PHYSICAL PRINCIPLES OF MAMMOGRAPHY

Abstract

An outline is given of the underlying physical principles that govern the selection and use of systems for X-ray mammography. Particular attention is paid to screen-film mammography as some aspects of digital mammography are considered in another lecture. The size and composition of the compressed female breast and of calcifications are described and the magnitude of photon interaction processes in breast tissues discussed. The physical performance measures contrast, unsharpness, dose, noise and dynamic range are outlined and used in a treatment of the various components of the mammographic system. The selection of photon energy is a compromise between contrast and/or signal-to-noise ratio on the one hand, and breast dose on the other. For screen-film imaging the contrast achieved is considered to be the most important image measure and the performances of different mammographic target/filter combinations (including Mo/Mo, Mo/Rh, Rh/Rh and W/Rh) are compared on this basis. For digital imaging, the signal-tonoise ratio is the most important image measure, and the optimal X-ray spectra are then different to those for screen-film mammography. The relationship between image unsharpness and focal spot size and image magnification is explored. The importance of breast compression is stressed and the advantages of compression listed. The contrast in the image is degraded by scattered photons recorded by the image receptor and the magnitude of this effect and the reduction achievable using mammographic anti-scatter grids considered. The performance of mammographic screen-film receptors is described and analyzed, paying attention to unsharpness, noise and receptor DQE.