Radiographic Radiographic PhotographyPhotographyDr Hussein Ahmed HassanDr Hussein Ahmed Hassan

Invisible X-ray Invisible X-ray imageimage

1.1. FormationFormation

2.2. CharacteristicsCharacteristics

X-ray tube

Object

Plot of incident x-ray beam intensity

Plot of transmitted x-ray beam intensity

Invisible x-ray image

Invisible x-ray imageInvisible x-ray image

B2 T1 T2 T3

E

E B1 E B2

ET1EM

ET2

ET3

EA

kV mA Sec FFD

BB1 Air

Supporting tissue (m)

EM

Invisible X-ray image consists of different x-ray intensities

CharacteristicsCharacteristics

Subject contrastSubject contrast SharpnessSharpness NoiseNoise ResolutionResolution

Subject contrastSubject contrast The difference in the x-ray The difference in the x-ray

intensities transmitted through the intensities transmitted through the subjectsubject

It is the shortened form of the It is the shortened form of the radiation contrast of the subjectradiation contrast of the subject

Causes of subject contrastCauses of subject contrast Differential attenuationDifferential attenuation Scattered radiationScattered radiation

Differential attenuationDifferential attenuation Differential attenuation is the result of Differential attenuation is the result of

the attenuation caused by the attenuation caused by Photoelectric Photoelectric absorptionabsorption and and Compton scattering.Compton scattering.

Depends onDepends on Thickness of the anatomical structureThickness of the anatomical structure Effective atomic number of the body Effective atomic number of the body

tissuestissues Physical density of the body tissuesPhysical density of the body tissues Presence of radiological contrast Presence of radiological contrast

mediummedium X-ray tube kilovoltage employedX-ray tube kilovoltage employed X-ray beam filtrationX-ray beam filtration

Effective atomic number & Effective atomic number & Subject contrastSubject contrast

For a given Photon energy the photo For a given Photon energy the photo electric absorption is higher when the electric absorption is higher when the atomic number is high ( bone absorbs more atomic number is high ( bone absorbs more radiation than soft tissue)radiation than soft tissue)

E.g. if the three tissues A,B,C have E.g. if the three tissues A,B,C have effective atomic numbers as Zeffective atomic numbers as Z11 > Z > Z2 2 > Z> Z33

AZ1

BZ2

CZ3

Incident intensity

Transmitted intensity

Subject contrast A-C

Subject contrast A-B

Subject contrast B-C

X-ray tube kilovoltage & X-ray tube kilovoltage & subject contrastsubject contrast

Photo electric absorption predominates Photo electric absorption predominates at low kilovoltages, therefore at low at low kilovoltages, therefore at low kilovoltages the subject contrast is high, kilovoltages the subject contrast is high, and when the kilovoltage is increased and when the kilovoltage is increased the subject contrast tend to be reduced.the subject contrast tend to be reduced.

At high kilovoltages approaching 150kV At high kilovoltages approaching 150kV the contrast is mainly caused by the the contrast is mainly caused by the compton effect which mainly depends compton effect which mainly depends on the density difference of the on the density difference of the anatomical structures.anatomical structures.

kV & subject contrastkV & subject contrast

B2 T1 T2 T3

E

E B1 E B2

ET1EM

ET2

ET3

EA

Low kV

BB1 Air

Supporting tissue (m)

EM

Higher differences

kV & subject contrastkV & subject contrast

B2 T1 T2 T3

E

E B1 E B2

ET1EM

ET2ET3

EA

High kV

BB1 Air

Supporting tissue (m)

EM

Lower differences

X-ray beam filtration & X-ray beam filtration & Subject contrastSubject contrast

Filtration reduces the low energy Filtration reduces the low energy components of the x-ray beam. Hence components of the x-ray beam. Hence increasing the filtration has the effect increasing the filtration has the effect of increasing the effective photon of increasing the effective photon energy of the beam. This influences the energy of the beam. This influences the photoelectric absorption in a similar photoelectric absorption in a similar way as increasing the tube kilovoltage.way as increasing the tube kilovoltage.

