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.
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.
The last time I saw anyone I went to high school with was at the reunion, but thanks to Facebook I can see what some of them are up to these days.
It’s unlikely I’ll be able to make it back home this year if there ends up being a 30 year reunion thing going on. I hope that if something happens, everybody has a good time and posts pictures of the festivities.
Stopped at my local Radio Shack again to see if it was still open and much to my surprise, it was, although today was the last day.
There wasn’t much left in the way of components, but I still found a few more things to pick up. Hard to turn things down when they’re marked down 90%. A few packs of 1/8 W resistors, 3 packs of terminal strips, 4 spools of speaker wire, some RF chokes and 4 PIR sensors.
After a bit of hemming and hawing over whether I should go back for more (didn’t want to seem greedy or anything), I headed back for round 3. At some point, someone else had stopped there and bought out the rest of the resistors, so I settled on (nearly) cleaning out the capacitors. I’m already pretty stocked up on resistors from the first trip anyway. Also picked up the rest of the heat sinks, IC sockets and a couple of telescoping antennas.
I probably could have grabbed a few more things, but they weren’t things that I would have easily found a use for.
Now to start setting things up so I can build things…
Still working on getting the hang of getting pictures of the sun with my little solar telescope. A little blurry, but a couple of sunspot groups can be seen pretty easily. At first I thought they were specks of dirt or dust on the telescope optics, but a check of spaceweather.com showed there were indeed two groups of sunspots (2644 and 2645) on the surface of the sun.
Used the little Barlow lens with the camera adapter for this shot. Gives a larger image of the sun but not quite as bright, and the smaller FOV means spending more time hunting for the sun.