Experimental investigation of the dose and image quality characteristics of a digital mammography imaging system

Authors:
  • ASAnthony M. Sajewicz
    Anthony M. Sajewicz
  • KOKent M. Ogden
    Kent M. Ogden
  • DDDavid R. Dance
    David R. Dance
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Abstract
Our purpose in this study was to investigate the image quality and absorbed dose characteristics of a digital mammography imaging system with a CsI scintillator, and to identify an optimal x‐ray tube voltage for imaging simulated masses in an average size breast with 50% glandularity. Images were taken of an ACR accreditation phantom using a LORAD digital mammography system with a Mo target and a Mo filter. In one experiment, exposures were performed at 80 mAs with x‐ray tube voltages varying between 24 and 34 kVp. In a second experiment, the x‐ray tube voltage was kept constant at 28 kVp and the technique factor was varied between 5 and 500 mAs. The average glandular dose at each x‐ray tube voltage was determined from measurements of entrance skin exposure and x‐ray beam half‐value layer. Image contrast was measured as the fractional digital signal intensity difference for the image of a 4 mm thick acrylic disk. Image noise was obtained from the standard deviation in a uniformly exposed region of interest expressed as a fraction of the background intensity. The measured digital signal intensity was proportional to the mAs and to the kVp5.8. Image contrast was independent of mAs, and dropped by 21% when the x‐ray tube voltage increased from 24 to 34 kVp. At a constant x‐ray tube voltage, image noise was shown to be approximately proportional to (mAs)−0.5, which permits the image contrast to noise ratio (CNR) to be modified by changing the mAs. At 80 mAs, increasing the x‐ray tube voltage from 24 to 34 kVp increased the CNR by 78%, and increased the average glandular dose by 285%. At a constant lesion CNR, the lowest average glandular dose value occurred at 27.3 kVp. Increasing or decreasing the x‐ray tube voltage by 2.3 kVp from the optimum kVp increased the average glandular dose values by 5%. These results show that imaging simulated masses in a 4.2 cm compressed breast at ∼27 kVp with a Mo/Mo target/filter results in the lowest average glandular dose.

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