Class Notes 09.22.08

Class notes 09.22.08

Heel effect on density • The heel effect refers to a reduction in the x-ray beam intensity toward the anode side of the x-ray. • For a given field size, the heel effect is less prominent with a longer sourceto-image distance. Processing – effect on density Radiographic density will increase: 1. Increased developer time 2. Increased developer temperature 3. Increase in chemical concentration 4. Extraneous light fog 5. Increase in chemical fumes Radiographic contrast Contrast is the difference in Optical Density between adjacent anatomical structures contrast is the result of difference in attenuation of the x-ray beam as it passes through various tissues of the body. KVp is the primary controlling factor of contrast. kVp vs. mAs • Both affect film density • KVp affects film contrast ○ Low kVp  High contrast  Short scale • Black and white ○ High kVp  Low contrast  Long scale Primary factor – radiographic contrast 1. KVp is the primary factor in the control of radiographic contrast. 2. KVp provides the quality of radiation that is needed to penetrate the anatomic part. 3. KVp is formulated based on anatomic part thickness. a. 2 kVp X cm thickness + 30 kVp  high contrast b. 2 kVp X cm thickness + 40 kVp  low contrast 4. Radiographic contrast will effect radiographic density. a. ↑ kVp 15% = 2x the density b. ↓ kVp 15% = ½ the density Changes in kVp can be made to maintain radiographic density; however, changes in kVp will affect radiographic contrast which may change the visibility of detail. Therefore: ↑ 15% of original kVp, decrease the mAs by ½ ↓ 15% of original kVp, increase the mAs by 2x

Class notes 09.22.08 Reason for any changes in kVp are principally based on reducing patient motion by decreasing exposure time. Subject Contrast The differences in the anatomic part thickness & tissue density will determine the amount of radiant energy that will be attenuated. The attenuation of the material being irradiated will produce the radiographic contrast that will provide the visibility of detail. Factors: subject contrast Patient thickness (part thickness) Tissue mass density (tissue density) Effective atomic number Object shape Kilovolt peak (kVp) kVp Underpenetrated – optimum – overpenetrated High contrast/ short scale/ low kVp Low contrast/ long scale/ high kVp mAs Over exposed/ under exposed Influencing factors of radiographic contrast 1. Density 2. Distance 3. Intense screens 4. Grids 5. Beam limit device 6. Patient consideration 7. Processing 8. Contrast media