Histogram analysis in digital radiography refers to how the imaging system processes the shades of gray in an exposure.
The computer starts at the center of the detector and works outward to locate collimated borders.
It collects all shades of gray in the exposure into a histogram, then compares it to a reference histogram stored for that specific anatomy—in this case, a lateral hand.
In the example image, the hand appears too light due to a histogram analysis error.
Large areas of black surrounding the anatomy were included in the histogram.
These black areas represent unattenuated X-ray photons striking the detector without passing through anatomy.
The computer interpreted this excess black as overexposure, causing the image to be lightened excessively during rescaling.
When a histogram analysis error occurs:
The exposure indicator will be incorrect and should not be used for evaluation.
The problem is not due to technical factor settings but due to processing.
Identify that the poor image is due to a histogram analysis error.
Do not change your kVp or mAs.
Improve collimation to limit unattenuated areas from being included in the histogram.
Repeat the exposure with the same technical factors but proper collimation for accurate rescaling.
