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PRINCIPLE OF CONVENTIONAL TOMOGRAPHY
BIBASH SHAHI
ROLL NO-148
BSC MIT 1ST YEAR
IOM MMC
INTRODUCTION
• In normal radiograph , the anatomical structure of interest
is very often partially or completely obscured by the
shadows cast by overlying or underlying structures.
• This drawback lead to introduction of conventional
tomography which records an image of the plane or layer
of interest on its own with obsuring effect of other layers.
2
• Previously known as body section radiography until, in
1962 International Commission on radiologic units and
measurement adopted the term “Tomography”
• Also called as planigraphy, stratigraphy and
laminography
• In tomography, whole plane is in focus not just a point
object
• Not a method of improving sharpness of any part of
radiographic image but it is process of controlled blurring.
• Used to inner ear studies and still used at MCG for IVPS.
• Popularity decreasing due to introduction of CT and MRI
INTRODUCTION
3
HISTORY
• A need to see beyond 2D image was gradually increased
• First approach was to view orthogonal radiograph and
make 3D perception.
• The origin of tomography’s credit goes to JEAN KIEFFAR, a
radiological technologist in US, who developed the special
radiographic technique to demonstrate a form of TB that
he had in 1928.
• His process was termed ‘laminography’ by J.ROBERT
ANDREWS,assisted in construction of tomographic device
called ‘laminograph’.
4
TERMINOLOGY
FULCRUM
• is the pivot point about which lever arm rotates.
• Only point in system which remains stationary
• The plane of interest within the patient is positioned at
level of fulcrum & all points above and below are blurred
• two mechanically different types of fulcrum
 First one= the relationship between x ray tube, fulcrum &
film is fixed, patient is moved up and down on adjustable
table
 Second one=has adjustable fulcrum that is moved to height
of desired plane while patient remains still on the table.
5
6
TERMINOLOGY
FOCAL OR OBJECT PLANE
• plane of tissue in focus on a tomograph
• object plane is at the level of fulcrum & is parallel with
table top
FOCAL PLANE LEVEL
• It is the height of focal plane above tabletop
7
TERMINOLOGY
AMPLITUDE/TOMOGRAPHIC ANGLE(ARC)
• Distance travel by x ray tube and film around the patient is
considered as amplitude
• It is measured in degrees and is called tomographic angle
EXPOSURE ANGLE
• Angle through which x ray beam (or central ray) moves
during exposure
8
9
TERMINOLOGY
• TOMOGRAPHIC SECTION
 The thickness of tissue that will be imaged
 The larger is the tomographic angle , the thinner is the
tomographic section.
10
11
TERMINOLOGY
SECTIONAL INTERVAL
It is the distance between fulcrum levels
12
PRINCIPLE
• The principle of tomography is that it selects a level in the
body & blurs out structure below and above that plane
leaving a clear image of selected part
• Synchronous movements of the x-ray tube and the image
receptor in opposite directions around stationary fulcrum
(pivot point) during the exposure.
13
PRINCIPLE
• Tomography is concerned with layer rather than plane.
• The fulcrum area is sharp.
• The farther is the anatomical structure from the object
plane , the more blurred its image is.
