Full Field Digital Mammography - System Development and Technical Performance

Martin J. Yaffe,* Donald B. Plewes,* Gordon E. Mawdsley,* Jeff Critten,* Stephen A. Feig,# Andrew D.A. Maidment,# Michele Picarro**

*Imaging Research Program, Sunnybrook Health Science Centre, University of Toronto, Toronto, Ont. Canada
# Department of Radiology, Thomas Jefferson University Hospital, Philadelphia, Pa, U.S.A.
** Fischer Imaging Corporation, Denver, Co, U.S.A.
e-mail: yaffe@srcl.sunnybrook.utoronto.ca

A prototype digital mammography system has been under development at the University of Toronto. This system is based on the scanned-slot principle, where the x-ray beam is confined to a long narrow field. This geometry allows for highly efficient rejection of scattered radiation and for simplicity of detector design as well as cost savings due to the smaller total detector area required. The detector consists of an x-ray absorbing scintillator, which can be either a settled phosphor screen or CsI(Tl), coupled through demagnifying fibre optic tapers to multiple CCD arrays. These arrays have a rectangular format, where the number of columns corresponds to the long direction of the x-ray slot and the signal is acquired using time delay integration (TDI) of the charge down the rows of each column.

The University of Toronto design uses three tapers and a detector format of 3.2 mm x 24 cm at the detector entrance surface. TDI acquisition occurs over the 64 rows of the CCD by moving the x-ray beam and detector array across the breast from chest wall to nipple during continuous x-ray exposure, while shifting the charge down CCD columns at equal speed, but in the opposite direction to the detector motion. While the total imaging time is 3-4 seconds, any image element is acquired in approximately 64 ms, so that motion blurring is not a problem. Because of the ability to enhance display contrast, it is possible to use a higher energy x-ray spectrum than is normally employed in film-screen mammography. A tungsten anode x-ray tube is used with kilovoltage and filtration designed to provide optimization of signal-to-noise ratio, radiation dose and imaging time.

A commercial clinical imaging system is being produced by Fischer Imaging Corp. This operates according to the same principles, however several design details are different. For example the scanning direction is orthogonal to the Toronto design and the slot width is greater. In this presentation, the design of the two systems will be described and details of imaging performance including spatial resolution, noise, contrast sensitivity and dynamic range characteristics will be presented.