Question for Papa

newstudent · Joined: 18 Dec 2008 Posts: 7

Hey, Papa, i just wanted to ask you a further question with regards to my other thread that you helped me with on radiographic image formation, and I hope you don't mind, but you explained things so well last time.

Re the other thread, it was assertained that bone absorbs more radiation that soft tissue, so casts a whiter shadow onto the photographic plate.

My question is regarding soft tissue absorbtion.

1: Is the reason soft tissue shows up on radiographs as differing shades of grey only because it simly absorbs less radiation than bone? or...

2: because radiation is able to penetrate soft tissue easier than bone, so when radiation hits soft tissue within the patient, some of it is absorbed, but some of it also also penetrates all the way through that soft tissue, hitting the plate turning it black - so in effect giving soft tissue a light/dark greyish image?

If 2 is right, is it correct in assuming that soft tissue not only aborbs radiation (albeit it less than bone), but radiation normally penetrates right through soft tissue aswell hiting the plate and turning it black. So in effect soft tissue is a mix of grey absorbtion (mAs) and black penetration (kV) all rolled into one?

Also, one last question , I promise!

A typical chest X-Ray:

http://www.southernhealth.org.au/imaging/images/29_chest.jpg

The outer ribs are whiter than the inner ribs. Why, if ribs are bone, do the outer portions of the ribs absorb more radiation than inner portions? Shouldn't each part of the rib aborsbed the same amont of radiation giving clear white ribs on the radiograph?

Also, looking at the right clavicle on that radiograph. It is a shade of white due to absorbtion, but also displays shades of dark grey within(inside) it. Are those shades of drak grey/black inside the clavicle due to some radiation actually penetrating the bone, and hitting the photographic plate, turning it black, or because the clavicle doesn't contain as much calcium etc as say, the sternum, so appears more trransparent?

Many Thanks for reading, and I hope you can reply when you can.

papa · Posted: Sun Dec 21, 2008 6:05 pm Post subject:

newstudent · Joined: 18 Dec 2008 Posts: 7

Hey Papa, thanks for the reply!

I'm not 100% with you though.

You said YES to:

1: Is the reason soft tissue shows up on radiographs as differing shades of grey only because it simly absorbs less radiation than bone? or...

2: because radiation is able to penetrate soft tissue easier than bone, so when radiation hits soft tissue within the patient, some of it is absorbed, but some of it also also penetrates all the way through that soft tissue, hitting the plate turning it black - so in effect giving soft tissue a light/dark greyish image?

but NO to:

If 2 is right, is it correct in assuming that soft tissue not only aborbs radiation (albeit it less than bone), but radiation normally penetrates right through soft tissue aswell hiting the plate and turning it black. So in effect soft tissue is a mix of grey absorbtion (mAs) and black penetration (kV) all rolled into one?

Maybe it was the way I worded it:

Using your bush analogy:

A:Light/Radiation that penetrates a bush (human body) that passes through the gaps in the bush/human body untouched (no interaction) will reach the plate and turn it black once processed.

B:Branches and twigs/bones and soft tissue) that absorb the light/radiation will show up as white/grey respectively.

C:Am in correct in saying that bones normally permit no penetration because it's been absorbed/soaked up by the bone, hence casting a white shadow onto the plate.

D:Softer tissue (whilst aborbing a lesser amount of radiation due to it's lower atomic number) absorbs all that it can, and then casts a greyer shadow onto the plate.

E:Is it correct in saying that bone and soft tissue normally permit no penetration (no rays passing through them to hit the plate), only varying degrees of absorbtion whose shadows are then cast onto the plate and recorded?

F:The only rays that actually penetrate the boy are those that pass all the way through untouched reaching the plate which in turn turns it black, hence once the radiograpjh is processed, we have black penetration, and differing shades of grey and white abrosbtion? Absorbtion meaning that bo rays have passed right through the bone or soft tissue, jus rays that have been soaked up to to varying levels (contrast)?

Re my clavicle and rib questions, it's not homework, it's just me trying to put all this into perspective, and understand it for myself, as I worry that if I can't understand this, then I'm gonna make a useless radiograopher lol

Ok,

The outer ribs are whiter than the inner ribs on a radiograph because the rays have been had to be absorbed by both posterior and anterior ribs, so cast a greyer shadow onto the plate?

Re the claivicle qustion,,.,u gotta help me ou there... Wink

papa · Posted: Tue Dec 23, 2008 10:37 am Post subject:

i think you're focusing too much on the absorbtion of the radiation, and not enough on the penatration.

try not to think about the absorbtion so much; that's probabally what's confusing you. let's replace the word "absorbtion" with "blocked". yes, i know, the x-rays are really absorbed into the tissues which will factor in with the dose the patient recieves. however, the act of "absorbtion" of the x-rays doesn't affect the image. the fact that the x-rays have been "blocked" does.

your questions:
a. yes
b. i think this is where you're getting confused. think "blocked", not "absorbed".
c. again, think blocked
d. see, you're still hooked on that absorbtion.
e. yeah, i really think you're confused with the whole absorbtion thing.
f. getting there.

don't worry, the light bulb over your head will suddenly turn on, and you'll get it. i'm just trying to find an analogy that'll flip that switch for ya.

all that we're really talking about is overlaping tissues. i'm gonna guess and say that you've just started your program. it might help me better explain this to you if i knew what you did before radiology.
_________________
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

Hello!, I must have just missed you, as seen you've just replied.

