Trial of a proposed protocol for constancy control of digital mammography systems.

摘要

PURPOSE: Evaluate the utility of tests in a proposed protocol for constancy control of digital mammography systems. METHODS: The protocol contained tests for image acquisition, mechanical function and safety, monitors and printers, and viewing conditions. Nine sites with digital systems from four equipment manufacturers were recruited. Dedicated PMMA test objects and Excel spreadsheets were developed. Quantitative measurements were done on processed images for systems where these images were the ones most readily available. For daily assessment of the automatic exposure control system, a homogeneous PMMA phantom was exposed under clinical conditions. The mAs and signal to noise ratio (SNR) were recorded, the deviation from a target value calculated, and the resulting image inspected for artifacts. For thickness tracking, the signal difference to noise ratio obtained for three thicknesses was calculated. Detector uniformity was assessed through comparison of SNR values for regions of interest in the center and corners of an image of a homogeneous test object. Mechanical function and safety control included a compression test, a checklist for mechanical aspects, and control of field alignment. Monitor performance was evaluated by visual inspection of the AAPM TG 18 QC test image [E. Samei et al., "Assessment of display performance for medical imaging systems," Task Group 18 (Madison, WI, April 2005)]. RESULTS: For quantitative parameters, target values and tolerance limits were established. Test results exceeding the limits were registered. Most systems exhibited stable mAs values, indicating that the tolerance limit of +/- 10% was readily achievable. The SNR also showed little variation, indicating that the tolerance limit of +/- 20% was too wide. At one site, a defective grid caused artifacts that were visible in the test images. The monitor controls proved more difficult to implement due to both difficulties importing and displaying the test image, and the radiographic technologists not getting necessary access to the reading stations. CONCLUSIONS: The proposed tests could easily be performed by trained radiographic technologists and within a time frame comparable to similar programs for analog systems. Tests with quantitative measures were more readily performed than procedures that required a subjective evaluation. Several of the proposed tests revealed equipment performance that required intervention, and which would otherwise have gone unnoticed. They therefore defend a place in a vendor-independent constancy control protocol.