Sensitometry & characteristic curve

exposure factor and curve of exposure with related to image contrast and how this curve helpful in good quality images with exposure.

1. SENSITOMETRY & CHARACTERISTIC CURVE Radiology information by Rmidha

2. Describing photographic perfomance To compare and to evaluate one film or film screen system with another. Method - Characteristic curve Essential tool in sensitometry , scientific study of response to exposure of photosensitive materials.

3. Optical density Opacity = I0/ It Optical density – expressing opacity in logarithmic terms. Optical density= log 10 opacity = log10 (I o / I t ) D=Density I =Light incident on film I t =Light transmitted by the film

4. Log relative exposure Typical radiographic image – range of exposure at different areas of film. Expressed as logarithmic values. Range of 1 – 1000 becomes a range of 0 – 3.

5. Characteristic curve It is a graph which shows way in which a film or film screen system responds to different levels of exposure. Plots optical density (D)against log relative exposure (log E ). D log E curve Hurter and Driffield / H and D curve

6. STAGES OF CURVE 1. Exposing and processing the film. 2. Measuring the densities produced. 3. Plotting the curve

7. 1.Exposing and processing film Irradiate film with a series of exposures. Exposure series can be achieved in 2 ways : Time scale sensitometry – each area on film is exposed to same intensity but duration of exposure is varied. Intensity scale sensitometry _ Each area on film is exposd for same duration of time but intensity varied X- ray exposure : Al step wedge calibration done to find wedge factor. Wedge factor :- relationship with one exposure with next exposure. Visible light exposure : Sensitometer

8. 2. Measuring the densities produced Densitometer – Optical density of each exposed step on film strip Density is read out from a digital display or from an analogue display using pointer & calibrated scale.

9. 3. Plotting curve

10. Features of curve Curve is divided into 3 regions : 1. Region to left of toe Base density Fog Threshold

11. Region to left of toe Region of underexposure Base density Due to absorption of light as it is transmitted through base Fog : density produced by development of silver halide grains which have received no intentional exposure. Types : Storage fog, Safelight fog Threshold : As exposure increases , film emulsion begins to respond to exposure and density begins to rise above gross fog.

12. Region between toe and shoulder Contrast Latitude Speed & Sensitivity

13. Contrast Density difference and image contrast maximum where curve is steepest. Slope/ Gradient ( G) of curve - measure of contrast .

14. Gradient ( G) at a point on curve – slope of the tangent to curve. It angle of slope is A , G = tan A . Gamma - Value of G is constant along straightline . Point of Inflection : Point where curve changes from concave to convex Max gamma – tangent of angle of slope of curve at its point of inflection.

15. Average gradient G̅ = D Y – D X log E Y -log E X FACTORS : Film emulsion characteristics Film processing conditions Characteristics of intensifying screen For some direct exposure films, Point of inflection occurs at density above midrange value used in diagnostic imaging.

16. Latitude It refers way in which a film/ film screen system is able to record successfully wide range of exposures. FILM LATITUDE EXPOSURE LATITUDE

17. Film latitude =log E Y - log E X Average gradient = D Y - D X = Useful density range log E Y - logE x Film latitude

18. Exposure latitude Represents tolerance of a film / film screen system to errors in selection of exposure factors when radiographic exposure is made. Exp latitude = Film latitude – log exposure range

19. Speed & Sensitivity It is an expression of xray exposure required to produce a given image density. Higher speed of system, less exposure to produce given density. ( curve will be further to left ) Speed of system A = mAs for system B Speed of system B mAs for system A

20. How is relative speed assesed from curve ? Log exposure values corresponding to a net density of 1 is 1.4 and 1.8 Log E B – log E A = .4 E B /E A = antilog ( .4) = 2.5 System A is 2 .5 times speed of system B

21. Region to right of shoulder Maximum Density Reversal

22. Maximum density As film/ film screen is subjected to greater exposure ,a point is reached where all of silver halide grains in emulsion are reduced to silver.

23. 1. Silver coating weight 2. Processing conditions

24. Reversal Reduction in image density due to increase in exposure.

25. Testing for reversal property Film removed from cassette Exposed to bright light Film processed Normal films – black but film which exibits reversal – almost transparent.

26. References Chesney’s Radiographic Imaging – John Ball and Tony Price