Image CharacteristicsImage Characteristics

What is an image?What is an image?

Dictionary meaningDictionary meaning

An optical appearanceAn optical appearance A form of semblanceA form of semblance A mental representationA mental representation An idea or conceptionAn idea or conception

The term image may be The term image may be applied to:applied to:

1.1. A picture such as a photograph, a A picture such as a photograph, a painting or a sketch which has a painting or a sketch which has a physicalphysical existence existence

2.2. An idea or concept which has a An idea or concept which has a mentalmental existence existence

(When we see an object like an apple (When we see an object like an apple or a picture of an apple we can or a picture of an apple we can imagine the taste of apple)imagine the taste of apple)

Visual images are of two Visual images are of two typestypes

Real imagesReal images – those having physical – those having physical existence such as pictures which are existence such as pictures which are accessible to scientific accessible to scientific measurements and measurements and objectiveobjective study study

Mental imagesMental images – those generated as – those generated as mental pictures within our minds and mental pictures within our minds and which are accessible to which are accessible to subjectivesubjective studystudy

Real ImagesReal Images Real images consist of patterns of light Real images consist of patterns of light

intensity and possibly variations of intensity and possibly variations of colourcolour

The patterns of light intensity are The patterns of light intensity are created in one of three wayscreated in one of three ways

1.1. Viewing by reflected light from a surfaceViewing by reflected light from a surface2.2. Viewing by light transmitted by Viewing by light transmitted by

semitransparent layersemitransparent layer3.3. Viewing by light emitted by a fluorescent Viewing by light emitted by a fluorescent

layerlayer

Viewing by reflected light from a Viewing by reflected light from a surfacesurface

Viewing by light transmitted by Viewing by light transmitted by semitransparent layersemitransparent layer

More light transmitted

Less light transmitted

Viewing by light Emitted from a Viewing by light Emitted from a fluorescent layerfluorescent layer

X-rays of different intensities

Pigmented layer

Light of different intensities

Base

High

Low

Image characteristicsImage characteristicsReal images display four essential Real images display four essential

characteristicscharacteristics NoiseNoise ContrastContrast SharpnessSharpness ResolutionResolution

NoiseNoise

Real images consists of two Real images consists of two componentscomponents

A A meaningful patternmeaningful pattern which carries which carries information about the object - information about the object - SignalSignal

A A spuriousspurious chaoticchaotic pattern carrying pattern carrying no information about the object - no information about the object - NoiseNoise

Effect of noiseEffect of noise The presence of noise limits the The presence of noise limits the

amount of information which can be amount of information which can be extracted from the image. Especially extracted from the image. Especially the finer details of structure may be the finer details of structure may be lost by being swamped by the effect of lost by being swamped by the effect of noise.noise.

Example :- fogging on the radiographic Example :- fogging on the radiographic imageimage

Good image fogged imageGood image fogged image

High signal to noise ratio Low signal to noise ratio

Signal –to- noise ratioSignal –to- noise ratio When under the optimum conditions When under the optimum conditions

the magnitude of the signal is very the magnitude of the signal is very much greater than the magnitude of much greater than the magnitude of the noise, the signal –to-noise ratio is the noise, the signal –to-noise ratio is said to be high and much information said to be high and much information is gained.is gained.

When under the adverse conditions When under the adverse conditions the signal-to-noise ratio is low much the signal-to-noise ratio is low much information is lost.information is lost.

ContrastContrast Contrast is the difference of appearance of Contrast is the difference of appearance of

a feature or a structure in an image from a feature or a structure in an image from its surrounding.its surrounding.

It may be described as the difference It may be described as the difference between the shades of gray or degree of between the shades of gray or degree of luminance on an imageluminance on an image

Eg. Optical Density (degree of blackening) Eg. Optical Density (degree of blackening) difference on a radiograph.difference on a radiograph.

