Lung ultrasound is the one examination where you are deliberately interpreting artifacts rather than anatomy. Air scatters ultrasound completely, so normal aerated lung produces no real image at all — what you see is a set of reverberation patterns, and the diagnosis comes from which pattern is present and where.

Once that reframing clicks, lung ultrasound becomes one of the fastest and highest-yield studies you can perform.

Probe Selection

Curvilinear (2–5 MHz): Best all-round choice. Good depth for effusions and consolidation, adequate pleural detail.
Linear (7–12 MHz): Superior for pleural line detail — pneumothorax assessment and subtle pleural irregularity.
Phased array: Fits between ribs easily, useful when combining with echo.

Orient the probe longitudinally, perpendicular to the ribs, so you see two rib shadows with the pleural line between them. That is the "bat sign" — the ribs are the wings, the pleural line is the body — and it confirms you are at the pleura and not looking at something else.

The Normal Lung

Lung Sliding

The visceral and parietal pleura slide against each other with respiration, producing a shimmering, granular movement at the pleural line. Present lung sliding at a given location excludes pneumothorax at that location. This is the single most useful negative finding in the study.

M-mode confirmation: Place the M-mode cursor through the pleural line. Normal lung gives the seashore sign — motionless horizontal lines above the pleura (the "waves"), granular speckled pattern below (the "sand").

A-Lines

Horizontal hyperechoic lines below the pleural line, repeating at intervals equal to the skin-to-pleura distance. They are reverberation artifacts from the pleura itself.

A-lines plus lung sliding = normally aerated lung. This is the pattern you want in a dyspnoeic patient you are trying to clear of pulmonary oedema.

B-Lines

Vertical hyperechoic artifacts that arise from the pleural line, extend to the bottom of the screen without fading, erase A-lines where they pass, and move with lung sliding. All four features matter — vertical artifacts that fade out or do not move are not B-lines.

Interpretation depends on number and distribution:

0–2 per intercostal space: Normal, particularly in dependent zones
3 or more in a single intercostal space: A positive zone, indicating interstitial syndrome
Bilateral, symmetric, diffuse positive zones: Suggests cardiogenic pulmonary oedema or volume overload
Focal or patchy, with an irregular pleural line: Suggests pneumonia, contusion, ARDS, or fibrosis

The clinically useful distinction: cardiogenic oedema tends to produce smooth pleural lines with symmetric, gravity-dependent B-lines. Inflammatory processes tend to produce irregular, thickened pleural lines with patchy, asymmetric distribution and spared areas. That pattern difference does real diagnostic work in an undifferentiated dyspnoeic patient.

Pneumothorax

Lung ultrasound is more sensitive than supine chest radiography for pneumothorax, and the logic is a sequence of findings.

1. Absent lung sliding. Necessary but not sufficient — sliding is also absent in pleural adhesions, main-stem intubation, apnoea, severe COPD with bullae, and ARDS. Absent sliding alone is a reason to look further, not a diagnosis.

2. Absent B-lines. B-lines arise from visceral pleura. If they are present, the visceral pleura is in contact with the chest wall at that point, which excludes pneumothorax there.

3. Barcode (stratosphere) sign on M-mode. Horizontal lines both above and below the pleural line, with no granular pattern. The "sand" is gone.

4. Lung point. The location where sliding lung meets non-sliding lung within a single image, alternating with respiration. This is essentially pathognomonic for pneumothorax and also marks the edge of the collapse, giving a rough sense of size. Its absence does not exclude a large pneumothorax — in a complete collapse there is no transition point to find.

Scan the least dependent areas in a supine patient — anterior chest, second to fourth intercostal spaces mid-clavicular line — because that is where free air collects.

Pleural Effusion

Ultrasound is markedly more sensitive than chest radiography here, detecting volumes as small as roughly 20 mL under good conditions.

Position: Sitting up is ideal. In a supine patient, scan the posterolateral chest in the dependent zone.

Landmarks: Find the diaphragm and the liver or spleen first, then look above the diaphragm.

Findings

Anechoic or hypoechoic space above the diaphragm
Spine sign (vertebral line sign): the thoracic vertebral bodies become visible above the diaphragm. Normally aerated lung blocks them entirely, so seeing spine above the diaphragm indicates fluid. Genuinely useful when the effusion is small or you are unsure.
Quad sign: the effusion bounded by the pleural line, two rib shadows, and the lung line
Sinusoid sign on M-mode: the lung line moves toward the pleural line with inspiration in a sinusoidal pattern, confirming free-flowing fluid
Plankton sign: swirling internal echoes, suggesting exudate, haemothorax, or empyema

Characterizing the Fluid

Anechoic: usually transudative
Complex non-septated: internal echoes without septations
Complex septated: fibrinous strands — suggests exudate, empyema, or organizing haemothorax, and predicts that simple drainage may not suffice
Homogeneously echogenic: haemothorax or empyema

Septations are one of the things ultrasound sees that CT frequently underestimates. Report them clearly, because they change the drainage plan.

Volume Estimation

Several formulas exist and all are approximations. A commonly used bedside estimate multiplies the maximum interpleural distance at the lung base (in mm, measured at end-expiration in a supine patient) by roughly 20 to give an approximate volume in mL.

Treat these as order-of-magnitude estimates. The clinically important reporting elements are the maximum depth, whether the collection is free-flowing or loculated, and whether there is a safe window for drainage.

Consolidation

When alveoli fill with fluid or pus, lung starts transmitting ultrasound and you finally see real tissue.

Hepatization: consolidated lung takes on a tissue echotexture resembling liver
Air bronchograms: hyperechoic branching foci within the consolidation. Dynamic air bronchograms, which move with respiration, suggest patent airways and favour pneumonia over obstructive atelectasis, where bronchograms are static.
Fluid bronchograms: anechoic tubular structures — check with Doppler to distinguish from vessels
Shred sign: irregular, ragged border between consolidated and aerated lung, typical of pneumonia
Compressive atelectasis: a wedge of tissue-like lung floating within a large effusion, often re-aerating as fluid is drained

A Practical Scanning Sequence

For a dyspnoeic patient, scan a defined set of zones on each side rather than wandering:

Anterior superior — pneumothorax, A-lines vs B-lines
Anterior inferior — B-line burden
Lateral — B-lines, early effusion
Posterolateral (PLAPS point) — effusion, consolidation

Comparing the same zones on both sides is what turns a collection of images into a diagnosis. Asymmetry is usually the finding.

Reporting Language

Pulmonary oedema: "Bilateral symmetric B-lines with three or more per intercostal space in anterior and lateral zones, most pronounced in dependent regions. Pleural lines smooth and regular. Lung sliding present throughout. Small bilateral pleural effusions, maximum depth 2.1 cm on the right. Findings consistent with interstitial syndrome; pattern favours cardiogenic pulmonary oedema — correlate clinically."

Complex effusion: "Large right pleural effusion with maximum interpleural distance 6.4 cm at the lung base. Multiple internal septations and mobile echogenic debris. Compressive atelectasis of the adjacent lower lobe with static air bronchograms. Septated appearance raises concern for empyema or organizing haemothorax; correlate with clinical picture and consider diagnostic aspiration."

SonoBuddy is a reference tool for sonographers — not a substitute for clinical correlation and physician interpretation.

Lung and Pleural Ultrasound: A-Lines, B-Lines, and Effusion Assessment