A computed tomography (CT) scan of the lungs provides detailed cross-sectional views of the chest cavity. This technique allows physicians to examine the lungs, heart, blood vessels, and surrounding structures. A lung CT scan is routinely used to investigate persistent respiratory symptoms and to monitor the progression of known diseases. Understanding what this technology reveals clarifies its central role in modern pulmonary medicine.
The Basics of Computed Tomography
Computed tomography generates a three-dimensional view of internal structures using X-ray technology. The CT scanner is a large, ring-shaped machine that rotates a focused X-ray beam around the patient’s body as they pass through the opening. Detectors measure the amount of radiation absorbed by different tissues from thousands of unique angles.
A powerful computer processes these measurements to create a series of cross-sectional images, often called “slices,” representing the chest cavity. Unlike conventional X-rays, CT images display tissues based on density. This allows for clear differentiation between air (black), soft tissue (shades of gray), and bone (white).
Key Structures and Features Visualized
A lung CT scan provides a map of the thoracic anatomy, allowing for evaluation of normal structures and the detection of abnormalities. Radiologists examine the large airways (trachea and main bronchi) and the pulmonary parenchyma—the functioning tissue of the lungs. The major blood vessels, the mediastinum (the space between the lungs), and the pleural space are all clearly delineated.
Normal lung tissue is primarily air, giving it a dark or black appearance on the scan, with only the fine network of blood vessels visible. A radiologist looks for abnormal findings, beginning with nodules and masses, which are rounded opacities of tissue. Nodules are lesions under three centimeters in diameter; anything larger is classified as a mass. The characteristics of these lesions, such as their shape, border, and internal density, provide diagnostic clues.
Opacities are described by their appearance on the image. A ground-glass opacity looks like a hazy increase in lung density, but the underlying blood vessels remain visible. When the air in the lung’s air sacs is completely replaced by fluid or soft tissue, the resulting white appearance is called consolidation. The scan also highlights vascular features, such as the size of pulmonary arteries and veins. Fluid accumulation, known as a pleural effusion, appears as a collection of gray density pooling around the lungs.
Specific Conditions Diagnosed
The detailed images produced by a lung CT scan are used for confirming and staging pulmonary diseases. For lung cancer, CT is the primary tool for initial detection, especially in screening programs for high-risk individuals. It precisely measures the size of the primary tumor (“T” component) and assesses the spread to regional lymph nodes (“N” component) by looking for enlargement in specific stations within the chest.
The scan plays a role in diagnosing vascular conditions like pulmonary embolism (PE), a blockage in a pulmonary artery usually caused by a blood clot. CT Pulmonary Angiography (CTPA) uses intravenous contrast dye to highlight the blood vessels. A PE appears as a “filling defect”—a dark area where the contrast-filled blood is unable to flow due to the presence of a clot.
CT is effective in characterizing infections and inflammatory diseases. Acute bacterial pneumonia typically manifests as consolidation, often affecting a segment or lobe of the lung. For chronic conditions, CT allows for the diagnosis of Interstitial Lung Diseases (ILD), which can produce patterns like reticulation or cystic air spaces known as honeycombing. In emphysema (a form of Chronic Obstructive Pulmonary Disease, or COPD), the scan clearly shows areas of low density where the air sacs have been destroyed.
Preparing for the Scan and What to Expect
Preparing for a lung CT scan depends on whether a contrast agent will be used. Patients are typically asked to remove all metal objects, including jewelry, as metal interferes with the X-ray beams and can create image artifacts. If a full diagnostic scan is ordered, an intravenous contrast dye containing iodine may be injected to enhance the visibility of blood vessels, tumors, and areas of inflammation. When contrast is administered, patients may be asked to fast for a few hours beforehand, and they should inform the technologist of any allergies or kidney problems.
During the procedure, the patient lies flat on a motorized table that slides into the center of the large, doughnut-shaped scanner. The technologist monitors the patient from an adjacent room but communicates through an intercom system. The scan itself is very fast, often taking less than a minute, though the total appointment time may be around 30 minutes. The technologist will ask the patient to briefly hold their breath for a few seconds to ensure the images are sharp and clear. If contrast dye is used, the patient may experience a momentary warm sensation or a metallic taste in their mouth, which is a normal reaction. After the scan is complete, the images are sent to a radiologist for interpretation, and the patient can typically resume their normal activities immediately.