mA Linearity Test

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Objective:     To determine if the X-ray unit produces the same radiation output linearity for the same kVp and mAs regardless of the mA station used.


Materials:      Dosimeter

Lead sheets




1.         Place the dosimeter on the table on a sheet of lead rubber to absorb back-scatter.


2.         Ensure that the SID is 100cm to the top of the dosimeter.


3.         Take 4 exposures using 80 kV and 20 mAs.  Each exposure should have a different mA and time combination to equal 20mAs.  (as listed below)


i.        80 kv   200 mA   100 ms

ii.        80 kV   100 mA   200 ms

iii.        80 kV    250 mA   80 ms

iv.        80 kV   180 mA   110 ms


OR any other variables to produce the same mAs.



4.         Record the dosimeter readings from each exposure.  Remember to press the “reset” button after each x-ray exposure.


5.         Divide each recording by 20 mAs to get the mR/mAs. (you may use the table below to keep track of your results)




Small focal spot


Small focal spot


Large focal spot


Large focal spot

mA 180 80 180 80
ms 110 200 110 200
Dosimeter   reading 72 mR 80 mR 78 mR 80 mR
Step 5   results = mR/mAs 3.6 mR/mAs 4 mR/mAs 3.9 mR/mAs 4 mR/mAs



6.         From step 5, determine the minimum, maximum and average mR/mAs.

Minimum= 4 mR/mAs, Maximum= 3.6 mR/mAs, Average= 3.8 mR/mAs


7.         Calculate the Linearity variance using the equation on page 97 of Papp.


Linearity variance =



Linearity variance =  =


Linearity variance = 0.05




1.         According to SC 35A, what are the standards for mA linearity?  (include the section number in your response))

According to SC 35A (2.5.2 #2; P. 25) Radiation Output Linearity- 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.


2.         What are the standards stated in Papp?

According to Popp (p. 97; Procedure) the reading should be within an optical density value of ± 0.1, (10%).


3.         Does the unit you tested pass?

The unit that I tested pass, because ,according to Popp the reading should be within an optical density value of ± 0.1 .My linearity variance was 0.05.


4.         Explain why it is important for the equipment to pass this test.

Variation in X-ray output and quality during a diagnostic exposure can be undesirable and may result in unnecessary dose to the patient. mAs is equal to the product of the milliamperage and the exposure time in seconds and determines the quantity of x-rays. kVp the highest kilovoltage used in producing a radiograph and determines the quality.This analysis is significant to certify that the x-ray device is producing a steady radiation output.  This will aid to reduce patient dose.



9 thoughts on “mA Linearity Test

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