A Computed Tomography (CT) scan uses a series of X-ray images taken from different angles to create cross-sectional views of the body’s internal structures. While CT does not directly measure the molecular or cellular inflammatory response, it detects the physical consequences of that response. Inflammation causes visible structural changes in tissues and organs. A CT scan is a powerful tool for diagnosing inflammation by visualizing these abnormalities.
Visualizing Physical Changes
The fundamental way a CT scan detects inflammation is by measuring changes in tissue density, reported in Hounsfield Units. Inflammation causes edema, or swelling, due to the accumulation of fluid and inflammatory cells. This fluid alters how the tissue absorbs X-rays, making the affected area appear denser than healthy tissue on the scan.
Inflammatory processes lead to structural changes highly visible on CT images. For example, in conditions like colitis or diverticulitis, the wall of the affected bowel segment may become visibly thickened. This thickening results from tissue swelling and cellular infiltration within the wall layers.
Severe inflammation can also lead to the formation of localized fluid collections or abscesses. These collections of pus and fluid appear distinctly different from surrounding organs, often presenting as areas of high or mixed density with irregular margins. Identifying physical changes, such as wall thickening or a defined fluid pocket, allows clinicians to pinpoint the exact location and extent of the inflammatory process.
Enhancing Detection with Contrast
While non-contrast CT scans reveal structural changes, the use of intravenous (IV) contrast material significantly improves inflammation detection. The contrast agent, typically an iodine-based compound, is injected into a vein and travels through the bloodstream. Inflammation causes a localized increase in blood flow, called hyperemia, as the body sends immune cells to the affected site.
This increased blood flow delivers a higher concentration of the contrast agent to the inflamed area. Additionally, small blood vessels in the inflamed tissue often become “leaky” due to increased capillary permeability. This allows the contrast agent to move out of the blood vessels and accumulate in the surrounding extracellular space.
The iodine element is highly effective at attenuating X-rays, causing the inflamed tissue to “enhance” or appear much brighter on the CT image. This enhancement makes it easier to distinguish subtle inflammation from normal tissue, providing clearer diagnostic margins. Contrast enhancement is useful for identifying early inflammatory processes that might otherwise be missed on a non-contrast scan.
Practical Diagnostic Scenarios
CT scanning is frequently the preferred imaging method for diagnosing acute inflammatory conditions within the abdomen and pelvis. It is the standard for confirming acute appendicitis, clearly showing a thickened appendix and surrounding inflammatory changes in the fat tissue, known as fat stranding. CT is also excellent for diagnosing diverticulitis by identifying pockets of inflammation, associated bowel wall thickening, and abscess formation.
The modality is highly effective for assessing acute pancreatitis, revealing an enlarged pancreas and fluid collections around the organ. Because CT provides rapid, detailed cross-sectional images, it is indispensable in emergency settings for locating deep-seated infections, such as internal abscesses, that require immediate drainage. Its ability to image bone structures also makes it valuable for detecting bone infections, or osteomyelitis.
Despite its strengths, CT is less effective for detecting subtle, diffuse, or microscopic inflammation. Conditions like early inflammatory bowel disease, where tissue changes are minimal, may be better evaluated using other modalities, such as Magnetic Resonance Imaging (MRI). CT also provides less detailed differentiation of soft tissues compared to MRI, meaning it may not be ideal for assessing inflammation in joints or certain rheumatological conditions.