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 mAs).
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.
Some time ago, I came across an image acquired using a pinhole array that showed very nicely how the effective focal spot changes across the image receptor due to the x-ray tube anode angle. I don’t recall if it was in a textbook or a paper, but it’s something I’ve been wanting to replicate for myself to include in my teaching file.
I found some ~1 mm thick sheet lead left over from from some past experiments and punched a bunch of holes in it on a 10 mm grid using a push pin.
After some experimenting to find a decent x-ray technique to use, I ended up with these two images for the large and small focal spots.
I’ve chosen to invert the grayscale to use a black background instead of the normal white to make the focal spot images easier to see.
The pinholes are a little bit on the large side (~1 mm diameter) so the focal spot images aren’t as well defined as what I’d have gotten using a pinhole camera (which has a ~0.1 mm diameter hole), but these are good enough for demonstration purposes.
What’s going on here?
In all x-ray tubes, the tungsten anode is angled about 12-17° from the perpendicular relative to the anode-cathode direction, as shown in the image below (taken from Review of Radiologic Physics by Walter Huda).
When most people think about the focal spot of the x-ray tube, they’re thinking about the effective focal spot (F). The focal spot size of a tube is specified along the central axis of the beam perpendicular to the image receptor. If you were to look up from the image receptor to the x-ray tube (along F), you’d see a tiny little rectangle where the x-rays come from.
Now, consider the situation where we move away from the perpendicular to some other location along the image receptor. Now if you look back at the x-ray tube, the effective focal spot size has changed (G and H).
The effective focal spot gets larger as you move toward the cathode, and smaller moving toward the anode. In addition, the shape of the focal spot changes as well. This is most easily seen in the large focal spot image above.
This effect has some interesting ramifications when it comes to talking about focal spot blurring. Because the effective focal spot size changes across the image receptor, this means the amount of focal spot blurring also changes across the image receptor. Fortunately, focal spot blurring is relatively small compared to other sources of blurring in medical imaging, so even though focal spot blurring varies across the image, it’s not a huge thing to worry about.
When the focal spot of an x-ray tube needs to be evaluated, the usual method is to use a slit or pinhole. The slit camera is typically made from two pieces of tungsten or other dense material arranged so that there’s a very small gap between the two, typically 0.1 mm wide or smaller. Place the slit camera up against the collimator and make an exposure. The resulting image is a line that represents the detected radiation from the focal spot. Given the distance from the focal spot to the camera and to the image receptor, the size of the focal spot can be calculated from the width of the line. Turning the slit camera 90° and acquiring a second image gives the focal spot dimension in the other axis.
The pinhole camera works in much the same way, except using a very small hole instead of a slit. You also get an image of the actual focal spot itself and can measure both dimensions of the focal spot directly. It’s the same as using a pinhole camera for visible light pictures.
A long time ago, I learned that if you don’t have a pinhole camera, you can sort of fake it by closing the collimator blades almost all the way down until you can see just a pinprick of light when the light field is turned on. Since it can be difficult to know the distance between the hole and the focal spot, doing quantitative measurements of the focal spot isn’t easy, but it can be useful for demonstration purposes, to show a class of rad tech students or residents.
Every now and then, you get focal spot images showing up in images unintentionally, like in this image from a portable x-ray unit I was testing the other day
I circled the artifact to make it easier to notice. Here it is cropped and enhanced to show some of the features that make it identifiable as a focal spot image.
The double band (double banana) is characteristic of most focal spot images taken with a pinhole camera. A normal pinhole image of a focal spot is dark in appearance, but this artifact is lighter than the surroundings.
The most likely source of the artifact is a tiny dense particle, probably a metal shaving or something similar, that’s landed near the x-ray tube window or in the collimator housing somewhere. The metal shaving is dense enough to absorb some of the radiation, acting as kind of a “reverse pinhole”, producing a light pinhole image instead of a dark one.