User Manual

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4343W X-ray Detector
REFERENCE MANUAL
10.5.3 Gain Calibration
To compensate for non-uniformities in the X-ray Detector, a gain reference image (flat field) is
used by the Corrections module to correct all images. The flat field image must be captured by
the Varex Smart Panel (VSP) 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 X-ray Detector to dose. Normalization is
achieved by applying the flat field image acquired during the Gain calibration to all images
corrected by the VSP. 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 X-ray Detector, or a low signal-to-noise ratio.
Flat field images acquired near or exceeding the saturation point will cause normalization
failures with all images acquired until a Gain calibration with the correct dose is performed.
This range will ensure that Gain calibration will meet both the upper and lower dose
requirements under all modes of operation. Dose requirements are determined by the settings of
the generator X-ray source.
To reduce the effects of noise, the average of each pixel in the flat field image is calculated by
accumulating a number of frames into an internal memory buffer, then dividing the sum of each
pixel by the number of frames acquired.
The number of calibration frames used during Gain and Offset calibrations can be adjusted
under the Mode Settings pull down menu.
Note
The Gain calibration process will include automatically running an Offset
calibration to 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 X-ray Detector, but lower than the saturation
point of the X-ray Detector.
Note
It is recommended that flat field images be acquired with a median count of
approximately 13000 - 14000.
Note
Gain calibration requires X-ray dose. Certain precautions must be taken by
the human operator.