Thoracic Imaging

What Is the Right Initial Imaging for Chronic Dyspnea with Suspected Small Airways Disease?

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What Is the Right Initial Imaging for Chronic Dyspnea with Suspected Small Airways Disease?

A 55-year-old woman with a history of rheumatoid arthritis presents with nine months of progressive, exertional dyspnea. She is a never-smoker, and her physical exam reveals faint end-expiratory wheezes. Pulmonary function tests (PFTs) show a mixed obstructive and restrictive pattern with evidence of air trapping, but her symptoms are out of proportion to the findings. You suspect a process affecting the small airways, possibly related to her underlying autoimmune condition. You need to decide on the most appropriate first imaging study to evaluate her lungs without exposing her to unnecessary radiation or contrast. This article details the American College of Radiology (ACR) Appropriateness Criteria workflow for this specific clinical question, starting with the initial imaging step. For this scenario, the ACR rates a chest radiograph as Usually Appropriate.

Who Fits This Clinical Scenario for Suspected Small Airways Disease?

This guidance applies to adult patients presenting with chronic dyspnea (lasting weeks to months) where the clinical evidence points toward a primary pathology of the small airways—the bronchioles. This suspicion is not based on a vague symptom but is typically supported by objective findings.

Inclusion criteria for this scenario include:

  • Clinical Suspicion: The history, physical exam, or PFTs suggest bronchiolar disease. This may include a history of toxic inhalation, organ transplantation (lung or hematopoietic stem cell), or an underlying connective tissue disease.
  • PFT Findings: Pulmonary function tests may show air trapping (an elevated residual volume), a disproportionately reduced Forced Expiratory Flow (FEF) 25-75%, or an obstructive/mixed pattern that doesn’t fit a classic asthma or Chronic Obstructive Pulmonary Disease (COPD) diagnosis.
  • Non-Cardiovascular Origin: A cardiac cause for the dyspnea has been reasonably excluded.

This workflow is NOT for patients who:

  • Have a clear diagnosis of COPD: These patients have a distinct clinical pathway. This scenario is for when COPD is not the suspected diagnosis. See the ACR variant for Chronic Dyspnea, Suspected COPD.
  • Present with dyspnea of completely unclear etiology: If there is no localizing evidence from the exam or PFTs, the initial workup is broader. See the ACR variant for Chronic Dyspnea, Unclear Etiology.
  • Have signs pointing to the pleura or chest wall: Symptoms like pleuritic chest pain, palpable chest wall abnormalities, or suspected paralysis would direct the workup differently. See the ACR variant for Chronic Dyspnea, Suspected Disease of the Pleura or Chest Wall.

What Diagnoses Are You Working Up in Suspected Small Airways Disease?

When you suspect small airways disease, you are considering a group of conditions that cause inflammation and/or fibrosis of the bronchioles. The differential is broad, and imaging is key to narrowing the possibilities and guiding further testing, such as bronchoscopy or lung biopsy.

A primary consideration is constrictive (obliterative) bronchiolitis. This is a serious condition characterized by progressive concentric fibrosis of the small airways, leading to luminal narrowing. It is a well-known complication following lung or hematopoietic stem cell transplantation (as a manifestation of chronic rejection) but can also be idiopathic or associated with connective tissue diseases, post-infectious states, and toxic fume inhalation.

Another important diagnosis, particularly in patients with a smoking history, is respiratory bronchiolitis-interstitial lung disease (RB-ILD). This condition involves the accumulation of pigmented macrophages in and around the respiratory bronchioles, causing inflammation and mild interstitial thickening. While classified as an interstitial lung disease, its pathology is centered on the small airways.

Hypersensitivity pneumonitis (HP), especially in its subacute form, is a crucial mimic. Caused by an immunologic reaction to an inhaled antigen (e.g., mold, bird proteins), HP often has a prominent small airways component. Imaging findings of centrilobular nodules and mosaic attenuation due to air trapping are classic signs of bronchiolar involvement in HP.

