Much of lung ultrasound is based on the interpretation of artefacts produced at the pleural surface. Most of these relate to the way ultrasound and air interact. Ultrasound can only be used to interpret the characteristics of the surface of an air interface but cannot penetrate gas. This means pathology lying deep to air cannot be visualised. Fortunately much pulmonary pathology does involve the pleural surface to some extent.
Ultrasound can be used to explore most of the pleural surface, with the exception of the mediastinal reflections. Probe selection and technique vary depending on clinical suspicion and circumstance.There is frequently overlap in the sonographic appearance of differing conditions and integration with the clinical picture is particularly important when interpreting lung ultrasound.
Cardiac assessment with echocardiography frequently gives important additional information to the clinician’s assessment.
- Pleural Effusion (techniques)
- The clinician must adapt the ultrasound examination to the clinical scenario and the pathology they are searching for.
- Trauma protocols mandate the patient is supine. This is convenient for pneumothorax detection, where free pleural air gathers at the most apical point of the thoracic cavity. Detection of haemothorax which is dependent is more challenging with a posterolateral approach required. Approaching from below the diaphragm using the liver or spleen as an acoustic window is often an effective alternative.
- Intubated patients are also generally managed supine. Anterior, lateral and posterolateral approaches give a great deal of information, and if a true posterior approach is required the patient may be rolled one way and then the other.
- Conscious patients in acute respiratory distress usually sit up to optimise their ventilation. This allows access to the entire chest wall, but when exhausted the patient may need support to allow access to their back.
- Where the patient is not acutely distressed and is mobile sitting on a chair or stool will achieve maximal chest wall accessibility. Getting the patient to hug themselves, or putting their arms out forwards, will move the scapula laterally around the chest wall helping access the upper posterior lung when that is required.
The curvilinear transducer
- For most clinicians this transducer has become the go to probe for lung ultrasound.
- Like the phased array this gently curved transducer also operates at relatively low frequency (1-5MHz). This means that it has good penetration where thoracic imaging is required to look further into the chest – for example through a large pleural effusion.
- The larger footprint enables examination of a larger field of the pleural surface. If the probe is oriented across intercostal spaces rib shadows are seen, sliding readily determined, and orientation is easily achieved. The costophrenic angle is also most easily seen with this transducer. Placing the transducer in the line of an intercostal space allows an extended view of the pleural surface.
- This transducer is ideal when searching the entire pleural surface for pathology. It can also be used from the upper abdomen to explore the diaphragmatic surface of the lung. If a higher resolution transducer is required once pathology is found, a change to the linear probe is reasonable.
The linear transducer
- This transducer operates at a higher frequency, it is good at interrogating a pleural surface with a maximal depth of 4cm or so. One selects this probe when wanting the highest resolution examination of an abnormality at the pleural surface. It is also the probe of choice for examining the ribs and sternum for fractures.
The phased array transducer
- Much of the early work in lung ultrasound was done in Intensive Care Units using a relatively low frequency phased array transducer (1-3MHz), which is also used for echocardiography. As much of lung ultrasound relies on artefacts rather than image resolution, this was a reasonable choice. The pleural surface usually lies within several centimetres of the skin and detailed assessment of pathology close to the transducer is not possible.
- Proponents claim an advantage is that the small probe footprint means achieving a view in a small intercostal space is easier. It also means that at any one position only a tiny field of the pleural surface is examined and it is easy to miss localised pathology.
- A phased array transducer is a reasonable selection for lung ultrasound where gross and diffuse pathology is suspected. This would include pulmonary oedema and large pleural effusion or pneumothorax detection.
- Modern ultrasound machines use numerous techniques to minimise the presence of ultrasound artefacts. Lung ultrasound however often relies on the user recognising typical artefacts and patterns caused by ultrasound and air interactions at the pleural surface.
- A “lung” preset should turn off the various smoothing and artefact minimisation algorithms routinely used. Turn compound imaging and harmonic imaging off and reduce dynamic range. This is particularly important if you are uncertain whether lung sliding is present or absent, or if you want to characterise and quantify B-lines – both these rely on artefact creation.
- To highlight lung sliding and B-lines your focal point should be at the pleural surface.
- If you are wanting to optimise imaging of solid areas within lung, such as consolidation or tumour; or you want pleural fluid to be as well defined as possible an alternative preset will be more appropriate. An “abdominal” preset is generally a safe choice.