Luminance (brightness) difference on TV Luminance (brightness) difference on TV screenscreen

High Low OptimumHigh Low OptimumContrast ContrastContrast Contrast Contrast Contrast

SharpnessSharpness Sharpness is concerned with how Sharpness is concerned with how

suddenly blackening changes at the suddenly blackening changes at the boundary between adjacent parts.boundary between adjacent parts.

SharpnessSharpness

A B

Distance (mm)

Densi

ty

D1

D2

0 5 10

15

20

The boundary between two areas A & B appears very sharp

UnsharpnessUnsharpness

A

Distance (mm)

Densi

ty

D1

D2

0 .2

.4 .6 .8

The boundary between two areas A & B appears unsharp

B

The steeper the slope the more sharp the image appears. The shallower the slope the more blurred the image

Sharpness, unsharpness & lack Sharpness, unsharpness & lack of sharpnessof sharpness

No image is perfectly sharpNo image is perfectly sharp Every image has a certain lack of Every image has a certain lack of

sharpnesssharpness UnsharpnessUnsharpness is an objective concept is an objective concept

which can be measuredwhich can be measured SharpnessSharpness is our subjective is our subjective

perceptionperception of of unsharpnessunsharpness, and , and depends on depends on contrastcontrast and and unsharpnessunsharpness

Contrast & perception of Contrast & perception of unsharpnessunsharpness

We judge We judge one imageone image boundary to be boundary to be sharpersharper than another, even though than another, even though they are both they are both equally unsharpequally unsharp, if the , if the contrastcontrast of the of the first imagefirst image is is greater greater..

Specifying unsharpnessSpecifying unsharpness

Image unsharpness can be expressed Image unsharpness can be expressed by means of transfer functions such by means of transfer functions such asas Edge spread functionEdge spread function Point spread functionPoint spread function Line spread functionLine spread function

OROR As a simple numerical quantityAs a simple numerical quantity

Edge spread functionEdge spread function Using the microdensitometer trace of an Using the microdensitometer trace of an

image of a perfectly sharp object edge. image of a perfectly sharp object edge. The density values should be expressed in The density values should be expressed in relative terms.relative terms.A

Distance (mm)

Densi

ty

0

1

0 .1

.2 .3 .4

B

X Y

Unsharpness is expressed as the distance over which the density is changing (XY) = 0.15 mm

Point spread function (PSF)Point spread function (PSF) This is the microdensitometer trace This is the microdensitometer trace

of a point structure in an object (e.g. of a point structure in an object (e.g. a minute hole in a sheet of lead)a minute hole in a sheet of lead)

Object

Image

0

1

0.5

Rela

tive o

pti

cal

densi

ty

Distance

Measure the density along this line

This is the microdensitometer trace This is the microdensitometer trace of a line structure in an object (e.g. a of a line structure in an object (e.g. a thin slit in a sheet of lead)thin slit in a sheet of lead)

Line spread function (LSF)Line spread function (LSF)

Object

Image

0

1

0.5

FWHM

The unsharpness can be expressed as a simple numerical value as the FWHM measurement ( Full Width at Half Maximum)

Rela

tive o

pti

cal

densi

ty

Distance

Perception of unsharpnessPerception of unsharpness In radiography it is said that an image whose In radiography it is said that an image whose

unsharpness is <0.1mm will appear to be unsharpness is <0.1mm will appear to be sharp under normal viewing conditionssharp under normal viewing conditions

The perception of unsharpness is more The perception of unsharpness is more determined by the density gradient than by the determined by the density gradient than by the simple measure of the unsharpnesssimple measure of the unsharpness

Density gradient depends both on unsharpness Density gradient depends both on unsharpness and density difference (Contrast)and density difference (Contrast)

Thus an image of high contrast appear sharper Thus an image of high contrast appear sharper than one of low contrast, even though both than one of low contrast, even though both images have the same measured unsharpnessimages have the same measured unsharpness

