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Using the +/- Density Settings on the Phantom

Comments:

The +/- density control settings on the mammography equipment should not be used to keep the phantom chart in limits.  Only if a change in film optical density is confirmed to be due to a change in the film emulsion batch, and if the magnitude of the change is within the expected batch-to-batch variation for that film type, then an adjustment of the density control setting to bring the phantom background optical density back into control is an appropriate corrective action.  If for any other reason your phantom falls out of limits, then the service provider must be called to correct the problem.  See discussion below.

 

                          Question:  

Can I use the density control function to keep my phantom chart within limits?

 

Answer:

Only if you know that the change is due to a change in emulsion batch. The density control function should not be used to mask processing issues. 

Referenced from the Policy Guidance Help System

900.12(e)(2)(i),(ii),(iii),(iv):  (2) Weekly quality control tests. 

FDA realizes that, due to differences in emulsion batches, a phantom image test with films from a new box may show variance in optical density and density difference greater than the allowed limits (when measured against the operating level established with films from the previous box). In such a case, facilities are advised to first check the whole imaging chain including the processor performance (facilities may wish to contact their medical physicist for help with this process). If no problems are detected, the facility may assume the change is due to different film emulsion batches.  They may then adjust their typical clinical technique factors to meet the phantom optical density requirements.  

Question 12:  If the optical density (OD) for the weekly phantom test falls below 1.20  (and/or changes by more than +/- 0.20 from the established operating level), must the unit be recalibrated or can we adjust the density setting to obtain a 1.20 OD? 

If the OD at the center of the phantom image falls below the required minimum of 1.20 (and/or changes by more than +/- 0.20 from the established operating level), the facility should follow pathway A, B, or C; below, based on the situation at the facility: 

  1. If the film is of a different type (e.g., switch from Min-R 2000 to Min-R E) from the previous week’s passing test, the facility should establish new phantom QC operating levels. 

  2. If the film emulsion batch is unchanged from the previous week’s passing test: 

  1. Ensure that the phantom is exposed using typical clinical conditions and that the position of the phantom and, where appropriate, the position of the AEC detector have not changed from that used for prior images. 

  2. Reevaluate the daily processor performance and make sure the processor is properly optimized according to the film manufacturer’s specifications. 

  3. If the facility has been tracking mAs, check the function of the mammography unit by comparing the mammography unit’s current mAs output with values obtained for previous phantom images.  If the mAs has changed by more than 15%, and the facility has been using the same kVp, the same mammography unit density setting, and the processor is operating within its action limits, then the medical physicist should be called to check the entire imaging chain, including the mammography unit.  If the mAs has not changed by more than 15%, then proceed with step 4. If the facility has not been tracking mAs, the facility should consult with its medical physicist for what to do next. 

  4. If no problems are found in steps 2 and 3, adjust the density control setting to obtain an optical density of at least 1.20 at the center of the phantom image (or obtain an optical density within +/- 0.20 of the established operating level). 

  5. Adjust the density control setting used clinically to be consistent with the changes made in step  4.

  1. If the film is of the same type but of a different emulsion batch from the previous week’s passing test, the facility should follow the steps as described in B 1 through 5.  

If the optical density again falls below 1.20 (and/or changes by more than +/- 0.20 from the established operating level) the next time the weekly phantom test is performed, the facility should follow the appropriate pathway (based on the film emulsion used) from the following three options:  

  1. If film of a different type (e.g., switch from Min-R 2000 to Min-R E) is used, the facility should establish new phantom QC operating levels. 

  2. If film of the same emulsion batch is used (assuming the same kVp and mammography unit density settings are used, and the processor is operating within its action limits), the facility should consult with its physicist and check the entire imaging chain before performing mammograms.   

  3.  If film of the same type (but not of the same emulsion batch) is used, the facility should repeat steps B 1 through 5. 

©FDA DMQRP 

You may also refer to the ACR manual pg 186 which states “If the change in film OD is confirmed to be due to a change in the film emulsion batch, and if the magnitude of the change is within the expected batch to batch variation for that film type, then an adjustment of the density control setting to bring the phantom BG OD back into control is an appropriate corrective action.” 

 

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