Mon 12 Oct 2009
Radiology Student Chat Zone :: Help understanding ionization chambers
Posted by heaven under UncategorizedComments Off
Author: wvaio
SubjectStep two
Posted: Wed Oct 07, 2009 8:19 pm (GMT -4)
Topic Replies: 6
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
_________________
Whether you think that you can, or that you can't, you are usually right. --- Henry Ford
SubjectStep two
Posted: Wed Oct 07, 2009 8:19 pm (GMT -4)
Topic Replies: 6
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
_________________
Whether you think that you can, or that you can't, you are usually right. --- Henry Ford