The last Radio Shack in the Charleston area is closing (well technically there is one more, but it’s a franchise store and I have no idea if it’s going to keep the Radio Shack name).
I happened to be in the area and stopped by to check and saw the signs. Not sure when their final day is, but judging from how picked over their inventory was, I’d guess it’s pretty soon.
All their component bin inventory was hanging up on the shelves, and was listed for $1/package. Not quite as good a deal as the $0.15/package I got when the Radio Shack near me closed out. Most of the other inventory was listed at 70-90% off.
There wasn’t much left in the store, but I did find a few things that I thought was a good deal at $1/package. Picked up the rest of the SMA edge connectors and DPDT mini toggle switches. Also picked up a heat sink and a few BNC barrel and T adapters for 90% off. The batteries were marked down a fair bit and at 2 for 1 also, so I picked up a few N cells to use in my HP 28S.
I’d been wanting to add a SSD drive to the system for a while now, and finally bought a 250 GB Samsung EVO 850 to function as the boot drive.
Installation was pretty easy. The hard part was fishing out a spare power cable and finding a spot to secure the drive. The case is technically out of free drive bays, but SSDs are pretty thin, and I was able to find enough space between the CD drives and one of the hard drives to secure the SSD to.
Once the drive was installed, the computer got a fresh Fedora 25 install with the SSD as the boot drive. As expected, once all the packages were downloaded, installation went quickly. Really quickly. For a fresh install I’m used to leaving my computer alone for an hour or so while it’s installing packages. With the SSD, everything was installed and I was rebooting in under 30 minutes.
Boot times for my computer are around 1/3 or so (haven’t really timed it) of what it used to be booting off the spinning disk. A couple months in, I haven’t noticed any significant change in performance and there’s still plenty of room left on the SSD.
The original boot partition has been changed to a /var mount point and now the computer is on Fedora 26 Alpha.
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.