Contrast, Density gradient & Contrast, Density gradient & UnsharpnessUnsharpness

Distance (mm)

Densi

ty

D1

D2 D4

D3

Unsharpness = X Unsharpness = X

Image A Appear sharper than image B

Distance (mm)

Image A Image B

Contrast of A > Contrast of B

ResolutionResolution The resolution of a system is its ability to The resolution of a system is its ability to

demonstrate closely spaced structures in demonstrate closely spaced structures in the subject as separate entities in the the subject as separate entities in the imageimage

E.g. The trabeculae pattern of boneE.g. The trabeculae pattern of bone The resolution of an image refers to its The resolution of an image refers to its

ability to show small structures separatelyability to show small structures separately The smaller the structures visible the The smaller the structures visible the

higher the resolution of the image/systemhigher the resolution of the image/system

Measurement of resolutionMeasurement of resolution

The resolution is expressed in terms The resolution is expressed in terms of the smallest spacing /highest of the smallest spacing /highest spatial frequency of high and low spatial frequency of high and low dense structures visible in the imagedense structures visible in the image

This is done subjectively by This is done subjectively by radiographing a test object consist of radiographing a test object consist of a grid of closely spaced lines, a grid of closely spaced lines, alternately radiopaque and alternately radiopaque and radiolucent.radiolucent.

Resolution test object Resolution test object Each radiopaque line and Each radiopaque line and

corresponding space is known as a corresponding space is known as a line pairline pair

The spacing of line pairs is expressed The spacing of line pairs is expressed as line pairs per millimetre (lp/mm)as line pairs per millimetre (lp/mm)Line pairs

Resolution test objectResolution test object

Comparison of resolutionComparison of resolution The resolution of different imaging The resolution of different imaging

systems may be compared by systems may be compared by reference to the reference to the Modulation Transfer Modulation Transfer FunctionFunction ( (MTFMTF) of each system) of each system

* Modulation = frequency of change * Modulation = frequency of change (of structure , etc.)(of structure , etc.)

MTF is a method of assessing the MTF is a method of assessing the success with which modulations of success with which modulations of structure (detail) in an object are structure (detail) in an object are transferred into modulations of transferred into modulations of density or luminance in the imagedensity or luminance in the image

Modulation in the imageModulation in the image MTF = ------------------------------------MTF = ------------------------------------

Modulation in the objectModulation in the object

MTF = 1 means that image MTF = 1 means that image reproduces exactly the variations reproduces exactly the variations in the object.in the object.

It is a characteristic of imaging It is a characteristic of imaging systems that, as the detail in the systems that, as the detail in the object becomes finer, the ability of object becomes finer, the ability of the system to record that detail the system to record that detail becomes progressively reduced.becomes progressively reduced.

As the spatial frequency increases As the spatial frequency increases MTF decreases.MTF decreases.

Modulation transfer function Modulation transfer function curve of a film-screen systemcurve of a film-screen system

System 1

System 2

Factors limiting resolutionFactors limiting resolution The resolution of a radiographic The resolution of a radiographic

image is influenced at every stage in image is influenced at every stage in the process of image production. the process of image production.

If we consider each step in this If we consider each step in this process as links in a chain, then the process as links in a chain, then the quality of the final image can be no quality of the final image can be no better than that of the weakest link .better than that of the weakest link .

We must identify the weakest links We must identify the weakest links and try to improve them.and try to improve them.

But we must realize that in But we must realize that in improving one aspect we may improving one aspect we may weaken another.weaken another.

There is a complex interrelationship There is a complex interrelationship between many of the factors between many of the factors concerned, including all of the image concerned, including all of the image characteristics discussed here.characteristics discussed here.

Only when we have studied all the Only when we have studied all the ramifications of image production will ramifications of image production will we be able to understand fully how we be able to understand fully how resolution may be optimized.resolution may be optimized.

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Production of & Characteristics of the Production of & Characteristics of the invisible x-ray imageinvisible x-ray image