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.