Therefore increasing the filtration will Therefore increasing the filtration will decrease the subject contrastdecrease the subject contrast

Scattered radiation & Scattered radiation & subject contrastsubject contrast

Scattered radiation

Primary beam

Scattered radiation & Scattered radiation & subject contrastsubject contrast

When the primary beam from x-ray tube When the primary beam from x-ray tube interacts with matter scattered interacts with matter scattered radiation is produced.radiation is produced.

Scattered radiation travels in different Scattered radiation travels in different paths from the primary beam and will paths from the primary beam and will reduce the subject contrast of the reduce the subject contrast of the invisible x-ray image.invisible x-ray image.

Not only the subject contrast but it will Not only the subject contrast but it will reduce the signal to noise ratio also.reduce the signal to noise ratio also.

Scatter reduces the subject Scatter reduces the subject contrastcontrast

B2 T1 T2 T3

E

E B1 E B2

ET1EM

ET2ET3

EA

BB1 Air

Supporting tissue (m)

EM

Scatter Lowers the differences

How to minimize the effect How to minimize the effect of scatter on subject of scatter on subject

contrast?contrast? Reduce the amount of scatter produced Reduce the amount of scatter produced

at the object (patient) by:at the object (patient) by: Collimating the primary beamCollimating the primary beam Reducing the proportion of forward scatter Reducing the proportion of forward scatter

using low kVusing low kV Reducing the tissue thicknessReducing the tissue thickness Avoiding other sources of scatter, such as Avoiding other sources of scatter, such as

bucky traybucky tray Protecting the image receptor byProtecting the image receptor by

Use of secondary radiation gridUse of secondary radiation grid Employing an air gapEmploying an air gap

Use of gridUse of grid

Lead strips

Radiolucent inter-space

Image receptor

Employing Air gapEmploying Air gap

Percentage of oblique ray reaching the image receptor plane is reduced at image plane 2

Object

Image plane 2

Scatter

Image plane 1

Air gap

Sharpness of Invisible x-ray Sharpness of Invisible x-ray imageimage

The sharpness is determined first by The sharpness is determined first by the geometry of image formationthe geometry of image formation

The size of the source of radiation is The size of the source of radiation is of primary concernedof primary concerned Infinite size (Point source)Infinite size (Point source) Finite size ( larger than a point)Finite size ( larger than a point)

When the size of the x-ray source When the size of the x-ray source (Focus) is large the sharpness of the (Focus) is large the sharpness of the image is less image is less

Image GeometryImage GeometryPoint source Finite

source

Unsharpness (penumbra)

Image plane

Intensity distribution at Intensity distribution at previous situationsprevious situations

Inte

nsi

ty o

f x-

rays

at

imag

e p

lan

e

Inte

nsi

ty o

f x-

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at

imag

e p

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Distance across image plane Distance across

image plane

U U

Geometric unsharpnessGeometric unsharpness

The formation of unsharpness due to a The formation of unsharpness due to a penumbra is a direct consequence of the penumbra is a direct consequence of the finite sizefinite size of the x-ray source. of the x-ray source.

This form of unsharpness is known as This form of unsharpness is known as Geometric unsharpness (UGeometric unsharpness (UGG))

It can be shown thatIt can be shown that

focal spot size focal spot size xx object- object-image distanceimage distance

Geometric = Geometric = --------------------------------------------------------------------------------------

Unsharpness focus-object Unsharpness focus-object distancedistance

Evaluation of Geometric Evaluation of Geometric unsharpnessunsharpness

A B

O

C D

Source

Image plane

Object

Triangles OAB & OCD are similar.

