With traditional PCF, the high-voltage generator creates the X-ray pulses. However, long power cables from generator to X-ray tube behave like big capacitors, strongly affecting the form of the electric signal they transport.
Therefore pulses are not precisely rectangular, but rounded, having rising edges (ramping) and falling tails (trailing).
This results in unusable extra dose for the patient (low-energy X-rays, often called soft radiation). This radiation should be avoided because it contributes to patient dose but does not have enough energy to reach the detector, and therefore it does not contribute to the imaging process.