1. Chapter 10 Exposure Mathematics

2. Exposure Mathematics u We have seen in Chapter Seven that there are several exposure factors that are predetermined before making an x- ray exposure. u These factors are supplied to us by exposure charts (which usually require slight alterations for each particular machine).

3. Exposure Mathematics u However, there are times when, for one reason or another, we may want to change a factor in a technique that has consistently produced good results. u For example, we may want to shorten the exposure time if there is danger of motion or we may want to increase the film-anode distance to eliminate distortion.

4. Exposure Mathematics u In cases like these we must compensate for changing one factor by changing a second.

5. The Inverse Square Law u The Inverse Square Law states that the intensity of radiation decreases in proportion to the square of the distance. u Original Intensity of Radiation = New F.A.D. 2 New Intensity of Radiation Old F.A.D. 2

6. The Inverse Square Law u A simple example of this is that doubling the FAD results in one-fourth of the original intensity of radiation and tripling the FAD results in one-ninth of the original intensity. u Many of the rules that follow have their basis in the Inverse Square Law and can be better understood if studied with the rule in mind.

7. Milliamperage-Distance Relationship u The milliamperage (MA) requirements are directly proportional to the square of the focal film distance (D). Original MA = Original D 2 New MA New D 2 u For example: l To compensate for doubling the film anode distance, we could increase the milliamperage four times.

8. Milliamperage-Time Relationship u The milliamperage (MA) requirements are inversely proportional to the time of exposure (S). Original MA = New S New MA Original S u For example: l To compensate for using one-half of the charted exposure time, we could double the mlliamperage.

9. Time-Distance Relationship u The exposure time (S) requirements are directly proportional to the square of the focal film distance (D). Original S = Original D 2 New S New D 2 u For example: l To compensate for doubling the film anode distance, we could increase the time of exposure four times.

10. Milliamperage-Time-Distance Relationship u The milliamperage and time (MAS) requirements are both directly proportional to the square of the focal - film distance (D). Original MAS = Original D 2 New MAS New D 2 u This is actually a combination of the three relationships described above.

11. Milliamperage-Time-Distance Relationship u For example: u To compensate for doubling the film anode distance, we would be required to increase the MAS by four times (by either increasing the milliamperage, increasing the exposure time, or increasing both to provide the require MAS).

12. Kilovoltage-Milliamperage-Time Relationship u There is no definite mathematical relationship between the kilovoltage and the MAS. u There are available various charts to follow when altering these factors, but the film latitude makes the following rough estimations practical:

13. Kilovoltage-Milliamperage-Time Relationship u A decrease of 10 KV requires twice the original MAS u A decrease of 15 KV requires four times the original MAS u A decrease of 28 KV requires 10 times the original MAS u An increase of 10 KV requires 1/2 the original MAS u An increase of 15 KV requires 1/4 of the original MAS

14. Kilovoltage-Milliamperage-Time Relationship u One point that should be remembered is that an increase in kilovoltage will cause a decrease in contrast even though the MAS may be altered to maintain average density. In the same manner a decrease in kilovoltage results in an increase in contrast even though the MAS is changed to maintain correct density.