Image quality


There are certain qualities of an image that affect each other and determine the quality of the displayed image:

  1. Contrast
  2. Resolution
  3. Noise

As well as:

  1. Unsharpness
  2. Magnification
  3. Distortion
  4. Artefacts

1. Contrast

Contrast is the difference in the displayed or image signal intensity between two areas of interest e.g. a lesion and background tissue. A high contrast image has a greater difference but a smaller range of greys. A low contrast image has a smaller difference (i.e. it's more difficult to make out different areas) but a larger range of greys.

High and low contrast

Subject contrast

Subject contrast is the ratio of the radiation intensities in different parts of an image due to the quality of the subject being imaged. The contrast is due to the differential attenuation by the tissues.

 

c ∝ (μ1 - μ2) x t
where:
c = contrast
μ = attenuation coefficient of object 1 and 2 in the material being imaged
t = thickness of the structure

 

From the above equation you can see that a higher contrast is achieved with:

  • Thicker structure being imaged
  • Greater difference between the attenuation of the two objects

 

Subject contrast

 

In the diagram tissue A absorbs 50% of the radiation incident upon it, B absorbes 90%. If there are 1000 photons for every element of the image then 500 photons will emerge from A and 100 from B (a ratio of 5:1).

As optical densities (the displayed shade in the image) vary with the log of the exposure log500 = 2.7 and log100 = 2.0 so the subject contrast has a difference in the logs of 0.7.

Factors affecting contrast

Linear attenuation coefficient of subject

The linear attenuation coefficient depends on the Compton and the photoelectric linear attenuation coefficient (LAC).

 

Compton LAC = ρ / E

Photoelectric LAC = ρZ3 / E3
where:
ρ = density
E = energy (kV)
Z = atomic number of material

 

From the equations above we can see contrast can be improved by:

  • Decreasing the energy (tube potential kV)
  • Increasing the difference in Z (atomic number) (e.g. use of iodine or barium as a contrast medium)
  • Increasing in ρ (density) (e.g. use of barium or gas as a contrast medium)
Overlying tissue

If there is overlying tissue over both A and B, subject contrast is not changed as the same ratio of photons is absorbed in tissue A and B.

Scatter

Supose scatter contributes an additional 50 photons to each element in the image, there will now be 150 photons in the film under tissue A and 70 under tissue B. The ratio of signals is now 2.14 (150/70) and the difference in logs is 0.33 (was 0.7) i.e. a reduced contrast.

Scatter is reduced by:

  • Using an anti-scatter grid
  • Using a larger air gap

Summary

Improved contrast
  • Thicker structure
  • Greater attenuation between objects
  • Decreasing kV
  • Increasing difference in Z of objects
  • Increasing difference in density of objects
Reduced contrast
  • Increased scatter
No effect
  • Overlying tissue

 

Image contrast

In film screen systems

This is the difference in optical densities of the film. It depends on the subject contrast and the gamma of the film (refer to Screen Film Radiography chapter). The greater the gamma, the steeper the curve and the greater the difference in optical densities. BUT, increasing the gamma leads to reduced latitude and fewer optical densities can be displayed adequately.

In digital imaging systems

Digital windowing

 

This is achieved by windowing on the viewing monitor. Images are presented at a certain width and centre of Hounsfield units displayed. The larger the width, the larger the range of shades displayed and, therefore, the smaller the difference and contrast between each shade. The window is adjusted for the Hounsfieid unit of the tissues that need to be assessed.

Windowing explanation

 

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2. Spatial resolution

Resolution is the measure of how far apart two objects must be before they can be seen as separate details in the image. There are several ways to measure spatial resolution.

Measuring spatial resolution

Line spread function

This is a measure of how spread out the image of a sharp object becomes. However, this is difficult to calculate and it is easier to look at the image in terms of spatial frequency content.

Spatial frequency


Spatial frequency in line pairs per mm

 

This is measured in line pairs per mm (lp/mm). An image with a high lp/mm is a high spatial frequency image as there are many alternating light and dark regions in a single millimetre. We, therefore, need a system that can reproduce the image with the appropriate frequency.

How close a system is able to represent the object spatial frequency is expressed as the modulation transfer function (MTF). The lp/mm of different radiographic techniques can be found in the appendix.

