What a field day for the heat
A thousand people in the street
Singing songs and carrying signs
Mostly saying, "hooray for our side"

Wednesday, September 3, 2014

Thing I've learned in the reboot, part 3

"I've had so many x-rays I'll probably glow in the dark now."

"X-rays don't work that way."

I told the truth, and I lied. X-rays don't deposit radiation that way, it's not like you accumulate radioactive materials and then become an emitter (that's a different type of radiation). So I told the truth for what it was worth. But while the beam is on, you emit x-rays through scattering. So I lied.

Some quick notes. X-rays are high speed photons. So far, we haven't been able to detect matter in the beam (like we can with visible light). X-rays can't be reflected. They can't be focused. They travel in straight lines. You can think of them as very fast bundles of energy. They lose that energy quickly. We no longer think of x-rays and gamma rays as being different, it only depends on the source and a little bit of the energy levels.

Here's how we create x-rays. There's this tube, think of it like a large arc light. On one side we accumulate electrons (mA) and for a (usually) small amount of time (s, mA * s = mAs, this is how many x-rays we're going to make). Then we hit that filament with a high voltage charge (kV, this is how energetic and penetrative the x-rays will be) forcing those electrons across the gap to the anode. Then several things happen, but mostly we make a hellalotta heat and a little bit of x-rays (something like 98+% to less than 2%). See, there's a lot of friction when the electrons slam into the Tungsten (used for general diagnostic x-rays, mammogram machines use molybdenum - the metal is chosen for the specific binding energy of the K-shell/inner shell electrons and the ruling energy profile of the beam and their ability to handle high heat). Most of that friction is transferred to heat.

Here's where the fun part begins.

The x-ray beam is heterogeneous in energy. Remember kV? Okay, energies of the x-ray will start around 10 kV and go up to the maximum kV we selected (most machines can go up to 120 kV). A small percentage of electrons, however, either are slowed down by the nucleus of the target material and that energy loss is transferred into creating an x-ray photon (this is called Bremsstrahlung or "braking"). These x-rays are typically of the higher energy levels. The electron could knock an outer shell electron out of it's orbit. This will create an x-ray of the same energy as the binding energy of the electron (lower in outer shells) and propel the electron with the force of the remaining energy. These are typically around 10 kV or so and aren't useful for diagnostic x-rays (sometime called Grenz Rays). The original x-rays could knock a K-shell (the innermost shell) electron out of orbit. For tungsten this creates an x-ray in the 50-70 kV range (what we use for most diagnostic imaging). Then, as electrons cascade down the shells we create even more x-rays at lower energy levels.

Okay, so we now have a whole bunch of heat (which will be handled by the tube vacuum, the oil the tube sits in, the rotation of the anode, etc., but this is also important, it is possible to overload the tube this way and then Bad Things Maynerd™) and a heterogenous bunch of x-rays heading out in all directions. Now, the tube case handles much of the x-rays heading in directions we don't want them to (lead lined, although there is an allowable "leakage"). There's a window at the bottom of the tube which lets the x-rays out in the direction we want. We then have aluminum shielding to remove all the lower energy rays (10 to 50 kV). These can't give us anything diagnostic so if we let them in you would be receiving a high skin dosage of radiation without any benefit (here we talk about half value layers, and you don't want to know). After this filtering we now have collimation (best thing ever) which helps us "cone down" the beam to where we need it. That is, no need to expose a full 8"x10"cassette if we're just x-raying your finger. We "cone down" to the finger and that's the area where the x-rays are (mostly, we'll get to that in a moment).

The x-rays now enter your skin and start doing a whole bunch of other interactions. As you change the beam, this is called "attenuation" and it's what gives us the image. Most x-rays will pass right through you without interacting. Your body can do a number of things with the x-ray that don't pass right though. They can absorb the entire x-ray photon (photoelectric effect). This then can produce scattering. It's basically the same results as creating x-rays. Some will "break", some will eject k-shell electrons, some will raise electrons to higher energy states. And here is where I lied. These interactions can also create new x-rays which typically travel within a 15° cone of the original x-ray path (classical scattering), but others (depending on energy levels and interaction states) can travel back the way the original x-ray came (back scatter) or head off in new directions (Compton scattering, this is the most dangerous for the x-ray techs and why we can't be in the room with you or we have to wear lead shielding). The ones that continue through then hit the cassette or digital imaging plate (CS, direct, and indirect). And here's the thing, most of the x-rays will pass through that without interacting as well. But some of them will hit luminescing crystals (indirect digital and film screen - for these types of devices 90% of the exposure will be from this photoluminescence and 10% from direct x-ray exposure) or the PSP (CS, or digital cassettes, this raises electrons to higher energy states which we read to make the image) or direct digital receptors. After that we have lead backgrounds in the cassettes or in the floors or somewhere and the x-rays (most of them) are exhausted.

For those x-rays that continue (and scatter) they conform to the Inverse Square Law. That is, the intensity of the beam diminishes by the inverse square of the distance. Or, in the space it takes the x-ray to travel from 2' to 4' the beam is 1/4 the intensity than it was at 2'. At a distance of 6' the beam is pretty weak.

Still with me? Okay, so we mentioned scatter. This is why we do all our protection schemes. This is why we'll use lead shields when we can. It's also why we need to leave the room if we can. Not because of the central ray (the main beam), but the scatter from the patient. This is where the majority of our occupational exposure comes from.

And you aren't the only thing that produces scatter. Air is not a vacuum. Air between the tube and the patient will produce a small amount of scatter. You need to think in volumes here. The table will produce scatter. Anything that is lighter than lead will produce scatter.

Lead is used for shielding because of it's high atomic weight (x-rays are absorbed by the photoelectric effect without producing scatter) and it's pretty dense. Note, there are regulations on the thickness of lead that is needed, blah blah blah. It's also cheap. There are newer materials (plastics and nano materials), but they're pretty expensive. There's even a cream that's being tested to see if it'll protect doctor's hands.

But, when the beam is not on, there is no radiation (normally, there's leakage things, blah blah, so small it doesn't matter). There is no scatter.

So, does that explain it? X-rays will not make you radioactive, but you will actually produce other x-rays while the beam is on. Once the beam ceases, no more radiation.

Of course, this is the high level overview (and I think I got all my terms correct).

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