Focal spot blooming demonstration

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.

Focal spot blooming
Demonstration of focal spot blooming at low tube current. Acquired at 50 kV, 50 mA, 100 ms, 181 cm SID

Now here’s an image acquired at 50 kV, 500 mA and 10 ms (5 mA).

Focal spot blooming
Demonstration of focal spot blooming at high tube current. Acquired at 50 kV, 500 mA, 10 ms, 181 cm SID

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.

An array of focal spots

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.

Pinhole Grid
1 mm pin holes on a 10 mm grid punched into a sheet of lead

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.

Large focal spot 81 kV 1mAs
Large focal spot. Acquired at 81 kV, 1 mAs, 181 cm SID
Small focal spot 81 kV 1mAs
Small focal spot. Acquired at 81 kV, 1 mAs, 181 cm SID

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).

Line focus principle
Diagram illustrating the line focus principle

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).

Effective focal spot size from different locations
Effective focal spot size from different locations

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.

Has it really been 30 years?

This  year is 30 years since high school. Hard to believe it’s already been 10  years since my 20 year high school reunion.

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.

One last visit

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.

Radio Shack haul part 1
Radio Shack haul part 1

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.

Radio Shack haul part 2
Radio Shack haul part 2

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…

Little black spots

On the sun today

Sun 20170329
Sunspots on the Sun 20170329

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.