Normally,
ultrasounds are not transmitted through the aerated lung.
When US beam impacts the surface of aerated lung,
the most of it is reflected; as a result,
a hyperechoic band-like image is generated,
called “pleural line”.
Pleural line is characterized by a dynamic sign,
the lung sliding,
and some artifacts (A lines,
B lines etc),
that resulted to be very helpfulf in the diagnosis of some pathologies,
such as diffuse lung diseases and pneumothorax (Fig.
1).
Fig.: 1. Transthoracic US - Pleural line is a hyperechoic band like image constantly visible deep the intercostal muscle between the ribs
For this reason,
US study of the lung is chiefly based on analysis of these specific signs arising from pleural line,
rather than on direct depiction of lung structures.
It is therefore essential to be familiar with the US signs of normal and pathologic pleural line.
Longitudinal sonographic lung scan displaying the so-called “bat sign”,
wich appears when the pleural line is surmounted by the two adjacent lateral ribs,
resembling a flying bat (Fig.
2).
Fig.: 2. Transthoracic US - "Bat sign" (white arrows) and pleural line (grey arrow).
In real time we are able to see a to-and-from movement at the pleural line, caused by the normal sliding of the two layers of the pleura.
This dynamic sign is called “lung sliding” (Fig.
3).
Fig.: 3. Transthoracic real time US - "Lung sliding".
Lung sliding can be objectified using the time-motion mode (M-mode),
which highlights a clear distinction between a “wave-like pattern” located above the pleural line,
generated by the motionless superficial layers,
and a “sand-like pattern” below the pleural line.
This situation is called the “seashore” (Fig.
4).
Fig.: 4. Transthoracic real time M-mode US - "Seashore".
Two main artifacts arising from the lung-wall can be described and are called A and B lines.
Only one of these can be observed at one point of the lung.
The A lines consist in horizontal artifact described as hyperechoic lines parallel to the pleural line.
This lines are produced by the intense reflection of the ultrasound beam in the surface of contact between elements with different acoustic impedance (soft tissue and the surface of air-filled lung) (Fig.
5).
Fig.: 5. Transthoracic US - A lines
A similar scan displaying the B lines (asterisks) which are typical vertical artefact,
originally called “comet tail artifacts”.
The B lines are characteristically well-defined and spread to the edge of the screen without fading,
erasing the A lines.
Since this artifacts are generated by the visceral pleura,
they moves with lung sliding (Fig.
6).
Fig.: 6. Transthoracic US - B lines(asterisks) and pleural line (arrow)
Representative cases are shown to highlight the important diagnostic role in the assessment of thoracic ultrasound.
We retrospectively analyzed the iconography related to patients admitted to our Emergency Department,
Intensive Care Unit,
Thoracic Surgery Ward and Respiratory Diseases Ward.
We selected the key images about,
pulmonary edema,
lung interstitial diseases,
pneumothorax,
ARDS,
pleural effusions of various causes.
We documented the ultrasound patterns related to these conditions and compared these findings with corresponding chest radiography and CT images.
Lung diseases are characterized by a change in the relationships between air and water.
Depending on the location and magnitude of such changes,
a distinction can be made between three different sonographic syndromes:
- the pleural syndrome (pleural effusion,
pleural masses,
pneumothorax),
characterized by the presence of fluid,
abnormal tissue in the pleural space or air.
- the interstitial syndrome (pulmonary edema,
interstitial pneumonia, pulmonary fibrosis),
characterized by slight increase of fluid in the interstitium and reduced air in the alveolar spaces.
- the alveolar syndrome (ARDS,
pneumonia,
atelectasis,
contusion,
cancer,
infarction),
characterized by fluid with complete loss of air in the alveolar spaces,
leading to consolidation.
PLEURAL SYNDROME
Pleural effusion
The differential diagnosis between exudate and transudate is not possible with chest X-ray; ultrasound can spread between exudative and transudative effusion,
and permits to demonstrate the presence of septa.
Fig.: 7A,B. Transthoracic US - Large pleural effusion with multiple, thick septae suggesting inflammatory effusion; despite the presence of multiple thick fibrous bands, the fluid appears relatively clear.
