X-rays?

[BatteryCharger]

This is just something I've always wondered about. Today I had to get a chest X-ray at the doctor. I vaguely understand the theory of how X-rays work, but the machine itself...I have no idea.

Usually when I see an X-ray machine there is some really large electrical boxes in the room as if it uses a ton of power. The machine sounds like it's "spooling up" when they turn it on to take a picture, and then slows down to a stop. What is spinning? How is the X-ray light generated, and how much juice does it use?

 

[unit311]

Im starting school in the fall to become an X-ray technician. I'll tell you then.

 

[ASheep]

As far as I know/understand, the X-rays are produced in a vacuum tube, by firing an electron beam from the cathode to a (normally) tungsten anode. The high energy electron beam causes X-ray photon emission, usually at quite low efficiency. The electron beam requires huge voltages to operate (up to 150kV and 1Amp) hence the "large electrical boxes", they are probably the switching power supplies/transformers.

As for the picture generation, they used to use photographic film, but I believe they now use digital receivers, either from reflected X-rays or penetrated X-rays, depending on application.

That's just my precursory knowledge gleaned from friends who are radiologists, and my own interest in the engineering of it. Hope it helps

Edit: I also remember hearing something about certain types of X-Ray generators using a rotating anode system for angiograms and CT scan devices, but I'm not sure of other applications.

Alex

 

[JCD]

How Stuff Works

 

[geepondy]

I work on the HV supply units for cat scanners and Airport baggage scanners. Yes, the x-rays are generated from a vacuum tube. Voltage is applied to the tube filament which is pretty much a heater. As it heats up electrons are boiled off. The high voltage between the anode and the cathode is necessary to accelerate these boiled off filament electrons which is referred to as the beam current. The beam current then strikes a tube plate as mentioned, generally mostly a tungsten material. This causes the x-rays to be fired off, which also has been mentioned, in a not very efficient manner. Only a small percentage of the fired off x-rays make it to the intended target. What some people don't realize is the x-ray itself is just a high frequency wavelength photon, hence the ability to penetrate soft tissue. The radiation is a nasty by-product of the tube that also is generated when the beam current strikes the plate.

 

[BentHeadTX]

That sound you hear is the tungsten/copper anode spinning up to speed. Since the anode is getting hammer by up to 150 KV and 1.25 amps, it uses a large copper disk coated with tungsten that spins at thousands of RPM to prevent damage from the massive heat that must be absorbed.

Generally speaking, the larger units run at 480V 3-phase with an auto transformer adjusting the voltage. The other cabinets are used for automatic exposure control, digital imaging (if equipped) table control and various other functions.

Worked on x-ray for 25 years, interesting but not exactly my cup of tea. A CT scanner is basically a rotating tube that fires multiple shots (called slices) into sensor arrays that are processed digitally to create an image. They use very large tubes that are oil cooled with radiators mounted on them to remove heat. That big donut looking thing is were the tube spins around with large fans forcing cooling air through the spinning radiators.

One way to think of it is a digital camera with a flash unit pointing at the lens. Sorta... in a way...

 

[BatteryCharger]

Very interesting! So to put it simply - it's basically a ~150kw flash bulb? :huh: My doctor's still using an old film machine and they position me in front of it by telling me to hang onto a wood post. :laughing:

Honestly I was mostly just staring at the giant power supply and the wires hanging out of the back thinking "that can't be up to code"...:laughing:

 

[geepondy]

I have wondered what kind of power is necessary and how it is generated for the big particle accelerator in Europe. With the cat scanners and other medical xray devices, the high voltage power supply generation becomes tricky as their are size constrictions but I imagine it's much less so with the particle accelerator. I would guess it must involve massive transformers at least at the front end.