Sensitometer

how the film emulsion responds to the xray exposure.

1. SENSITOMETRY DEEPAK NEGI MRIT TMU

2. INDEX SENSITOMERTY AND HISTORY DENSITY (TRANSPARECY AND OPACITY) ,OPTICAL DENSITY XRAY EXPOSURE, RELATIVE EXPOSURE, LOG RELATVE EXPOSURE THE CHARACTERSTIC CURVE HOW THE CURVE IS PRODUCED? PLOTTING THE CURVE TOE OF THE CURVE(BASE DENSITY,FOG,THRESHOLD) REGION BETWEEN TOE AND SHOULDER (CONTRAST, LATITUDE, SPEED) SHOULDER OF THE CURVE APPLICATION OF SENSITOMETRY

3. SENSITOMETRY The scientific study of the response to exposure of the photo sensitive material. FERDINANAD HURTER and VERO CHARLES DRIFFIELD .( 1890) A characteristic curve shows the representation of the sensitometry . Photo sensitive material ( xray films) are used to record the invisible xray images. The performance of the image recording system is tested that how well an emulsion is responding to the exposure we are giving.

4. 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 OPACITY OPTICAL DENSITY

5. TRANSPARENCY The number of transmitted light passing through a film. TRANSPARENCY = intensity of transmitted light intensity of incident light

6. OPACITY It is the ability to stop light. OPACITY = intensity of incident light intensity of transmitted light NOTE:- The action of radiation is to blacken rather then lighten the film. Higher density means a blacker film.

7. OPTICAL DENSITY Difference in optical density is called contrast, where optical density is the no of shades of grey in an Image .(degree of blackening) Expressed as the ratio of incident light to transmitted light. O.D varies from 0.20 -3.5 Density increases with increase in exposure .

8. Log concept:- C onveniently express large differences in numbers on a small scale . ex= like density of 1000 =3 (when converted by log concept) The human eyes respond different tones in a way which is approximately logarithmic . ex= a density of 2 looks twice as dark as 1 . Deals with the addition or superimposition of films.

9. 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 RELATIVE EXPOSURE LOG RELATIVE EXPOSURE

10. RELATIVE EXPOSURE In this lt 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 xray 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

11. LOG RELATIVE EXPOSURE The range of exposure reaching different areas of the film screen system is very great . To make it easy we use log concept.

12. 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 H and D curve after the 2 pioneers in sensitometry . FERDINANAD HURTER AND VERO CHARLES DRIFFIELD

13. How the curve is produced? Expose and process a film. Measuring the densities produced. Plotting the curve .

14. Exposing and processing The film is irradiated with a series of exposure and the progress is recorded in steps . WEDGE FACTOR:- the relationship between one exposure and the next is known as the WEDGE FACTOR. The smallest exposure made shall have no measurable effect. The heaviest exposure made shall activate all the silver halides. Exposure values are achieved in 2 ways . Time – scale sensitometry . intensity-scale sensitometry .

15. Time – scale sensitometry Each area on the film is exposed to same intensity but duration of exposure is varied. tube kv , focal spot setting, FFD and mA INTENSITY- scale sensitometry Using variation along intensity scale.

16. X-RAY EXPOSURE To plot the curve we need intensity variation and this is achieved by a steeped wedge made up of Al. The different attenuation caused by the wedge provides the required range of intensities for the graph. Visible light exposure = sensitometer

17. DENSITOMTER The optical density of each exposed area on the film is measured by a densitometer. Density is read out from a digital display using pointer and calibrated scale.

18. Plotting the curve

19. The characteristic curve The graph has 2 sharp curves T he 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, film latitude, exposure latitude, speed and sensitivity The region to the right og the shoulder is determined by Maximum density, Reversal

20. The T oe 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 and threshold

21. Base density The density produced by exposure when it transmits the polyster base. FOG The density produced by development process which has no intentional exposure. Causes:- Ageing of the film Poor conditions for storage chemical fog Net Density = Gross density – Gross fog

22. Threshold The region where the film emulsion begins to respond to the exposure.

23. The region between toe and shoulder The straight line part of the curve . The most important feature is the change in density as the exposure changes . 3 majors factors that affect this region are. Contrast , Latitude , Speed and sensitivity.

24. Contrast Contrast generates difference in image density . The greatest density difference is between toe and shoulder region. The slope or gradient of the curve is used as a measure of contrast. G= tan A where tan A is the trigonometric tangent of A Gamma = the tangent of the angle of slope of the straight line part of the curve . Maximum gamma = The point of inflection in the curve from being concave to convex.

25. Average Gradient The area of the slope represented by X and Y is the area most important for diagnostic purpose. X has a net density of 0.25. Y has a net density of 2. G= Dy-Dx (2-0.25=1.75) This is the useful density value as most of the radiographs fall under this value. Factors affecting Average Gradient :- Film emulsion single or double coated film Film processing intensifying screen

26. AVERAGE GRADIENT

27. latitude The expression of the tolerance of the system to extreme conditions of exposure . shows the ability to record successfully a wide range of exposure. It of 2 types Film latitude Exposure latitude

28. FILM LATITUDE

29. EXPOSURE LATITUDE The tolerance of a film to errors in the selection of the exposure factors ( kVp , mA, mAs ). It also provides room for error on selection of exposure factors. Depends on film latitude and subject contrast . 2 factors that define Exposure latitude are:- a- The median of log exposure value= This represents the difference between maximum and minimum log exposure values and depends on the total quantity of the xrays . b-The range of the log exposure values:- This represents the difference between maximum and minimum log exposure values and depends on the quality of the xray and subject contrast.

30. EXPOSURE LATITUDE

31. Continue:-

32. SPEED AND SENSITIVITY Sensitivity refers to the exposure required by a film to produce a net density of 1. A high sensitive or high speed system requires less exposure . The higher the speed of the system , the further to the left the curve appears. Affected by:- immersion time , solution temperature, chemical activity.

33. Speed and Sensitivity

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

35. 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:- 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 T emperature variations . Note- This maximum density of the film is used in mammography .

36. MAXIMUM DENSITY

37. 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. T esting 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 .

38. REVERSAL

39. Use of characteristic curve Information G ross 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 C omparision of screen systems Duplication of Radiographs

40. References Chesney’s Radiographic imaging . Research papers:- Analysis of Sensitometry in Radiographic films using Optical Density measurements. Sensitometric curve of radiographic films by xray fluorescence.