Short answer: On lung ultrasound, A-lines are repeating horizontal reverberation artifacts below the pleural line. B-lines are bright vertical artifacts that arise from the pleural line, extend to the bottom of the image without fading, and interrupt or erase A-lines along their path. A-lines usually reflect aerated lung at the scanned location; B-lines reflect a change in the air–fluid or air–tissue mixture near the pleural surface. Neither pattern is a diagnosis by itself.
Interpretation depends on image quality, lung sliding, the pleural line, the number and distribution of artifacts, other lung-ultrasound findings, and the complete clinical context. “A-lines equal normal” and “B-lines equal pulmonary edema” are useful memory shortcuts only until they become unsafe oversimplifications.
- A-lines run horizontally and repeat at regular depth intervals.
- B-lines run vertically from the pleural line to the far field without fading.
- Authentic B-lines erase A-lines along their course and move with the pleura when lung sliding is present.
- A-lines do not prove that the lung is normal.
- B-lines are not specific to one disease. Their distribution and associated findings matter.
- One intercostal space does not represent both lungs. Use an application-specific scanning protocol and integrate the result with the patient assessment.
A-lines vs B-lines: the quick comparison
| Feature | A-lines | B-lines |
|---|---|---|
| Direction | Horizontal | Vertical |
| Origin | Repeating reverberation of the pleural line | Arise from the pleural line |
| Depth behavior | Repeat at regular intervals related to the probe-to-pleura distance and usually become less bright with depth | Extend to the bottom of the displayed field without fading |
| Relationship to A-lines | They are the repeating horizontal pattern | Erase or interrupt A-lines along their path |
| Respiratory movement | The lines themselves are generally static; assess the pleural line separately for sliding | Move with the pleural line when lung sliding is present |
| What the pattern supports | Aerated lung beneath the scanned location | Reduced aeration or increased density near the pleural surface |
| Main pitfall | A-lines can occur in both normal and abnormal clinical states | B-lines have multiple causes and are sensitive to technique and settings |
The one-sentence distinction: A-lines are horizontal repeats of the pleural line; B-lines are vertical artifacts from the pleural line that reach the far field and erase A-lines along their course.
Practice lung-ultrasound patterns inside a complete case.
Use POCUS Simulator to connect the probe and window to image recognition, interpretation, and feedback. Educational rehearsal only; not a clinical diagnostic or competency tool.
Start with the pleural line, not the artifact
Lung ultrasound is different from many other ultrasound applications because much of the information comes from artifacts created at the chest wall–pleural–lung interface. Air scatters and reflects ultrasound strongly, so a normally aerated lung is not displayed as a detailed picture of the lung parenchyma. Instead, the operator evaluates the pleural line, its motion, and the artifact pattern beneath it.
First locate an intercostal space. The ribs appear as bright curved interfaces with posterior acoustic shadows. Between them, the closely apposed parietal and visceral pleura usually appear as one bright horizontal line. This rib–pleural-line arrangement is often called the bat sign.
The ACEP Sonoguide lung chapter emphasizes that accurate lung-ultrasound interpretation depends on recognizing these artifacts and their relationship to the pleural interface. If the pleural line is not confidently located, do not label a bright streak as an A-line or B-line yet.
A quick image-quality check
- Are one or both rib shadows visible?
- Is the pleural line identified between the ribs rather than confused with a superficial fascial line?
- Is the depth sufficient to show the expected far-field behavior of the artifact?
- Is the gain balanced rather than washing out subtle lines or manufacturing excess brightness?
- Is the beam reasonably perpendicular to the pleural surface?
- Is this a moving clip when pleural motion is part of the question?
A recent Society of Hospital Medicine position statement on POCUS image quality includes the pleural line, rib shadows, balanced gain, sufficient depth, and perpendicular beam alignment among the quality elements used for common lung images. An artifact should not be interpreted separately from the quality of the view that produced it.
What are A-lines on lung ultrasound?
A-lines are horizontal reverberation artifacts that appear beneath the pleural line. They occur because sound reflects repeatedly between the transducer and the strong pleural–air interface. The ultrasound system places each delayed returning echo deeper in the image, creating several copies of the pleural line.
