Image Manipulation - Computed Tomography - Lecture Slides, Slides of Computed Tomography

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Image

Manipulation in

CT

Image Manipulation Defined

 “Those techniques (operations) or processes which

modify an image or group of images to enhance the

visibility of useful information while suppressing

‘noise’ or non-useful information”

 No additional or new information produced

 Content of interest emphasized

 Overall information content reduced

Windowing

 Manipulation allows customization of visibility

 soft tissues  brain  dense structures  bone

Window Width & Level

 Window width

 range of CT #’s imaged  determines maximum # of gray shades which could be displayed on monitor

 Window level

 center or midpoint of CT # range

Window Width & Level

151- 101- 51- 1- (-49)- (-99)-(-50) (-149)-(-100) (-199)-(-150) <(-199)

Window: 400 Level: 0

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Small Window Width

Window: 400 Level: 0

 Short gray scale

 Small block of CT #’s assigned gray levels

 Small transition zone of white to black

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Large Window Width

Window: 2000 Level: 0

 Long gray scale

 Large block of CT #’s assigned gray levels

 Large transition zone of white to black

Large Window Width

Window: 2000 Level: 0

Used where large

latitude required

Used to

simultaneously display tissues of greatly differing attenuation

Another Window Example

WL = 40 WW = 200

All pixels with CT #’s > 40 + (200/2) = 140: White

All pixels with CT #’s < 40 - (200/2) = -60: Black

200 40

Still Another Window Example

WL = 0 WW = 400

All pixels with CT #’s > 0 + (400/2) = 200: White

All pixels with CT #’s < 0 - (400/2) = -200: Black

400 0

Window Width & Contrast

 As WW increases

 contrast decreases  latitude (range of CT #’s imaged) increases

 As WW decreases

 contrast increases  latitude decreases

 Clinical goal:

 Largest available contrast at the latitude required by study

Window Width & Image Contrast

 Large window width

 different structures more likely to have same gray shade

 Narrow window width

 Gray shade differences more likely visible between structures

 Very narrow window width

 Small differences in attenuation seen as black & white

Specialized CT Image Manipulation

Programs

 Region of interest (ROI)

analysis

 average CT #  standard deviation

Specialized CT Image Manipulation Programs Image annotation grids arrows notes Bad Thingee