Understanding the CT Grayscale and Density
A computed tomography (CT) scan uses X-rays and computer processing to generate detailed cross-sectional images of the body. These images are displayed exclusively in shades of gray, representing the varying densities of internal tissues. The CT scan measures how effectively different materials absorb the X-ray beam.
The degree of X-ray absorption is quantified using the Hounsfield Unit (HU) scale, a standardized measurement. This scale is anchored by two fixed points: pure water is defined as 0 HU, while air is set at -1000 HU. Materials denser than water have positive HU values and appear brighter, or white, while materials less dense than water have negative HU values and appear darker, or black.
Three terms describe the appearance of tissue relative to its surroundings: hyperdense, hypodense, and isodense. Hyperdense tissues, such as bone or metal, absorb a high amount of X-ray radiation, resulting in high positive HU values (often over +100) and appearing bright white. Hypodense tissues, like fat (-50 to -115 HU) or air, absorb very little radiation and appear dark gray or black. Isodense tissue has a density and HU value similar to the surrounding structures, making it difficult to distinguish without context.
The Appearance of Acute Hemorrhage
When fresh blood, known as an acute hemorrhage, is present, it registers as a bright white or hyperdense area on the CT scan. This appearance results from the high physical density of clotted blood compared to the surrounding soft tissue. Normal unclotted blood measures between +13 and +50 HU, but clotting significantly increases its density.
The clotting process concentrates solid components, specifically hemoglobin. This high concentration of protein and cellular material absorbs a large amount of X-ray energy. This pushes the Hounsfield Unit value into the hyperdense range, typically between +50 and +75 HU for clotted blood. An acute hemorrhage is generally defined as a bleed that has occurred within the past one to three days.
The brightness of acute hemorrhage makes it highly visible against less dense soft tissues, such as brain matter (+30 to +40 HU). This contrast is important for diagnosing conditions like acute subdural hematomas or intracerebral hemorrhages. The intense hyperdensity indicates that a recent bleeding event has occurred.
How Blood Appearance Changes Over Time
The appearance of blood on a CT scan evolves significantly as the body breaks down the hematoma, allowing radiologists to estimate the age of the bleed. This process is driven by the gradual breakdown of dense protein structures and the reabsorption of water. The initial hyperdense appearance transitions through subacute and chronic phases.
Subacute Phase
During the subacute phase, which lasts from one to three weeks, the density of the blood clot decreases. As cellular components are broken down, the water content increases, causing the Hounsfield Unit values to drop. This makes the blood become isodense, meaning its shade of gray is similar to the surrounding brain tissue, making the hemorrhage temporarily difficult to see.
Chronic Phase
The chronic phase begins after several weeks and can last for months. By this time, the clot has fully dissolved, and the collection is primarily composed of low-density fluid, such as serum or cerebrospinal fluid (CSF). The HU values fall into the hypodense range, often near 0 HU. This causes the area of the former bleed to appear dark gray or black, indicating an old, resolved hemorrhage.
Context Matters: Identifying Bleeding Location
While density rules govern the appearance of blood, the precise location of the bleeding is a fundamental factor in interpretation. An intracranial hemorrhage is easily identified in the acute phase because the bright white blood sharply contrasts with the gray matter of the cerebrum. Identifying a bleed in the abdomen or chest is more complex due to the varying densities of nearby organs and fluids.
Challenges in Extracranial Bleeding
In the abdomen, a hemorrhage near organs like the liver or spleen can complicate distinction, as their soft tissue densities may be close to that of blood. Furthermore, the use of intravenous contrast agents, which are iodine-based dyes, increases the density of normal tissue. These contrast-enhanced areas can appear bright white, requiring specialized interpretation to distinguish active bleeding from surrounding enhanced organs.
Interpreting blood on a CT scan requires more than just measuring the Hounsfield Unit. It involves analyzing the shape, volume, and location of the collection relative to adjacent anatomical structures. This contextual analysis allows the radiologist to determine the age of the hemorrhage, its potential cause, and its overall effect on the patient’s physiology.