invisible x ray-image
TRANSCRIPT
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-
rays
at
imag
e p
lan
e
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
at
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.