Principle of conventional tomography-Bibash Shahi ppt..pptx

before the computed tomography, it had been widely used.

1. PRINCIPLE OF CONVENTIONAL TOMOGRAPHY BIBASH SHAHI ROLL NO-148 BSC MIT 1 ST 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

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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

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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

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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

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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

17. TOMOGRAPHIC BLURRING 17

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

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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 I n 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

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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

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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 fitted to edge of x ray table nearer to tube stand and linkage arm 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

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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

39. EQUIPMENT 39

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 Selection of tube speed S election of angle of exposure Warning lamp to indicate equipment is energized 40

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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

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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) 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 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 Y

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 unsharpnes s 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

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62. QUESTIONS 1) What are the components of tomographic attachments in conventional tomography? 2) What are the principles of conventional tomography? What are the blur controlling factors in conventional tomography? What is fulcrum in conventional t omography ? 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