Investigation of the line-pair pattern method for evaluating mammographic focal spot performance

The latest American College of Radiology (ACR) Mammography Quality Control Manual contains a new method for evaluating focal spot performance, which this paper refers to as the "line-pair pattern test." The ACR describes a variety of methods for performing this test, and does not advocate one method over another. The authors of this paper conducted an investigation to compare the optional ways for performing the test. Resolution measurements were obtained using a prototype line-pair resolution phantom imaged with a GE DMR mammography unit. Measurements were made with the line-pair pattern 4.5 cm above the breast support platforms in both conventional (contact) and magnification geometries. Both 4.5 cm of air and Lucite were tested as attenuators between the line-pair pattern and the breast support platform. Image receptors that were employed included film alone, screen-film, and screen-film that was not allowed to wait the recommended 15 min before exposure. kVp was varied as was the orientation of the line-pair pattern relative to the chest wall. For the air attenuator case, the screen degraded the measured resolution by 1-3 lp/mm when compared to the direct film. The Lucite attenuator reduced the resolution by an additional 1 1p/mm. Increasing kVp improved the resolution slightly for the conventional mode, but decreased it slightly for the magnification mode. Based upon the results of this study, recommendations are made for improving the test protocol. For a test of focal spot performance, one should use the no-attenuation with direct film detector setup. For a measure of the resolution of the entire imaging chain, one should use the Lucite attenuator with screen-film detector setup.