RADIATION PROTECTION IN DIAGNOSTIC RADIOLOGY

1. RADIATION PROTECTION IN DIAGNOSTIC RADIOLOGY Chapter 3 Part 1 : Radiography

2. Content • Intensifying screen structure and characteristics • Screen film combination • Radiographic film structure and characteristics • Anti scatter grid • Darkroom • Viewing Box

3. Overview • To become familiar with basic knowledge of the component that form the radiographic chain.

4. Topic 1 : Intensifying screen structure and characteristics

5. Primary beam attenuation and latent image Film, fluorescent screen or image intensifier Scattered radiation « Latent » radiological image Bone X Soft tissue Air Primary collimation Antiscatter Grid Beam intensity at detector level

6. Intensifying screen Layer of material used in conventional radiography to : • Convert the incident X-rays into radiation more suitable for the radiation-sensitive emulsion of the radiographic film (X-ray  light photons) • Reduce the patient exposure needed to achieve a given level of film blackening • Reduce the exposure time as well as the power of the x-ray generator (cost savings) • Increase photoelectric effect  better use of the beam energy (image formation)

7. Intensifying screen structure (I) • Supporting Base (mainly polyester material) • chemically neutral, resistant to X-ray exposure, flexible, perfectly flat • Reflecting layer (Titanium dioxide - TiO2) • a crystalline compound reflecting backward photons to sensitive emulsion • Fluorescent layer (polymer) • crystals dispersed in a suspension of plastic material • Protective overcoat • colorless thin film avoiding abrasions of fluorescent layer due to the use of screen

8. Intensifying screen structure (II) (Incident X-ray beam) Supporting Base (240 m) Screen Reflecting layer (25 m) Fluorescent layer (100 to 400 m) Protective overcoat (20 m) (Light-sensitive film)

9. Intensifying screen structure (III) • The fluorescent layer (luminophor crystals) should : • be able to absorb the maximum quantity of X-rays • convert the X-ray energy into light energy • match its fluorescence with the film sensitivity (color of emitted light) • Type of material : • Calcium Tungstate (CaWO4) (till 1972) • Rare earth (since 1970) (LaOBr:Tm) (Gd2O2S:Tb) more sensitive and effective than(CaWO4)

10. Intensifying screen characteristics (I) • IF (Intensifying Factor): ratio of exposures giving the same film optical density, with and without screen • 50 < IF < 150 (depending on screen material and X-ray beam energy) • QDE (Quantum Detection Efficiency): fraction of photons absorbed by the screen • 40% for CaWO4 < QDE < 75% for rare earth (depending on crystal material, thickness of fluorescent layer and X-ray spectrum) •  (Rendering coefficient): ratio of light energy emitted to X-ray energy absorbed (%) • 3% for CaWO4 < < 20% for rare earth • C (Detection Coefficient): ratio of energy captured and used by the film to energy emitted by the crystal (%) • Cis maximum for screens emitting in UV color wave length  90%

11. Intensifying screen characteristics (II) Sensitivity of a Conventional Film BaSO4:Eu,Sr YTaO4:Nb BaSO4:Pb Relative Sensitivity of Film CaWO4 250 300 350 400 450 500 550 600 UV Blue Green

12. Intensifying screen characteristics (III) Intensifying factor: ratio of exposures giving the same film optical density, with and without screen 175 150 125 100 75 50 25 0 Gd2O2S LaOBr Intensifying factor CaWO4 kV 50 60 70 80 90 100 110 120

13. Topic 2 : Screen film combination

14. Screen film combination • Sensitivity (screen film): The quotient K0/Ka, where K0 = 1 mGy and Ka is the air kerma free-in-air for the net density D = 1.0, measured in the film plane • Screen film system: A particular intensifying screen used with a particular type of film • Sensitivity class: Defined range of sensitivity values of a screen film system • Singleemulsion film: One coated film used with one intensifying screen • Double emulsion film: A double coated film used with a couple of intensifying screens • Screen film contact    Quantum mottle

15. Screen film combination performance • Spatial Resolution: capability of a screen film combination to display a limited number of line pairs per mm. It can be assessed by the Hüttner resolution pattern. • Modulation Transfer Function (MTF): description of how sinusoidal fluctuations in X-ray transmission through the screen film combination are reproduced in the image • Noise spectrum: component of noise due to intensifying system (screen film) • Quantum noise, Screen noise, Granularity • Quantum Detection Efficiency (QDE): the quotient of signal to noise ratios (SNR) of radiographic image and “latent” image

16. Screen film combination performance • Identification of screen by type and format • type mismatch (use of different types of screens) FOR THE SAME FORMAT is not ADVISABLE • Screen film contact • loss of spatial resolution • blurred image • Cleanliness • Inter cassette sensitivity

17. Topic 3 : Radiographic film structure and characteristics

18. Radiographic film(structure and characteristics) • Protective layer (outer surface) • Sensitive layer (~20µ) • Base material (transparence and mechanical resistance) (~170µ) • Binding (base-sensitive layer) or anti cross-over layer • Filtering layer • Sensitivity class

19. Protective layer Film emulsion Binding layer Anti cross-over (optional) Filter Supporting Base Single emulsion film Radiographic film structure

20. Film Developing 1)نورباعث يونيزه شدن AgBr شده و يون Ag+ ايجاد مي كند. 2) وقتي فيلم در Developer قرار گرفت يونAg+ احيا شده وذرات Ag كه سياه رنگ مي باشند بدست میآیند. 3)فيلم در fixer كه محلول سخت كننده است قرار مي گيرد، ذرات AgBr شسته شده تا ديگر نور روي فيلم اثر نگذارد.

