Radiology Myth-Busters: CR Myths #1

An increase in exposure creates an increase in density: False

Scenario: you are performing a knee x-ray using the table bucky and you use 75 kVp and 10 mAs, a technique that you have been using with the same equipment for the last 5 years on patients of similar size.  Upon processing the image, it appears to have low density on the monitor – more lightly shaded pixels than the appropriate darker shades.  What do you do?

If you come from the film/screen era, your first instinct might be to increase your mAs.  You go to the control panel, double your mAs and re-shoot your image only to find out that the image is lighter than the original one… what the heck is going on?

The truth is, the original image probably had the correct technical factors, but other factors come into play that may not be apparent.  Typically, when a CR image is too light, this is the computer software’s response to an over-exposure, rather than not enough technique.  The computer thinks the image is over exposed, and adjusts the entire image to be lighter in order to compensate.  The default processing algorithm does not take into account the different scenarios that may have caused more exposure on the image plate than what you intended such as:

Pathology: the patient could have a destructive pathology causing a once-good technique to be overexposed.

Image plate fog: it was a slow weekend and the knee x-ray was the first exposure made on the image plate in 3 days.  Excess background radiation has accumulated on your PSP causing what looks like overexposure to the software.

Collimation error: meaning a lack thereof.  If you don’t collimate, the software will compute an “average” pixel grayscale value and shift the brightness of your image to the lighter side.

There are a few additional errors that may cause this effect depending on manufacturer, but use the tools at hand to determine if you need to repeat, and if so, what technical factors you should be using.  The exposure indicator should be within the proposed range, and we need to make a habit of viewing it during our image QC.  Choosing the correct processing algorithm is a must.  Ensure that image plates are erased daily, or at least every other day.  And pay attention to how you collimate.

You may find your system has errors that seem unexplainable at the time… create documentation of these errors so that your QC team can evaluate more than one of them in the future.  Is it only one particular piece of anatomy and one particular view?  The processing algorithm may need adjustment.  Is it one particular cassette that does it?  Perhaps there is a flaw and it needs replacement.  Is there one particular kVp or mAs setting that the error occurs at?  Your x-ray tube and/or generator may need to be serviced.

CR and DR imaging can still be difficult to adjust to, especially if you have been a radiographer using film/screen for a majority of your career and if you have not had the appropriate education.  Just remember to utilize the tools at your disposal, and take some additional time to observe what you can before charging back into the exam room for a repeat using a method that would have worked with film/screen.

Stay tuned for more CR myths debunked!
*Related post:  The CR Image Plate