- An initial depth setting on the curvilinear transducer of around 14cm is reasonable. At that depth setting one should see the pleural surface and have enough depth below the surface to characterise the vertical artefacts such as acoustic shadowing from the ribs, and B-lines from air / fluid interactions at the pleural surface. Depth may need to be increased to assess pathology passing from the pleural surface deeper into the thorax, or to assess the inferior diaphragmatic surface of the lung. Depth may need to be reduced to better interpret the closer pleural surface.
What to look for/consider in each view
In each view consider:
The chest wall
- Look for ribs and intercostal muscles to ensure orientation and exclude surgical emphysema, then focus on the pleural line.
- Pleural reflections, particularly the costophrenic angle and diaphragm are key to image orientation. If a pleural reflection lies within your field make sure you identify it as misinterpretation of the reflection is a common error.
The pleural line
- Is there lung sliding or lung pulse? Hold the transducer very still and look for movement at the pleural line.
Vertical artefacts below the pleural line
- Are there comet trials, B-lines, and if so how many? Are they confluent? Think clinically. Is the presentation acute of chronic, are those B-lines likely to represent fluid or fibrosis?
Horizontal artefacts (A-lines) below the pleural line
- Are these present? Are they interrupted by vertical moving artefacts cast by the pleural line? Remember they are most clearly seen when the pleural line is perpendicular to the line of interrogation of the ultrasound beam.
Pathological non-aerated areas allowing ultrasound to penetrate deeper into the chest
- These may include pleural effusion, pulmonary consolidation, infarction or collapse, or lung tumours.
- Determine whether the abnormality lies within the pleural space – effusion or pleural tumour; or whether it lies within lung.
- Assess for vascularity using colour Doppler.
- Describe the number and size of lesions.
- Describe the contour or outline (smooth, irregular, “shredded”, lobulated, well defined, poorly defined)
- Describe the echogenicity (hypoechoic, isoechoic, hyper echoic or heterogenous)
- Describe the internal echotexture (fine, course, lobulated, varied…)
- Describe any regions of special interest (cystic areas, air fluid levels, air bronchograms, the pattern of vascularity…)
PLEURAL EFFUSION (Tips and TRICKS)
The supine patient
- If the patient is supine or lying back against a sloped bed the posterior chest wall is relatively inaccessible. To examine for pleural effusion directly explore the costophrenic angle at the posterior axillary line with the probe in longitudinal position. A reasonable volume effusion will be evident there.
- Another option to assess the dependent pleural cavity in a supine patient, is to examine from below. Use the liver or spleen as an acoustic window and examine upwards through the liver and diaphragm to view the posterior costophrenic angle.
- Where appropriate the supine patient may be rolled to assess the posterior chest wall sonographically, but be aware the fluid will move as the patient is moved.
The erect patient
- If a patient can sit forward assessing for pleural effusion is far simpler.
- Explore the posterior chest wall first as effusion is most commonly evident there. Ideally have the patient’s arms up and forward – putting them on an elevated tray table works well as it rotates the scapulae laterally and away from the posterior chest wall, and gives the patient something to lean on.
- With the probe in longitudinal plan begin high approximately 10cm lateral to the spinous processes. Slide the transducer downwards crossing ribs and focussing on the pleural line until you encounter the diaphragm. A tiny pleural effusion will just fill the costophrenic angle with hypo echoic fluid. As it gets bigger is will outline the postero-inferior angle of the lower lobe of the lung, and as is gets larger still compress it causing atelectasis.
Characterise the effusion
- Describe its size. A depth in centimetres between the parietal pleural surface and the lung is useful. A depth between diaphragm and the lower border of the lung is also informative.
- If drainage is planned note the depth of the chest wall, position of ribs, and ideally locate and avoid the intercostal vessels.
- Describe the character of the fluid. It it anechoic, if not is there fine swirling echogenic debris, or is there echogenic material (such as blood clot) within the effusion, is there a hydropneumothorax?
- Describe any locations; are they fine fibrinous mobile locations, or thickened and immobile with solid components?
- Describe any pleural deposits either on the lung or on the diaphragm or inner chest wall. Assess them for vascularity with colour Doppler.
- Describe any underlying lung changes, atelectasis and collapse being the most frequent.