7C. Chest X-ray of the same patient allows only to visualize effusion located on right hemithorax.
Fig.: 8A. Transthoracic US - Pattern of exudative pleural effusion with homogeneously echogenic spots compatible with empyema.
8B. Chest CT scan of the same patient shows the "pleural split sign", consisting in a thickening of the pleural leaflets surrounding the pleural effusion, typical of empyema.
Fig.: 9A. Transthoracic US image showing heterogeneous hyper-hypoechoic collection.
9B. Chest CT scan of the same patient also effectively shows massive pleural effusion with inhomogeneous hyperdensity in left basal location suggestive for hemorrhagic effusion in organisation phase.
Mesothelioma
Fig.: 10A,B,C,D. In this patient we can observe how the US images allows to visualize the pleural involvement better than chest X-ray and contrast-enhanced CT.
Pneumothorax
Transthoracic ultrasonography constitutes an important diagnostic tool in the diagnosis of pneumothorax (PNX).
In various comparative studies,
ultrasound presents high rates of sensitivity and speciality.
Signs which allow the diagnosis of pneumothorax are:
- absence of lung sliding: Lung sliding is a slight and bright horizontal movement of the pleural line which can be checked in a few seconds and is more evident during active and passive respiration.
Separate detection of lung sliding on the anterior–inferior areas of the two hemithoraxes in the supine patient confidently allows exclusion of PNX with a negative predictive value of 100%;
- absence of B lines: B lines are particularly important for the diagnosis of PNX.
Visualization of B lines rules out PNX with a true negative rate of 100%;
- absence of lung pulse: lung pulse is a vertical movement of the pleural line synchronous to the cardiac rhythm;
- detection of lung point: lung point is the point on the chest wall where a respiratory pattern (i.e.,
lung sliding and/or B lines) is visualized again and intermittently replaces the motionless pleura; the lung point allows confirmation of PNX with 100% specificity.
This technique is particularly suited to bedside use in the intensive care unit,
due to low sensitivity of supine chest X-ray and to the fact that computed tomography is not easy to perform in such condition.
Diagnosis of pneumothorax is difficult on supine AP chest X-rays (30-50% of mild to moderate grade PNX are not recognized).
US allows the diagnosis of presence of very small pneumothorax,
such as chest CT.
Fig.: 11A. Clinosthatic chest X-ray doesn’t detect pneumothorax.
11B. Orthostatic chest X-ray allows to view left apical small PNX.
11C. Chest CT scan confirms the diagnosis of PNX.
Fig.: 12. Transthoracic real time US demonstrating lung point and the absence of lung sliding at cranial scans.
That shows how US is more sensible than supine chest X-ray for the diagnosis of pneumothorax.
Indirect signs of pneumothorax in chest clinosthatic X-ray are "deep sulcus sign" and "double diaphragm sign".
These signs are detectable only in large pneumothoraxes (Fig.
13).
Fig.: 13. Supine chest X-ray illustrates the "deep sulcus sign" (arrowhead), with abnormal deepening and lucency of right lateral costophrenic angle, diagnostic for pneumothorax on the right side.
Fig.: 14. Transthoracic real time M-mode US - The "stratosphere sign" in M-Mode US corresponds to the "lung point" in B-mode US.
In this case transthoracic B-mode US demonstrated lung point in caudal scans,
suggestive for extended PNX.
INTERSTITIAL SYNDROME
The interstitial syndrome of the lung includes several heterogeneous conditions with diffuse involvement of the interstitium and impairment of the alveolo-capillary exchange capacity,
which leads to more or less severe respiratory failure.
Such conditions are either chronic (eg.
pulmonary fibrosis) or acute (eg.
acute pulmonary edema,
interstitial pneumonia).
Sonographic diagnosis of interstitial syndrome relies on the detection of multiple and diffuse comet tail B lines at all lung scans.
CT represent the gold standard examination which provides accurate information for the differential diagnosis of this heterogeneous group of diffuse pulmonary diseases,
unfortunately not easy to perform in intensive care unit.
