When performing the weekly phantom image test must we monitor kVp and/or mAs?
No. The only requirements on the weekly phantom image test are that the phantom image achieve at least the minimum phantom scores established by the accreditation body and must be within the action limits established for the three optical density requirements. FDA is aware that many facilities are monitoring kVp and/or mAs as part of their weekly phantom QC testing. This is not required. If a facility uses the Full-Auto mode and monitors kVp and/or mAs, it will probably observe that, over time, the Full-Auto mode leads to small variations in the kVp selected by the unit for the phantom exposures. Even small variations in kVp may lead to significant variations in the mAs values obtained. While small variations in kVp are to be expected when using the Full-Auto mode, large variations in kVp (greater than 1 kVp of the value usually obtained) may indicate a problem and should be further evaluated. Facilities using the Full-Auto mode that wish to monitor kVp and/or mAs may want to establish baseline mAs values corresponding to the specific kVp values usually encountered during phantom testing. In this way, they can account for the mAs variability that may be caused by small changes in kVp.
Mobile facilities should be aware of the following if they are monitoring mAs as part of their post-move-pre-exam testing. Performing the post-move-pre-exam test in the Full-Auto mode may be problematic (due to the variability of kVp and mAs as previously mentioned). In these cases, the facility may:
1. Use the AEC mode to perform the post-move-pre-exam test, even if they use the Full-Auto mode for their patients with the standard breast. Note: The weekly phantom QC test must be performed using the same clinical conditions that the facility uses for its patients with the standard breast.
2. Use the Full-Auto mode and establish baseline mAs values corresponding to the specific kVp values usually encountered during phantom testing. If the mAs value is within 10% of the baseline value for the post exposure kVp value, the unit has passed that portion of the post-move-pre examination test.