Chest Ultrasound

Introduction

In the last decades, technology advances have greatly improved the imaging capacity of chest ultrasound (CUS). Major advantages of chest sonography include the lack of ionization radiation, low cost, exibility, reproducibility, short time examination, and bedside availability for reduced mobilization of patients. CUS is notably helpful for critically ill patients [1] because of its portability, simplicity, and for the follow-up of the diseases [1]. Transthoracic scanning has proved to be a reliable imaging tool for the evaluation of thoracic pathologies. Studies have proven ultrasonography’s superiority to chest radiography [2] and computed tomography (CT) scan [3] for detecting pleural effusion, pneumothorax, lung consolidation, or interstitial syndrome. Furthermore, sonography offers the possibility to

R. Tazi Mezalek (*)

Bronchoscopy Unit and Interventional Pulmonology, Hospital Universitari Germans Trias i Pujol, Barcelona, Spain

Gerència Metropolitana Nord, Institut Català de la Salut, Barcelona, Spain

e-mail: rtazi.germanstrias@gencat.cat

P. Trias Sabrià

Bronchoscopy and Interventional Pulmonology Unit, Department of Respiratory Medicine, Hospital Universitari de Bellvitge, Hospitalet de Llobregat, Barcelona, Spain

e-mail: ptrias@bellvitgehospital.cat

guide needle aspiration or biopsy with an increased success rate and reduced risk of complications [4, 5]. Certainly, ultrasounds did not provide a complete overview of the chest but only a section of it for a specifc problem under investigation. Intrapulmonary processes can be detected by sonography only when they extend up to the chest wall or through a sound-­conducting medium such as uid or consolidated lung. Most ultrasound emitted by the transducer are repelled at the interface between the pleura and the lung due to the large difference of acoustic impedance between the soft tissue and the air. Ribs also act as a natural barrier to the ultrasound beam.

The Technique

Thoracic sonography can easily be performed by chest physicians with several modality systems as B mode, M mode, color Doppler, and spectral analysis curve. Brightness mode or B mode imaging is the traditional two-dimensional grayscale cross-sectional imaging mode. Motion mode or M mode ultrasound studies the velocity of a specifc organ or structure relative to the probe position and it is displayed in one dimension. Both B and M modes are often complementary for chest disease characterization. Color Doppler is useful for a qualitative study of parenchymal vascularization and distinction between artifacts from respiratory and cardiac move-

ments. Finally, spectral curve analysis studies the arterial ow signal patterns in the specifc area of investigation, very helpful to discern pathologies [6–8].

Physicians who practice with ultrasounds should be familiar with the basic controls, including the freeze, depth, gain functions, and focus...

Freeze function creates still images allowing measurements of the structures and printing images for the clinical report. Depth function is a digital zoom that defnes what portion of the scanned image is displayed on the monitor at a certain magnifcation (close or far from the probe), and it is adjusted in function of operator’s interest with a scale in the vertical axis. Gain function allows an adjustment of the amplifcation of echoes and determines the brightness of the image, very useful for an optimal contrast between adjacent tissues. Focus function should be positioned at the pleural line in order to enhance the quality of the image; but it should be moved deeper when the main target is less superfcial.

The choice of transducer depends largely on the area to study and the depth of the pathology in question. Low-frequency probes (2 to 5 MHz) with a curvilinear shape are suitable for scanning deeper structures, while high-frequency linear shape probes (5–10 MHz) are used for chest wall diseases, diagnosis of pneumothorax, and for a refned assessment. In fact, higher frequency allows a superior resolution close to the probe but at the cost of reduced penetration [6, 9]. Small sector scanners with a reduced footprint may on occasion be useful for visualizing lesions through a small acoustic window, for example in the case of a narrow intercostal space. For daily practice, the best combination is one curved probe of 3.5–5 MHz with a 5–8 MHz small linear probe.

