Establishing and/or Changing the Phantom Image Test Operating Levels
Objective: The objective of this article is to provide help to facilities that perform screen-film (S-F) mammography for initially establishing the operating levels for their phantom image test and for changing the operating levels in a “for cause” situation.
Note: The following suggestions apply to facilities using S-F mammography units. For full-field digital mammography (FFDM) units, facilities should refer to the unit’s quality control manual.
Establishing operating levels for the first time
Prior to performing mammography with new equipment for the first time, your medical physicist (MP) must evaluate your mammography unit and film processor. During or immediately following the mammography equipment evaluation, when the mammography unit and processor are operating under optimal conditions, your MP should be able to assist you in establishing baseline operating levels for the background density on your phantom images. He/she may recommend an exposure technique that will result in:
- a background density of 1.80 or higher, and
- a density difference (contrast) with which you can expect to get a minimum of 0.40 under optimized technique factors and processing conditions.
Typically, most current mammography S-F combinations are designed to produce a density difference value of approximately 0.50 or higher when the background density is approximately 1.80. If the density difference value on your phantom image is below 0.40, you should contact your MP regarding how to proceed. Using these baseline values will help in optimizing the image quality of your mammogram films. Additionally, it is also helpful to determine baseline values from phantom images produced over a five-day period; that will help stabilize the operational levels. An average of the values from the five phantom images should produce the most appropriate operating limits.
Once you start performing mammography after setting your baseline operating levels for background density and density difference, you should ask your lead interpreting physician whether the density and contrast of the mammograms are acceptable. If the interpreting physician feels the mammograms are of adequate quality, your operating levels are sufficient and you should continue to use them. However, if your lead interpreting physician recommends changes, you may want to contact your MP for help in determining new operating levels.
Changing operating levels for background density and density difference
Once you have set your operating levels, you should keep them the same unless your lead interpreting physician suggests changes to achieve a difference in density or contrast. Any significant changes occurring in the equipment might also require you to reevaluate the operating levels. Some examples that would lead you to consider reestablishing your existing operating levels are:
- Lead interpreting physician decision to increase clinical image optical density
- The replacement of the mammography x-ray unit or film processor
- Changes in manufacturer or type of film, screen, or processing chemistry
Note: You should not reestablish the operating levels on the recommendation of processor maintenance personnel without consulting with your MP, as the MP is responsible for the technical quality of the image.
Before considering a change in operating levels due to replacement of the mammography x-ray unit or the film processor, consult with your MP about keeping the same operating levels. A small change in system settings, such as the density control, may allow you to continue to use your present operating levels. If your MP recommends new operating levels, inform your lead interpreting physician of the change. If new operating levels are established, ask him/her whether the new levels are producing an acceptable density and contrast for quality mammograms.
If you change the manufacturer or type of film, screen, and/or processor chemistry, you may need to set new operating levels. If you are considering a change in film or screens, you should contact your MP before making the change. During your discussion, your MP may recommend a change in operating levels for your new S-F combination that will work better with a higher background density and density difference. As mentioned earlier, you should always notify your lead interpreting physician about any intended change(s), and immediately following a change, confer with your interpreting physician regarding the quality of the mammograms.
Cautions about changing operating levels
You should not “automatically” change operating levels when problems arise with any of your mammography equipment. If the background density or the density difference start to fluctuate or drift toward or exceed the operating level action limits, contact your MP for help in determining the source of this change. If the change is caused by a problem with a component of your imaging system, you need to fix the problem so that your facility can perform high quality mammography. Adjusting or repairing equipment is a much better approach than frequent changes in the operating levels to accommodate a problem that could adversely affect the quality your facility’s mammograms. When you consider changing operating levels, you should be sure that the change is best for quality mammograms.