Background and Indications for Examination
Thoracic ultrasonography is useful for the evaluation of pleural, pulmonary, and chest wall pathologies. The major advantages of thoracic ultrasound include bedside availability, absence of radiation, and real-time guidance for therapeutic interventions. Traditionally, thoracic ultrasound was limited to exploration of pleural effusion, but with the availability of modern and smaller handheld units, the range of applications has broadened. Thoracic ultrasound is superior to standard chest radiography in the detection and characterization of pleural effusion, and it is indispensable for the guidance of pleural interventions at the bedside.
Thoracic ultrasound can quickly diagnose pneumothorax not only in the emergency department, but also in intensive care units and procedure suites where it is encountered as a complication of interventions such as central line placement or thoracentesis. In addition, bedside thoracic ultrasound is helpful in differentiating between consolidation, atelectasis, and pleural effusion, all of which may be visualized as an indeterminate opacity on a chest radiograph. It is also useful in the evaluation of pleural and chest wall pathology. The presence of pleural artifacts (B-lines or “lung rockets”) indicates interstitial fluid and may help determine if there is pulmonary edema or acute respiratory distress syndrome (ARDS). In critical care units, thoracic ultrasound allows for earlier and more frequent assessments, and in many cases eliminates the transport risk associated with off-site CT scan.
Probe Selection and Technical Considerations
Higher-frequency transducers provide better resolution at the cost of depth. They are used for visualization of chest wall structures, which tend to be more superficial.
Lower-frequency probes with smaller footprints should be used for the evaluation of pulmonary and pleural pathology. In particular a phased-array probe, which generates a sector image from a point using an electronically steered beam, may be ideal for getting a window between the ribs. These probes have an acceptable compromise between near-field resolution and depth of penetration.
Time-motion mode (M-mode) displays motion as a one-dimensional line over time on the x-axis. It is useful for detecting lung sliding which is absent in the presence of a pneumothorax (the “seashore sign”).
Color-flow Doppler imaging can be used to detect vascular flow within chest wall lesions and consolidated or atelectatic lung. Color Doppler helps to distinguish blood vessel flow from other structures that may be present in lung tissue. Power Doppler (angiography) has also been described as identifying pleural sliding (the “power slide” sign).
An abdominal setting is often appropriate when imaging a simple pleural effusion. In the imaging of lung artifacts, a lower dynamic range/lower persistence, as is typically found in a cardiac setting (image appears more contrasted), will often display long rockets more effectively. Tissue harmonic imaging, when available, may also help. When using color flow or power Doppler, the scale or pulse repetition frequency (PRF) will need to be lowered in order to pick up flow without artifact, and gain may need to be increased.
Normal Ultrasound Anatomy
The ribs are the first structures seen in thoracic ultrasound and help to serve as landmarks for other important structures. Similar to other bones, ribs block the ultrasound signal from getting through to more distal structures. They appear as semicircle hyperechoic structures anteriorly, and therefore appear at the top of the image on the monitor. Due to the inability of the beam to penetrate the ribs, a shadow is created behind the rib, which is seen as an anechoic area emanating from the rib surface posteriorly. The ribs and their shadows serve as landmarks in identifying the parietal and visceral pleura (Fig. 7-1).
Higher-resolution probes of 7.5–10 MHz provide more detailed views of the various layers of the chest wall and pleura. Ultrasound examination of the pleura is affected by the presence of ribs and aerated lungs. Ribs absorb sound waves yielding a shadowing artifact, while air is a strong ultrasound reflector, making it impossible to directly visualize normally aerated lung. The normal pleura appears as an echogenic line between the chest wall and the aerated lung (see Fig. 7-1). In a longitudinal view, the succession of upper rib, pleural line, and lower rib outlines a characteristic pattern, the “bat sign.” The visceral and parietal pleural layers cannot be distinguished from one another due to their thickness of only 0.2–0.4 mm. The entire costal pleura can be visualized using proper exam technique. The mediastinal and majority of the diaphragmatic pleura cannot be visualized due to the interposition of aerated lung. If there is a pleural effusion, or the interposed lung is not normally aerated, as in atelectasis or consolidation, examination of these areas is often possible. In addition, some parts of the heart or great vessels may be visible through the non-aerated lung or a pleural effusion. The diaphragm is visualized as an echogenic structure with respiratory movements.