• The blurring produced in tomography is motional
unsharpness and is produced due to movement in tube and
film during the exposure
14
15
TOMOGRAPHIC BLURRING
• It is distortion of definition of object outside the focal
plane
• Purpose of tomography is to distort or blur the object
that might interfere with our perception of particular
tomographic image
 Blur margin-it’s the edge of a blurred image
16
TOMOGRAPHIC BLURRING
17
TOMOGRAPHIC BLURRING
• Blurring depends on following factors
Amplitude of tube travel
Distance from focal plane
Distance from film
Orientation of tube travel
18
TOMOGRAPHIC BLURRING
Amplitude of tube travel
-Blurring is a direct linear function of number of degrees
of tube travel
- As amplitude of tube travel increases, blurring also
increases
19
TOMOGRAPHIC BLURRING
Distance from focal plane
-Farther the distance from focal plane ,more will be
blurring
-no control over this distance in diagnostic radiology
20
TOMOGRAPHIC BLURRING
Distance from film
-objects far away from film are blurred more than
objects close to film (if they’re at same distance
from focal plane)
21
22
Orientation of tube travel
-When body parts are long and narrow then, the
longitudinal axis of an object is oriented in same direction as
that of x ray tube travel so the image of object will not be
blurred even though it lies outside focal plane
-Maximum blurring occurs when the long axis of the part to
be blurred is perpendicular to the direction of tube travel
TOMOGRAPHIC BLURRING
23
MOTION BLURRING
• All points that are parallel
to table top & are on object
plane are displayed in sharp
focus in tomograph
• All object on object plane
move exactly as same distance
as tomographic film so they are
not blurred
24
MOTION BLURRING
• For object on two different
plane when tube moves, only
image of object plane remain
in sharp focus as only it moves
exactly as film
• objects below & above the
object plane doesn’t move as
film, so the image becomes
blurred
25
IMAGE QUALITY
• In term of contrast & sharpness, the quality of
tomograph isn’t as good as that of radiograph
CONTRAST
• when thin layer being recorded inherent contrast is low
• Inherent contrast of tomographic structure should be
high for better contrast of tomographic image
26
UNSHARPNESS
• Geometric unsharpness;
it increases with increase in focal spot and pivot to film
distance
• Movement unsharpness;
it is due to extended exposure time
27
GRID
• During tomography, linear grid must be used &
• the grid line must be oriented in same direction as x-ray
tube movement that results no grid cut off with better
image quality
28
EQUIPMENT
Tomogram is performed in following ways :
• The x ray tube and film moved through equal and opposite
direction
• The fulcrum determines the layer depth in which the
radiography is done
• The preselected height is in between [0 to 20 cm]
29
EQUIPMENT
• Equipment of tomography may be of two categories
 Tomography attachments to a standard radiographic table
and tube support
 Special tomography table pre selection
30
EQUIPMENT
 The components of tomography attachments to standard x
ray units are
 Linkage mechanism
 Pivot unit
 Mechanical drive
 Drive control
31
EQUIPMENT
32
EQUIPMENT
 Linkage mechanism
• It is long telescopic steel rod coupling together the x ray
tube and bucky carriage by means of clamps and locking
handles
• The link rod may have telescopic structure
33
EQUIPMENT
34
 Pivot unit
• It is small turret like structure & also called as fulcrum tower
Functions of pivot unit
• A pivot for the opposite movements of x ray tube and bucky tray
• A means to alter the height of the pivot point
1) fitted to edge of x ray table nearer to tube stand and linkage arm
2) A scale mounted on adjacent aspect of the tower is calibrated in
cm or inch and position of pivot is shown on scale by means of
suitable indicator
EQUIPMENT
35
EQUIPMENT
• It includes switch assembly; one initiate x-ray exposure & the
other terminate it which operates as linkage arm travel from one
side of tower to the other side,
• So value of exposure depend on time required by the linkage
arm to travel between two station
• & is in turn depends on speed & angle of tube movement
36
EQUIPMENT
37
EQUIPMENT
Mechanical drive
• It allows the movement of tube during exposure
• It makes the use of small motor for the movement of
tube instead of moving tube with hand during
exposure
• As a result reducing the radiation hazard to the
technologist
38
EQUIPMENT
39
EQUIPMENT
 DRIVE CONTROL
• It is control unit for tube drive
• It is present in separate wall mounted box
• It consist of switch which permit
1. Selection of tube speed
2. Selection of angle of exposure
3. Warning lamp to indicate equipment
is energized
40
EQUIPMENT
41
EQUIPMENT
 Tomographic table
• Tomographic tables are those planned especially for
