Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Chapter 8: Mammography 223TABLE 8-6. DgN CONVERSION FACTOR (mRAD PER ROENTGEN) AS A FUNCTIONOF HVL AND kVp FOR Mo TARGET/FILTER: 4.5-CM BREAST THICKNESS OF 50%GLANDULAR AND 50% ADIPOSE BREAST TISSUE COMPOSITION*HVL (mm) 25 26 27 28 29 30 31 320.25 1220.26 126 1280.27 130 132 1340.28 134 136 138 1390.29 139 141 142 143 1440.30 143 145 146 147 148 1490.31 147 149 150 151 152 153 1540.32 151 153 154 155 156 158 159 1600.33 155 157 158 159 160 162 163 1640.34 160 161 162 163 164 166 167 1680.35 164 166 167 168 169 170 171 1720.36 168 170 171 172 173 174 175 1760.37 174 175 176 177 178 178 1790.38 179 180 181 182 182 1830.39 184 185 186 186 1870.40 189 190 191 192*Adapted from ACR QC Manual, 1999.kVpmrad/R for 6-cm thickness in Table 8-6). However, the lower DgN for the thickerbreast is more than offset by a large increase in the entrance exposure necessary toachieve the desired optical density on the film.Factors AffectingBreast DoseMany variables affect breast dose. The speed of the screen-film receptor and the filmoptical density preferred by the radiologist are major factors. Breast thickness andtissue composition strongly affect x-ray absorption. Higher kVp (higher HVL)increases beam penetrability (lower ESE and lower average glandular dose), butdecreases inherent subject contrast (Fig. 8-29). Antiscatter grids improve image600'0 Film00 = 1.80~ 500 .s(J)(Jl0 400"0roMo targetI Mo filter50% glandular--50% adipose tissue"S"0C'""6l(J)Cl~(J)~3002001000 2 3 4 5 6Breast thickness (cm)FIGURE 8-29. Approximateaverage glandular doses (25to 35 kVp, Mo targetiO.03-mm Mo filter)for an opticaldensity of about 1.80 and a7 8 50:50 glandular/adipose tissuecontent are plotted as afunction of breast thickness.