GE Revolution CT, Руководство пользователя

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6.5 Pixels and CT Numbers
Besides anatomic location and area, each CT pixel also represents a CT number, which in turn
indicates tissue density.
• An ROI averages the values of the enclosed pixels, and displays the resulting Mean value.
•
Standard Deviation describes the difference between the minimum and maximum ROI
value.
• A large ROI provides a larger, more accurate statistical sample than a small ROI.
An image pixel represents a three dimensional volume, or voxel. It represents anatomy with a
location, an area, and a pixel (density) value. The system flattens the 0.625, 1.25, 2.5, 3.75, and
5 mm scan thickness into a two dimensional screen image. If a pixel represents a variety of
tissues, the system averages the contents to produce an averaged, rather than accurate, pixel
value. Uniform tissues (within the voxel) produce fairly accurate pixel values.
CT pixel shading shows relative density. Denser materials weaken X-ray and produce whiter
pixels. (Assumes Inverse Video OFF)
Reformat displays non axial planes created from contiguous pixels extracted from multiple
images. 3D locates similar pixel values within contiguous images, and generates a
mathematical model to produce images that appear three dimensional. BMD samples pixel
values to estimate bone or tissue density.
Reconstruction assigns one value to every image pixel. CT uses pixel values of -32767 to
+32767. MR uses pixel values of +16,000. The screen pixel translates the assigned value into
one of the 256 shades of gray. Vary the gray scale window width and level to select anatomy for
display. Window Width determines the quantity of gray pixel values. Window Level selects the
center Window Width pixel value.
Example: Two windows may contain identical widths of 100 values, but display completely
different anatomy, because one has a level of -100 and the other has a level of 150.
6.6 Window Width
The system uses 256 gray shades to display 63,486 CT pixel values. The Window Width
selection determines the number of CT values represented by each shade of gray. A narrow
window assigns fewer pixels to each gray level than a wide window.
Example: WW = 256 System assigns one pixel value to each gray shade WW = 2560 System
assigns ten pixel values to each gray shade.
6.7 Window Level
The Level equals the CT number value of pixel in the center of the Window Width range. The
Level value receives the middle shade of gray. The system displays pixel values that fall
between the center and upper window level as gray to off white. It displays pixel values that fall
between the center and lower window values as gray to charcoal. When you change the level,
the window width moves up and down the CT number line. The CT values change with Window
Level, but the Window Width and number of pixels per gray level don’t change.
Inverse Video reverses display conventions. Dense or high numbers are portrayed as black
rather than white.
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
658 6 Image Display