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Image Display

Image Display. Chapter 4. Display Monitors. CT images are usually displayed on a black-and-white or color monitor Display devices can be either A cathode-ray tube (CRT) Some form of flat panel, such as TFT LCD Monitors consist of The display device

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Image Display

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  1. Image Display Chapter 4

  2. Display Monitors • CT images are usually displayed on a black-and-white or color monitor • Display devices can be either • A cathode-ray tube (CRT) • Some form of flat panel, such as TFT LCD • Monitors consist of • The display device • Circuitry to generate an image from electronic signals • An enclosure

  3. Display Monitors (cont’d) • CRT monitors are heavier, bulkier, hotter, and less durable than the newer LCD monitors • LCD monitors produce higher luminance and high spatial resolution • Digital-to-analog converters change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor

  4. Cameras • In some instances CT images are transferred to film • The camera may be a multiformat camera, although most modern CT systems include a laser camera • Multiformat cameras transfer the image displayed on the monitor to film • Laser cameras bypass the video system entirely • Film used in CT consists of a single emulsion

  5. Window Settings • Adjusting the window width and window level will change the way an image is viewed on the monitor

  6. Window Settings (cont’d) • Many studies require each image to be viewed at two or more window settings • “Ideal” window settings are somewhat subjective • Imaging departments typically have an established setting for each type of examination • However, because many factors have an effect, technologists must adjust settings in individual situations

  7. Window Settings (cont’d) • Images cannot be displayed with a different shade of gray for each HU because • There are more 2,000 Hounsfield values; most monitors can only display 256 shades of gray • The human eye can differentiate only a fraction of those shades • As a general rule, the human eye cannot appreciate contrast differences of less than 10%, whereas CT scanners can easily demonstrate differences of less than 1%

  8. Gray Scale • The gray scale is used to display CT images • This system assigns a certain number of HU to each shade of gray • By convention, the gray scale assigns higher HU values lighter shades of gray

  9. Window Width • The window width determines the number of HUs represented on a specific image • Values higher than the selected range will be white • Values lower than the selected range will be black • Increasing the window width assigns more HU to each shade of gray

  10. Window Width • To illustrate, assume • we have just 10 shades of gray available • 300 is selected as the window width • 300 density values will be represented on the image as a shade of gray • All others will be either black or white

  11. Window Width/Window Level • If the window width is set at 300, which 300 Hounsfield values, from the more than 2,000 possible, will be displayed? • Window width selects the quantity of HU • Window level selects the range of HU

  12. Window Level • Selects the center CT value of the window width • Window level is also called window center • Window level determines which HUs are displayed as shades of gray

  13. Window Level (cont’d) • Continuing with the previous example, assume • 0 is selected as the level • The HUs that are represented as a shade of gray on this image will range from –150 to 150

  14. Review • The selected window width is 400 • The selected window level is 50 • Which HUs are displayed on the image as shades of gray? • –200 to 200 • –150 to 450 • –150 to 250 • 350 to 450

  15. Answer • –150 to 250

  16. General Rule for Setting Window Level • The window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

  17. Setting Window Width • In general, wide window widths (500–2,000) are best for imaging tissue types that vary greatly, when the goal is to see all of the various tissues on one image, such as lung • Wider window widths encompass greater anatomic diversity, but subtle density discrimination is lost • Because wider window width settings decrease image contrast, they suppress the display of noise on an image

  18. Review • A window width of 90 is most likely used to display an image of the • Lung • Femur • Brain • Abdomen

  19. Answer c. Brain

  20. Region of Interest (ROI) • An ROI is most often circular, but may be elliptic, 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

  21. HU Measurement and Standard Deviation • HU measurements may be affected by volume averaging or image noise • A cursor (+) displays a measurement of the HU of the pixel that the cursor covers • Conversely, an ROI provides an averaged measurement of all of the pixels within the ROI • When an ROI is used, the standard deviation is also displayed

  22. Other Image Display Options • Distance measurements • Image annotation • Reference image • Image magnification • Multiple image display • Histogram • Multiplanar and three-dimensional reformatting

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