tomography
• Tomographic tables can be categorized into 3 groups
Group I
Group II
Group III
42
TUBE TRAJECTORY
• There are 6 basic types of tube motion(patterns of tube
and film motion as seen from above)
 Linear movement
 Circular movement
 Elliptical movement
 Spiral movement
 Lissajous or figure of 8
 Hypocycloidal movement
43
44
PHANTOM IMAGE
• It is something that appear in sight of image but have no
physical existence
• It is less dense and less sharp than the real image but
causes difficulty in film interpretation
• It is produced by blurred margins of structure outside
the focal plane
• most likely to occur in circular tomography and narrow
angle technique
45
COMPARISON BETWEEN LINEAR AND CIRCULAR TOMOGRAPHY
LINEAR TOMOGRAPHY CIRCULAR TOMOGRAPHY
1) Equipment is inexpensive 1) Equipment is very expensive
2) Section thickness is dependent on
orientation of body parts (no true
section thickness)
2) Section thickness is
independent of orientation of
parts (produces uniform section
thickness)
3) Blur margins are tapered and
indistinct
3) Blur margins are abrupt
and sharply defined
4) Objects outside the focal plane
may be incompletely blurred,
producing ‘parasite’ streaks
4) Objects outside the focal
plane are uniformly blurred ,no
‘parasite’ streaks
5) Does not produce phantom images 5) Likely to produce
phantom images (with
narrow angle
tomography)
46
TYPES OF TOMOGRAPHY
• There are 7 types of tomography
1. Wide angle tomography
2. Zonography
3. Auto tomography
4. Pan tomography
5. Axial tomography
6. Book tomography
7. Computed tomography
47
WIDE ANGLE TOMOGRAPHY
• It enable us to see object that are completely obscured by
overlying shadows on general radiography
• It decreases contrast and sharpness of image
• It produces thinner slice thickness
• It is most effective in studying tissue that have a great deal
of image contrast such as bone
• It is extensively used to examine inner ear
48
ZONOGRAPHY
• It is also known as narrow angle tomography
• It employ tomographic angle of less than 10º
• Requires multidirectional tube motion
• It is not efficient for linear tomography, usually circular
tomography is in practice for zonography
• It produces sharply defined image of object on focal plane
• It has got tendency to produce phantom image
• It is used when subject contrast is so low that thin section
radiography would result in poor image
49
ZONOGRAPHY
• It gives image of thick section
• Its application is in chest and renal radiography
50
•COMPARISON OF WIDE ANGLE TOMOGRAPHY AND
NARROW ANGLE TOMOGRAPHY
51
WIDE ANGLE TOMOGHRAPHY NARROW ANGLE TOMOGRAPHY
1)Tomographic arc of more than 10
degree(usually 30 to 50 degree)
1)Tomographic arc of less than 10
degree
2)Less section thickness 2)Greater section thickness
3)Considerable unsharpness of focal
plane images
3)Very little unsharpness of focal
plane images
4)Maximum blurring of objects
outside focal plane
4)Minimum blurring of objects
outside focal plane
5)Best for tissue with high
contrast(Bone)
5)Best for tissues with low contrast
(lung)
6)Can be done either with linear or
circular motion
6)Usually done with circular
tomograpic motion
7)Unlikely to cause phantom images 7)Frequently causes phantom images
8)Long exposure times 8)Short exposure times (with
properly designed equipment)
AUTO TOMOGRAPHY
• tomography involving movement of the patient instead
of x-ray tube
• The only structure remain in focus are those along the
axis of rotation & other structure are blurred
52
PANTOMOGRAPHY
• Provides a panoramic view of the arc from TM joint to TM
joint.
• Beam is tightly collimated to produce a vertical fan.
• The film holder has a vertical collimation slit to allow the
cassette to roll past the slit and capture the image.
53
AXIAL TOMOGRAPHY
• Gets axial section of the patient by using motion of
patient and film
• X-ray beam remains stationary
BOOK TOMOGRAPHY
 It is used to form image through multiple objective
planes
54
COMPUTED TOMOGRAPHY
• It is used to generate an image of the tissue density in a slice as
thin as 1-10mm in thickness
55
DISADVANTAGES OF TOMOGRAPHY
• High patient dose with multiple cuts
• High cost of circular tomography
• Long exposure time (determined by time it take to move
the tube 3 to 6 sec)
• Motion artifacts with long exposure times are common
56
COMPARISON OF TOMOGRAPHY AND
RADIOGRAPHY
CHARACTERISTIC TOMOGRAPHY GENERAL RADIOGRAPHY
1) Image Cross-section layer Plane
2) Image blurring Increase image quality
(need of tomography)
Deteriorate image
quality(blurring can not be
overcome)
3) Radiation dose High Low
4) Exposure time More Less
5) Immobilization of
patient
Long time Comparatively less time
57
SUMMARY
• Technique of controlled blurring that selects a level in the body
& blurs out the structure below and above that plane leaving a
clear image of selected part.