Patient position for scanning is important to obtain the best images possible, and it depends on the location of the diseases under investigation [9, 10]. Chest physicians often use available imagery (chest X-rays or CT scans) to identify the area of interest and determine the patient’s position for an optimal examination [6, 9]. Scanning the posterior chest is better to do it with the patient sitting upright, while the anterior part

of the chest is in the decubitus position. Lateral exploration can be done in both positions depending on each patient and on the extension of the disease. The caudal parts of the lung may be accessed more easily via a subcostal approach, using the liver or spleen as an acoustic window. Parasternal and supraclavicular approaches may be used for the assessment of the mediastinum and lung apices [11, 12, 13]. Raising the arm above the patient’s head increases the rib space distance, and facilitates a wider ultrasound window. It also elevates the scapula for a better posterior chest exploration. Scanning along the intercostal spaces and avoiding the shadow of the ribs make possible a maximum visualization of lung and pleura. A quiet or suspended respiration of the patient can help to exanimate in more detail some structures. The probe should be held by the sonographer as a pen with generous application of gel in contact with the skin. Physicians should be trained to use both hands to scan with and reserve the dominant hand for interventional procedures guided by ultrasounds. Contralateral side should be always being explored and used as a control. Ultrasound exploration is better done in a reduced lighting environment and the operator should be able to describe echogenicity (compared with that of the liver) and to characterize all the fndings in a specifc ultrasound report. Saving images and short clips of exploration in a database are highly recommended.

The Normal Thorax

The different layers of the chest wall are easy to identify with a high-frequency ultrasound probe. The intercostal muscles appear as hypoechoic linear structures containing echogenic fascial layers. On a vertical (or longitudinal) scanning, ribs appear as convex structures with a typical posterior acoustic shadowing and on a horizontal (or oblique) view, the anterior cortex appears as an uninterrupted echogenic line. Both visceral and parietal pleura usually appear as a single highly echogenic line no more than 2 mm wide representing the pleuropulmonary interface [6, 9, 10], but they can be seen as two distinct echo-

genic lines on high-resolution scanning. In B mode, sliding of the two layers of pleura during respiration gives rise to the “gliding pleura” or “sliding lung sign.” The amplitude of sliding is greatest at the lung bases and minimal at the lung apices [14], and it is best appreciated on horizontal scanning and its presence has a high negative predictive value for the diagnosis of a pneumothorax [15–17]. Normal lung parenchyma cannot be visualized on transthoracic ultrasonography because the ultrasound beam undergoes complete re ection at the interface between the pleura and the aerated lung. However, this large acoustic difference­ impedance creates hyperechoic reverberations artifacts, easy to identify, and essential to know for the physician.

“A lines” are brightly and static echogenic horizontal artifacts which run parallel to the pleural surface (Fig. 30.1). They represent reverberation signals between the pleural surface and the outer surface of the chest wall. They are visualized in a multiplicative distance between the skin surface and the pleural line.

“B lines” are vertical lines that arise from pleural line and spread uninterrupted up to the edge of the screen, also known as “lung rockets” or “comet tail sign,” and can be seen at the lung bases in healthy volunteers representing uid-­

flled subpleural interlobular septa (Fig. 30.2). They are usually hyperechoic and multiple in one longitudinal scan. Less than three B lines may be normal in the lower and lateral part of the lung [18, 19]. They are dynamic artifacts and move synchronously with the lung sliding and tend to erase A lines at the point of intersection. In fact, distribution of B lines is helpful in assessing alveolar and interstitial lung diseases [18]. Thickening of the interlobular septa due to interstitial edema results in multiple and regular 7 mm spaced B lines. More closely (3 mm) spaced B lines and con uent to the pleural line are seen in case of alveolar edema, and have been correlated with elevated pulmonary capillary wedge pressure [22]. An absence of B lines is typically seen in the pneumothorax. Thus, the presence of a single B line is enough to rule out the diagnosis of pneumothorax [20, 21]. Occasionally, ultrasound parasites appear as vertical lines which should never be confused with B lines. They are called “Z line” and have no known meaning. They arise from the pleural line but they are ill-­ defned and fade after a few centimeters. They are not hyperechoic as B lines but gray at their onset with respect to the pleural line and they don’t erase A lines. Finally, they don’t move with lung sliding [23].