21. Optical Density • سياهي امولسيـون فتوگرافي دانسيته نام دارد. • وقتي فيلم اكسپوز شده روي Viewingbox قرار مي گيرد Li شدت نور وارد شده به فيلم و Ltشدت نور عبور كرده از فيلم مي باشد و دانسيته D از لگاريتم نسبت انتقال بدست مي آيد. • اگر شدت نور از فيلم عبور كند دانسيته برابر Log10 =1 مي شود. • اغلب اطلاعات تشخيصي دانسيته بين 0.5 تا2 را دارند. • اختلاف بين دانسيته دو نقطهDD بنام كنتراست راديوگرافي ميباشد. اين كنتراست باعث تشخيص تغييرات توسط راديولژيست ميشود.

22. Characteristic curve of a radiographic film Optical Density (OD) Saturation D2 Visually evaluable range of densities  = (D2 -D1) / (log E2 - log E1)  The  of a film : the gradient of the «straight line» portion of the characteristic curve D1 Normal range of exposures Base + fog Log Exposure (mR) E1 E2

23. Film sensitometry parameters • Base + fog: The OD of a film due to its base density plus any action of the developer on the radiographically unexposed emulsion • Sensitivity (speed): The reciprocal of the exposure value needed to achieve a film net OD of 1.0 • Gamma (contrast): The gradient of the straight line portion of the characteristic curve • Latitude: Steepness of a characteristic curve, determining the range of exposures that can be transformed into a visually evaluable range of OD

24. Comparison of characteristic curves (OD) (OD) Film A Film A Film B Film B Log Exposure (mR) Log Exposure (mR) Film A is faster than Film B Film A and B have the same sensibility but different contrast Film A and B have the same contrast

25. 11 12 13 16 17 18 20 21 10 14 15 19 1 2 3 4 5 6 7 8 9 Sensitometric strip Sensitometry: A method of exposing a film by means of a light sensitometer and assessing its response to exposure and development

26. Topic 4 : Anti-scatter grid and grid performance parameters IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

27. Anti-scatter grid (I) • Radiation emerging from the patient • primary beam : contributing to the image formation • scattered radiation : not reaching the detector but contributing to the patient dose • the grid (between patient and film) eliminates most of scattered radiation • stationary grid • moving grid (better performance) • focused grid • Potter-Bucky system

28. Anti-scatter grid (II) Source of X-rays Patient Scattered X-rays Lead strip Film and cassette Useful X-rays

29. Grid performance parameters (I) • Grid ratio • Ratio of the height of the strips to the width of the gaps at the central line • Contrast improvement ratio • Ratio of the transmission of primary radiation to the transmission of total radiation • Grid exposure factor • Ratio of total radiation without the anti-scatter grid to that with the anti-scatter grid placed in the beam for a similar density

30. Grid performance parameters (II) • Strip number • The number of attenuating lamella per cm • Grid focusing distance • Distance between the front of a focused grid and the line formed by the converging planes

31. Example of anti-scatter grids (grid ratio) Grid : C Grid : A Grid : B D h    1 h Grid ratio : r = = 5 < r < 16 tg  D • Grid A and B have the same strip number • Grid B and C have the same interspace between the lamella

32. Grid selectivity (II) 100 90 80 70 60 55 50 45 40 35 30 25 20 15 10 5 0 • A grid with r = 12 transmits 5% • of scattered radiation • A grid with r = 16 transmits 3.8% • N.B. : slight difference % of scattered beam transmitted 5% 3.8% r 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

33. Grid focusing error(virtual increasing of grid shadow) X-Ray source (too far) X-Ray source (too close) Grid Film and cassette grid shadow deformation (applicable to both cases)

34. Grid focusing error(leading to 25% of beam loss)

35. Grid out of center(virtual deformation of grid shadow) Lateral shift X-Ray source Film and cassette Grid Grid shadow

36. Grid focusing error due to lateral shift(leading to 25% loss of X-ray beam)

37. Topic 5 : Darkroom and viewing box

38. Darkroom characteristics • Safelight • number (as low as possible), distance from the table • type and colors of filters • bulb color (red) • power (< 25 W) • External light tightness • Room temperature < 20° • Film storage conditions

39. Viewbox characteristics Since the viewing conditions are essential for a good interpretation of the diagnostic images, the viewing conditions must be optimal • Cleanliness of external/internal surface • Brightness • homogeneity of different viewing boxes: 1300 - 2000 cd/m2 • homogeneity within the same viewing box • Coloring • color mismatch must be avoided • Environment • ambient light level : 50 lux maximum

40. 5 7 0 0 5 8 1 0 5 6 1 0 6 1 1 0 6 2 0 0 6 1 3 0 5 9 2 0 5 8 6 0 6 0 9 0 5 9 2 0 Viewbox brightness EXAMPLE OF MEASUREMENTS CORRECT CONFIGURATION (cd/m2)

41. BLUE COLOR WHITE COLOR 5 7 0 0 5 8 1 0 3 5 1 0 3 8 7 0 6 2 0 0 4 1 6 0 5 9 2 0 5 8 6 0 2 1 5 0 3 1 1 0 Viewbox color and brightness WRONG CONFIGURATIONS (cd/m2)

42. Where to Get More Information • Physics of diagnostic radiology, Curry et al, Lea & Febiger, 1990 • Imaging systems in medical diagnostics, Krestel ed., Siemens, 1990 • The physics of diagnostic imaging, Dowsett et al, Chapman&Hall, 1998