This produces three useful recognition features:
- A-lines are parallel to the pleural line.
- The interval between successive A-lines relates to the distance from the transducer to the pleural line.
- The repeated lines often become less bright with depth as energy is lost.
The RSNA review Lung Ultrasound: The Essentials explains that A-lines allow the examiner to infer aerated lung beneath the pleural interface. That is a description of the artifact at one scanned location—not a conclusion that the entire lung or patient is normal.
Do A-lines mean normal lung?
Not automatically. A-lines commonly predominate over normally aerated lung, especially when accompanied by a regular pleural line and lung sliding. They may also appear in clinical states where the lung remains aerated, including obstructive disease, and they can be present when pleural air prevents the visceral and parietal pleura from remaining apposed.
That is why an A-line pattern must be described with other observations:
- Was lung sliding present, absent, or not confidently assessed?
- Was the pleural line regular or abnormal?
- Were the findings focal or widespread?
- Were consolidations, pleural fluid, or other signs present?
- Was the examination technically adequate and complete for the focused question?
Safe wording: “An A-line-predominant pattern is present at this location” is more accurate than “the lung is normal.”
What are B-lines on lung ultrasound?
B-lines are bright vertical artifacts associated with a change in the mixture of air and fluid or tissue near the pleural surface. Their exact physical origin is complex and remains an active area of research, so it is safer to recognize the established image features than to rely on one simplified physics explanation.
A line should meet the full pattern before you call it a B-line:
- It arises from the pleural line.
- It is bright and predominantly vertical.
- It extends to the bottom of the displayed field without fading.
- It erases or interrupts A-lines along its path.
- It moves with the pleural line when lung sliding is present.
Depending on the lung, scanning zone, probe, and settings, B-lines may be discrete, numerous, or confluent. A few isolated B-lines can occur in otherwise normal lungs, particularly in dependent regions. A larger or more widespread B-line pattern indicates reduced aeration or increased lung density, but it does not identify one cause on its own.
Do B-lines always mean pulmonary edema?
No. B-lines can occur with cardiogenic pulmonary congestion, but they can also appear with other causes of reduced aeration or increased density near the pleural surface, including inflammatory, infectious, fibrotic, and traumatic processes. The distribution of the B-lines, appearance of the pleural line, presence of spared regions or consolidations, associated cardiac findings, and clinical context all influence interpretation.
The European Association of Cardiovascular Imaging consensus on lung ultrasound in heart failure uses multiple, diffuse, bilateral B-lines as part of the assessment of pulmonary congestion while also emphasizing integrated cardiac and lung evaluation. A B-line is a sonographic artifact; “heart failure” is a clinical diagnosis.
Is there one abnormal B-line count?
No single count works across every protocol, transducer, patient position, device, and clinical question. An older beginner rule often labels three or more B-lines in one intercostal space as abnormal, but isolated B-lines may be normal and modern scoring approaches use protocol-specific zones, percentages, spacing, or pattern categories.
If you are monitoring change over time, use the same validated protocol and keep the probe, preset, depth, gain, patient position, and scan zones as consistent as possible. Do not convert an isolated count into a diagnosis without considering the full distribution and context.
How to distinguish B-lines from common mimics
Not every vertical bright streak is a B-line. Before labeling one, return to the pleural line and ask where the artifact begins, how deep it travels, whether it fades, whether it erases A-lines, and how it moves.
| Finding | Recognition clue | Common error |
|---|---|---|
| B-line | Begins at the pleural line, reaches the bottom without fading, and erases A-lines along its path | Calling any vertical streak a B-line |
| Short vertical or Z-line artifact | Usually fades before the bottom and does not erase A-lines | Counting it as evidence of a B-line pattern |
| Superficial vertical artifact | Originates above the true pleural line, such as from subcutaneous tissue or air | Mistaking a fascial interface for pleura |
| Edge or processing artifact | Changes markedly with angle, focal position, gain, preset, or post-processing | Assuming machine-generated brightness is a stable lung sign |
Use moving clips whenever respiratory movement matters. A still frame may show shape and origin, but it cannot demonstrate how the pleural line or vertical artifact behaves through respiration.
