Ray Photon Energy keV

Fig. 3.1. X-ray spectra for 30 kVp operating potential for Mo/Mo (a), Mo/Rh (b), Rh/Rh (c), and W/Rh (d) source/filter assemblies (Barnes, 1999).

Tungsten targets, though used for mammography at one time, are now recognized as being inadequate to accomplish the subtle imaging tasks modern mammography involves. However, when used at an appropriately low operating potential and with suitable k-edge filters, tungsten target tubes have some advantages in mammography, particularly when imaging patients with dense breasts and large compressed breast thicknesses (Beaman and Lillicrap, 1982; Beaman et al., 1983; Bushong, 1992; Desponds et al, 1991; Jennings et al, 1981; Kimme-Smith et al., 1989a; Sabel et al., 1986; Stanton and Villafana, 1989). For tungsten target and k-edge filter combinations where the k-edge is >20 keV, the average beam energy will be higher than for a Mo/Mo combination and the dose will consequently be lower. In addition, the physical characteristics of tungsten also led to some advantages. The higher atomic number of tungsten (74), as compared with molybdenum (42) or rhodium (45), results in more efficient x-ray production and thus, in higher output exposure rates under otherwise identical conditions. In addition, the higher melting point of tungsten allows the use of smaller focal-spot sizes or higher tube currents. These factors can result in reduced blur due to improved geometric unsharp-ness or shorter exposure times leading to reduced motion unsharpness. One tungsten target unit is commercially available and it employs a 60 pm molybdenum filter, or a 50 pm rhodium filter with the latter used at higher tube potential settings for thick-dense breasts (Haus, 1991). The important characteristics of tungsten, molybdenum, and rhodium as target materials are compared in Table 3.3.

In a dedicated mammographic unit, the x-ray tube is generally oriented with the anode-cathode axis at right angles to the chest-wall edge of the image receptor with the cathode nearer the chest wall. The collimation is arranged in what is called a "half-field geometry" so that only the anode half of the x-ray field is utilized. In this geometry, the central ray, the ray perpendicular to the

TABLE 3.3—Physical properties of molybdenum, rhodium and tungsten (adapted from Lide, 2004).

Element

Atomic Number

Density (g cm-3)

Melting Point (°C)

Molybdenum

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