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. ┬áFortunately, 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.