Element of Image Sharpness

1. Element of Image Sharpness • Recorded detail is the sharpness of the lines of the image. • Distortion is the misrepresantation of of the true size or shape of the image compared to the object.

2. What impacts the recorded detail? • Focal spot size • Object to Image Distance (OID) or Object to Film Distance (OFD) • Source to Image Distance (SID) or Focal Film Distance (FFD) • Screen to Film Contact & Screen Speed • Motion

3. What impacts Distortion? • Object Image Distance (OID) or Object Film Distance (OFD) • Source Image Distance (SID) or Focal Film Distance (FFD) • Object Alignment • Film Alignment • Central Ray Alignment

4. Selecting the Focal Spot • The operator has total control over the focal spot. • The “S” indicates the small focal spot is selected.

5. Selecting the Focal Spot • Selecting “S” will mean the voltage will be applied to the small filament. • Selecting “L” will apply the voltage to the large filament.

6. Focal Spot Size Selection • The x-ray beam is more tightly focused when the small focal spot is selected. • The edge sharpness is improved because of less shadow or penumbra.

7. Small Focal Spot • Advantages • Reduced penumbra • OFD can be increased with minimal loss of detail. • Disadvantages • Lower mA selection due to tube heating • Longer exposure times that can cause patient motion

8. Testing the Focal Spot Size • Part of Quality Control is testing the affective focal spot. • Using the same SID, mAs and kVp, exposures are made with the small and large focal spots.

9. Testing the Focal Spot Size • The film is processed • Three horizontal and vertical bars will be seen on the film. • The number next to the bars will be the line pairs/ millimeter.

10. Testing the Focal Spot Size • The small should produce 10 to 11 lp/mm. • The large focal spot should resolve 5 to 6 lp/mm.

11. Screen and Film Speed • The type of film and materials used in the screens will determine the speed of the film & screen combination. • Small phosphors in the screens and grain in the film will produce sharper images.

12. Screen and Film Speed • Speed is determined by the system’s RSV or Relative Speed Value. • High RSV needs less radiation but have lower recorded detail.

13. Focal Spot and OFD • When the object is far away from the film more shadows of the edges are produced resulting in a loss of sharpness. • The shadow is greater with the large focal spot. • Using the small focal spot will reduce the unsharpness.

14. Inverse Square Law • The intensity of the beam is inversely proportional to the square of the distance. • This law can be used to compute patient or staff radiation exposure. • If you double your distance from the source, the exposure is reduced 75%.

15. Inverse Square Law • The intensity of the beam is inversely proportional to the square of the distance. • To adjust the mAs for a different SID, the following formula is used. Old mAs x New Distance2 • New mAs = ------------------------ Old distance 2

16. Inverse Square Law Exercise If 5 mAs was used at a 40” SID, what mAs would be used at 20” SID? Old SID: 40” New SID: 20” Old mAs: 5 New mAs: ?

17. Inverse Square Law 5 mAs x 20” 2 New mAs =------------------------- 40” 2 5 x 400 mAs = --------------------- = 1.25 mAs 1600

18. SID and Sharpness • Longer SID= Less Unsharpness

19. OFD & Focal Spot Exercise • A P-A view of the hand is taken at a 20” SID with the hand resting on the cassette. • Notice how close the tube is to the hand. Would the skin dose be more than at 40” SID?

20. OFD & Focal Spot Exercise • A second exposure is made with the hand raised 3.5” above the film. The SID remains at 20”. • Would the patient exposure be increased or decreased? • Increased!!!

21. OFD & Focal Spot Exercise • A films is taken with the large focal spot with the two Object to Film Distances. • A film is taken with the Small Focal Spot. • The images are compared.

22. OFD & Focal Spot Exercise • First lets compare the films taken at 20” SID to the same views at 40” SID. • Notice how the image is larger when the hand is farther away from the film.

23. OFD & Focal Spot Exercise • 1. Did the reduced distance have an impact on image resolution with the large focal spot? • Yes

24. OFD & Focal Spot Exercise • 2. What happened when the hand was moved away from the film or the OFD was increased? • The image lost resolution.

25. OFD & Focal Spot Exercise • 3. Did the reduced distance have an impact on image with the small focal spot? • Yes be the impact was less.

26. OFD & Focal Spot Exercise • When comparing the large and small focal spots, look at the images with increased object to film distance.

27. Unsharpness Factor • Using this formula we can compute the loss of geometric unsharpness caused by focal spot and Object to Film Distance problems. Focal Spot X OID • Unsharpness = ---------------------------- SOD

28. Unsharpness Factor Image A Small Focal Spot short OID 20” SID Unsharpness Factor = 1mm x 5 mm /470mm = 0.010 Image B Small Focal Long OID 20” SID Unsharpness Factor = 1mm x 87mm/390mm =0.223

29. Unsharpness Factor Image A Large Focal Spot Short OID 20” SID Unsharpness Factor = 2mm x 5 mm /470mm = 0.0212 Image B Large Focal Long OID 20” SID Unsharpness Factor = 2mm x 87mm/390mm =0.4461 Very poor image

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