Less common but important considerations include follicular bronchiolitis, an inflammatory condition characterized by lymphoid hyperplasia along the bronchioles, often seen in patients with connective tissue disease (especially rheumatoid arthritis and Sjögren’s syndrome) or immunodeficiency.

Why Is a Chest Radiograph the Recommended Initial Study for This Presentation?

For an adult with chronic dyspnea and suspected small airways disease, the ACR panel rates both Radiography chest and CT chest without IV contrast as Usually Appropriate. The standard clinical workflow, however, begins with the chest radiograph as the initial step.

The primary role of the chest radiograph in this context is not necessarily to diagnose the small airways disease itself, but to serve as a crucial screening tool. The small airways are below the resolution of plain radiography, so a chest X-ray is often normal or may show only non-specific secondary signs like hyperinflation. Its real value lies in its ability to quickly and efficiently exclude other common causes of dyspnea. It can reveal overt interstitial lung disease, consolidation, a large mass, significant pleural effusion, or cardiomegaly—diagnoses that would fundamentally alter the clinical pathway. Given its widespread availability, low cost, and very low radiation dose (☢ <0.1 mSv), it is the ideal first-line examination. While the chest radiograph is an excellent starting point, high-resolution computed tomography (HRCT) of the chest is the definitive non-invasive modality for evaluating the lung parenchyma and small airways. This is why a CT chest without IV contrast is also rated Usually Appropriate. An HRCT protocol can directly visualize subtle signs of bronchiolar pathology, such as centrilobular nodules, tree-in-bud opacities, and, most importantly, mosaic attenuation. The addition of imaging during full expiration is critical to confirm that the low-density areas seen on inspiratory scans represent true air trapping, a hallmark of small airways obstruction.

Why are other studies rated lower?

  • CT chest with IV contrast is rated Usually Not Appropriate. For this specific clinical question, intravenous contrast does not add diagnostic information about the lung parenchyma or small airways. It increases the cost, carries a risk of allergic reaction and contrast-induced nephropathy, and increases the radiation dose (☢☢☢ 1-10 mSv) without benefit.
  • US chest is also Usually Not Appropriate. While valuable for assessing the pleura for effusions or pneumothorax, ultrasound waves cannot penetrate the air-filled lung to visualize the deep structures like bronchioles.

What’s Next After Initial Imaging? Downstream Workflow

The results of the initial chest radiograph will dictate the next steps in the diagnostic algorithm. The workflow is designed to escalate from a low-cost, low-radiation screening test to a more definitive, higher-resolution study only when necessary.

If the chest radiograph is abnormal and suggests an alternative diagnosis:
If the X-ray reveals findings like dense interstitial opacities, a large mass, or significant pleural effusion, the workup pivots. The focus is no longer on small airways disease but on the new leading diagnosis. Further imaging or procedures (e.g., CT with contrast for a suspected mass, thoracentesis for an effusion) would be guided by these specific findings.

If the chest radiograph is normal or shows non-specific findings (e.g., hyperinflation):
This is a very common outcome in true small airways disease. A normal radiograph does not exclude the diagnosis. In a patient with persistent symptoms and ongoing clinical suspicion, the next step is to proceed to the other Usually Appropriate study: a CT chest without IV contrast. It is essential to order this as a high-resolution (HRCT) protocol and to specifically request both inspiratory and expiratory phase imaging to assess for mosaic attenuation and air trapping.

If the HRCT is positive for small airways disease:
The pattern of findings on HRCT (e.g., centrilobular nodules vs. diffuse air trapping) combined with the clinical history can often narrow the differential diagnosis significantly. This may be sufficient to make a presumptive diagnosis (like RB-ILD in a smoker) or may prompt further investigation, such as serologic testing for connective tissue disease, specific antibody testing for hypersensitivity pneumonitis, or bronchoscopy with bronchoalveolar lavage (BAL) or transbronchial biopsy.

