FRCR: Physics Lectures

Diagnostic Radiology

Lecture 3 Image quality, scatter and contrast agents

Dr Tim Wood

Clinical Scientist

Overview

• What is image quality?• Contrast

– What is contrast?– Contrast agents– Scatter vs contrast

• Spatial Resolution– What is limiting spatial resolution?– What determines spatial resolution?

• Noise– What is noise?– Quantum noise vs patient dose

The story so far…

Interaction Processes

• Elastic scattering

• Photoelectric effect

• Compton effect

The Mass Attenuation Interaction Coefficient

X-ray tube design - basic principles

• Electrons generated by thermionic emission from a heated filament (cathode)

• Accelerating voltage (kVp) displaces space charge towards a metal target (anode)

• X-rays are produced when fast-moving electrons are suddenly stopped by impact on the metal target

• The kinetic energy is converted into X-rays (~1%) and heat (~99%)

The diagnostic X-ray tube

• Tube Current (mA)/Exposure Factor (mAs)– Double the mA/mAs, double the intensity– Beam quality not affected

• Tube Voltage (kVp)– Intensity α kVp2

– Penetrating power increases with kVp– Higher kVp reduces skin dose

• Filtration (mm Al)• Focus-to-skin distance

Factors Affecting Patient Dose

Filtration

Focus-to-skin Distance:The Inverse Square Law

• For a point source, and in the absence of attenuation, intensity decreases as the inverse of the square of the distance

• This is a statement of the conservation of energy

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1

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r

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Image Quality

What is ‘image quality’

• Image quality describes the overall appearance of the image and its fitness for purpose– Remember: There is always a play-off between

image quality and patient dose– We only need images that are of diagnostic quality (fit

for purpose) – not pretty pieces of art!

• The main factors to consider are:– Contrast– Spatial resolution– Noise

Contrast

• Most medical images presented as shades of grey from black to white (greyscale)

• Contrast resolution is the ability to distinguish between regions of the image

• The amount of contrast between tissues is intrinsically linked to their properties and the imaging modality being used

• Also influenced by the technique and specification of the equipment being used, and how it is displayed– Digital imaging techniques allow image processing to

be applied to improve the visibility of details

Image contrast

• The final contrast in the image will depend on a number of factors, such as;– Subject contrast – an inherent property of the patient

being imaged that will depend on the attenuation coefficients of the tissues (or contrast media), the thickness of structures, the nature of any overlapping tissues and the incident X-ray spectrum (kVp, filtration, etc – discussed previously)

– Detector properties – film and digital detectors each have different implications for the contrast in the final image (these will be discussed in the next lectures)

– Scattered radiation – scatter can degrade image contrast if it reaches the detector as it conveys no information about where it came from. Scatter rejection techniques may be used to remove this.

Subject contrast

Subject contrast• Visibility of object structure – requires physical

contrast between the object to be imaged and the surrounding tissues

• The contrast may be based on a difference in densities or atomic number (chemical composition), coupled with the thickness of the particular material through which the radiation has to pass– Proportional to object density x object thickness

• Also remember, higher kVp = less photoelectric absorption = poorer contrast

Material Effective Atomic Number Z

Density g cm-3 Contrast Index

Density x thickness = 4

Soft Tissue

- Fat- Water- Muscle

7

6

7.4

7.5

1

0.9

1

1

4cm

4.4cm

4cm

4cm

Air 7.6 0.00129 3100cm

Bone 14 1.85 2.1cm

Calcium 20 1.55

Tooth 2.4 1.7cm

Iodine 53 4.9

Barium 56 3.5 1.1cm

Barium Contrast Media 56 4.25

Lead 82 11.3 0.35cm

Subject contrast

Subject contrast

Subject contrast

• Contrast between bone and muscle is large at low kV, but decreases with increasing kV

• Contrast between low atomic number soft tissues is low, even at low kV

• The contrast between air and tissue (similar Z) is large due to the difference in density

Balancing Contrast & Patient Dose

• Penetration, contrast and patient dose depend on the x-ray beam spectrum. The ‘best’ spectrum provides adequate penetration and contrast, whilst keeping the patient dose as low as possible

• The spectrum produced is dependent upon the target material, inherent and added filtration, and kilo-voltage

Low Energy (kV) = Low Penetration = High Contrast = High Patient Dose

High Energy (kV) = High Penetration = Low Contrast = Low Patient Dose

Optimal Situation = kV that gives adequate contrast for an acceptable patient dose – will vary for the clinical application

