Computed tomography (CT) scans are effective diagnostic tools that provide detailed images of the body’s internal structures. Unlike standard X-rays, which produce two-dimensional images, CT scans create cross-sectional slices that reveal information based on tissue density. Understanding how these scans visualize different tissues is important for interpreting the images, particularly when identifying the presence of blood.
How CT Scans Visualize Tissues
CT scans operate by directing X-rays through the body, measuring how much the X-ray beam is absorbed by various tissues. Denser tissues absorb more X-rays, while less dense tissues allow more X-rays to pass through. This differential absorption translates into a grayscale image, where denser structures appear brighter and less dense areas appear darker.
The quantitative measure of tissue density on a CT scan is expressed in Hounsfield Units (HU). Distilled water is 0 HU, and air is -1000 HU. Tissues with higher density, like bone, have HU values over +1000, appearing very bright white. Conversely, fatty tissues range from -50 to -100 HU, appearing dark.
The Appearance of Blood on a CT Scan
When blood appears on a non-contrast CT scan, its appearance varies significantly depending on its state. Acutely clotted blood, indicative of a fresh hemorrhage, appears bright white, or hyperdense. This high density is due to concentrated protein and cellular components within the clot.
The Hounsfield Unit range for acute blood on a CT scan is between 60 to 80 HU. The bright appearance of acute hemorrhage makes it readily detectable, standing out against the less dense surrounding brain tissue, which ranges from 25-40 HU.
Why Blood’s Appearance Changes Over Time
Blood’s appearance on a CT scan changes predictably as a hemorrhage ages and the clot breaks down. Initially, the blood is hyperdense due to intact red blood cells and clot formation. Over days to weeks, biological processes alter the clot’s composition and density.
As the clot ages, protein denaturation and the degradation of hemoglobin occur. This process leads to a decrease in the overall density of the hematoma. Subacute hemorrhages, occurring between 3 to 14 days after the initial bleed, may become isodense, meaning they appear similar in density to the surrounding brain tissue, making them potentially harder to identify. Eventually, in the chronic phase (after two weeks), the blood products are reabsorbed, and the area may become hypodense, appearing darker, often resembling fluid or cerebrospinal fluid.
What Finding Blood on a CT Scan Signifies
Identifying blood on a CT scan provides important diagnostic information for medical professionals. The presence, location, and age of the blood can indicate various underlying conditions, guiding immediate medical management. For instance, CT scans are a key tool for diagnosing different types of strokes, particularly hemorrhagic strokes, where bleeding occurs within the brain.
Blood on a CT scan also helps diagnose traumatic injuries and can reveal the presence of conditions like ruptured aneurysms. The specific appearance of the blood, whether acute, subacute, or chronic, assists clinicians in determining the timeline of the event. Interpreting these findings requires medical expertise, and CT scans are an important component of emergency and diagnostic medicine.