This is a function of Fluoroscopy, which controls the overall performance of the fluoroscopic image by automatically adjusting kVp, mA or both. By this, the image brightness is kept fixed on the monitor. It is a feed back circuit that measures the light intensity or video camera signal of the output screen.
Image A photomultiplier tube or a photocathode is used to monitor the light output of the intensifier tube. When the corresponding brightness changes, it is sent to the generator for adjustment. The generator controls the exposure rate by changing kVp, mA or both. The brightness of the central area of the output screen is measured for this adjustment.
Brightness can be adjusted with both kVp, mA, which affects both contrast and parient dose. Three methods of adjustment are as follows -
- Change of kV at constant mA
- Change of mA at constant kV
- Change of both kV and mA
When the II tube moves from the thin part of the patient to the thick part, then increasing the kV reduces the patient dose and contrast. In addition, when mA is increased, contrast is obtained better and patient dose is also higher.
Generally mA is first adjusted to adjust the input dose rate of the II tube. The kV is adjusted to the input dose rate when mA reaches its maximum limit.
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