Balancing Contrast & Patient Dose

Contrast agents• Soft tissue contrast is a problem in radiography• Two possible solutions;

– Use lowest possible kVp e.g. mammography– Use a high Z contrast agent in the tissue of interest

• The high Z of contrast agents maximises the photoelectric absorption of X-rays

• Ideally, absorption edges lie just to the left of the major part of the X-ray spectrum

• Two common contrast agents are Iodine (Z = 53, EK = 33 keV) and Barium (Z = 56, EK = 37 keV)

• Air (similar Z, but low density) can also be used, but much less common now

Contrast agents

Contrast agents in clinical practice

http://health.siemens.com/AX/Artis_zee_cardiology_image_quality/clinicalresults.swf

Contrast agents in clinical practice

Clinical Examples

Clinical Examples

Clinical Examples

Contrast and scatter

• Primary radiation carries the information to be imaged

• Scatter obscures this as it carries no information about where it came from

• The amount of scatter (S) may be several times the amount of primary (P) in the same position

• The ratio S/P depends on thickness of patient– For typical PA chest = 4:1 (20% of photons carry

useful information)– For typical lateral pelvis = 9:1 (10% of photons…)

Contrast and scatter• Scatter is generally quite uniform across the

image• Reduces contrast that would otherwise be

produced by the primary by a factor of (1+S/P) – up to 10 times!

P

S

S+P% contrast reduced

S+P contrast = (10-9)/10 x 100 = 10%

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8

10

Position

Primary contrast = (2-1)/2 x 100 = 50%

Scatter = 4 x Primary (PA Chest)

Intensity

Scatter reduction• Scatter and patient dose may be reduced by;

– Reducing the field area with collimation (reduce volume of tissue generating scatter)

– Compress the tissue to minimise overlying structures (reduce volume of tissue generating scatter)

• Scatter may be reduced at the expense of increased patient dose by;– Reducing kVp – less forward scatter is produced and

is much less penetrating (less scatter produced)– Use an anti-scatter grid to remove scatter from the X-

ray beam– Use an air-gap to reduce the intensity of scatter

reaching the detector

Anti-scatter grids

• Series of thin strips of high Z metal (e.g. Pb) sandwiched between low attenuation spacer material (e.g. C fibre)– Typically 0.05-0.07 mm wide with 30-80 strips per cm

(the line density)• X-rays that hit the grid at oblique angles (i.e. not

along the path of the primary beam) are absorbed• High proportion (but not all) of the primary beam

passes straight through to the film• The grid ratio is the depth of the channels divided

by its width – typically 8:1– Larger grid ratios = smaller angle of acceptance and

better rejection of scatter

Anti-scatter grids

depth

width separation

Anti-scatter grids

• Large fields tend to require higher grid ratios (12:1 or 16:1)

• Grids generally not used for thin parts of the body or children as less scatter is generated (and increase in dose not justified)

• Contrast improvement factor is defined as contrast with a grid divided by contrast without– Typically between 2 and 4

Anti-scatter grids

• Parallel lead strips = unfocussed grid– Away from the centre of the beam, primary X-rays will

strike the grid obliquely and be attenuated giving grid cut-off

– Use longer FFD or lower grid ratio to avoid this• Focussed grid uses strips that are angled to

match the divergence of the X-ray beam– All strips point to the tube focus– Ensure grid is the right way round!– Only to be used at a specified distance (or range of

distances)– Tube must be centred on grid with no tilt, otherwise

get cut-off

Focussed anti-scatter grids

Anti-scatter grids

• Grid lines will result from the shadow of the lead strips for a stationary grid

• If sufficiently high line density, may not be visible without magnification– However, stationary grids used in digital imaging may

produce interference patterns

• Alternatively, may use a moving grid to blur out the grid lines during exposure– Movement must start before exposure– Must be fast for blurring to occur on shortest

exposure times

Anti scatter grids

• The use of grids necessitates increase exposure factors (mAs) due to the attenuation of primary beam

• The grid factor is the ratio of exposure necessary with grid to that without– Typically in the range 3-5– Hence, corresponding increase in patient dose!