Lung sliding is the key finding in a lung without pneumothorax. While it may initially be subtle to novice observers, with practice it will be reliably seen as a “shimmering” movement at the pleural line. Lung sliding is a respirophasic movement of the lung and represents the visceral pleura moving against the parietal pleura, and is seen in both spontaneous breathing and mechanically ventilated patients.
Because the lung is filled with air, it is not directly visualized unless there is consolidation. Instead, artifacts arising from the pleural interface are used. While artifacts often interfere with other imaging, they can be diagnostic in thoracic sonography.
There are two main types of lung artifacts that are commonly visualized: A-lines and “comet-tail” artifacts; both are types of reverberation artifacts (see Table 7-1). A-lines, also known as horizontal artifacts, are echogenic lines that appear between rib shadows that represent the repetition of the pleural line (Fig. 7-2). The presence of A-lines is considered normal, and should not be seen when B-lines or “lung rockets” are visualized.
|Horizontal artifact (A-lines)||Horizontal echogenic line between rib shadows that represents the repetition of the pleural line. Seen in normal lung.|
|Comet-tail artifact (Z-lines)||Tapering vertical echogenic line that does not reach the edge of the screen. Often seen in normal patients.|
|Comet-tail artifact (B-lines)||Vertical echogenic reverberation artifact arising from the pleural line, spreads to the edge of the screen without fading, moves with the lung. Presence excludes pneumothorax. A single B-line or several in the dependent portion of the lung may be normal. Multiple B-lines (“lung rockets”) are pathologic.|
Comet-tail artifacts are a type of reverberation artifact that occurs at a small air–fluid interface. They appear as vertical hyperechoic rays that arise at the pleural line. There are two major types of comet tail, often known as Z-lines and B-lines. Z-lines are tapering hyperechoic lines that do not reach the far side of the screen and are often seen in normal individuals. B-lines are comet-tail artifacts that arise from the pleural line, are well defined, spread to the edge of the screen without fading, erase A-lines, and move with lung sliding. A single B-line, or multiple B-lines in a dependent portion of the lung, is often found in normal subjects. Multiple B-lines, especially in the anterior/superior portion of the lung, are often termed “lung rockets,” and are pathologic. The use of B-lines in diagnosis is discussed below (Fig. 7-3).
Imaging Tips and Protocol
Thoracic ultrasound is usually goal-directed. In emergency department patients, thoracic ultrasound is commonly used to diagnose a pneumothorax, pleural effusion, or in the real-time guidance of a thoracentesis. The critical care physician uses thoracic ultrasound for similar applications, but in addition it is often used to further characterize an opacity seen on chest radiograph or CT scan. The optimal positioning of the patient depends on such factors as what the physician is trying to image and if the patient is compliant and stable.
The anterior and posterior chest can be examined with the patient placed in a sitting position with both arms elevated. In the critically ill and in patients who are unable to sit upright, thoracic ultrasound may be performed in the supine or lateral position with one or more assistants helping to hold the patient.
By convention, the probe indicator is directed cranially and the corresponding mark on the screen can be seen in the upper left corner. Thus, the orientation of a typical thoracic ultrasound image is cephalad to the left and caudad to the right. The probe should be moved in longitudinal and then transverse directions to visualize the lung surface through the intercostal spaces, thereby avoiding the ribs. The parts of the lungs adjacent to the diaphragm may be examined from an abdominal window similar to a FAST examination of the right and left upper quadrants.
In the evaluation of a pneumothorax, the patient is best examined in the supine position where the most sensitive spot is in the evaluation of the lung apices. The probe should be placed in a sagittal orientation with the indicator pointing cephalad beginning at the second intercostal space midclavicular line (Fig. 7-4). The sliding of the parietal and visceral pleura should be evaluated in this view.