Why probe choice and settings change what you see
Probe selection depends on the focused question. A high-frequency linear probe can provide detailed assessment of the superficial pleural line. Curvilinear and phased-array transducers provide greater depth and are commonly used when the examination also includes deeper lung, pleural space, diaphragm, or adjacent structures.
The same chest region may not look identical on every system. Transducer bandwidth, preset, depth, gain, focus, harmonics, compounding, and other processing can change the visibility, width, and number of vertical artifacts. This is one reason serial B-line assessment should use a standardized method rather than an improvised count.
For the physics behind reverberation, frequency, depth, gain, and artifacts, review 10 ultrasound physics principles for better POCUS images.
A safer four-question interpretation sequence
Use the artifacts inside a structured interpretation rather than treating them as a binary test.
1. Is the view adequate?
Confirm ribs, pleural line, depth, gain, and beam alignment. If you cannot locate the pleural interface, the artifact cannot be classified reliably.
2. What is happening at the pleural line?
Assess lung sliding when the focused question requires it. Note pleural-line regularity and any subpleural abnormality. Do not infer motion from a still image.
3. Which artifact pattern is present?
Describe A-line predominance, discrete B-lines, multiple B-lines, confluent vertical artifacts, another artifact, or an indeterminate view. Avoid forcing a technically limited image into one category.
4. What is the distribution and context?
State where the pattern was found, whether it is focal or diffuse, and which associated lung or cardiac findings were assessed. Then integrate it with the focused question and complete patient evaluation.
The 2025 focused update of the international point-of-care lung-ultrasound recommendations, published in 2026, emphasizes the strengths and limitations of lung ultrasound as a standalone tool and its eventual integration with other organ-specific ultrasound and complementary diagnostic information.
A 10-minute A-line vs B-line practice drill
This drill is for cognitive rehearsal with reviewed clips. It does not replace supervised image acquisition or clinical interpretation.
- Find the pleural line. Point to the ribs and pleural interface before naming an artifact.
- Judge the view. Decide whether the depth and gain allow the artifact’s full behavior to be evaluated.
- Commit before revealing the label. Choose A-line pattern, B-line pattern, another artifact, or indeterminate.
- Defend the answer. State direction, origin, far-field behavior, A-line effect, and respiratory movement.
- Add the limitation. Explain what the clip cannot establish by itself.
- Review the miss. Record whether the error involved anatomy, image quality, artifact criteria, distribution, or overinterpretation.
If a machine is not available, use the full off-machine POCUS practice plan. When you return to supervised scanning, take one acquisition goal with you, such as obtaining a perpendicular pleural view or keeping settings consistent across zones.
Practice lung patterns with immediate feedback.
Rehearse probe choice, positioning concepts, ultrasound patterns, and decisions in POCUS Simulator, then pair the review with supervised hands-on scanning.
Common A-line and B-line interpretation mistakes
Assuming A-lines mean everything is normal
A-lines support aeration at the scanned point. They do not exclude every pulmonary or pleural abnormality and do not replace assessment of sliding, pleura, distribution, other signs, and clinical context.
Assuming B-lines mean heart failure
B-lines are nonspecific vertical artifacts. A diffuse bilateral pattern may support pulmonary congestion in the appropriate context, but other processes can also produce B-lines.
Counting every vertical line
Check that the artifact begins at the pleural line, reaches the bottom without fading, erases A-lines, and moves appropriately when sliding is present.
Interpreting one intercostal space as the whole examination
Lung findings can be regional. Use the scan zones and completeness criteria of the validated protocol taught for your clinical question.
Comparing counts obtained with different settings
Probe, preset, depth, gain, processing, patient position, and scan location can all alter artifact appearance. Standardize serial examinations.
Using a still image to judge motion
Lung sliding and respiratory movement require cine review or real-time assessment. A still frame cannot show temporal behavior.
Ignoring image quality
A poorly aligned beam or incorrect depth can hide A-lines, truncate vertical artifacts, or create misleading streaks. Technical adequacy comes before interpretation.
Safety and limitations
- This guide teaches artifact recognition, not a complete lung-ultrasound examination.
