Question for wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

Hi, wvaio, you helped me out a little before about ionisation chambers, but I am still a little confued, so wondered if you can help me out.

First question:

I think what I'm confused with is the actual definition of a bucky. Is the bucky the same thing as the grid or are they two separate things? I get confused with people at placement saying out of bucky or out of grid etc...

I thought that the word bucky was just another name for a grid, but maybe I'm wrong? Or is the word bucky just another name for the ionization chambers?

I know what a grid is (an anti scatter device) but I thought that if you're using a grid, you're automatically using a bucky, but maybe I'm wrong here. Could you clear this up for me?

Second question:

At placement we use both digital radiography and CR.

Say you did a chest X-Ray using CR with no AEC, with 96kV and 3.5mAs - Is it the ionization chambers' job to detect when 3.5mAS has been used up before termination the exposure or have I got that wrong?

IE, what is the difference betwween an ionization chamber and an automatic exposure device? Do AED's have ionzation chambers?

Just confused here, and could do with some help

Thanks and hope you can help

traineetech · Joined: 06 Oct 2009 Posts: 15

Another question before you hopefully reply...

Say you set a manual technique of 55kV and 3.2mAs, it would peak at 3.2mAs and cut off.

But if you use an AEC with the same expsure factors, it would also cut off at 3.2 mAs

So what is the difference between the two? Why use one over the other if you see what I mean?

Unless the words 'manual' refer to the fact that no chamber/aec device is used, and that the machine itself will cut off at 3.2 for a manual technique? Whereas on an AEC technique, it's the chambers that detect the 3.2 before cutting off?

I'm getting confused!

wvaio · Joined: 22 Sep 2007 Posts: 82

First off "manual" means that there is NO AEC used. You choose a set kVp and mAs. The tube produces that exposure. The system calculates the exposure by determining a set mA and time, equaling mAs. If you set 3.2 mAs manually, the system will produce 3.2 mAs, not peak. If you shoot 10 times at 3.2 mAs, theoretically you would have 10 exposures of 3.2 mAs.

Secondly, The term "bucky" refers to the reciprocating grid that is housed inside of the table or upright wall unit. The Bucky can be used with AEC or with a manual technique. Either way, milliseconds before the start of the exposure, the grid begins reciprocating to blur the grid lines and clean up scatter. The Bucky is completely separate from the AEC system. The term "bucky" is loosely used as a term to describe the drawer that holds the cassette. It is actually called a bucky drawer. The wall unit as a whole can also be referred to as a bucky because it houses a bucky unit within it. The actual bucky moves in unicine with the bucky drawer, so that they are always positioned together.

The AED or AEC(one in the same) uses the ionization chambers. They calculate the number of photons that have passed through the patient and scattered by the patient.

After reading this, reread the prior post on ionization chambers again and see if it starts to make more sense.

Feel free to keep asking questions. I am more than happy to answer them all. It keeps me sharp!

Wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

thanks, wvaio. I now understand it all apart from one part:

'When a pre-determined number of photons hit the cassette, the aec tells the machine to stop'

Who sets the pre-determined number? How is it set? How does the aec know when enough photons have hit the cassette to produce optimum density and tell the machine to stop?

That's the part I don;t understand?

I mean, how does the machine know whether the patient is going to be fat or thin, and therefore know when to stop exposing?

I hope you understand what I mean, and thanks for your help so far...

wvaio · Joined: 22 Sep 2007 Posts: 82

This is the final key to the puzzle.

The way that the AEC determines whether the person is thin or thick. Think of the possible interactions with matter. Some x-rays will pass right through the body and others will be attenuated by the body. The AEC is only going to count the x-rays that are passed completely the patient and those that are scattered from the patient. It can't count the photons absorbed by the body, or those that are scatter in different directions other than right towards the detectors. Lets say that on a thin patient 8 out of 10 x-rays per second will pass through the patient (or scatter toward the detectors). On a thicker patient only 4 out of 10 pass through per second. If the predetermined number was 80, and if the machine shoots 10 x-rays per second the thin patient would be exposed for 10 seconds. The thick patient would have to be exposed for 20 seconds. These are not real world numbers of course.

That predetermined number is set by the person that calibrates the machine. I don't know any more than that. It could be set by the exposure fairy for all I know. That is the realm of the radiology technician.

Make sense?

Wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

Ah, I See what you mean now ( I think)

So, on a thinner patient, the AEC would detect that the cassette would have reached its 'pre-determined' density quicker than that of a larger person due to them having less 'bulk' so to speak for the x-rays to pass through, and for the AEC to detect?

Meaning that the pre-determined (optimum density) is reached regardless of patient size, but for larger patients, this may mean that exposure time is a little higher (unless, of course, the tech ups the kV which would balance things out a little) ?

I hope I've got this now!?!

Do you think that the AEC's pre-determined density level was set by the manufacturer of the unit, or maybe some group of boffins somewhere after much testing?

wvaio · Joined: 22 Sep 2007 Posts: 82

Yes you have it!
I will ask the technician at work and get back to you on the predetermined density. I would imagine that it is set by the manufacturer but is calibrated by the physicist that does the QA. I will find out and get back to you. I never tracked it back any further than what I have explained in the posts.

Wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

Great thanks! I'll look forward to hearing back from you about te pre-determind thing.

Also, one last question about all of this:

Does this mean then that theoretically, the AEC's pre-determined denisty (mAs) figure is pre-set at the same level for each patient regardless of patient size, and that the mAs figure after exposure should reveal identical levels of mAs given for differing patients, but with differing ms times?

IE, If the AEC's pre-determined mAs level for a CXR was pre-set at 5mAS, will every patient's mAs reading after exposure read 5mAs, but display differing ms times due to differing patient sizes?