kVp Accuracy

In every day’s use of radiographic equipments, technologists must select optimistic kVp for all examinations according to patient size, diagnostic purpose, and etc. In diagnostic radiography, kVp is very powerful. A variation of approximately 4% is necessary to influence image optical density and image contrast.  kVp can significantly affect image quality, special resolution, tube load, and most importantly, the patient dose. Therefore, x ray generator must be calibrated periodical and scheduledly by quality control technologists.

This quality control test can be performed in rooms 3, 4, 5, or 6. My group decided to do it in room 5. First of all, we need to change the mAs from 15 into 16 in our lab manual, because the x ray console does not allow us to select 15 mAs. Secondly, when we collimate the x-ray field to the kVp meter during the test, we need to make sure we collimate to include the sensor (田 showed in the middle of meter), but not the whole meter.

We change the kVp setting into 60 kVp (5mAs), 81kVp (5mAs), and 102kVp (5mAs), then for the same kVp settings we use 16 mAs. For each setting, we expose 3 times. Then record the reading from kVp meter for each setting. All the collected data is in the following chart:

Nominal kVp	mAs	Measured kVp  1	Measured kVp 2	Measured kVp 3	Average kVp
60	5	59.6	59.6	59.5	59.57
60	16	59.6	59.6	59.7	59.63
81	5	82.3	82.4	82.3	82.33
81	16	81.9	81.9	81.9	81.9
102	5	102.9	102.9	102.9	102.9
102	16	102.4	102.5	102.4	102.43

According to SC35, the loading factors must not deviated from the selected value, for any combination of loading factors, by more than 10% for x-ray tube voltage.

According to HARP standards, the average peak kilovoltage corresponds to the selected value to within + 8%

Therefore, the x-ray equipment in room 5 is working properly.

By looking at the chart, we will find that the reading from each measurement at the same kVp and mAs setting is slightly different sometimes. Is the reading a true reading? The answer is yes. They are all correct in their own ways. The reason why there is a difference is because there is an inherent, random variation in all measurements.