In fluoroscopic systems equipped with automatic brightness control (ABC) and under the table x-ray tube, as the intensifier is moved away from the patient:

In fluoroscopic systems with automatic brightness control (ABC), the primary function of the system is to maintain a consistent brightness level in the resulting image by automatically adjusting the x-ray output. When the image intensifier is moved further away from the patient, the x-ray beam disperses over a larger area. This increase in distance results in a reduction of the number of x-rays that reach the image intensifier, which ultimately leads to a degradation in image quality if the system were not to compensate.

As the intensifier moves away, the x-rays that manage to reach the image intensifier will become less concentrated, causing more x-rays to be needed to achieve the desired brightness level. ABC compensates for this by increasing the x-ray output to maintain image brightness. However, this increase in x-ray output results in an enhanced dose to the patient, since the machine compensates for the distance-induced attenuation of the beam.

Therefore, as the intensifier is moved away, the patient experiences an increased dose due to the mechanisms of compensation for maintaining image quality and brightness.

This understanding of the relationship between the position of the image intensifier, patient dose, and the function of ABC highlights the complexities involved in fluoroscopic imaging.