20 Questions
What is the term for the center of the grayscale range?
Window Level
What is the effect of a wider Window Width on a CT image?
Decreases image contrast
What is the purpose of defining a Region of Interest (ROI)?
To measure and display specific image data
What is the result of image magnification?
Enlarged image size
What is a histogram in the context of CT imaging?
A graphical representation of frequency changes in CT numbers within an ROI
What is the effect of Window Level on a CT image?
It changes the overall image brightness
What is the purpose of the Region of Interest (ROI) shape?
To define a specific area for measurement and display
What is the term for displaying multiple images in one frame at a time?
Multiple Image Display
What is the result of a narrower Window Width on a CT image?
Increased image contrast
What type of tissue is best displayed in a narrow Window Width?
Tissue with similar densities
What is the primary goal of image manipulation techniques in CT scans?
To enhance the visibility of useful information while suppressing noise
What is the term used when raw data are manipulated to create pixels that are then used to create an image?
Reconstruction
How many shades of gray can a typical monitor display?
256
What is the minimum contrast difference that the human eye can appreciate?
10%
What is the primary purpose of contrast enhancement (windowing) in CT scans?
To manipulate the CT image gray scale
What determines the range of the grayscale that can be displayed?
Window width
What is the result of adjusting the window width and window level in CT scans?
Optimum display of different structures
What is the purpose of reconstruction in CT scans?
To create pixels that are then used to create an image
How many different Hounsfield values are there?
More than 2,000
What is the limitation of the human eye in terms of differentiating shades of gray?
Fewer than 40
Study Notes
Image Processing
- Image manipulation refers to the techniques or processes that modify an image to enhance the visibility of useful information while suppressing noise.
Gray Scale
- In CT scans, the image is displayed with a different shade of gray for each CT number.
- Although there are over 2,000 different Hounsfield values, the monitor can only display 256 shades of gray.
- The human eye can differentiate only a fraction of those shades, typically fewer than 40.
Contrast Enhancement (Windowing)
- Windowing is the most common processing technique in CT, allowing the technologist to alter the contrast of the displayed image.
- Windowing involves manipulating the CT image gray scale using CT numbers of the image.
- Adjusting the window width (WW) and window level (WL) produces an optimum display of different structures, such as the chest, abdomen, bone, etc.
Window Width (WW) and Window Level (WL)
- The window width (WW) is the range of the grayscale that can be displayed.
- The center of the grayscale range is referred to as the window level (WL).
Effects of Windowing
- The window level (WL) changes the overall image brightness.
- The window width (WW) defines image contrast, with a wider WW decreasing image contrast and vice versa.
- Tissue types with similar densities are best displayed in a narrow WW.
Region of Interest (ROI)
- The region of interest (ROI) is a display function that defines an area on the image.
- The ROI can be circular, elliptical, square, or rectangular, or may be custom drawn by the operator.
- Defining the size, shape, and location of the ROI is the first step in many display and measurement functions.
Image Magnification and Multiple Image Display
- Image magnification only uses image data, enlarging the existing image without improving resolution.
- Multiple image display allows more than one image to be displayed in one frame at a time.
Histogram
- A histogram is a graphical representation of frequent changes of a range of CT numbers within an ROI.
Learn about image processing techniques, including gray scale and windowing, and understand how to modify and reformat images.
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