RADIOGRAPHIC DENSITY

1. RADIOGRAPHIC DENSITY Sergeo Guilbaud School of Radiologic Sciences

2. Radiographic Density Density is one of the two photographic properties that comprise visibility of detail.

3. Definition of Density • 1. Degree of overall blackness of the radiograph. • 2. Easiest of the prime technical factors to evaluate and adjust.

4. Definition of Density • 3. Major consideration in assessment of density is: a. Proper densities are visible throughout anatomical area of interest. b. Densities must be well within range of human visibility. (1) OD 0.25 - 2.50

5. Definition of Density • c. If choice must be made between insufficient density or excessive density, produce a darker image.

6. Controlling Factor of Density • 1. Milliampere seconds • A. Milliampere measures quantity of current passing through X-ray tube each time of exposure. • B. Determines quantity of X-rays produced during the exposure.

7. Controlling Factor of Density • 3. MA setting determines how hot filament gets controlling the number of electrons the filament releases. That number determines amount of X-rays produced during an exposure. • 4. Direct relationship between a number of X-rays produced and density of radiograph. • 5. Milliampere seconds affects only density.

8. Controlling Factor of Density • There are a wide variety of factors that affect radiographic density. • Factors are classified as: • Controlling • Influencing

9. Factors Controlling Radiographic Density • Milliampere-seconds • Relationship b/w mAs and exposure is directly proportional. • Recall that density is determined by the amount of silver deposited in the film emulsion due to film type, exposure conditions, exposure in (mR) and processing.

10. Factors Controlling Radiographic Density • The D log E sensitometric curve expresses the relationship b/w exposure and density with log-relative exposure plotted on the X axis and (D) plotted on the Y axis. • The important part is the portion b/w the toe and shoulder.

12. Factors Controlling Radiographic Density • Common misnomer in radiography is that doubling the exposure will double the density. • Not true b/c if the exposure to a film is increased, the density to that film will increase to a point.

13. Factors Controlling Radiographic Density • B/C density is determined by the amount of exposure a film receives, and since the exposure is directly proportional to mAs, mAs is used as the primary controller of radiographic density. • Hence as mAs increases, X-ray exposure increases proportionally and radiographic density also increases.

14. Factors Influencing Radiographic Density • kVp • Alters the intensity of the beam reaching the image receptor. • kV controls the energy of the beam and therefore the strength of the electrons striking the target of the tube for any given mAs. • Thus a change in kV alters the intensity of the beam when the mAs and other factors remain the same.

15. Factors Influencing Radiographic Density • Both the quantity and quality of the X-ray beam will vary significantly with changes in kilovoltage. • Kilovoltage has a tremendous impact on radiographic density. • A visible change in density can be detected with a 4-5% kVp change in the lower kVp range (30-50) • 8-9% kVp change in the 50-90 kVp range. • 10-12% kVp change in the 90-130 kVp range.

16. Factors Influencing Radiographic Density • In order for radiographers to have an orderly means of manipulating kVp as it relates to density, the 15% kV rule has been developed to maintain density.

17. Factors Influencing Radiographic Density • Distance (SID) • Alters the intensity of the beam reaching the image receptor, according to the inverse square law. mAs1 = (D1)2 mAs2 = (D2)2

18. Factors Influencing Radiographic Density • The inverse square law affects exposure in inverse proportions to the square of the distance. I1 = (D2)2 I2 = (D1)2

19. Factors Influencing Radiographic Density • Screens • Both film and screens alter the density. • When the intensifying screen’s phosphors convert X-ray photons to the light photons that will expose the silver halides in the film, they attenuate the intensity of the light that has become the imaging beam.

20. Factors Influencing Radiographic Density • Beam Limiting Devices • Limitation of the beam reduces the primary beam field size thus reducing the number of photons available. • This serves to reduce the amount of photons available for Compton’s interaction. • The effect of beam restriction depends on the number of photons reaching the image receptor not the amount of Compton’s scatter created.

21. Factors Influencing Radiographic Density • Beam Limiting Devices • Only under the following conditions are changes in technique required for beam limitation: • Large anatomical areas/parts • High kilovoltage techniques • Low grid efficiency • Non-grid examinations

22. Factors Influencing Radiographic Density • Grids • Absorb scatter which creates scatter on the image. • As the grid efficiency increases, the less density that is produced on the image. • Note: • 1. Grid errors also reduce the overall density on the radiographic image. • 2. It is not recommended to change the kVp b/c it may change the scale of contrast in the opposite direction thus negating the reason for grid use.

23. Factors Influencing Radiographic Density • Anode Heel Effect

24. References Bushong, S. Radiologic Science for Technologists, Physics, Biology, and Protection, 6th Edition, Mosby, 1997. Bushberg, et al. The Essential Physics of Medical Imaging, Williams & Wilkins, 1994. Carlton et al. Principles of Radiographic Imaging an Art and a Science, 3rd Edition, Delmar, 2001. Selman, J. The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles Thomas, 1994.