Fig.: 15. “Black Lung” - Transthoracic US scan is negative for interstial syndrome (absence of B lines).
Fig.: 16. “White lung” - Transthoracic US shows multiple B lines arising from pleuric line.
The presence of numerous comet tail artifacts distributed across all areas of the lung is a sign of diffuse thickening of subpleural septa,
common in all cases of interstitial syndrome.
Pulmonary edema
Fig.: 17A. Transthoracic US demonstrating B lines diffusely affecting all lung areas in a patient with pulmonary edema.
17B. Corresponding AP chest X-ray showing bilateral alveolar opacities in the same patient.
17C. Chest CT scans revealing asymetric bilateral pulmonary edema.
Fig.: 18A. Transthoracic US - All the scans demonstrate the presence of B line artifacts, in a patient with pulmonary edema.
18B. Chest X-ray of the same patient shows pulmonary edema.
Fig.: 19A. Transthoracic US - All the scans demonstrate the absence of B line artifacts, in the same patient of fig 18, who has undeergone diuretic therapy, demonstrating how sonographic control can detect pulmonary edema resolution.
18B. Chest X-ray of the same patient shows pulmonary edema resolution.
Systemic sclerosis
The presence of B lines raises the suspicion of pulmonary fibrosis in the very early stages.
Fig.: 20A,B. Transthoracic US scans of a patient with early pulmonary fibrosis (systemic sclerosis) showing the presence of a few B lines, suggestive for initial pulmonary fibrosis.
20C. Chest CT scan of the same patient confirms the diagnosis.
Pulmonary fibrosis
Fig.: 21A. Transthoracic US demonstrating the presence of B lines , suggestive for pulmonary fibrosis.
21B. Chest CT scan confirm the diagnosis in the same patient.
Pulmonary proteinosis
Fig.: 22A. Transthoracic US scans demonstrates pulmonary alveolar proteinosis presenting as B lines positive in all scans.
22B,C. Chest CT scans of the same patient reveal widespread bilateral air-space disease with geographic pattern.
ALVEOLAR SYNDROME
Acute Respiratory Distress Syndrome (ARDS)
Fig.: 23A. Transthoracic US scans - The presence of several B lines in all scans, besides thickening and irregularity of the pleural line, suggests the diagnosis of ARDS.
23B. Chest X-ray showing ARDS.
23C,D. High-resolution axial CT scan and sagittal reconstruction confirm the diagnosis of ARDS.
In fig.
23C and 23D we can see extensive ground-glass opacities (defined as a hazy increase in lung attenuation with preservation of bronchovascular markings) with superimposed smooth septal thickening and intralobular lines (“crazy paving” pattern).
Also noted are bilateral areas of consolidation (defined as a homogeneous increase in lung attenuation with obscuration of the bronchovascular structures,
in which an air bronchogram may be present).
Pneumonia
In early congestive stage of pneumonia,
the echogenic texture of the consolidated lung is similar to the liver.
However,
a marked tree-shaped bronchoaerogram,
and a large number of lens shaped echo reflections measuring a few millimeters in size are frequently observed up to the pleura.
Fig.: 24A. Transthoracic US reveals loss of normal aeration of pulmonary parenchyma and air broncogram, suggesting consolidation of the lung with unobstructed bronchi.
24B. Chest CT of the same patient, showing an extended area of parenchimal consolidation with air broncogram.
24C. Chest X-ray of the same patient, demonstrating parenchimal consolidation and air broncogram.
Ab ingestis pneumonia
Fig.: 25A. Transthoracic US reveals loss of normal aeration of pulmonary parenchyma with similar echogenicity and texture to liver, besides fluid broncogram; these image suggests consolidation of the lung.
25B. The corresponding chest CT scan reveals the presence of bronchial stagnation in the same patient affected by ab ingestis pneumonia.
Alveolar consolidation
Fig.: 26A,B. Transthoracic US scans allow to demonstrate alveolar consolidation and rule out the presence of pleural effusion.
26C,D. Chest X-ray shows opacization of inferior right hemithorax.
26E. Chest CT scan reveals brochiolo-alveolar carcinoma.