Image Display; Ct Number - GE Revolution CT User Manual

6 Image Display

The left monitor LCD has two viewports in a matrix of 512 x 512 picture elements, or pixels. The
right monitor LCD displays a matrix of 1024 x 1024 pixels. The 1024 display can be further
divided into up to 4 viewports of 512 x 512 pixels. Each pixel displays one of the 256 available
shades of gray.
The Revolution CT system reconstructs axial and continuous images of 512 x 512 pixels.
Images from other scanners may display 64, 128, 320, or 1024 pixel image matrices.
The amount of anatomy represented by each pixel equals the Display Field of View diameter in
mm divided by the matrix width/height.
The system assigns a unique CT number value, originally called a Hounsfield Unit, to each
pixel. The two dimensional pixel represents a three dimensional portion of patient tissue. The
pixel value represents the proportional amount of X-ray beam that passed through anatomy and
entered the detector.

6.1 CT Number

Image reconstruction supports two ranges of pixel CT Numbers, the "normal range" and an
"extended range".
• Normal Range is -1024 to 3071
•
Extended Range is -31743 to 31743
However, the system display supports pixels with a range of -32,767 to +32,767.
The system references CT number zero to water and CT number -1000 to Air. Lung and fat
have negative pixel values and normally appear black. A CT number over 200 represents dense
material like contrast agent, calcium, bone, and normally appears white.
Inverse Video reverses video white to black, but pixel values remain the same.
Variables that can affect CT Number accuracy:
•
Partial volume effects of anatomy
•
Scans acquired with IV or oral contrast agents
• X-ray tube deterioration
Chapter 21 General Information
Illustration 5: Monitor examples
Revolution CT User Manual
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