Yes, I think maybe I am getting a little confused.

Prior to starting my program, I was in sales lol

I wish somewhere there was a video that showed a 3d video simulatation of x-rays passing through matter, and showing the respective formation of the image. Someone should do this, as it's important for students to understand this fully, and I don't think it's covered enough from the outset of the program which is a shame.

I think some of my mis-understanding has come from the word 'penetration' Penetration can, afterall, mean to pass into (into the bone/tissue) or to pass through (through the bone/tissue to hit the plate). When I've been speaking of penetration, I've been using the word as if to mean that the rays have gone through the matter as opposed to being passed into it.

Does radiographic penetration refer to passing into body matter, or all the way through body matter?

So I can understand this, is it right in suggesting that for the most part:

a) bone and tissue block radiation (through varying degrees of absorbtion), but may actually allow some rays to pass right through them and hit the plate on the other end?

or

b) bone and tissue never normally let x-rays to actually go right through them, hitting the plate on the other end?

I'm presuming b would be correct due to total blockage, whicn in turn produces the varying degrees of 'blockage shadows@ on the film when processed.

papa · Posted: Wed Dec 24, 2008 11:13 am Post subject:

actually, "a" is correct.

let's try this approach: " Very Happy

the happy little x-ray and friends Very Happy

"

ok, let's start at the x-ray tube. "Ray" and a bunch of his friends want to have a race. the finish line is sitting behind this person who happens to be lying on a table. Ray and his friends know that they can only travel in a straight line, so they have to go through this person lying on the table to get there.

Ray and his friends see the spark in the tube which tells them to go! as fast as they can, they race towards the finish line. suddenly, they come to this person. Steve, who is just as strong as Ray is, says "c'mon Ray, we're strong enough to go right through!" Steve procedes to enter the person. Ray and all his friends follow.

both Ray and Steve are among the stronger racers. as the racers race through the patient, they look at all the atoms that make up this person. some of the racers get destracted and get too close to the atoms. the atoms make some of the racers change direction, and some of them they make stop. but not Ray and Steve. they want to get all the way to the finish.

suddenly, Steve sees some bone in his way. "oh no!" cries Steve. "all my friends are not going to be able to get through this!" and as Steve passes through the bone, almost all of his friends have to stop. he doesn't have many friends left.

now, Ray sees something in his way. "what is that," he says. "a kidney? we shouldn't have any problem going through here!" so in he goes, and out the other side he comes, along with most of his friends. Frank and Junior aren't there. they were kind of slow to begin with.

now, since Ray and Steve are about the same strength, they exit out of the patient at the same time. it's going to be a tie! it's a photo finish!

Ray and Steve look around at their friends. Ray still has most of his friends and each of them hold up a black flag (work with me, here). Steve's friends hold up their black flags, too.

the judges, who just happen to be standing across the room, take the photo that was made by Ray, Steve, and their friends with the black flags. since Ray had more friends with him, his part of the photo has more black flags. Steve's part of the photo doesn't look as dark, because he has fewer friends with black flags.

Ray is the winner!

THE END
_________________
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

Thanks Papa. I think I'm 'getting there'

Wink

papa · Posted: Sat Dec 27, 2008 8:09 pm Post subject:

...as the light bulb slowly turns on.

another reason you may not understand this is that you're just starting your schooling. you may want to check with your instructor as to wether or not this stuff will be addressed in the future.

good luck to ya!
_________________
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

wvaio · Joined: 22 Sep 2007 Posts: 85

Everyone's thought process is different, so I would like to take a whack at a layman's explanation.If it doesn't make sense feel free to disregard.

X-rays are polyenergetic. They contain many different energies. The more photons that hit the image receptor the more black the image will become. Bone has a higher probability of blocking X-rays than soft tissue.

Let say we send 100 imaginary xrays through a finger. Of those 100 xrays, there will be energies ranging from 1 to 10. The bone will absorb the energies 1-8. Allowing only 9 and 10 to pass through.The soft tissue will block energies 1-3 allowing 4-10 to hit the image receptor. The image will be made of the photons that hit the image receptor. Every time a photon hits, more black is produced.
Lets say that at 50 kVp and 3 mas there are 100 photons. 10 1's, 10 2's, 10 3's, 10 4's and so on. If you raise the kVp, then you would have less 1's and 2's and 3's, but more 8's, 9's and 10's. 2 1's, 2 2's, 2's but 15 7's 15, 8's, 15 9's.
If you raise the mAs, you will have more of all energies 1-10. 20 1's, 20 2's, 20 3's, 20 4's etc. The more you raise the mAs the more chances you have of photons hitting the film because of volume. The more you raise the kVp the more chances you have at hitting the film because the energies are higher.
Both kVp and mAs contribute to the image. kVp is the PEAK energy, the maximum energy on the scale. Not the only energy on the scale. mAs is the volume of available photons distributable on the scale.