IMAGING CHARACTERISTICS OF X-RAY CAPILLARY OPTICS IN DIGITAL MAMMOGRAPHY

Computed radiography (CR) has shown promise in digital mammographic sc reening due to its good low spatial frequency MTF and its relatively w ide exposure latitude. The CR image format has not gained acceptance c linically because of reduced high spatial frequency resolution as comp ared to film-screen images. X-ray capillary optics, aligned between th e breast and CR phosphor imaging plate, will capture primary x-ray pho tons almost exclusively. Due to the very small angle of acceptance, sc attered photons angled more than about 1.6X10(-3) radians from primary trajectory will not be accepted at the capillary optic entrance. The virtual elimination of detected scatter means almost 100% of the possi ble primary contrast should be visible in the image. In addition, the image can be magnified without focal spot blurring. Effective resoluti on of CR images can be increased by a factor equal to that magnificati on. Clinical implementation of future capillary optics are expected to be either in the form of a large, stationary, post-patient optic that accepts primary from the entire breast or a fan-shaped optic that is scanned across the breast. Measurements of a test capillary optic show ed a reduction of scatter fraction to 0.018. Images of a lucite contra st detail phantom revealed a corresponding increase in image contrast when compared to anti-scatter grid and no grid methods. Spectral trans mission measurements using a high-purity germanium detector showed goo d primary transmission (45%-50%) in the mammographic energy range. The MTF measurements of both stationary and scanned capillary optics show ed improvement at the 5% MTF level to 8.4 mm(-1) for scanned optics an d 9.2 mm(-1) for stationary optics representing a 68% and 84% respecti ve increase over the CR MTF without magnification or capillary optics. (C) 1996 American Association of Physicists in Medicine.