1. CHARACTERISTIC CURVE DR. Muhammad Ibrahim Department of Radiology, FTH Gombe 19/07/2022 1 1

2. SENSITOMETRY AND HISTORY SENSITOMETRY AND HISTORY PHOTOGRAPHIC DENSITY PHOTOGRAPHIC DENSITY X-RAY EXPOSURE X-RAY EXPOSURE THE CHARACTERISTIC CURVE THE CHARACTERISTIC CURVE HOW THE CURVE IS PRODUCED? HOW THE CURVE IS PRODUCED? PLOTTING THE CURVE PLOTTING THE CURVE PARTS OF THE CURVE PARTS OF THE CURVE APPLICATION OF THE SENSITOMETRY APPLICATION OF THE SENSITOMETRYOutline 2 2

3. SENSITOMETRY AND HISTORY Ferdinand Hurter and Vero Charles DriffieldSensitometry as a photographic science has been credited to two Americans, Ferdinand Hurter and Vero Charles Driffield, who first published the characteristic curve in England in 1890. The main interest in radiography is how the film will react to (apparently) x-radiation, although usually the real interest is how the film will react to the light produced by the Intensifying screen. 3 3

4. SENSITOMETRY Sensitometry is the scientific study of light-sensitive materials. The performance of the image recording system is tested on how well an emulsion is responding to the exposure we are giving. Sensitometer is an instrument that can be used to drive the characteristic curve. And used in measuring the sensitivity of radiographic films exposed with fluorescent intensifying screens. 4 4

5. Photographic Density When a film is exposed and processed a blackening effect is produced, Which is referred to as the density of the film. TRANSPARENCY (TRANSMISSION) OPACITY(reciprocal of transparency) OPTICAL DENSITY 5 5

6. TRANSPARENCY/TRANSMITTANCE The number of transmitted light passing through a film. If the intensity of the light measured with no film in the densitometer is given by I o, and the intensity measured at a particular location on a film is given by I, then the transmittance (T) of the film at that location is defined as follows: 6 6

7. OPACITY 7 7

8. OPTICAL DENSITY 8 8

9. 9 9 9

10. Radiographic Density 10 In routine radiograph, the useful range varies from 0.25 to 2.5 density. Density below 0.25 is due to the base and fog on the film

11. 11 Relationship between Optical Density (OD) and transmission (t) applicable to diagnostic radiology applications 11

12. 12 Relationship between Opacity, OD and Percentage of Light Transmitted by X-Ray Films of Various Densities 12

13. DENSITOMTER The densitometer is a simple device that shines white light onto each exposed area of the developed film and electronically measures the amount of light reaching the other side. 13

14. 14 The densitometer has a small sensitive area (aperture), typically about 3 mm in diameter, and measurements of OD correspond to that specific area of the film. Density is read out from a digital display using pointer and calibrated scale. Reads on a scale of 0-4 0 meaning that all light is transmitted 4 meaning that no light transmits through the film Typical diagnostic densities range between 0.25 to 2.5 DENSITOMTER 14

15. Densitometer 15

16. 16 Densitometer 16

17. Why is density expressed as a logarithm? There are three primary reasons 1st, logarithms conveniently express large differences in numbers on a small scale. 2nd the physiologic response of the eye to differences in light intensities is logarithmic. –E.g., A density of 2 looks twice as dark as 1. 3rd, Deals with the addition or superimposition of films. If films are superimposed, the resulting density is equal to the sum of the density of each film. 17

18. X-RAY EXPOSURE In conventional, x ray interacts with the recording system. A film screen system has its intensifying screen exposed to xray. A non screen system has its film directly exposed to x ray. So sensitometer is used to measure the rate of exposure and effect on emulsion. –2 types of exposure are used for the illustration of the characteristic curve 1. RELATIVE EXPOSURE 2. LOG RELATIVE EXPOSURE 18

19. RELATIVE EXPOSURE It is considered that if an exposure time of 0.01s is used to expose an area A and 0.02s is used for area B, than the area B is considered to receive twice the x-ray exposure to area A. The relative exposure is 2 at B compared with 1 at A. Different exposure of varying kVp and mAs are made. The smallest area of exposure is used as Baseline 19

20. LOG RELATIVE EXPOSURE Use of a logarithmic scale allows a very wide range of exposures to be expressed in a compact graph. 20

21. THE CHARACTERISTIC CURVE A graph which illustrates the way in which a film or film screen system responds to different levels of exposure. (describing sensitometric behavior of a photographic recording system) The curve is a plot of Optical density against log relative exposure. Also known as; Sensitometric curve or H and D curve or after the 2 pioneers in sensitometry or D log E curve 2121

22. How the curve is produced? Characteristic curves are derived by Giving a film a series of exposures and processing it. Measuring the densities produced. Plotting the resulting density against the known exposure. 22

23. Exposing and processing The film is irradiated with a series of exposure and the progress is recorded in steps. Exposure values are achieved in 2 ways. 1. Time – scale sensitometry. 2. intensity-scale sensitometry. 23

24. Time – scale sensitometry Tube kVp mA mAs. –Each area on the film is exposed to same intensity but duration of exposure is varied. (e.g., with constant kVp and mA, doubling the time of exposure will double the mAs 24

25. Intensity- scale sensitometry Using variation along intensity scale. Intensity variation is achieved by a steped wedge made up of Aluminium. The different attenuation caused by the wedge provides the required range of intensities for the graph. An aluminium step-wedge. 25

