Tuesday, February 5, 2013

CR Quality Control #4 – Contrast Evaluation


Perform this test to see if the CR’s ability to visualize scale of contrast is comparable to the initial baseline image.  You can evaluate the contrast for hard copy film, the CR QC station, and the radiologist’s reading station. 

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. Use the same technical factors that produced the baseline image (check monthly log for exposure factors) and make an exposure
  7. Annotate exposure indicator number and which CR reader was used
  8. Visually compare the contrast areas on the new exposure of the phantom with the contrast areas on the baseline image in your archive (*note – the brightness and contrast settings on your viewing monitors should be the same as when the baseline image was evaluated)

Evaluation:

The contrast area on the left (for my phantom) consists of two circles of Pb (lead) equivalent material.  The outer circle is 0.15 mm Pb, while the inner is 0.5 mm Pb.  




The contrast area on the right consists of an empty hole, while the inner circle is made of 0.7 mm Cu (copper).




A simple visual comparison is enough to determine a pass/fail result.  Keep a few things in mind when looking at the images:
The monitors will degrade over time, which can provide some display inconsistency.  It is advised to observe both the QC monitor and the radiologist’s monitor for variation.  If only one of the monitor’s display appears to be sub-par, while the other appears optimum, consider replacing the sub-par monitor.  However, if both monitors appear sub-par, it is more likely due to a reduced contrast performance of the reader, and a service engineer should be notified.
If you are comparing using hard-copy film, you can use a densitometer to get a precise density difference measurement from each contrast area, comparing the outer circles to the inner.


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Saturday, February 2, 2013

CR Quality Control #3 - Exposure Indicator Calibration

Two primary things can cause the exposure indicator to vary during regular operating procedures; the photomultiplier tube can be out of specifications as well as the exposure output of the x-ray tube.  In order to identify early variances, we must perform monthly tests to ensure proper calibration.

Supplies needed:
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. Use the same technical factors that produced the baseline image (check monthly log for exposure factors) and make an exposure
  7. Annotate exposure indicator number and which CR reader was used
  8. Compare to baseline exposure indicator number
Results: 

The exposure indicator should match or be very close to that of the baseline reading.  If it does not match, you need to calculate the percentage of difference.  Acceptable variation is +/- 20%.  Use this formula to calculate:


If results are outside 20% variation, repeat the exposure.  If the second exposure is outside variation, there could possibly be a problem with the generator or the CR reader which would warrant a service call. Once the problem has been identified and corrected, a new baseline must be established, and monthly testing needs to continue with comparisons to the new baseline.  Note the new exposure indicator and date on the image and in the QC log.

Note - "exposure indicator" refers to a general term that varies between manufacturer (S#, EI, LgM, etc.)


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