image quality radiographic resolution. demelza green jan 2004 technologies for imaging
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Image QualityRadiographic Resolution.
Demelza Green Jan 2004
Technologies for Imaging.
Radiographic Resolution
The resolving power of a radiograph.
How close structures can be and still appear separate - fine detail.
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
Noise
Noise - contributes no useful diagnostic information and serves only to detract from
the quality from the image.
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.
Unsharpness
• Three Types–Geometric.–Movement.–Photographic.
Unsharpness
• SOD - source object distance
• SID - source image (receptor) distance
• OID - object image (receptor) distance
Geometric Unsharpness (Ug)
• Size of focal spot.
• Image magnification -
• Geometric unsharpness =
• Poor film/screen contact.
SIDSOD
Focal spot size x OID
SOD
Movement unsharpness (Um)
• Exposure time.
• Subject motion - voluntary and involuntary.
• Immobilisation.
Photographic Unsharpness (Up)
• Screen and film crystal size
• Duplitised screens/ emulsions (crossover)
• Reflective screen layers.
Unsharpness
Total unsharpness = √ Ug
2 + U m 2+Up2
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.
Radiographic Contrast
Subject Contrast Film/Receptor Contrast
KVp Film Type
Tissue composition Direct exposure/ intensifying screens
Contrast Medium Processing Conditions
Contrast
• High Contrast film– Black/white– CXR for bony mets or rib fractures.
• Low Contrast film– Shades of grey– CXR for lung lesions or chest infections.
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
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.
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
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.
Contrast
• Kilovoltage.
Low KVp decreased penetration
more absorption
more density differences - high contrast
Contrast
• Kilovoltage
High KVp increased penetration. less absorption. less density
differences - low contrast. Increase in scatter -
unwanted interactions with film.
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
Scatter radiation
Digital Imaging
• Define spatial resolution.
• Describe factors influencing spatial resolution.
• Fully describe the concept of windowing.
• Define the terms window width and window level.
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.
Digital Resolution
• Typical TV monitor 2.5 lp/mm
• Radiographic Film 10 lp/mm need a tv monitor martix of at least
2048x2048 - fine line monitors - expensive.
• Resolution poor.
Windowing
Grey
Levels
+ 1000
- 1000
0
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.
Windowing
• Window Width (WW) = number of grey levels displayed.
• Window level (WL) = mid point of the range chosen.
Windowing
Soft Tissue.
white
Black
+100
0
-100
WW = 200
WL = 0
Windowing
Lung Tissue.
white
Black
-300
-600
-900
WW = 700
WL = -600
Windowing
Bone.
white
Black
+800
450
+100
WW = 700
WL = 450
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.
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.
Noise
• System Noise.
• Less of a problem than image noise.
• Arises from the processing.
• Effected by - phosphor conversion fluctuations,
laser beam scanning,
ADC.
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