26. Plotting the c haracteristic curve 26

27. The characteristic curve The graph has 2 sharp curves –The Toe and The shoulder The region of the toe is determined by –Base density, Fog, Threshold The region between the toe and the shoulder is determined by –Contrast, Gradient, latitude, Speed and Sensitivity The region to the right of the shoulder is determined by –Maximum density, Reversal 27

28. 28 The characteristic curve 28

29. The Toe region It is the underexposed area. Shows values from 0-0.6. There is hardly any change in the density to produce any photographic effect. The density of the toe is based on 3 things Base density Fog Threshold 29

30. Base density If a sheet of fresh unexposed film is taken out of the box and processed, it will have an optical density in the range from about 0.11 to 0.15. This optical density is that of the film base. Greater if the base has been tinted during manufacture. 30

31. FOG The density produced by development process which has no intentional exposure. In this case, unexposed sheet of film after processing may have an OD in the 0.13 to 0.18 range. Causes:- Ageing of the film Poor conditions for storage Chemical fog Safelight fog 31

32. Cont. Note that the total density of base plus fog is often referred to as basic fog or gross fog. Its value should not be greater than about 0.2 Net Density = Gross density – basic fog 32

33. Threshold The region where the film emulsion begins to respond to the exposure. On a correctly exposed chest radiograph, the subphrenic region usually receives an exposure which is at or below threshold. 33

34. The region between toe and shoulder The straight line or linear region of the curve. The most important feature is the change in density as the exposure changes. This is the important part of the curve in radiography. 3 majors factors that affect this region are. Contrast Film gamma and average gradient Latitude Speed or sensitivity 34

35. CONTRAST Contrast is the difference in density existing between various regions on the film. Radiographic contrast depends on subject contrast and on film contrast. The contrast of a radiographic film is related to the slope of the H & D curve. Regions of higher slope have higher contrast, and regions of reduced slope (i.e., the toe and the shoulder) have lower contrast. 35

36. 36 Film Gamma and average gradient 36

37. The average gradient is the slope of a straight line connecting two well- defined points on the H & D curve. The average gradient is measured between two specific optical densities on the curve. 37 Average gradient

38. 38

39. 39 Average Gradient cont. Factors affecting Average Gradient :- Film emulsion Single or double coated film Film processing Intensifying screen 39

40. LATITUDE Latitude refers to the range of exposure that will produce a diagnostic or usefully distinguishable range of OD on a film. Latitude is inversely proportional to contrast. The wider the latitude of a film, the greater the range of object densities visualized. Wide latitude has a wide gray scale or low contrast. Because many densities between totally black and totally clear are recorded. Such films are useful when both osseous and soft tissue must be viewed. Narrow latitude has a short scale or high contrast. 40

41. 41 Film Latitude 41 Latitude is of 2 types Film latitude Exposure latitude

42. The tolerance of a film to errors in the selection of the exposure factors (kVp, mAs). It also provides room for error on selection of exposure factors. Depends on film latitude and subject contrast. 42

43. 43 Two factors define Exposure latitude. Both represents the difference between maximum and minimum log exposure The median of log exposure value:- depends on the total quantity of the x-rays. The range of the log exposure values:- depends on the quality of the x-ray and subject contrast. Exposure latitude 43

44. 44 H & D curve illustrates screen-film systems A and B, which differ in their contrast. The slope of system A is steeper than that of system B, and therefore system A has higher contrast. The compromise of a high- contrast screen-film system (i.e., system A) is that it has reduced latitude. 44 Contrast

45. SPEED AND SENSITIVITY Ability of an IR to respond to low exposure The speed of a film-screen system is defined as the reciprocal of the exposure in roentgens required to produce a density of 1.0 above base plus fog density. Speed= 1/Roentgens A high sensitive or high speed system requires less exposure and result in lower patient doses. The higher the speed of the system, the further to the left the curve appears. Affected by:- –immersion time, solution temperature, chemical activity. 45

46. 46 Speed 46 68 140 The x-ray exposures required to reach this OD level are indicated for each system (vertical arrows). System A requires less exposure than system B to achieve the same OD; therefore, it is the faster, more sensitive of the two screen-film systems.

47. The region of the shoulder The upmost part of the graph. The region of overexposure. 2 factors that affect this region are:- Maximum Density Reversal 47

48. MAXIMUM DENSITY When the exposure to the film is greatest and a maximum level of density is achieved. Image contrast is 0. Depends on 2 things:- 1. Silver coating weight :- the greater the amount of silver halide the greater the density. Non-screen direct exposure film has higher Dmax than screen type xray film. 2. Processing condition:- Exhaustion of the developing agent and Temperature variations. Note- This maximum density of the film is used in mammography. 48

49. Maximum density 49 a.Characteristic curve showing the region of maximum density (Dmax). b. Characteristic curve showing the increase maximum density of direct exposure film.

50. Reversal It is a strange phenomena in which the film respond in the opposite way to normal, producing a reduction in image density as a result of increase in exposure. Testing of the reversal properties:- The film is removed from the cassette and exposed to bright light. A film which exhibits reversal will be almost transparent. 50

51. Reversal 51 Characteristic curve showing reversal properties

52. Use of characteristic curve Informations Gross fog (Basic fog) Threshold Contrast Latitude (film and exposure) Speed and sensitivity Maximum Density Reversal Uses Selection of films Quality control Selection of exposure factors Comparision of screen systems Duplication of Radiographs 52

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