• Synchronous movement of x ray tube & IR in opposite direction
around stationary fulcrum during exposure.
• Fulcrum determines the layer depth in which the radiography is
done .
• Thickness of section that is in focus depends on amplitude of
tube travel(larger the amplitude, larger the section).
• Tomographic blurring depends on amplitude of tube travel,
distance from focal plane, distance from film & orientation of
tube travel.
• Equipments of tomography comprises of linkage mechanism,
pivot unit, mechanical drive, drive control & tomographic table.
• 6 types of motion i.e. linear, circular, elliptical, spiral, lissajous &
hypocycloidal.
58
SUMMARY
• Wide angle tomography is best for tissue with high
contrast like bone & narrow angle foe tissues with low
contrast like lung.
• Disadvantages like high patient dose, high cost for circular
tomography, motion artifacts & long exposure time are
there.
• Now introduced CT which generates an image of tissue
density as slice which is about 1 to 10 mm thick.
59
REFERENCES
 X-ray equipments for radiographers by DN and MO
Chesney.
 Christensen’s physics for diagnostic radiology
 Radiologic science for technologist by Stewart Carlyle
Bushong
 Various internet researches
60
61
QUESTIONS
1) What are the components of tomographic attachments in
conventional tomography?
2) What are the principles of conventional tomography?
3) What are the blur controlling factors in conventional
tomography?
4) What is fulcrum in conventional tomography?
5) How does section thickness vary with amplitude of x ray
tube travel?
6) What are the types of movements in tomography?
7) What are the disadvantages of tomography?
62

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Principle of conventional tomography-Bibash Shahi ppt..pptx

  • 1. PRINCIPLE OF CONVENTIONAL TOMOGRAPHY BIBASH SHAHI ROLL NO-148 BSC MIT 1ST YEAR IOM MMC
  • 2. INTRODUCTION • In normal radiograph , the anatomical structure of interest is very often partially or completely obscured by the shadows cast by overlying or underlying structures. • This drawback lead to introduction of conventional tomography which records an image of the plane or layer of interest on its own with obsuring effect of other layers. 2
  • 3. • Previously known as body section radiography until, in 1962 International Commission on radiologic units and measurement adopted the term “Tomography” • Also called as planigraphy, stratigraphy and laminography • In tomography, whole plane is in focus not just a point object • Not a method of improving sharpness of any part of radiographic image but it is process of controlled blurring. • Used to inner ear studies and still used at MCG for IVPS. • Popularity decreasing due to introduction of CT and MRI INTRODUCTION 3
  • 4. HISTORY • A need to see beyond 2D image was gradually increased • First approach was to view orthogonal radiograph and make 3D perception. • The origin of tomography’s credit goes to JEAN KIEFFAR, a radiological technologist in US, who developed the special radiographic technique to demonstrate a form of TB that he had in 1928. • His process was termed ‘laminography’ by J.ROBERT ANDREWS,assisted in construction of tomographic device called ‘laminograph’. 4
  • 5. TERMINOLOGY FULCRUM • is the pivot point about which lever arm rotates. • Only point in system which remains stationary • The plane of interest within the patient is positioned at level of fulcrum & all points above and below are blurred • two mechanically different types of fulcrum  First one= the relationship between x ray tube, fulcrum & film is fixed, patient is moved up and down on adjustable table  Second one=has adjustable fulcrum that is moved to height of desired plane while patient remains still on the table. 5
  • 6. 6
  • 7. TERMINOLOGY FOCAL OR OBJECT PLANE • plane of tissue in focus on a tomograph • object plane is at the level of fulcrum & is parallel with table top FOCAL PLANE LEVEL • It is the height of focal plane above tabletop 7
  • 8. TERMINOLOGY AMPLITUDE/TOMOGRAPHIC ANGLE(ARC) • Distance travel by x ray tube and film around the patient is considered as amplitude • It is measured in degrees and is called tomographic angle EXPOSURE ANGLE • Angle through which x ray beam (or central ray) moves during exposure 8