Pitfalls to Avoid (and When to Get Help)

Navigating the workup for suspected small airways disease requires careful attention to clinical and imaging details. Here are common pitfalls to avoid:

  • Stopping the workup after a normal chest X-ray: This is the most common error. Remember that a normal radiograph is an expected finding in many cases of pure small airways disease. If clinical suspicion remains high, proceeding to HRCT is the correct next step.
  • Ordering a standard CT instead of an HRCT: A routine chest CT may not use the thin slices or reconstruction algorithms needed to visualize subtle bronchiolar changes. Always specify “high-resolution” or “interstitial lung disease protocol.”
  • Forgetting to order expiratory views: Air trapping is a key diagnostic feature that can only be confirmed on expiratory CT images. Omitting this part of the protocol can lead to a non-diagnostic or misleading study.
  • Misinterpreting mosaic attenuation: Mosaic perfusion can be caused by vascular disease (e.g., chronic thromboembolic pulmonary hypertension) in addition to airway disease. The clinical context and presence of other findings are critical for correct interpretation.

If the HRCT findings are complex, indeterminate, or discordant with the clinical picture, escalation to a multidisciplinary discussion involving pulmonology, radiology, and potentially pathology is the best course of action.

Related ACR Topics and Tools

This article covers one specific scenario. For a comprehensive overview of all clinical variants related to this topic, please consult the parent guide. For tools to help with ordering, protocoling, and patient communication, see the resources below.

Frequently Asked Questions

Why is a chest X-ray considered ‘Usually Appropriate’ if it’s often normal in small airways disease?

The primary role of the initial chest radiograph is not to definitively diagnose small airways disease, but to act as a highly effective, low-radiation screening tool. It helps exclude more common or obvious causes of chronic dyspnea, such as significant interstitial lung disease, consolidation, large tumors, or pleural effusions. A normal result in the face of high clinical suspicion is a key finding that justifies moving to the next step, a high-resolution CT.

If HRCT is the best test, why not just start with that?

While HRCT is the most sensitive non-invasive test for this condition, the principle of starting with the simplest, lowest-dose imaging guides the workflow. A chest radiograph is significantly less expensive and involves a fraction of the radiation dose of a CT scan (☢ <0.1 mSv vs. ☢☢☢ 1-10 mSv). In some cases, the radiograph may reveal an unexpected alternative diagnosis, making the CT unnecessary. This stepwise approach ensures appropriate resource utilization and minimizes patient radiation exposure.

What is the difference between a standard chest CT and an HRCT for this workup?

A high-resolution CT (HRCT) uses very thin image slices (typically 1-1.5 mm) and a special high-spatial-frequency reconstruction algorithm. This technique provides much greater detail of the lung parenchyma, including the tiny bronchioles and interstitium, compared to a standard chest CT which uses thicker slices. For suspected small airways disease, an HRCT protocol is essential for accurate diagnosis.

Is there any role for MRI in evaluating suspected small airways disease?

No, for this specific clinical question, MRI is rated ‘Usually Not Appropriate’ by the ACR. Standard chest MRI has poor spatial resolution for the fine details of the lung parenchyma and suffers from artifacts related to breathing and cardiac motion. While research into advanced MRI techniques for lung imaging is ongoing, CT remains the gold standard for evaluating the airways and interstitium.

What if my patient has a history of prior COVID-19 infection?

If the primary suspicion is that the chronic dyspnea is a sequela of a prior COVID-19 infection, that constitutes a different clinical scenario with its own specific ACR recommendations. While small airways disease can be a post-COVID finding, the workup should follow the dedicated pathway for post-COVID dyspnea, which may have different imaging considerations. You should consult the ACR variant for ‘Adult. Chronic dyspnea. Known or suspected prior COVID-19 infection. Initial imaging.’

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 30, 2026