Fun (but needlessly confusing) question

[Febrifuge]

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A classmate just asked an awesome question, which points out one of those things that seems easy but then gets complicated when you think about it.

What we call an "AP view" of the chest is actually an AP projection; the beam goes from front to back, and that's where the cartridge is. No problems there. But we really shouldn't call it an AP view.

The thing is (and this was the basis of the question), if you think about the completed image as a window, if you read that beam's information back, it's like you're standing behind the patient.

The image is two-dimensional, but to point out why this is confusing, imagine it's a thin prism, like a 1cm-thick rectangle of gel. We put the spine closest to the film because that's what we care about, and therefore we want to put the spine closest to us when we read it. So it seems backwards to call it an AP "view." We're not viewing it that way at all.

The real answer, I think, is to remember the image really is two-dimensional, and flip it over on the light-box, and stop worrying. But it caused us some head-scratching. Silly students.

What else can you think of that seems simple... until you try to explain it to someone new?

 

[docB]

We put the spine closest to the film because that's what we care about, and therefore we want to put the spine closest to us when we read it.

Usually what we really care about in a chest Xray, as far as what's closer or farther away from the film, is the heart. In plain films stuff that's closer to the source and farther away from the film gets magnified which can lead to misinterpretation. We get a PA view, where the front of the patient and the heart are up against the film and the source is behind them. The reason we often get the AP view is not because we care about the spine (unless we're getting a Tspine series). It's because it's more convenient for a portable xray to put the film behind the patient and shoot from the front. It's a poorer quality study, but is quicker and easier to get.

As for stuff that's easy conceptually but hard to explain I vote for WPW with orthodromic and antidromic excitation. Once you get it it makes sense intuitively but when you try to teach it to someone you can see their eyes glaze over.

 

[AmoryBlaine]

I had a (sort of) friendly argument with a one-month-from-graduation FP resident about how 2 view chest films are shot. We were looking at a PORTABLE chest on the PACS system for one of her inpatients.

FP: "Wow! Look at the size of that heart!"
AB (not impressed with heart size): "Ummm, well it IS a portable"
FP: "So?"
AB: "well the heart is going to be magnified on an AP view, get a 2 view if you're worried about it though."
FP: "A 2 view is a lateral and an AP view."
AB: "umm, no, it's PA. That's sort of the whole idea as to why portables aren't the best films..."
FP: "I am graduating in a month, I should know, 2 view chest films are shot AP."
AB (drops it, imagines how many echos this person is going to needlessly order).

 

[docB]

FP: "I am graduating in a month, I should know, 2 view chest films are shot AP."
AB (drops it, imagines how many echos this person is going to needlessly order).

Feb brings up a good point about this being a conceptually simple issue if you get it and yet being tough to explain.

I recommend the flashlight, finger and wall approach. Stand next to a wall and point your light at the wall. Explain that the wall is the film and the light is the xray machine. Hold your finger an inch away from the wall and say "This is the PA. The finger is its normal size." Hold your finger a few inches from the light without moving the light. "This is the AP. The finger looks huge even though my finger hasn't changed." If you have to do this more than once switch to the appropriate finger.

 

[Powdermonkey]

If you have to do this more than once switch to the appropriate finger.

hahahahahahahaha

 

[Febrifuge]

FP: "A 2 view is a lateral and an AP view."
AB: "umm, no, it's PA. That's sort of the whole idea as to why portables aren't the best films..."

Sadly, I have confused myself enough that this has become unclear. Where's the emitter, and where's the cartridge?

I'm assuming the pt is lying on the cartridge, and the beam is traveling anterior-to-posterior through the body. So, in that situation, the beam travels AP, right?

And that would be called an "AP projection," yes? Would the same setup result in a "PA view?" (I think that's the source of the confusion.)

To add, I totally get the flashlight thing, at least.

 

[docB]

Sadly, I have confused myself enough that this has become unclear. Where's the emitter, and where's the cartridge?

I'm assuming the pt is lying on the cartridge, and the beam is traveling anterior-to-posterior through the body. So, in that situation, the beam travels AP, right?

And that would be called an "AP projection," yes? Would the same setup result in a "PA view?" (I think that's the source of the confusion.)

To add, I totally get the flashlight thing, at least.

Just remember that a typical portable gives you the poorer quality AP view and that's the patient lying with their back on the film and the emitter in front of them. A PA is usually done in radiology witht the patient standing with their chest up against the film and the emitter behind them.

 

[Toohotinvegas33]

AP/PA, the first letter is where the x-ray machine is and the 2nd letter is where the film is placed. You want the film to be closer to the area of interest. So for the heart you want a PA film and a spine shot an AP film.

 

[Febrifuge]

Thank god, because that's what I had been thinking.

I just got confused because of ABlaine's post about the portable, and the way I read the AP vs. PA stuff. Damn classmates have me questioning the small bank of knowledge I had when I got here. 😉

So, just to beat it thoroughly into the ground, the terms "AP view" and "AP projection" are used interchangably in the real world, right?

 

[Jeff698]

As for stuff that's easy conceptually but hard to explain I vote for WPW with orthodromic and antidromic excitation. Once you get it it makes sense intuitively but when you try to teach it to someone you can see their eyes glaze over.

Amen. My glaze only recently cleared.

Take care,
Jeff

 

[VeerTheTIGuy]

So, just to beat it thoroughly into the ground, the terms "AP view" and "AP projection" are used interchangably in the real world, right?

While I'm pretty sure people do, most just call it an "AP" or a "Portable Chest".

I used to get confused about how an AP and a PA could have the heart on the same side until a professor told me that they switch the film automatically in a PA to make the heart on the right of the film (so the left of teh patient)