Below is a list of the different window settings, which may vary slightly from institution to institution. However, they are usually quite similar. Window level: The window level refers to the HU center point displayed in the window setting. The lower the window level, the brighter the entire image becomes. When you increase the window layer, the brightness of the image decreases: when the window layer is increased, the image becomes darker. This is because as the level increases, higher HU values are required for a density to be represented in white. Window width: Window width refers to the number of different HU units, which are represented by different shades of gray. Any HU value less than the lower value of the window width will be displayed as BLACK during scanning, while any HU value greater than the upper value of the window width will be WHITE. If you decrease the width of the window, the contrast of the image will be INCREASED: If the width of the window decreases, a smaller change in density will cause a change in the color of the study images. This results in increased contrast, as structures close to each other in density are assigned different shades of gray/white/black. Advantages: This window is also used to evaluate Brian`s parenchyma, but it is better for assessing hypodensities that may occur during stroke (although other etiologies are also possible).
It has a similar level to the brain window above, but has a much narrower window that increases contrast and helps hypodensities appear larger. When confronted with a WW and a WL, one can calculate the upper and lower shades of gray, that is, the values greater than x are white and the values less than y are black. When viewing CT images, there are often “default” window settings that can be quickly used in the image viewer. These settings are quick to use and are often the most convenient way to quickly switch between window settings. They can also help teach concepts that are important for understanding what exactly window settings are. The following is an example of some default window settings found in a particular CT reading program. It`s important to remember the Hounsfield Unites (HU) when we think of window windows. The higher the HU value, the denser an object is.
Here are some important values to keep in mind on the HU scale. As a result, it is important to note that a significantly wide window displaying all CT numbers will mask different attenuations between soft tissues 1. In other words, windowing, also known as grayscale mapping, contrast stretching, histogram change, or contrast enhancement, is the process of manipulating an image`s CT grayscale component via CT numbers (Hounsfield units). Therefore, increasing the width of the window reduces the contrast of the image. The window plane (WL), often referred to as the center of the window, is the center of the range of CT numbers displayed. Advantages: Designed to assess the brain parenchyma (usually on a non-contrast CT scan), this window is particularly sensitive to detecting intracranial hemorrhage with a narrow window and a level close to the expected hemorrhage density in the brain. Although this varies somewhat from institution to institution and provider to provider, the width of the window and the centres are generally quite similar. The following values are specified as width and plane (W:x L:y) in Hounsfield units (HU). Window level (WL) refers to the HU value of the center or center of the window that is displayed in the window setting. Pros: As the name suggests, this window is used to evaluate the lungs.
A high window level near the density of lung tissue (due to its low density, the level is LOW) and is used with a wide window to achieve good resolution and visualize a variety of densities in the chest such as the lung parenchyma as well as adjacent blood vessels. Pros: As the name suggests, this window is useful for visualizing bones. A high window level near bone density (given its density, the level is HIGH) is used with a wide window to achieve good resolution. Learning things like window width and level may seem tedious, but adjusting these settings can have a significant impact on image interpretation. This is because the human eye can only see a finite number of colors on the CT scan, ranging from black to gray to white. These settings adjust how these colors are used and adjust the contrast and brightness of the image. Here`s an important summary of how changing each parameter changes the image: Here, the numbers shown in the figure represent two different values, the first is the window width (WW) and the second is the window plane (wl). When the window plane is reduced, the CT image becomes brighter and vice versa. The width of the window refers to the number of different HU units, which are represented by different shades of gray. The window parameter setting applies exclusively to these Hounsfield units and relates certain values to different shades of white, gray and black. This is explained in more detail in the following sections.
A larger window width (2000 U) therefore indicates a greater range of CT numbers. Therefore, the transition from dark to light structures occurs via a larger transition area to that of a narrow window width (<1000 HU). If you increase the width of the window, the contrast of the image decreases: as the width of the window increases, a greater change in density is required to change the gray tone that represents a particular HU unit. This results in a loss of contrast, as more structures look similar (despite different densities). This page discusses an important nuance of CT studies that is often overlooked: proper windowing during image analysis. The way a study is created has a significant impact on the ability to interpret the patient`s anatomy, as well as any pathology. For this reason, CT studies must be properly framed before image interpretation. Changing the window settings helps us to change/improve the appearance of the CT image and highlight some features it contains.
Now, when the window level is raised, the image becomes darker. This is because when the level decreases, lower HU values are required for a density to be represented in white, and therefore more white values can pass, making the image brighter. Pros: This window is similar to the abdominal window, but uses a narrower window to increase contrast in the liver parenchyma (to make it a little easier to look for liver damage). The window width (WW), as the name suggests, is the measure of the range of CT numbers that an image contains. Pros: As the name suggests, this window is used to evaluate the abdominal cavity and its contents. A window level near liquid/soft tissue is used with a medium-sized window that provides moderate contrast. The lower the window layer in an image, the brighter the entire image becomes. This topic can be confusing, but it`s important to remember that these window settings are simply image processing features for a scanner that help highlight important anatomy/results so that image interpretation can be made as easy as possible. An example is the use of the “bone window” to clearly visualize scanned bones to identify a fracture. Windowing, also known as grayscale mapping, contrast stretching, histogram modification, or contrast enhancement, is the process by which the CT image grayscale component of an image is manipulated via CT numbers; This changes the appearance of the image to highlight certain textures. The brightness of the image is adjusted via the window plane. The contrast is adjusted via the width of the window.
The numbers in the figure above represent two different values. The first value listed is the window width (w/w), while the second value represents the window level (w/l). Let`s quickly define what each of your values represents. Pros: As the name suggests, this window is used to evaluate soft tissue. The window level is adjusted to the density of soft tissue (50U) and a medium-sized window is used to create a balance between contrast and resolution. It is not enough to understand the difference between the width of the window and the plane of the window, so let`s see how changing these values can have a significant impact on the interpretation of the image. For example, a brain is W:80 L:40, so all values above +80 are white and all values below 0 are black. Window settings: (L:2000, L:500) or (W:3000, L:1000). Advantages: When evaluating a CT angiogram (or any phase of a CT scan with significant contrast in the vessels), this parameter can be used to try to clearly see vascular pathology. It serves a good balance with providing a signal in the target vessel without much brightness in the vessels that eliminate intraluminal pathology.
Home » Specialties » Radiology » Diagnostic Radiology » Radiological Studies Guide » Fundamentals of CT Studies » Fundamentals of CT Studies: Windows. Defined as 400-2000 HU, which is best used in areas with very different attenuation values, a good example is the lungs or cortical tissue, where air and vessels sit side by side. Hounsfield units (HU) are a dimensionless unit universally used in computed tomography (CT) to express CT numbers in a standardized and convenient form. Well, Hounsfield units are related to the composition and type of tissue and therefore represent the density of different tissues. Note that the higher the HU value, the denser a material is and vice versa. Defined as 50-350U, they are great for examining areas with similar attenuation, such as soft tissue. Below are some values on the HU scale to consider when analyzing CT images.