Phantom Images Exposed in a Fully Automatic Mode – Assigning a kVp
Many mammographic x-ray systems have more than one AEC mode of operation. A commonly used AEC mode requires the radiologic technologist to set a specific kVp value with the AEC automatically determining the mAs for the exposure (commonly called fixed kVp or Auto-mAs modes). The radiologic technologist can vary the exposure in this AEC mode by setting the kVp, adjusting the density control setting, or both. A more advanced AEC mode is where the system automatically controls the kVp, mAs, and in some cases, target-filter combination (commonly called the Full-Auto mode). In this latter mode, the radiologic technologist can only adjust the exposure by use of the density control. The actual names for the different AEC modes of operation will vary with the different make and model of mammographic unit.
For equipment testing involving the phantom, inspectors should use the same technique factors and AEC mode of operation that the facility uses for its patients with the standard breast (compressed breast thickness of 4.2 cm, with breast tissue consisting of approximately 50% adipose (fat) tissue and 50% glandular tissue in composition). When a facility typically uses the Full-Auto AEC mode for its clinical examinations, the inspector should make an exposure of the phantom using the Full-Auto AEC mode and record the kVp selected by the x-ray system. This same kVp should be used when the beam quality (HVL) testing is conducted in the manual mode of operation. In the event that the displayed kVp after the exposure with the phantom has a three-digit display (e.g., 25.7 kVp), but the manual mode only allows selection of two digits (e.g., 25 kVp), round up or down based on the final digit (example: for 25.1 to 25.4, use 25 kVp; for 25.5 to 25.9, use 26.0 kVp).
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.
Note about facility phantom QC: If the facility typically uses the Full-Auto AEC mode for its clinical examinations, it must use this same AEC mode for its weekly phantom QC test.