  • 9. 9
  • 10. TERMINOLOGY • TOMOGRAPHIC SECTION  The thickness of tissue that will be imaged  The larger is the tomographic angle , the thinner is the tomographic section. 10
  • 11. 11
  • 12. TERMINOLOGY SECTIONAL INTERVAL It is the distance between fulcrum levels 12
  • 13. PRINCIPLE • The principle of tomography is that it selects a level in the body & blurs out structure below and above that plane leaving a clear image of selected part • Synchronous movements of the x-ray tube and the image receptor in opposite directions around stationary fulcrum (pivot point) during the exposure. 13
  • 14. PRINCIPLE • Tomography is concerned with layer rather than plane. • The fulcrum area is sharp. • The farther is the anatomical structure from the object plane , the more blurred its image is. • The blurring produced in tomography is motional unsharpness and is produced due to movement in tube and film during the exposure 14
  • 15. 15
  • 16. TOMOGRAPHIC BLURRING • It is distortion of definition of object outside the focal plane • Purpose of tomography is to distort or blur the object that might interfere with our perception of particular tomographic image  Blur margin-it’s the edge of a blurred image 16
  • 18. TOMOGRAPHIC BLURRING • Blurring depends on following factors Amplitude of tube travel Distance from focal plane Distance from film Orientation of tube travel 18
  • 19. TOMOGRAPHIC BLURRING Amplitude of tube travel -Blurring is a direct linear function of number of degrees of tube travel - As amplitude of tube travel increases, blurring also increases 19
  • 20. TOMOGRAPHIC BLURRING Distance from focal plane -Farther the distance from focal plane ,more will be blurring -no control over this distance in diagnostic radiology 20
  • 21. TOMOGRAPHIC BLURRING Distance from film -objects far away from film are blurred more than objects close to film (if they’re at same distance from focal plane) 21
  • 22. 22
  • 23. Orientation of tube travel -When body parts are long and narrow then, the longitudinal axis of an object is oriented in same direction as that of x ray tube travel so the image of object will not be blurred even though it lies outside focal plane -Maximum blurring occurs when the long axis of the part to be blurred is perpendicular to the direction of tube travel TOMOGRAPHIC BLURRING 23
  • 24. MOTION BLURRING • All points that are parallel to table top & are on object plane are displayed in sharp focus in tomograph • All object on object plane move exactly as same distance as tomographic film so they are not blurred 24
  • 25. MOTION BLURRING • For object on two different plane when tube moves, only image of object plane remain in sharp focus as only it moves exactly as film • objects below & above the object plane doesn’t move as film, so the image becomes blurred 25
  • 26. IMAGE QUALITY • In term of contrast & sharpness, the quality of tomograph isn’t as good as that of radiograph CONTRAST • when thin layer being recorded inherent contrast is low • Inherent contrast of tomographic structure should be high for better contrast of tomographic image 26
  • 27. UNSHARPNESS • Geometric unsharpness; it increases with increase in focal spot and pivot to film distance • Movement unsharpness; it is due to extended exposure time 27
  • 28. GRID • During tomography, linear grid must be used & • the grid line must be oriented in same direction as x-ray tube movement that results no grid cut off with better image quality 28
  • 29. EQUIPMENT Tomogram is performed in following ways : • The x ray tube and film moved through equal and opposite direction • The fulcrum determines the layer depth in which the radiography is done • The preselected height is in between [0 to 20 cm] 29
  • 30. EQUIPMENT • Equipment of tomography may be of two categories  Tomography attachments to a standard radiographic table and tube support  Special tomography table pre selection 30
  • 31. EQUIPMENT  The components of tomography attachments to standard x ray units are  Linkage mechanism  Pivot unit  Mechanical drive  Drive control 31
