image display

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

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– Circuitry to generate an image from electronic signals– An enclosure


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


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


Window Settings• Adjusting the window width and window level will change

the way an image is viewed on the monitor


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


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%


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


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


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


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


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


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


Review• The selected window width is 400• The selected window level is 50• Which HUs are displayed on the image as shades of

gray?a. –200 to 200b. –150 to 450c. –150 to 250d. 350 to 450


Answer

a. –150 to 250


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


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


Review• A window width of 90 is most likely used to display an image

of thea. Lungb. Femurc. Braind. Abdomen


Answerc. Brain


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


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


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

image display

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