mA Linearity Test
For mA linearity test we located a loaded cassette (Film 1) on the x-ray table and the cassette was positioned on a piece of lead rubber to absorb back-scatter. Next we sited an aluminum step wedge (with 10 steps) on the cassette on first part (almost 1/8). We tried that the step 10 be clearly visible, and step 1 was not included. After that we covered the rest of the cassette with lead. In this way we took 8 exposures using 75 kVp, 100 cm SID, 75 mA and 0.04 seconds. In the next step we processed the film. When it was reedy we took density readings of film 1. We selected the step with the density closest to one and calculated the density of this step on all of the 8 exposures. The optical densities in order were: 1.01, 1.02, 1.01, 1.04, 1.05, 1.06, 1.09 and 1.25, their average the density readings was 1.06.
Same procedure was repeated on Film 2, using 100 mA and 0.03 seconds. We took density readings of Film 2 and used the same step number that was used for the primary film. The optical densities in order were: 0.98, 0.97, 0.97, 0.98, 0.99, 1.04, 1.09, and 1.22. Average the density readings of film were 1.03. So we got on first film 1.06 and second was 1.03
1. According to SC 35A for any preselected value of X-ray tube voltage, within an applicable range, the quotient of the average air kerma measurement divided by the indicated current time product obtained at two applicable settings must not be more than 0.10 times their sum. According to Popp the reading should be within an optical density value of ± 0.1.
2. Yes, because in film 1 (mA1= 75, time 0.04) the overage density was 1.06 and in film 2 (mA2= 100, time 0.03) the overage density was 1.03. So according to our mA linearly test the reading is within an optical density value of ± 0.1. In this procedure sequential increase in mAs , produced almost the same sequential increase in the exposure measured.
3. This test is important for the quality assurance of the exposed x-ray films, i.e. for a proof of the achieved density. The mA selector in a generator and exposure time is determining the quality of x-rays in x-ray beams.
Test is important to ensure that the x-ray machine is producing a consistent radiation output. This will help to minimize patient dose. Also, mAs determines the quantity of x-rays, kVp determines the quality.