 

Modulation transfer function
MTF = 1 Same range is obtained in the image
MTF < 1 Lower range in the image
MTF = 0 No information in the image

 

E.g for an imaging system that can fully change from black to white over 1 mm:
For images with 0.5 lp/mm, it gives an MTF of 1
For images with > 0.5 lp/mm, it gives an MTF of <1

MTF is calculated from the lines spread function using Fourier transform analysis. The total MTF is the product of the MTF of all constituent parts of the imaging system.

Factors affecting spatial resolution

 

System spatial resolution

 

  • If the object spatial frequency is too high for the system, the system will be unable to display the image adequately. The higher the object spatial frequency, the lower the MTF until the system cannot distinguish the line pairs at all resulting in a homogeneous grey i.e. MTF = 0.
  • If the object has low contrast the system will reach an MTF of 0 earlier as the smaller difference in the range of shades means that the image will reach a homogeneous grey much sooner than if it was a high contrast image (e.g. alternating bands of black and white).
  • Anything that increases the unsharpness will blur the edges and further reduce the spatial frequency.

Digital detectors

There are several things that affect the resolution of digital detectors.

 

Detector properties

 

Detector aperture

This signal is averaged over the area of the detector element. If object details are much smaller than the size of the element they are not visible unless they have enough contrast to have a significant effect on the average signal.

Sampling pitch

This is the centre-to-centre distance between individual detector elements. It determines the highest spatial frequency that can be imaged; the Nyquist frequency.

Nyquist criterion states that the sampling frequency must be at least twice the highest signal frequency. The highest signal frequency is also called the "Nyguist frequency" i.e. for a system to be able to accurately represent the spatial resolution of the object it must have the appropriate sampling pitch which is no less than double the object spatial frequency.

Sampling frequency = 2 x Nyquist frequency

Sampling frequency limit

 


3. Noise

There is random variation in the number of photons forming each part of the image, called noise, that can obscure the signal received from the subject. The amount of quantum noise produced increases with an increasing total number of photons. We usually express this random variation as the standard deviation which is best estimated by the square root of the average number of photons per area.

Quantum noise ∝ √photons

However, when we calculate the quantum noise as a proportion of the total signal we can see that the proportion of noise in the signal actually decreases with an increasing photon concentration and:

Noise ∝ 1/ √photons

Average number of photons absorbed by each detector (N) 1000 100
Noise

√1000 =
31.6

√100 =
10

Proportion of signal which is noise

31/1000 x 100 =
3%

10/100 x 100 =
10%

Signal to noise ratio (SNR) = N / √N 31 10

 

Reducing the proportion of noise in an image will improve the quality. The main way to achieve this is the increase the number of photons detected and used to form each image pixel / element. This can be done in several ways.

  • Increasing the dose (mA): higher number of photons and smaller proportion of noise
  • Using an image receptor with a greater attenuation coefficient: more photons are absorbed and converted into a signal
  • Make the image receptor thicker: again, more photons will be absorbed and converted into a signal
  • Using larger detector elements: more area to absorb photons per pixel. However, the spatial resolution will decrease

Factors that don't reduce noise

  • Amplification: attaining a higher signal from each absorbed photon, either by using a faster film-screen combination or gain of an image intensifier would just amplify the signal from noise as well
  • Using a narrower window to produce a high contrast image

4. Unsharpness

There are four causes of unsharpness:

  1. Geometric unsharpness
  2. Image receptor unsharpness
  3. Movement unsharpness
  4. Edge unsharpness

a. Geometric unsharpness

 

Geometric unsharpness

 

The boundaries between a dark and a light area may be ill-defined, resulting in a blurred edge. This is called "unsharpness". There are several causes and types of unsharpness as outlined below.

The focal spot is not infinitely small. There will be areas of the image that are:

  • High signal: all x-ray photons reach detector
  • Low signal: no x-ray photons have passed through the object to reach the detector
  • Intermediate: not all photons have passed through the object. The size of this area determines the unsharpness and is called the penumbra.

Moving an object closer to the focal spot will increase the penumbra and, therefore, the unsharpness.