  • 33. EQUIPMENT  Linkage mechanism • It is long telescopic steel rod coupling together the x ray tube and bucky carriage by means of clamps and locking handles • The link rod may have telescopic structure 33
  • 35.  Pivot unit • It is small turret like structure & also called as fulcrum tower Functions of pivot unit • A pivot for the opposite movements of x ray tube and bucky tray • A means to alter the height of the pivot point 1) fitted to edge of x ray table nearer to tube stand and linkage arm 2) A scale mounted on adjacent aspect of the tower is calibrated in cm or inch and position of pivot is shown on scale by means of suitable indicator EQUIPMENT 35
  • 36. EQUIPMENT • It includes switch assembly; one initiate x-ray exposure & the other terminate it which operates as linkage arm travel from one side of tower to the other side, • So value of exposure depend on time required by the linkage arm to travel between two station • & is in turn depends on speed & angle of tube movement 36
  • 38. EQUIPMENT Mechanical drive • It allows the movement of tube during exposure • It makes the use of small motor for the movement of tube instead of moving tube with hand during exposure • As a result reducing the radiation hazard to the technologist 38
  • 40. EQUIPMENT  DRIVE CONTROL • It is control unit for tube drive • It is present in separate wall mounted box • It consist of switch which permit 1. Selection of tube speed 2. Selection of angle of exposure 3. Warning lamp to indicate equipment is energized 40
  • 42. EQUIPMENT  Tomographic table • Tomographic tables are those planned especially for tomography • Tomographic tables can be categorized into 3 groups Group I Group II Group III 42
  • 43. TUBE TRAJECTORY • There are 6 basic types of tube motion(patterns of tube and film motion as seen from above)  Linear movement  Circular movement  Elliptical movement  Spiral movement  Lissajous or figure of 8  Hypocycloidal movement 43
  • 44. 44
  • 45. PHANTOM IMAGE • It is something that appear in sight of image but have no physical existence • It is less dense and less sharp than the real image but causes difficulty in film interpretation • It is produced by blurred margins of structure outside the focal plane • most likely to occur in circular tomography and narrow angle technique 45
  • 46. COMPARISON BETWEEN LINEAR AND CIRCULAR TOMOGRAPHY LINEAR TOMOGRAPHY CIRCULAR TOMOGRAPHY 1) Equipment is inexpensive 1) Equipment is very expensive 2) Section thickness is dependent on orientation of body parts (no true section thickness) 2) Section thickness is independent of orientation of parts (produces uniform section thickness) 3) Blur margins are tapered and indistinct 3) Blur margins are abrupt and sharply defined 4) Objects outside the focal plane may be incompletely blurred, producing ‘parasite’ streaks 4) Objects outside the focal plane are uniformly blurred ,no ‘parasite’ streaks 5) Does not produce phantom images 5) Likely to produce phantom images (with narrow angle tomography) 46
  • 47. TYPES OF TOMOGRAPHY • There are 7 types of tomography 1. Wide angle tomography 2. Zonography 3. Auto tomography 4. Pan tomography 5. Axial tomography 6. Book tomography 7. Computed tomography 47
  • 48. WIDE ANGLE TOMOGRAPHY • It enable us to see object that are completely obscured by overlying shadows on general radiography • It decreases contrast and sharpness of image • It produces thinner slice thickness • It is most effective in studying tissue that have a great deal of image contrast such as bone • It is extensively used to examine inner ear 48
  • 49. ZONOGRAPHY • It is also known as narrow angle tomography • It employ tomographic angle of less than 10º • Requires multidirectional tube motion • It is not efficient for linear tomography, usually circular tomography is in practice for zonography • It produces sharply defined image of object on focal plane • It has got tendency to produce phantom image • It is used when subject contrast is so low that thin section radiography would result in poor image 49
