Help understanding ionization chambers

traineetech · Joined: 06 Oct 2009 Posts: 15

Hello. I'm just looking for some help with regards to understanding ionization chambers. I've checked out google etc, but just wondered if someone could see if I'm on the right track here...

Scenario

You're about to perform an erect PA chest X-Ray. You select the kV and mAs, and take the X-Ray. Would I be right in saying that the ionization chamber(s) lie behind/are built into the bucky/behind the image receptor somewhere, so that once the ionization chamber(s) detect the kV and mAs that you set for the X-Ray, it then cuts off somehow so that the patient does not receive more kV or mAs than was originally set by yourself for the examination?

Just a little confused, and need this clearing up, if anyone can help? Very Happy

wvaio · Joined: 22 Sep 2007 Posts: 82

Ahh... I can explain this one for you!

Layman's version:

Directly behind the grid on the wall bucky there are three chambers. They basically count the number of photons that are going to hit the cassette. When a determined number of photons (enough to produce a certain density on the image) are produced they tell the machine to stop. If they don't count enough x-rays before the back-up timer goes off, the timer terminates the exposure. The chambers will count any type of photons including scatter. That's why it is necessary to collimate and place the part over the correct cells. You mainly want them to count the photons in the area of interest. If they read too many photons from unnecessary exposure, you can end up with quantum mottle. The chambers do not care what kVp you use. They only care about ionizations produced by photons.

Hope that helps.
Wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

thank you! that's a great help!

traineetech · Joined: 06 Oct 2009 Posts: 15

P.S So there'd also be 3 chambers on the bucky table when carrying out an X-ray of say, an abdomen when the patient is lying down?

So, just to clarify, an ionization chamber is bascially there to makre sure that no more mAs/photons reach the image receptor than originally set by the technician?

IE, if you set an mAs of 15, once the ion chambers detects 15mAs, the exposure is stopped?

traineetech · Joined: 06 Oct 2009 Posts: 15

P.P.S another thing which has been bugging me...

When referring to a bucky, I thought that a bucky was the same thing as a grid as in Potter Bucky Grid? or am I getting this wrong?

Just got confused when you said "Directly behind the grid on the wall bucky there are three chambers"

wvaio · Joined: 22 Sep 2007 Posts: 82

Now that you get the general idea, I will up the ante a bit. I had the same questions when I started clinicals. It took me a while to figure out exactly what was happening. Its good that you are asking these questions. Understanding the AEC system is important to performing well in this field.

Here it is:

When you set a manual technique,as in 55kVp at 3.2 mas, the xray machine will shoot 3.2 mAs at 55kilovolts PEAK. That is calculated by the mA x the time.
The ionization chambers come into play when you use AEC(Automatic Exposure Control). The AEC system is ran by the computer system of the unit. Its function is to create a predetermined density on the cassette placed into the Bucky tray. The AEC uses the ionization chambers to calculate that exposure. When more than one chamber is used, the results of all chambers are averaged.
Lets say you are shooting a KUB. The patient is supine on the table. At a lot of places the computer would do the following steps for you just by pressing Abdomen table bucky, but for the sake of this example you are setting the factors by hand. First, you would choose a kVp. Lets say 80 kVp. Then you would select the number of chambers and their position. You would want to use all three chambers because the area of interest will completely cover all of the chambers. Next you would pick the mA that you want to use. We'll choose 240mA. When you press the exposure button, the tube will start to produce X-rays at 80kVp at 240 mA. The ionization chambers will start counting the number of photons that pass through the patient and the ones scattered by the patient. When the computer thinks the cassette has had enough exposure, it will terminate the exposure. The computer has a back-up timer that will terminate the exposure if it takes too long for the chambers to read their required number of photons. This protects the patient from being over-exposed due to a system failure( or operator error). If the machine produced x-rays at 80 kVp at 240 mA, and the ionization chambers calculate that they have the necessary exposure after .167 seconds, the machine will have produced 40 mAs. You could then turn off the AEC and set a manual technique of 40 mAs at 80 kVp, and achieve the same image as you did with the AEC on that same patient.

Hope that helps,

Wvaio

traineetech · Joined: 06 Oct 2009 Posts: 15

Hi again. Yes, I think that has cleared it up for me, so thanks for writing that! I'll go over it again in my head later, and if any further questions, shall gvet back to you.

Thanks!