CT scans provide detailed cross-sectional views of internal structures, making them the most sensitive method for visualizing the lungs. However, these powerful tools are subject to technical limitations and human interpretation. This means the possibility of an ambiguous or non-perfect reading is an important consideration. Understanding how emphysema is identified and the factors that can mimic its appearance can help clarify the significance of a CT finding.
Understanding Emphysema on a CT Scan
Emphysema involves the irreversible destruction of the alveolar walls, the tiny air sacs where gas exchange occurs. This leads to permanently enlarged air spaces and a loss of the lung’s elastic recoil. High-Resolution CT (HRCT) is the preferred imaging technique because it provides the spatial detail necessary to visualize these subtle changes. On a CT scan, emphysema appears as areas of abnormally low attenuation, which look darker than healthy lung tissue due to decreased density. The key visual feature distinguishing true emphysema is the absence of visible walls surrounding these low-density areas.
Different types of emphysema present with slightly varied patterns on the scan. For instance, centrilobular emphysema, the most common form, shows small, ill-defined dark areas in the central part of the secondary pulmonary lobules, typically affecting the upper lungs. Conversely, panlobular emphysema, often linked to a genetic condition, demonstrates a more uniform, diffuse low attenuation across the entire lobule and is usually more severe in the lower lungs.
Technical Factors Leading to Misdiagnosis
A CT image suggesting emphysema can be influenced by how the scan was acquired and processed, rather than purely biological factors. One such technical issue is the partial volume effect, where the computer averages the density of lung tissue when a small structure only partially fills the slice thickness. This averaging can falsely lower the measured density, making a small area appear more emphysematous than it truly is.
The Hounsfield Unit (HU) is a quantitative measure of tissue density used in CT imaging. Improper calibration or quantitative software settings can shift the measured density thresholds, causing normal lung tissue to fall into the range designated for emphysema. Patient movement during the scan, known as a motion artifact, can also blur the image, creating ambiguous, low-density areas that a radiologist might mistakenly interpret as parenchymal destruction.
The way an image is viewed, particularly the window width and level settings, also plays a role in interpretation. These settings adjust the contrast and brightness of the image; if they are set incorrectly, the subtle density difference between healthy and mildly damaged lung tissue can be exaggerated. This visual misrepresentation can lead to an over-diagnosis of mild emphysema simply due to the viewing parameters used by the clinician.
Lung Conditions That Resemble Emphysema
Several lung conditions or anatomical variations can produce low-attenuation areas on a CT scan, mimicking true emphysema. A common mimic is air trapping, which occurs when obstructed small airways prevent air from being fully exhaled. While air trapping results in decreased density, it is a reversible functional problem, not the irreversible structural destruction seen in emphysema. It is best identified on CT scans taken during expiration.
Localized, thin-walled cystic spaces, such as large bullae or cysts, can also be confused with widespread emphysema. True emphysema involves diffuse destruction, while conditions like lymphangioleiomyomatosis (LAM) or Langerhans cell histiocytosis (LCH) present with multiple, discrete cysts that can be mistaken for severe, confluent emphysema. The distribution and wall thickness of these cystic spaces are typically the features that help distinguish them from widespread emphysema.
Even certain normal physiological changes associated with aging can be misinterpreted as disease. Very small, scattered areas of low density are common in older, non-smoking patients and do not represent clinically significant emphysema. In addition, issues with pulmonary vasculature, such as reduced blood flow to a specific area, can make that portion of the lung appear darker on a CT scan due to a lack of density from blood volume, giving the false impression of airspace enlargement.
Confirming the Diagnosis and Next Steps
A finding of emphysema on a CT scan is a structural observation, but the diagnosis of a clinically significant disease requires functional confirmation. The immediate next step for anyone with an ambiguous CT finding is to undergo Pulmonary Function Tests (PFTs). These tests, especially spirometry, measure the amount of air the lungs can hold and how quickly air can be expelled, directly assessing the airflow limitation and hyperinflation characteristic of emphysema.
A diagnosis of emphysema as a disease is not solely based on the image, but must correlate with the patient’s symptoms, medical history, and physical exam. For instance, a CT finding of mild emphysema in a patient with no symptoms and normal PFTs may not warrant a clinical diagnosis of emphysema. This clinical correlation is a necessary step to determine if the structural changes are causing functional impairment.
Seeking a second opinion from a specialized thoracic radiologist or a pulmonologist is a prudent measure to resolve any lingering diagnostic uncertainty. These specialists have extensive experience interpreting subtle imaging findings and integrating them with clinical data. Furthermore, follow-up imaging, such as a serial CT scan after a period of time, may be recommended to monitor if the low-attenuation areas are stable, progressing, or simply a benign finding.