  • 50. ZONOGRAPHY • It gives image of thick section • Its application is in chest and renal radiography 50
  • 51. •COMPARISON OF WIDE ANGLE TOMOGRAPHY AND NARROW ANGLE TOMOGRAPHY 51 WIDE ANGLE TOMOGHRAPHY NARROW ANGLE TOMOGRAPHY 1)Tomographic arc of more than 10 degree(usually 30 to 50 degree) 1)Tomographic arc of less than 10 degree 2)Less section thickness 2)Greater section thickness 3)Considerable unsharpness of focal plane images 3)Very little unsharpness of focal plane images 4)Maximum blurring of objects outside focal plane 4)Minimum blurring of objects outside focal plane 5)Best for tissue with high contrast(Bone) 5)Best for tissues with low contrast (lung) 6)Can be done either with linear or circular motion 6)Usually done with circular tomograpic motion 7)Unlikely to cause phantom images 7)Frequently causes phantom images 8)Long exposure times 8)Short exposure times (with properly designed equipment)
  • 52. AUTO TOMOGRAPHY • tomography involving movement of the patient instead of x-ray tube • The only structure remain in focus are those along the axis of rotation & other structure are blurred 52
  • 53. PANTOMOGRAPHY • Provides a panoramic view of the arc from TM joint to TM joint. • Beam is tightly collimated to produce a vertical fan. • The film holder has a vertical collimation slit to allow the cassette to roll past the slit and capture the image. 53
  • 54. AXIAL TOMOGRAPHY • Gets axial section of the patient by using motion of patient and film • X-ray beam remains stationary BOOK TOMOGRAPHY  It is used to form image through multiple objective planes 54
  • 55. COMPUTED TOMOGRAPHY • It is used to generate an image of the tissue density in a slice as thin as 1-10mm in thickness 55
  • 56. DISADVANTAGES OF TOMOGRAPHY • High patient dose with multiple cuts • High cost of circular tomography • Long exposure time (determined by time it take to move the tube 3 to 6 sec) • Motion artifacts with long exposure times are common 56
  • 57. COMPARISON OF TOMOGRAPHY AND RADIOGRAPHY CHARACTERISTIC TOMOGRAPHY GENERAL RADIOGRAPHY 1) Image Cross-section layer Plane 2) Image blurring Increase image quality (need of tomography) Deteriorate image quality(blurring can not be overcome) 3) Radiation dose High Low 4) Exposure time More Less 5) Immobilization of patient Long time Comparatively less time 57
  • 58. SUMMARY • Technique of controlled blurring that selects a level in the body & blurs out the structure below and above that plane leaving a clear image of selected part. • Synchronous movement of x ray tube & IR in opposite direction around stationary fulcrum during exposure. • Fulcrum determines the layer depth in which the radiography is done . • Thickness of section that is in focus depends on amplitude of tube travel(larger the amplitude, larger the section). • Tomographic blurring depends on amplitude of tube travel, distance from focal plane, distance from film & orientation of tube travel. • Equipments of tomography comprises of linkage mechanism, pivot unit, mechanical drive, drive control & tomographic table. • 6 types of motion i.e. linear, circular, elliptical, spiral, lissajous & hypocycloidal. 58
  • 59. SUMMARY • Wide angle tomography is best for tissue with high contrast like bone & narrow angle foe tissues with low contrast like lung. • Disadvantages like high patient dose, high cost for circular tomography, motion artifacts & long exposure time are there. • Now introduced CT which generates an image of tissue density as slice which is about 1 to 10 mm thick. 59
  • 60. REFERENCES  X-ray equipments for radiographers by DN and MO Chesney.  Christensen’s physics for diagnostic radiology  Radiologic science for technologist by Stewart Carlyle Bushong  Various internet researches 60
  • 61. 61
  • 62. QUESTIONS 1) What are the components of tomographic attachments in conventional tomography? 2) What are the principles of conventional tomography? 3) What are the blur controlling factors in conventional tomography? 4) What is fulcrum in conventional tomography? 5) How does section thickness vary with amplitude of x ray tube travel? 6) What are the types of movements in tomography? 7) What are the disadvantages of tomography? 62