Computed Tomography (CT) scans generate detailed cross-sectional images, offering capabilities far beyond standard two-dimensional X-rays. This advanced imaging technique collects quantitative data on how tissues interact with the X-ray beam. Every point, or pixel, in a CT image contains a numerical value that precisely measures the tissue density at that location. This objective measurement, known as the CT Number, standardizes and quantifies the subtle differences between structures within the body.
Defining the Hounsfield Scale
The CT Number is formally expressed in Hounsfield Units (HU), a dimensionless measurement created specifically for computed tomography. This scale provides a standardized system for representing a tissue’s measured X-ray attenuation coefficient. The scale is anchored by two constant reference materials, ensuring results are reproducible across different scanners and locations.
The foundational reference point is pure water, arbitrarily assigned a value of 0 HU. Materials denser than water, which absorb more X-ray radiation, receive positive HU values. Materials less dense than water receive negative values, such as air, which is set at approximately -1000 HU.
The Hounsfield scale can extend beyond these limits, with numbers representing the degree of attenuation relative to water. Dense materials, for instance, can have values reaching several thousand HU. This linear transformation allows clinicians to interpret a tissue’s physical density simply by looking at its assigned number.
Mapping CT Numbers to Body Tissues
Different biological tissues possess predictable CT number ranges, enabling clinicians to identify normal anatomy. Tissues primarily composed of fat, such as subcutaneous fat, are less dense than water and register in the negative range, typically between -50 HU and -150 HU. Air, found in the lungs or sinuses, appears darkest on the scan, with values around -1000 HU.
Soft tissues, including muscle, liver, and solid organs, are slightly denser than water, placing their values in a narrow positive range. Muscle tissue generally falls between +10 HU and +40 HU, while the liver is often around +40 HU to +60 HU. Fluid-filled structures like simple cysts or cerebrospinal fluid (CSF) are very close to the density of water, often measuring near 0 HU or slightly positive, around +15 HU.
At the opposite end of the spectrum, bone tissue absorbs a large amount of X-ray radiation due to its high concentration of calcium. Cortical bone, the dense outer layer, registers high positive values, often exceeding +1000 HU and sometimes reaching +3000 HU. These normal value ranges establish the baseline against which all abnormalities are measured.
How CT Numbers Aid in Disease Detection
The objective measurement provided by CT numbers is fundamental to identifying pathological changes, as diseases often alter the density of normal tissues. When a measured HU value deviates from the expected range, it signals a potential abnormality requiring further investigation. The numerical value helps distinguish between various types of lesions, which often have characteristic densities.
For example, acute hemorrhage contains proteins and hemoglobin that make it denser than surrounding brain tissue, registering high positive values, often between +50 HU and +75 HU. This high density allows a radiologist to differentiate an acute bleed from the brain parenchyma, which typically measures between +20 HU and +45 HU. As the hemorrhage ages and blood components break down, the HU value decreases, providing a timeline for the injury.
Conversely, a simple fluid-filled cyst exhibits a density close to 0 HU, distinguishing it from a solid tumor that would have a higher, soft-tissue density. Tumors can be characterized by their HU value; for instance, an adrenal mass measuring less than 10 HU is likely benign due to its low density indicating high fat content. The presence of calcifications, such as in kidney stones or arterial plaque, is confirmed by extremely high HU values, sometimes reaching +100 HU to +400 HU, which stand out sharply against the surrounding soft tissue.
The Concept of Windowing and Leveling
While CT numbers provide objective data, the human eye cannot discern the thousands of possible shades of gray corresponding to the full range of HU values. Therefore, a process called windowing is used to optimize the visual display for human interpretation. Windowing manipulates the range of HU values mapped to the grayscale spectrum, which runs from pure black to pure white on the monitor.
This process is controlled by two parameters: the window width (WW) and the window level (WL). The window width determines the total range of HU values displayed as shades of gray, controlling image contrast. A narrow width increases contrast, making subtle density differences more apparent, which is useful for soft tissues like the brain.
The window level, or center, sets the midpoint of the HU range being displayed, controlling the overall brightness. By selecting a specific window and level setting, a radiologist can create a “bone window” to highlight high-density structures, or a “soft tissue window” to better visualize organs, all while relying on the same underlying CT number data.