“Acoustic shadow” and “tadpole tail sign” are other signs that one should recognize. They are artifacts due to the attenuation of ultrasound beams. Ultrasound waves propagating through a low impedance structure within a higher impedance tissue are less attenuated than others which propagated through the surrounding area. Thus, they are displayed as a brighter signal, known as a “tadpole tail sign.” When the ultrasound beam is almost completely re ected by the high impedance structure (as bone or calculus), the posterior area does not receive ultrasound waves or only at a very low energy level and, fnally, they are displayed as an “acoustic shadow.”

Another basic sign to consider is the “bat sign” obtained in a longitudinal scanning. The bat sign is formed by the superior and inferior ribs and the pleural line between them. The periosteum of the ribs represents the wings and the bright hyperechoic pleural line represents the bats’ body. By rotating the probe until an oblique position, one can visualize a larger part of the pleural line which is not interrupted by the rib shadows. The pleural line appears as a horizontal line.

Chest Wall Pathology

Ultrasound represents often the frst-line radiological investigation for a palpable chest wall lump whatever in ammatory or neoplastic. Masses generally are easy to detect but have variable echogenicity and the sonographic fndings are too nonspecifc to determine precisely the etiology. In addition, CUS detects the tumoral necrosis area to avoid during the puncture for an optimal diagnosis Table 30.1.

Lymph nodes can be detected in the palpation by the examiner and which are the most clinically relevant fnding. In B mode and with a high-­ frequency probe, sonography helps to differentiate between malignant and benign lymph nodes. The vascularization pattern on color Doppler provides more information about the type of the node. Changes in size, shape, margins, echogenicity in conjunction with clinical information help to make a correct diagnosis. US characteristics of lymph nodes are similar to those established during endobronchial ultrasound (EBUS) examination. But the fnal assessment is made by the histological sample which can be guided by US as well.

Rib fractures can be identifed two to six times more frequently by sonography than with chest radiography [24, 25]. They are visualized as a clear disruption of the anterior echogenic margin of the rib with associated step or overlying hematoma. Subtle rib fractures give rise to a reverberation artifact, also known as the “chimney sign” or “light-house phenomenon.” Fracture healing may be demonstrated by the evidence of an echogenic callus with a marked cortical re ex and fne acoustic shadow [26].

Metastatic lesions within the ribs have variety sonography appearances: disruption of the bony cortex, hypoechoic rounded, and well-­demarcated space occupying lesions or expansive lesion with rib destruction. Paik et al. (2005) found that sonography helps to distinguish metastasis from traumatic rib lesions in cases of hot-uptake lesions on bone scintigraphy, by demonstrating mass effect and irregular bony destruction [27]. Color Doppler reveals a corkscrew-like neoformation of vessels [28].

Chest extension of lung tumor can be access by ultrasound with a high-frequency scanning probes (7.5–10 MHz) useful for a clear discrimination between chest wall tissue layers. Thoracic US is signifcantly helpful for diagnosing chest wall extent of lung cancer. Direct evidence on US

of infltration of wall structures and rib destruction are reliable criteria [29]. Sugama et al. have defned three US pattern of chest wall invasion: UP1 (Ultrasound Pattern) indicates that the tumor is in contact with the visceral pleura, but on US the visceral pleura line is intact and the movement of the tumor with respiration is unaltered. UP2 indicates that the tumor has extended beyond the visceral pleura and is in contact with the parietal pleura. On US, the visceral pleura line is invaded or interrupted and the movement of the tumor is disturbed. Finally, UP3 means that the tumor has extended to the chest wall through both visceral and parietal pleura. The visceral pleura line is invaded or interrupted and the movement of the tumor is not present on US [30].

Parietal subcutaneous emphysema can be detected as well by US with typical fndings. Characteristic “E lines” are present on the scanning. E lines (E for emphysema) are known as well as “stripe of emphysema.” They are ­hyperechoic, vertical, and aligned artifacts spreading to the edge of the screen like B lines. The difference is that they arise not from the pleural line but from a hyperechoic line horizontally located just above it. Keep in mind that no bat sign is visible because we are not in a lung ultrasonography but just above in the soft tissue.