CR Quality Control Summary

A consistent quality control program goes a long way toward maintaining a high standard of image quality within your radiology department.  The tests posted within this series are not all-inclusive for every CR system, but may provide as a good start for the beginning stages of any QC program.  Variations may occur if your department utilizes a hard-copy laser printer and/or measurement tools at the QC station (or to include the radiologists' work station).

As a general rule of thumb, most CR vendors recommend designating one person as the quality control technologist.  Other technologists may be trained in the event of an absence, but testing should be performed as consistently as possible to reduce variables.

Any QC test that does not meet specifications should first be repeated.  After a repeat test fails, a qualified service professional should be contacted to troubleshoot the problem.  Additionally, after any equipment repairs or replacement of parts, a new baseline should be established.

There may be additional troubleshooting to consider with equipment that might affect the QC testing results as well.  If you know of any aspect of the x-ray machine used to perform QC testing that is inconsistent, such as line voltage, mA station linearity problems, or timer malfunctions, these can also be researched prior to contacting the CR reader's service professional.  Ideally, the tests would be set up in the beginning to be performed on the most consistent equipment available.  Imaging departments may also consider annual contracts with service professionals if consistency may be an issue.

Your quality control manual should be kept in a safe place, but accessible in the event of a state or JCAHO inspection.  All results (even failing results) should be documented and maintained within the manual.  You may also consider keeping records of service calls and documents related to equipment repair on hand for future analysis.  Always watch for trends in test failures and use these trends to determine what service contracts and/or equipment repairs and replacement may best suit the needs of your department.

I hope you've found this series useful.  Here is a summary of all of the tests covered:


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

CR Quality Control #10 – Residual Image Testing

This test evaluates the CR reader’s erasure function and should be performed semi-annually.  Each exposed image plate should be adequately erased during the processing cycle to prevent a residual image on the next image.

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. Expose the phantom at using twice the mAs value of the baseline image and process the image (or perform immediately following the system linearity test after the cassette has been erased on the exposure that was made at double the baseline).
7. Turn the phantom 180 degrees and place a radiopaque object in the center of the phantom
8. Expose at the normal baseline technique
9. Process the IP under “Contrast” menu selection

Results:

The processed image should appear normal compared to the baseline image, except that the phantom will be displayed upside down with the radiopaque object.  There should be no evidence of an additional phantom pattern.  If a residual image is present, re-test and call a service professional if the re-test does not eliminate the residual image.

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

Shape Distortion

One of the most fun courses I have taken and taught is exposure principles.  This course typically introduces students to different types of distortion that can occur on the radiograph.  Size distortion, otherwise known as simple distortion or magnification, occurs when OID is increased.  The other type of distortion, shape distortion, can be broken down into two main categories; foreshortening and elongation.

Foreshortening is when the radiographic image measures shorter in one dimension than the actual object being radiographed.  The only way this can happen is if the central ray and image receptor are perpendicular, and the object being radiographed is angled.  This is demonstrated in the image below (courtesy of students in the Radiography Program at OCTS/KCTCS) by comparing the top left, undistorted image, to the middle two which were placed on 45 degree-angled sponges.  This may also result in unequal magnification, where one side of the object may appear larger than the other due to differences in OID from one end of the angled part to the other.

Elongation is when the radiographic image appears longer than the object being radiographed.  There are several situations where this can happen.  If the central ray is perpendicular to the part, but the IR is angled, there will be more elongation with a greater increase in IR angle.  If the part is parallel to the IR, but the x-ray tube is angled, elongation can occur as in the bottom left image below (45 degree tube angle to the part).  Elongation can also occur when the x-ray tube is off-center to the part, even though the part may be parallel to the IR.  This is caused by the divergence of the beam, and is demonstrated by the two images on the right below.

*note - there would be no difference between an off-centered x-ray tube with collimation opened to the part compared to a tube angle from the same starting point (as long as the tube does not move).  Check this post out for more on that.

The image below is a similar experiment performed in one of my exposure principals labs.  We observe the following using a quarter, a 2" sponge, and a 45 degree angled sponge:

1. Perpendicular CR to the coin (parallel to the IR) with a 2" OID 
2. Coin parallel to IR on 2" sponge, but 45 degree tube angle
3. Tube returned to perpendicular to IR, but quarter is angled 45 degrees
4. CR angled 45 degrees (perpendicular to angled quarter)
5. CR angled half the part angle (demonstrating an isometric principle)

You'll notice very slight elongation in image 2 compared to the first.  Image 3 is quite foreshortened, and image 4 has the most elongation.  Image 5 is very useful for trauma views, and is illustrated with exams like the axial calcaneous and AP sacrum/coccyx.  The tube is angled half of the part's angle to the IR.  This reduces the effects of elongation as much as possible, so our exposure 5 measures the same as exposure 1.

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