How to Measure the Usage of a CT X-ray Tube

The X-ray tube is the most important and at the same time, the most expensive part of every CT scanner. Therefore, it is important to be aware of all the important aspects of it that you should consider when buying a new one.
Firstly, a CT X-ray tube is described by how much heat it can endure. This is measured in MHU (Mega Heat Unit). Simply put, the higher the MHU number, the better, because the more heat your X-ray tube can take and consequently, the higher number of examinations possible.
The tube usage is then defined by effect * time = tube usage.
Now, you can easily get confused by the types of CT tube count measures used by different manufacturers. To understand the measures is essential in order to be able to predict the life expectancy and make the right buying decision, especially considering the high price. There are four types of tube measures, namely The Total Patient Exams, Clicks/Counts, Scan Seconds and Milliampere Seconds (mAs).
The Total Patient Exams
This measure reveals the total number of individual cases performed with the tube. However, since not all the CT studies are the same length and do not require the same energy, this method of measure is not giving very deep understanding of the tube and it is harder to determine its life expectancy.
Focusing on this method of measurement, you will get an understanding of how many exposures the tube has performed. From this number, it is possible calculate the amount of time the tube has been used for, based on gantry rotation speed, but again, the number does not reveal how much energy has been spent during each of the exposures. This term is frequently used by Canon Medical (Toshiba).
Scan Seconds
This measure is more precise than the two above since it reveals the duration of the tube exposures. Therefore, you can get an understating of what is the total time the tube has been used for. On the other hand, the number of Scan Seconds do not show the amount of heat that has run through the tube.
This measure is the most widely used among the manufacturers, including Philips, GE, Siemens, or Canon.
Milliampere Seconds (mAs)
This measurement can be usually on GE tubes. Out of the four types of tube measures, this one is the most accurate. Milliampere Seconds reveals both the duration of the exposures performed with the tube and the amount of power that has run through the tube.
As it is with any other machine, it is the best to have as many of these numbers as possible to fully understand the usage of the tube. Combining these numbers with the information of which techniques were performed on the system, is extremely helpful with predicting the life expectation of the tube. However, to get a basic understanding, on average, a 7 MHU tube can last at least 150 million mAs, and a 4 MHU tube about 70-100 million mAs. Nevertheless, it is important to remember that the tube lifetime is unpredictable and differs from a tube to tube.


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