• The grid, cassette holder and AEC system is often referred to as the ‘Bucky’

• Alternatively, can use slot scanning techniques, but this is at the expense of exposure time– e.g. CT and mammography

Air-gaps

• If the detector is moved 20-30 cm away from the patient, a significant proportion of the scatter will miss the detector

• Due to inverse square law, small reduction in primary intensity (hence, increase mAs to compensate)

• Also results in a magnified image

Air-gaps

Magnification• Magnification is the

result of the diverging X-ray beams travelling in straight lines

• Reduce by using longer focus-to-film distance or shorter object-to-film distance

• Magnification exploited in some modalities e.g. mammography

Spatial Resolution• Spatial resolution describes

the ability to see fine detail within an image

• Fine detail is clearer when the contrast is high– e.g. microcalcifications

• May be expressed as the smallest visible detail, but most common descriptor is the highest frequency of lines that can be resolved in a high-contrast bar pattern

Spatial Resolution• Bars of Pb or W, with width of

bar equal to space between them

• A bar and space make up a line pair (lp), and the spatial frequency of the pattern is given as lp mm-1 (or lp cm-1 for CT)

• Observer determines the highest spatial frequency that can be resolved = limiting spatial resolution

Limiting factors• Geometric Unsharpness – due to finite focal

spot size (discussed previously)• Movement Unsharpness – imaging moving

structures may result in blur e.g. heart, lungs, etc– Minimise by immobilisation e.g. compression in

mammography– Breath hold techniques– Use shortest possible exposure times (may be at the

expense of high mA)• Absorption Unsharpness – gradual changes in

absorption near tapered/rounded structures – Minimise by careful patient positioning

• Detector resolution – will be discussed in following lectures

Geometric unsharpness –The ideal point source

Object

Film/detector

Ideal point source of X-rays

FFD

OFD

Geometric unsharpness –A ‘real’ focal spot

Object

Film/detector

Focal spot of finite size, f

FFD

OFD

Penumbra

Noise

• Noise is a random, usually unwanted, variation in brightness or colour information in a visible image

• Noise is one of the most important limiting factors to contrast and spatial resolution

• The most significant source is quantum noise (or mottle) due to the low levels of radiation used to form an image

• Other sources include film grain and electronic noise in the image receptor

Noise

• Image noise is most apparent in image regions with low signal level, such as shadow regions or underexposed images

• Noise gives a grainy, mottled, textured or snowy appearance to an image

• Noise can mask fine detail in a radiograph• Noise reduces visibility of parts of a radiographic

image• Noise is particularly an issue with image details

that are already of low contrast

Types of noise

• Several types of noise contribute to the overall image quality, these are:– Quantum Noise – from statistical nature of the

interactions of the X-ray beam– Secondary Quantum Noise – associated with

secondary carriers at each conversion stage in image formation e.g. CR, Image Intensifiers, etc

– Structural Noise – films/screens/intensifier tube screens/digital receptors. Only tends to be important at high doses where all other sources are negligible

– Electronic Noise – circuit noise, thermal noise, external electrical device signals. Only obvious at low doses

Quantum noise (mottle)• X-ray beam is made up of individual quanta or

packets of radiation energy – Photons• Quantum Noise describes the random

statistical nature of the interactions of the radiation falling on a detector surface

• The number of photons absorbed varies from pixel to pixel– This variation is described by the standard deviation

• In all digital imaging modalities, quantum noise should be the main contributor to the overall noise at clinical dose levels– ‘Quantum Limited’

Quantum noise (mottle)• The higher the number of photons falling on a unit

area, the lower the deviation is relative to the mean number, and the lower the Quantum Noise level.

• Noise level (noise/signal) is inversely proportional to the square root of the exposure to the image receptor

• Signal-to-noise ratio (SNR) is proportional to the square root of dose Noise versus Photon Flux

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0 100 200 300 400 500 600

mAs

No

ise

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el

Quantum noise (mottle)• Therefore, to increase the concentration of

photons impinging on a unit area of a detector (to reduce the noise level) the radiation exposure needs to be increased

• Level of exposure required will depend on the clinical task and receptor sensitivity

• The consequences of increasing the exposure is to increase the dose to the patient

• As noise level is inversely proportional to the square root of dose, four times the dose will only half the noise level in the image

80kV 40mAs 80kV 0.5mAs

Striking the right balance

Reduced Exposure = Low Patient Dose = Increased Q-Noise = Reduced Image Quality

Increased Exposure = High Patient Dose = Reduced Q-Noise = Improved Image Quality

Optimal Situation = Exposure high enough to give a diagnostic image with acceptably low quantum noise, whilst maintaining an acceptable patient dose.