasound pulses and receives ultrasound pulses reflected from the boundaries between media (Figure 1.3). In addition, the ultrasonic processor evaluates the positions (distance from the probe) of boundaries between media based on the time between transmitting and receiving ultrasound pulses and converts the strength of returning pulses into the brightness of the image.

Figure 1.3 How an ultrasound image is made.

Figure 1.4 Difference in axial resolution between different frequencies.

Following these steps alone offers information about a body along a single line; thus, we obtain a two‐dimensional image by moving the ultrasonic transducer (mechanical scanning) or using a linear array of multiple ultrasonic transducers that sequentially emit and receive ultrasound pulses (electronic scanning). This method of ultrasound imaging is called B‐mode (B stands for brightness).

Resolution

Axial Resolution

Because an ultrasound pulse wave has a definite length, the boundary between media has a definite width on an ultrasound image. If the distance between the two boundaries of a medium is decreased, the pulse waves from the two boundaries will overlap, making it difficult to distinguish the two boundaries on the ultrasound image. The ability to distinguish objects on an ultrasound image is called resolution, and the resolution in the direction traveled by the ultrasound pulse is the axial resolution. Typically, higher the frequency, shorter is the ultrasound pulse; thus, distance resolution improves with higher frequencies (Figure 1.4).

Lateral Resolution

Resolution in the direction perpendicular to the direction traveled by the ultrasound pulse, in other words in the direction the probe moves or the direction of the...