AB/CD = OB/OC

Re-arranging

CD = AB x OC/OB

UG = focal size x OFD/FOB

Factors governing Factors governing geometric unsharpnessgeometric unsharpness

Focal spot sizeFocal spot size Small focus gives minimum geometric Small focus gives minimum geometric

unsharpnessunsharpness Object image (film) distance Object image (film) distance

Shorter OFD gives less geometric unsharpnessShorter OFD gives less geometric unsharpness Focus to object ( Focal film) distanceFocus to object ( Focal film) distance

Longer the FFD lesser the geometric Longer the FFD lesser the geometric unsharpnessunsharpness

Increase the FFD when OFD cannot be reduced, Increase the FFD when OFD cannot be reduced, to minimize the geometric unsharpnessto minimize the geometric unsharpness

Edge penetrationEdge penetration

Focal spot size & Geometric Focal spot size & Geometric unsharpnessunsharpness

Unsharpness increases, when apparent Unsharpness increases, when apparent focal area increases focal area increases

Apparent (effective) focal area = Actual Apparent (effective) focal area = Actual focal area x Sine of target anglefocal area x Sine of target angle

Therefore Unsharpness increases when Therefore Unsharpness increases when target angle increases for a given target angle increases for a given actual focal spot sizeactual focal spot size

Geometric Unsharpness can be reduced Geometric Unsharpness can be reduced by using small focus but that reduces by using small focus but that reduces the maximum tube loading capacitythe maximum tube loading capacity

Apparent (effective) focal area = Actual Apparent (effective) focal area = Actual focal area x Sine of target anglefocal area x Sine of target angle

Unsharpness due to Edge Unsharpness due to Edge penetrationpenetration

This is due to the This is due to the shape of the objectshape of the object

The edges of the The edges of the object absorb less object absorb less amount of radiation amount of radiation and the absorption and the absorption increases towards increases towards the centrethe centre

This creates a This creates a intensity gradient intensity gradient producing inherent producing inherent unsharpnessunsharpness

Inte

nsi

ty o

f x-

rays

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imag

e p

lan

e

Distance across image plane

Movement unsharpnessMovement unsharpness

Voluntary & involuntary movement of the Voluntary & involuntary movement of the organs or body parts or the patient as a organs or body parts or the patient as a whole will cause changes in the pattern whole will cause changes in the pattern of x-ray intensities forming the invisible of x-ray intensities forming the invisible x-ray imagex-ray image

This changes are referred to as This changes are referred to as movement unshrpness : Umovement unshrpness : UMM

If they occur during image recording they If they occur during image recording they will produce unsharpness in the final will produce unsharpness in the final imageimage

Noise in the invisible x-Noise in the invisible x-ray imageray image The kinds of noise present in the The kinds of noise present in the

invisible x-ray image areinvisible x-ray image are Fog due to scatter radiationFog due to scatter radiation Quantum noise – presence of less Quantum noise – presence of less

number of photons in the invisible x-number of photons in the invisible x-ray image, making the identification ray image, making the identification of gaps between individual photons of gaps between individual photons and finally making the recorded and finally making the recorded image looks grainy. image looks grainy. Quantum noise can be avoided by using Quantum noise can be avoided by using

adequate exposure factors producing adequate exposure factors producing high enough x-ray intensityhigh enough x-ray intensity

Resolution of invisible x-ray Resolution of invisible x-ray imageimage

The resolution depends on The resolution depends on contrast, contrast, sharpness and sharpness and noise.noise.

We must try to obtain maximum We must try to obtain maximum resolution at this stage because the resolution at this stage because the resolution becomes less and less in resolution becomes less and less in the next stages of image productionthe next stages of image production

ConclusionConclusion It is important to know the details of It is important to know the details of

production and characteristics of the production and characteristics of the invisible x-ray image because;invisible x-ray image because;

If the invisible x-ray image is of poor If the invisible x-ray image is of poor quality, it is extremely difficult to quality, it is extremely difficult to produce an adequate standard of final produce an adequate standard of final visible image.visible image.

It is during the production of the It is during the production of the invisible x-ray image that the invisible x-ray image that the radiographer has the greatest scope for radiographer has the greatest scope for control of image quality, particularly in control of image quality, particularly in conventional radiography.conventional radiography.