1313DXT-I/1515DXT-I/2520DX-I/3030DX-I X-ray Detectors
38
www.vareximaging.com
U
SER
M
ANUAL
3
An
Offset Calibration Process
window appears followed by
Offset Calibration Acquisition
Completion.
Figure 39
Offset Calibration Progress Window
10.5.3
Gain Calibration
To compensate for non-uniformities in the detector, a gain reference image (flat field) is used by
the Corrections module to correct all images. The VCP must capture the flat field image prior to
acquiring images. The process of capturing the flat field image is known as gain calibration.
Gain calibration is based upon the linear response of the detector to dose. Applying the flat field
image acquired during the gain calibration to all images corrected by VCP creates normalization.
Normalization will fail with pixels that are responding to dose in a non-linear manner. Pixels
responding to dose in a non-linear manner are usually caused by the saturation of the detector,
or a low signal-to-noise ratio.
Note
Every time a gain calibration is run, an offset calibration is required
beforehand; this will ensure that the detector is properly calibrated.
Note
It is critical to acquire the flat field image within a range that is large enough
to be higher than the background noise created by the X-ray source and
readout electronics of the detector, but lower than the saturation point of the
detector.
Caution
Flat field images acquired near or exceeding the saturation point cause
normalization failures with all images acquired until VCP performs a gain
calibration with the correct dose.
Note
Acquire flat field images with a median count of approximately
8000 – 16000
.
