1. Image Quality Radiographic Resolution.
Demelza Green Jan 2004
Technologies for Imaging.

2. Radiographic Resolution
The resolving power of a radiograph.
How close structures can be and still appear separate - fine detail.

3. Radiographic Resolution
Expressed as a measurement of line pairs per mm (lp/mm)
the number of line pairs resolved within a mm determines the quality of the recording medium.
A line pair is made of a line and a space.
Human eye is limited to resolve 5 lp/mm.
Radiographic film 10 lp/mm

4. Noise
Noise - contributes no useful diagnostic information and serves only to detract from the quality from the image.

5. Noise Quantum mottle = image noise.
Fluctuation in the quantity of photons that contribute to the image formation.
Image appear mottled or blotchy.
Result of fast screens that require less exposure - less photons.
Decreases image detail.

6. Unsharpness
Three Types
Geometric.
Movement.
Photographic.

7. Unsharpness SOD - source object distance
SID - source image (receptor) distance
OID - object image (receptor) distance

8. Geometric Unsharpness (Ug)
Size of focal spot.
Image magnification -
Geometric unsharpness =
Poor film/screen contact.
SID
SOD
Focal spot size x OID
SOD

9. Movement unsharpness (Um)
Exposure time.
Subject motion - voluntary and involuntary.
Immobilisation.

10. Photographic Unsharpness (Up)
Screen and film crystal size
Duplitised screens/ emulsions (crossover)
Reflective screen layers.

11. Unsharpness
Total unsharpness
= √ Ug2 + U m 2+Up2

12. Contrast What is contrast? The capacity of showing different densities
The whiteness of the bone against the blackness of the film and the range of greys in between.

13. Radiographic Contrast
Subject Contrast Film/Receptor Contrast
KVp Film Type
Tissue composition Direct exposure/ intensifying screens
Contrast Medium Processing Conditions

14. Contrast High Contrast film Low Contrast film Black/white
CXR for bony mets or rib fractures.
Low Contrast film
Shades of grey
CXR for lung lesions or chest infections.

15. Radiographic Contrast
Tissue Composition.
Composition and thickness of tissue affect absorption.
 create a range of densities (contrast)
anatomical structures with a wide range of tissue composition will produce high subject contrast.
Those with a similar range produce low subject contrast

16. Radiographic Contrast
Contrast media
for imaging areas of low subject contrast.
Positive contrast agents e.g. barium have high atomic no. and absorb more x-rays than the surrounding structures.
Negative contrast agents e.g. air have low atomic no. produce more density than surrounding structures.

17. Scatter.
when the x-rays transverse the body tissues either are absorbed or scattered or transmitted without interactions
scattered photons reaching the detector form a ‘fog’ on the radiographic which blurs the image and obscures the anatomical details  degradation of the image

18. Scatter
scattered photons can be absorbed by the body tissues  increase in patient’s dose
minimise the scattered radiation generated in the body tissues.
minimise the scattered radiation reaching the detector.

19. Contrast Kilovoltage. Low KVp decreased penetration more absorption
more density differences - high contrast

20. Contrast Kilovoltage High KVp increased penetration. less absorption.
less density differences - low contrast.
Increase in scatter - unwanted interactions with film.

21. Scatter Radiation
Using the minimum field size (collimation); the larger the irradiated volume the higher the scatter generated in the body; minimise the irradiated volume by correct collimation

22. Scatter radiation

23. Digital Imaging Define spatial resolution.
Describe factors influencing spatial resolution.
Fully describe the concept of windowing.
Define the terms window width and window level.

24. Spatial resolution
Matrix size - greater the matrix then smaller the pixel size improved spatial detail.
As with conventional radiography detail lost during imaging process e.g. on the plate.

25. Digital Resolution Typical TV monitor 2.5 lp/mm
Radiographic Film 10 lp/mm
 need a tv monitor martix of at least 2048x fine line monitors - expensive.
Resolution poor.

26. Windowing
There are 2000 levels of grey. Human eye cannot detect between small level changes.
Windowing is a contrast enhancing technique.
Allows us to pick a section within that scale and assign grey levels depending on what we are looking at.
+ 1000
Grey
Levels
- 1000

27. Windowing Window Width (WW) = number of grey levels displayed.
Window level (WL) = mid point of the range chosen.

28. Windowing
Soft Tissue.
white
WW = 200
WL = 0
+100
-100
Black

29. Windowing
Lung Tissue.
white
WW = 700
WL = -600
-300
-600
-900
Black

30. Windowing
Bone.
white
+800
450
+100
WW = 700
WL = 450
Black

31. Windowing
If we have a wide widow width contrast is poor, e.g. we will get a low contrast image.
A narrow WW gives grater contrast, e.g. a high contrast image.

32. Noise Quantum mottle = primary source of noise in CR/DR.
Is the visible density fluctuations on the image.
A result of fewer photons reaching the imaging plate to form the image.
Known as image noise.

33. Noise System Noise. Less of a problem than image noise.
Arises from the processing.
Effected by - phosphor conversion fluctuations,
laser beam scanning,
ADC.