A radiograph of my new Pixel 6 phone. The Pixel 6 is a big chunky phone with a lot of stuff in it.
This one was acquired using a portable x-ray unit at 80 kV, 1 mAs, and the small focal spot. It’s raised up about 25 cm above the image receptor for a bit of magnification (about 1.3x) and cropped in from the original image.
Radiograph of my Galaxy S7. 80 kVp, 2.8 mAs, Canon Aero DR detector.
The large rectangular gray block is the battery. Superimposed on the battery, the NFC and wireless charging coils are easily seen. Volume buttons are on the upper left side, and the power button is on the right side. The rear camera is the square object in the upper middle. The selfie camera is the white donut shaped object just above and to the right of the rear camera. There’s the micro-USB port at the bottom middle, and the headphone jack to the left of the USB port.
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.
Today, I was doing some testing on a mammography unit and acquired another x-ray of my Nexus 5. The mammography unit uses a much smaller focal spot, and will produce much sharper images. However, with the much lower kV mammography units use, it’s a lot harder to get adequate penetration through denser objects (like circuit boards and batteries).
This image was acquired at 34 kV and 120 mAs using the large (0.3 mm) focal spot. Its pretty easy to see that this one is a lot sharper than the other image (click the image to embiggen to all it’s glory).
The main circuit board area towards the top as well as the battery is harder to see through than the other version, because of the lack of penetration of the low energy x-rays. It’s a lot easier to see some of the detail in the circuit board at the bottom of the phone, because there’s less to go through there.