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.
Bedside thoracic ultrasound evaluation should be performed:
- In the patient with undifferentiated shortness of breath, hypoxemia, or chest pain
- In the patient with diminished breath sounds on lung exam
- To differentiate between effusion, infiltrate, and consolidation
- In the real-time guidance of thoracic procedures, such as thoracentesis
Linear Probe with a Frequency of 7.5–10 MHz
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.
Sector Scanning or Convex-Array Probe with a Frequency of 3.5–5 MHz
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)
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).