- A-lines and B-lines must be interpreted with the pleural line, lung sliding, distribution, associated findings, and clinical context.
- Do not diagnose, treat, or exclude a condition from one artifact or one intercostal space.
- Use a validated, application-specific scanning protocol taught by a qualified program.
- Follow local rules for scope, supervision, consent, infection control, documentation, archiving, and quality assurance.
- Use prudent output and exposure settings according to device guidance and the ALARA principle.
- Do not perform ultrasound-guided procedures based on self-directed image-recognition practice.
- When the view or interpretation is uncertain, obtain qualified review and any additional evaluation required by the clinical situation.
Frequently asked questions
What are A-lines in lung ultrasound?
A-lines are horizontal reverberation artifacts beneath the pleural line. They repeat at regular depth intervals because sound reflects between the probe and pleural–air interface. They support aerated lung at the scanned location but do not prove that the entire lung or patient is normal.
What are B-lines in lung ultrasound?
B-lines are bright vertical artifacts that arise from the pleural line, extend to the bottom of the image without fading, erase A-lines along their path, and move with the pleura when sliding is present. They indicate reduced aeration or increased density near the pleural surface but are not disease-specific.
Can A-lines and B-lines appear in the same examination?
Yes. Different lung zones can show different patterns. Within one view, discrete B-lines may erase A-lines along their course while A-lines remain visible elsewhere. Describe the location and distribution rather than forcing the entire examination into one label.
Do B-lines always mean fluid overload or pulmonary edema?
No. B-lines can occur with cardiogenic congestion and several other causes of reduced aeration or increased lung density. Distribution, pleural appearance, consolidations, associated cardiac findings, patient history, examination, and other testing determine the meaning.
Do A-lines rule out lung disease?
No. A-lines commonly occur over normally aerated lung, but they may also appear in abnormal clinical states where the scanned lung remains aerated. Pair the A-line pattern with lung sliding, the pleural line, other sonographic findings, scan distribution, and clinical context.
How many B-lines are abnormal?
There is no universal number for every protocol and device. Isolated B-lines may be normal, particularly in dependent regions. Use the threshold, zones, settings, and scoring method validated for the specific application rather than applying one count to every examination.
Why do B-lines look different on different ultrasound machines?
Transducer type, frequency, preset, depth, gain, focus, harmonics, compounding, and post-processing can change their width, brightness, and number. For serial assessment, standardize the device and technique as much as possible.
Can a still image show lung sliding?
No. Lung sliding is motion over time and should be assessed in real time or with a cine clip. A still image can show anatomy and artifact shape, but it cannot demonstrate respiratory movement.
Can I learn A-lines and B-lines with an app?
An app can support anatomy, artifact criteria, clip review, pattern recognition, and educational case reasoning. It cannot reproduce tissue contact, live image acquisition, patient variability, or supervised competency assessment. Use it to prepare for and reinforce hands-on education.
Read the pattern, then read the context
The fastest safe distinction is simple: A-lines repeat horizontally; B-lines travel vertically from the pleural line to the far field. The expert habit comes next—checking image quality, pleural motion, associated signs, distribution, settings, and clinical context before assigning meaning.
Continue learning: review ultrasound artifacts and image controls, use the off-machine practice plan, explore guided lung POCUS cases, or find supervised ultrasound training.
Selected authoritative references
- Volpicelli G, et al. International Evidence-Based Recommendations for Point-of-Care Lung Ultrasound: 2025 Focused Update of the 2012 Recommendations. Intensive Care Medicine. 2026.
- Demi L, et al. New International Guidelines and Consensus on the Use of Lung Ultrasound. Journal of Ultrasound in Medicine. 2023.
- American College of Emergency Physicians. Sonoguide: Lung Ultrasound.
- Defining Minimum Image Quality Criteria for Common Diagnostic Point-of-Care Ultrasound Images: A Position Statement of the Society of Hospital Medicine. 2026.
- Marini TJ, et al. Lung Ultrasound: The Essentials. Radiology: Cardiothoracic Imaging. 2021.
- Gargani L, et al. Lung Ultrasound in Acute and Chronic Heart Failure: A Clinical Consensus Statement of the European Association of Cardiovascular Imaging. 2023.
