Help re understanding Xray image Formation

newstudent · Joined: 18 Dec 2008 Posts: 7

Hi guys, I'm just trying to get my head around how the actual X-Ray imaged are formed.

I understand how different body structures and organs absorb more X-Rays than others (ie bone absorbs more due to its higher atomic number etc) but my question is this:

If bone blocks, and absorbs X-rays, how is the image of bone produced if none of the rays passed through the bone to get to the image receptor/recording device in the first place?

Can someone explain to me how the image production process works please?

Thanks!

papa · Posted: Thu Dec 18, 2008 12:27 pm Post subject:

(ok, if you laugh at this, try searching the forums for my chicken truck explination of mAs and kVp)

probabally the best way to describe it is to think of the film as a white sheet of paper, and the x-rays are black paint pellets coming out of a paintball gun. take an empty skeleton, and lay it on the paper. now splatter the skeleton with black paint balls. when you pick up the skeleton, you'll see an "inverted" image of the skeleton.

do you have that picture in your head? that's the raw basics of image production.

now, let's talk a little about the bony detail on say, a hand x-ray. "for this recipie we'll need some black ink, a sponge, some rice paper, and a white piece of paper" (forgive me, i was taking "inventory" of our kids christmas presents last night).

cut a hand out of the rice paper. it doesn't have to look like a hand, this is just an example. place the rice paper on the white piece of paper. rice paper is dense enough that it'll block a majority of the ink. dip your sponge in the ink, and pat the hand and the white piece of paper. then pick up the rice paper hand.

since the rice paper is dense enough to block a majority of the ink, you'll see the outline of the hand on the white piece of paper. however, you should also see the lines that make up the fibers of the rice paper. that's your detail.

bones will block a majority of the x-rays that pass through the body. the key in radiology to proper imaging, is using the proper technique. if you use too much radiation, too many x-rays will pass through the bone and you'll "black out" your image. if you use to little, your image will be "under-exposed", and you won't see the detail.

hope this helps, and if it doesn't, let us know, and we'll take another try Mr. Green

_________________
Mr. Green

papa RT(R)(CT) Mr. Green

Delusional Radiology
never meddle in the affairs of a dragon, for you are crunchy, and taste good with ketchup

hjhogle · Posted: Thu Dec 18, 2008 1:01 pm Post subject:

Boy, you're good at that, papa! Nice going.

hjh, RT

newstudent · Joined: 18 Dec 2008 Posts: 7

Hey Papa

Thank you for your explanation, and no i didn't laugh lol

I think I'm getting there..From what you've said, does this now make sense to you from what I've understood...

1: An X-Ray image is essentialy a shadow projected onto the film/receptor.

2: When X-Rays enter the body, bone will absorb alot of it, and so a shadow of white bone will be cast onto the plate which is recorded by the plate/receptor. Soft tissue will absorb slightly less, so will cast a greyer shadow of tissue structure onto the plate to be recorded. Even lighter tissue/air hardly absorbs anything so most of it penetrates all the way through the patient 'untouched' which reaches the plate and turns that black.

3: So when the recorded shawdows on the plate are processed, what we see is in actual fact varying absorbtion and penetration shadows (contrast) of for an example - a hand X-Ray image?

4: If that is right, is it also right in saying that the only X-Rays that actually reach the plate/receptor itself after penetreating right through the body with hardly any absorbtion/interraction are the rays that make the X-Ray image black, and therefore because of this blackness, we are able to differntiate between the varying densities and contrasts of body structure and tissue shades/shadows of white/grey etc?

5: Re your last point: "bones will block a majority of the x-rays that pass through the body. the key in radiology to proper imaging, is using the proper technique. if you use too much radiation, too many x-rays will pass through the bone and you'll "black out" your image. if you use to little, your image will be "under-exposed", and you won't see the detail"

Please take a look at this link picture:
[/url]http://img26.picoodle.com/img/img26/3/12/18/f_1m_eab3349.png[url]

If too much mAs is used, then why is the image dark? I thought that if more mAs was used then more X-rays would be absorbed which in turn would cast a whiter shadow onto the plate as opposed to a darker one?

I really hope you could reply to this, and clarify/expand on the points which I think I've understood above, and thanks again for your help!

P.S Please clarify/explain/expand on my points above in simple terms! Wink

Thanks again!

[/url]

papa · Posted: Fri Dec 19, 2008 12:13 pm Post subject:

*answers*
1. yep
2. you got it.
3. yes. this is why x-rays can be considered legal documentation in the right setting.
4. in a round about way, yes. the level of "density" or "blackness" is determined by the amount of x-rays that pass through the body. therefore, if it's blocked by tissue or bone, that will affect the "blackness" of the image.
5. that's where the black comes from. basically, you're "penetrating" the image with so many x-rays, the tissues that are supposed to be grey turn black, and those that are supposed to be white turn grey.

-the top image (lateral ankle in a cast) is what is called "over-exposed". too much radiation exposed the film. this could be from too much mAs, or too much kVp.

-the bottom image (AP knee) is "under-exposed". not enough radiation was used, or the kVp was set too low.

your kVp controls how powerful the x-ray beam is. the more powerful the beam, the more x-rays will be able to pass through denser material. check out my chicken truck theroy for a better explination for this.
_________________
Mr. Green

papa RT(R)(CT) Mr. Green

Delusional Radiology
never meddle in the affairs of a dragon, for you are crunchy, and taste good with ketchup

papa · Posted: Fri Dec 19, 2008 12:45 pm Post subject:

found it. here's the mack truck...

http://www.radiologyworkers.com/radiologyforums/viewtopic.php?t=4583&postdays=0&postorder=asc&start=0
_________________
Mr. Green

papa RT(R)(CT) Mr. Green

Delusional Radiology
never meddle in the affairs of a dragon, for you are crunchy, and taste good with ketchup

newstudent · Joined: 18 Dec 2008 Posts: 7

Thank you!

I now GET IT!

Wink