Scatter and kV
The images shown above were obtained at 50 kV (left) and 125 kV (right) at table top (i.e., with no scatter removal Bucky). The low kV image has superior contrast because of reduced levels of scatter. As the kV is reduced, the average photon energy is reduced; accordingly, the proportion of photoelectric interactions increases and the proportion of scatter events is reduced. Scatter is generally lower at lower kV values, especially when imaging bone which contains a high atomic number material (Calcium, Z = 20). In general, extremity imaging may be performed at table top because two factors help to reduce the amount of scatter: (a) thin body parts are easier to penetrate and therefore permit the use of lower x-ray tube voltages (kV); (b) the presence of bone (high Z) means that most interactions are photoelectric (not Compton) and thus scatter is reduced.
The image on the left (i.e., 50 kV), for a constant receptor radiation dose of 5 mGy (S ~ 200), required 15 mAs whereas the image on the right (125 kV) required only 0.5 mAs to achieve the same receptor dose. Increasing the x-ray tube voltage from 50 to 125 kV increases both the total x-ray tube output (air kerma) as well as the average photon energy (i.e., penetrating power). For both of these reasons, increasing the x-ray tube voltage by this amount required a 30 fold reduction in the mAs to maintain the same amount of radiation incident on the image receptor. The dynamic range of the image obtained at 50 kV was 160 (i.e., L = 2.2), whereas the dynamic range of the image obtained at 125 kV was markedly reduced to 50 (i.e., L = 1.7). In radiology, it is generally true that high x-ray tube voltages (i.e., increased photon energies) reduce the dynamic range in radiographic images, and vice versa.
The AP skull radiographs shown above were also obtained at the table top using 65 kV (left) and 125 kV (right). As expected, the low kV image is markedly superior to the high kV image because of the substantially lower levels of scatter. To achieve the same receptor dose, the mAs for the 125 kV image on the right was reduced by a factor of about 15 – the 125 kV image was obtained using 0.66 mAs, whereas the 65 kV image was obtained using 10 mAs, and where both resulted in an image receptor radiation intensity of 5 uR (i.e., S ~ 200).
The lateral skull radiographs shown above were also obtained at the table top using 60 kV (left) and 125 kV (right). Note that one can use a lower kV (60 kV) on the lateral skull than the AP (65 kV) because the lateral is generally thinner and therefore easier to penetrate. As expected, the low kV image is markedly superior to the high kV image because of the substantially lower levels of scatter.To achieve the same receptor dose, the mAs for the 125 kV image on the right was reduced by a factor of about ~25 – the 125 kV image was obtained using 0.5 mAs, whereas the 60 kV image was obtained using 12 mAs, with both resulting in an image receptor radiation intensity of 10 mR (i.e., S ~ 100).