Pixels; Pixel Coordinates - GE Revolution CT User Manual

•
Improperly calibrated system (poorly centered phantom, used wrong phantom, replaced
current calibration files with extremely old Cal files)
• Beam hardening due to patient anatomy, especially bone.
To reduce CT Number variations:
•
Warm up the X-ray tube whenever the system recommends it; make sure the tube design
matches the software configuration parameters
• Center the patient anatomy of interest in the gantry opening. Select an SFOV that
encompasses the patient.
•
Acquire comparable images with similar scan and reconstruction choices.
• Maintain consistent table height throughout the exam.
•
Test image quality on a regular basis to provide the numerical data to track system
performance over time.
To decrease the potential for misdiagnosis:
• Use ROI to compare pathology to surrounding tissue
•
Scan structures with slice thicknesses about one-half the thickness of the lesion or less.
Example: Prescribe scan thickness of 5 mm or less to scan a lesion with a 10 mm thickness.
(Display an axial image and use the Measure Distance and ROI functions to determine the size
of the pathology.)
Center ROI measurements over the midpoint of the pathology to minimize partial volume
effects.
The mixture of tissue types, such as fat with tissue within the same voxel (a pixel with depth),
varying patient sizes, differences between CT machines and X-ray tubes, all lead to CT number
variance. In a well calibrated scanner, water has a CT number that ranges from -3 to +3. The
CT number remains uniform across all kV settings. However, as the X-ray tube ages, kV
decreases and pixel values become less dependable.

6.2 Pixels

The anatomic image consists of rows and columns of small, square, picture elements called
pixels. The monitor displays 1,048,576 pixels in a matrix of 1024 horizontal rows of 1024 pixels.
Add number of viewports selected for viewing to determine the number of pixels used for display
in each viewport. The monitor pixel size remains the same, but the amount of anatomy the
pixels represent varies with the scan and display field of view (SFOV & DFOV). A pixel also
represents a specific anatomic area. The system identifies each two dimensional pixel by its
location, area and value.

6.3 Pixel Coordinates

Describe pixel location two ways.
•
Matrix Coordinates: Upper left pixel = (0,0);
Lower right pixel = (511,511);
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Revolution CT User Manual
Direction 5480385-1EN, Revision 1
6 Image Display