CR Quality Control #9 - Exposure Linearity

This test should be done semi-annually and tests for the CR system's ability to properly rescale images that are over or under exposed.  It also measures accuracy of the exposure indicator and its ability to increase/decrease in relationship to the exposure value.

Procedure:

1. Erase your dedicated test IP
2. Place a lead apron on the floor of your x-ray room (use same room that baseline image was performed in)
3. Place test IP on lead apron
4. Place phantom on cassette (same orientation as baseline image)
5. Raise the x-ray tube to maximum height, center, and open collimation about 1" past edges of test IP
6. For all following exposures, annotate the technical factors used, exposure indicator, date, and which reader the image was processed on.

Exposure 1 - should be set with the same kVp as the baseline technique, but half the mAs.  .

Exposure 2 - should be the same technical factors as the baseline exposure.

Exposure 3 - should be performed with the same kVp as the baseline technique, but double the mAs value.

Results:

The brightness of each of the three images should be the same.  If there is variation, you may have a problem with the automatic rescaling function (click here to read more about automatic rescaling).

Exposure 1 should display a change in baseline exposure indicator to represent 1/2 the exposure +/- 20% (S# = baseline x 2, EI = baseline - 300, LgM = baseline - 0.3).  Exposure 2 should be within +/- 20% of the baseline reading.  Exposure 3 should display a change in baseline exposure indicator to represent a double in exposure value +/- 20% (S# = baseline x 0.5, EI = baseline + 300, LgM = baseline + 0.3).

Use the following formula to calculate percentage of change (click here for detailed instructions how to calculate percentage of increase or decrease):

new exposure indicator - baseline exposure indicator  x 100
                  baseline exposure indicator 

If variance is outside the +/- 20%, service personnel should be contacted.

Other posts in this series:

CR Quality Control #1 - Screen Cleaning
CR Quality Control #2 - Creating a Baseline Phantom Image
CR Quality Control #3 - Exposure Indicator Calibration
CR Quality Control #4 - Contrast Evaluation 
CR Quality Control #5 - Sharpness
CR Quality Control #6 - Shading Correction
CR Quality Control #7 - Laser Jitter
CR Quality Control #8 - Image Artifacts and Noise
CR Quality Control #9 - Exposure Linearity
CR Quality Control #10 - Residual Image Testing