The geometric unsharpness (Ug) is determined as follows:

 

Ug = f x b / a
where:
f = x-ray focal spot size
a = distance from x-ray source to front surface of object
b = distance from object to detector

 

b. Image receptor unsharpness

  • Film screen images: in intensifying screens the absorbed x-ray is converted into light photons which then travel through to the film. If the screen is thick the light photons will be more spread out before reaching the film and produce a more blurred image.
  • Digital images: if a detector element lies across the border between a light and a dark area the pixel displayed will be an average of these two values creating a blurred border.

c. Movement unsharpness

If an object moves during the acquisition the edge will be blurred resulting in unsharpness.

d. Edge unsharpness

 

Edge unsharpness

 

If an object has a tapering edge the attenuation will gradually decrease along the object.


5. Magnification

 

Magnification artefact

 

Magnification (M) depends on the relative distance of the object between the x-ray source (focal spot) and the image receptor. The further from the detector the object is the more the image is magnified.

M = image size / object size

= d2 / d1


6. Distortion

Depending on the angle at which the x-ray beam passes through an object it can distort the shape and create a distortion artefact.


7. Artefacts

There are a variety of patient and system factors that can create artefacts:

  • Motion artefact
  • Double exposure
  • Grid cut off
  • Radio-opaque objects on or external to the patient

Σ  Summary

Contrast

  • Difference in attenuation (subject contrast) or displayed shade (image contrast) of an image

Subject contrast

  • contrast proportional to (μ1 - μ2) x t - where: μ = attenuation coefficient of object 1 and 2, t = object thickness
  • Improve contrast by:
    • Thicker object
    • Greater attenuation between objects
    • Decreasing kV
    • Increasing Z (atomic number) difference in objects
    • Increasing difference in density of objects
  • Contrast reduced by:
    • Increased scatter (no anti-scatter grid, smaller air-gap used)
  • No effect:
    • Overlying tissue

Image contrast

  • Film screen system: greater gamma gives greater contrast
  • Digital imaging system: achieved by windowing at the imaging monitor

Spatial resolution

  • Measure of how far apart two objects must be before they can be seen as separate details in an image

Measured as:

  • Line spread function: how spread an image of a sharp object becomes. Difficult to measure
  • Line pairs per mm (lp/mm)

Accuracy of system display of object spatial frequency = modulation transfer function (MTF)

  • MTF = 1 same range obtained
  • MTF < 1 smaller ranger obtained
  • MTF = 0 no information in image

Factors affecting spatial resolution:

  • Object properties:
    • Object spatial frequency: if it is too high for the system it will not be displayed accurately 
    • Object low contrast: lower contrast objects reach an MTF of 0 at lower spatial frequencies
  • Computed / digital radiography:
    • Detector element size: smaller element = higher spatial resolution
    • Distance between detector elements: smaller distance = higher spatial resolution
  • Others:
    • Anything that increases unsharpness

Noise

  • Noise proportional to √photons
  • Reducing noise: anything that increases number of x-ray photons (x-ray beam) produced and absorbed and the number of light photons (at the image receptor) produced:
    • Increasing dose (mA)
    • Using an image receptor with a greater attenuation coefficient
    • Making the image receptor thicker
    • Using larger detector elements

Factors that don't reduce noise:

  • Amplification
  • Using narrower window to produce a high contrast image

Others

Unsharpness

  • Geometric unsharpness
    • Focal spot not infinitely small, therefore, blurred penumbra produced at object edge
    • Ug = f x b / a - where f = x-ray focal spot size, a = distance from focal spot to object, b = distance from object to detector
  • Image receptor unsharpness
    • Film screen images: thicker screen causes light photons produced to spread out before reaching film 
    • Digital images: if detector element straddles light and dark area pixel displayed will be an average of these two values
  • Movement unsharpness
  • Edge unsharpness

Magnification

  • Greater magnification by moving object further from detector

Distortion

  • Due to the finite size of the focal spot an image may be distorted depending on the angle at which it is imaged

Artefacts

  • These may be due to patient or system factors

Digital radiography - special notes

Speed

  • Speed = 2000 / x  where x = dose incident on IP
  • S < 200 = improved SNR but increased patient doses

Spatial resolution

  • Measured by MTF
  • Detective quantum efficiency (DQE) = SNR2out/SNR2in
    • Greater DQE = more efficient detection of incident x-ray photons
  • Improved by:
    • Smaller diameter of readout laser beam
    • Smaller pixels
    • Smaller size of phosphor crystals
    • Thinner phosphor layer
    • No light reflection / absorption backing layer (produces scatter)

Next page: Quality assurance


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