Mammography

Mammography is a radiographic examination of the breast. An X-ray beam from a metallic target is directed toward the breast and the transmitted X-rays are detected by an image receptor. Most commonly, the image receptor is a fluorescent screen, held in intimate contact with a sheet of single-emulsion photographic film in a light-tight cassette. In mammography, the X-ray spectrum is most frequently emitted by a molybdenum target, although rhodium or tungsten may be used for imaging large or dense breasts. The spectrum usually has a maximum energy of 25-32 keV. The beam is prefiltered with a metallic foil, before becoming incident on the breast. Attenuation of X-rays by the breast is governed by the thickness and composition of the tissues through mechanisms of absorption and scattering. Because the scattered radiation degrades the shadow image of normal and pathological structures, a device, called a grid, is placed between the breast and the image receptor. This removes much of the scattered radiation, but unfortunately, also some of the useful, primary quanta. Equations (1) and (2) give a simplified description of the formation of the mammogram.

P TP

where

D(x', y') is the optical density (blackness) of the processed mammogram at a location (x', y') in the image receptor

L(x', y') is the quantity of light incident on the image receptor f (L) represents the characteristic response curve of the film to light

E is the energy of X-ray quanta

Emax is the maximum energy of quanta in the spectrum

(dN0/dE)(E) is the differential number of X-ray quanta at energy E that would be incident on a specified area of receptor in the absence of the breast and the grid

^(x, y, z, E) is the attenuation coefficient of the breast tissues at each X-ray energy E

T(x', y') is the total thickness of the breast along the path that projects to x', y'

SIP is the scatter-to-primary ratio of radiation at the image receptor with no grid

Tp, Ts are the transmission factors of the grid for primary and scattered quanta q(E) is the quantum interaction efficiency of the screen

G(E) represents the conversion efficiency and optical transmission of the screen

The film mammogram can be considered to be a map of optical density that reflects the composition of the breast, described in terms of the spatial distribution of The intensity of the X-ray spectrum, dN0(E)/dE, depends on the design of the X-ray tube, the target material, the applied voltage, the electron beam current (mA) and exposure time, the filter material and thickness, and the presence of other attenuators such as the breast compression plate and breast support table. Considering this and the other variables just enumerated, one can see that there are many factors that will influence the actual optical density of the mammogram. In addition, it is usual that a sensor is situated beneath the image receptor to record some of the radiation transmitted through the breast and the receptor. This is used to actuate an automatic exposure control (AEC) that terminates exposure when the sensor has received a predetermined amount of radiation. The purpose of the AEC is to provide an image on the processed film, which has a constant optical density, regardless of the thickness and composition of the breast. While the AEC is an important tool, contributing to high image quality, its use further weakens the absolute connection between the recorded signal D and the composition of the breast, unless all of the exposure factors are known. Therefore, much of the work on mammographic image analysis has not attempted to use the underlying physical variables of image acquisition, but instead to concentrate on subjective or quantitative features of the image itself.

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