Continuing on with my experiments with my pinhole grid, here’s a demonstration of focal spot blooming.
In a typical x-ray tube, you have electrons being emitted from the cathode filament and accelerated toward the tungsten anode. Being all the same charge, the electrons in this beam will naturally repel each other causing the beam to expand slightly before hitting the anode. When the tube current is low, there aren’t many electrons in the beam, so not a lot of expanding occurs before the anode is reached.
At high tube current, you have a lot of electrons coming off the cathode and going into the beam. Lots of electrons in the beam means more repulsion and you get much more expansion of the beam by the time it reaches the anode as a result.
Here’s an image I acquired using my pinhole grid at 50 kV, 50 mA and 100 ms (5 mAs). 50 mA is a pretty low tube current and about as low as most machines will go.
Now here’s an image acquired at 50 kV, 500 mA and 10 ms (5 mA).
Note how much larger the focal spot images are at high tube current. This is focal spot blooming, and can result in an increase in focal spot size by up to a factor of 2 depending on the tube current.