PRINCIPLE OF

TOMOGRAPHY & THEIR VARIOUS TYPES

M O D E R A T O R

MR S. C. BANSALLECTURER

P R E S E N T E D B Y

MANISH ANANDB. SC. MED. TECH. FINAL YEAR STUDENT

P O S T G R A D U A T E I N S T I T U T E O F M E D I C A L E D U C A T I O N A N D R E S E A R C H C H A N D I G A R H

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I N T R O D U C T I O N

The word tomography is derived from the two Greek words:

Tomos (=slice) and

Graphia (=describing).

So the tomography is describing of slices by imaging it.

Tomography is a radiographic technique that selects a level in

the body & blurs out structures below & above that plane

leaving a clear image of this selected anatomy.

Tomography concerned with layer rather than plane.

3

Need of Tomography:

As we know that tomography records cross-sectional image of selected layer, so by using tomography, we can evaluate the abnormality of that cross-section, that is usually superimposed by underlying and overlying structures in general radiography.

If we want to see the debris of potato we have to cut it into different slice, same in tomography if we want to see any underlying abnormality than we have to image the cut section .

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A Brief History Of Tomography Invention

It is a sobering thought that nearly 55% of what is done in

radiology today did not exist 20 years ago.

There were no MRI , no ultrasound or CT.

Since the introduction of radiology in medicine, a need to see

beyond the 2 dimensional image was gradually increasing, the

first approach was to view orthogonal radiographs and make a

mental 3D perception.

But real major break through was tomography which is also

called planigraphy or body section radiography.

5

History cont…

The origin of tomography cannot be attributed to any person,

but the major credit goes to JEAN KIEFFAR , a radiological

technologist in US ,who developed the special radiographic

technique to demonstrate a from of TB that he had in 1928.

His process was termed “Laminagraphy” by another American

J. ROBERT ANDREWS who assisted KIEFFAR in the

construction of this first tomographic device known as

LAMINAGRAPH.

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Basic Principle of Tomography:

• If there is synchronous movement b/w either of two among the three that is the patient ,the x-ray tube or the image receptor, than there is blurring of image causes.

• While movement only one thing is constant that is the fulcrum point of the tomographic equipment and the plane which posses this point is well demonstrated. It also results blurring of image of that structure which lie above and below to that plane.

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contd…

Now the synchronous movement can be achieved by any of the following methods:

The film remains stationary while the x-ray tube and the patient move.

The x-ray tube remains Stationary while the film and the patient move.

The patient remains stationary while the x-ray tube and the film move. This is the most accepted technique used.

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Terminology:

Before discussing blurring pattern and the concept of blurring, we have to know some relevant terms,

these are: – Tomographic angle – Exposure angle– fulcrum– Pivot point – Focal plane– Section thickness

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Tomographic Angle: -It is the amplitude of tube travel expressed in degrees.

Exposure Angle: - It is the angle through which

the x-ray beam moves during the exposure. The exposure angle and tomographic angle are not always equal to each other, mostly T. angle is greater than the exposure angle this is due to equipment malfunction.

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Section Thickness: - As we go apart from the focal plane the sharpness of image is decreases so the range of thickness that can be accepted by our eye is called the slice thickness.

Slice thickness inversely proportional to the tomographic angle

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Pivot point/Fulcrum: -

The only point of the system

that remains stationary, we can also

assume the axis around which the

equipment move .

Focal Plane:-

The plane of maximal focus

and represent the axis about which the

x-ray tube and film rotate.

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Tomography Blurring:

• Objects above or below fulcrum plane change position on film & thus blur

B BCCA ACA

A

B

C

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A'B’

C’

Distance travel by Film =X.

Tomographic Blurring Principle

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contd… A’ = distance travel by image A

B’ = distance travel by image B

C’ = distance travel by image C

X = film travel distance

Here A’>B’ = X<C’

To blur any image there should relative movement b/w the

film and the object higher the relative motion more the blur.

Hence the image of A and c is being blur , there is no blurring

of image B.

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Blurring depends upon:

Amplitude of tube travel α width of blur

Distance from focal plane α width of blur

Distance from film α width of blur

Orientation of tube travel = maximum blurring when

long axis of the part to be blurred is perpendicular to the

direction of tube travel.

contd…

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BLUR: Controlling Factors

1. Distance from objective plane:

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2. Exposure Angle: Increase in exposure angle,increase in movement,thinner focal plane and more blurring of above

and below structures.

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3. Object Image Receptor Distance (OID):Greater OID,increased blurring

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4. Tube Trajectory:Maximum blurring occurs when object is perpendicular to

tube travel.

F I V E P O S S I B L E T U B E T R A J E C T O R I E S

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Five Possible Tube Trajectories:

1. LINEAR

2. ELLIPTICAL 3. CIRCULAR

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4. SPIRAL

Five Possible Tube Trajectories: contd.

225. HYPOCYCLOIDAL

Five Possible Tube Trajectories: contd.

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Variable fulcrum: fulcrum level changes

Fixed fulcrum: table and patient move, changing fulcrum level.

Image Quality Of Tomographic image In terms of contrast and sharpness the quality of tomograph is not as

good as that of radiograph. CONTRAST :- Because of thin layer is being recorded, inherent

contrast is low, so the inherent contrast should high for tomographic structure for better contrast. e.g.- if there is bone with adjacent air cavity then the expectable contrast can be achieve by 1mm of thickness.

UNSHARPNESS:- Geometric unsharpness: Increases with increase in size of focal

spot and pivot to film distance. E.g.-in hypocycloidal movement the PFD is large so more than 0.6 mm focal spot is not allowed .

Movement unsharpness: Predominant in tomography due to extended exposure time.

During tomography, linear grids must be used and the grid lines must be oriented in the same direction as the X-ray tube movement that results no grid cut-off with better image quality. 24

NOTE:

25

F

FULCRUM

X-RAY FOCUS PATH

C I R C U L A R M O V E M E N T A N D P O S I T I O N O F G R I D

F

FF

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Magnification in Tomography

There is some magnification of tomographic image, that can be

calculated by:

This can be summarized as follows:

1. Using fixed pivot equipment, all layers are recorded at the same

magnification.

2. Using a moving pivot system, magnification increases with increase in

pivot height.

3. In simultaneous multisection tomography the magnification is the

same for all layers taken with a single exposure.

FOCUS-TO-PIVOT DISTANCE

__________________________FFDM=

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Phantom Image:

Phantom is defined by Webster as ‘some thing that appears to the sight but has no physical existence. It can be formed by 2 different mechanism.

1st type of phantom image is produce in narrow angle tomography as shown in fig.

It is produce by the blurred margins of structures outside of the focal plane, and they are most likely to occur with circular tomography and narrow angle tomography.

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2nd type of phantom image is produce by the displacement of the blurred image from an object outside the focal plane to simulate a less dense structure.

This type of phantom image most likely to occur when the shape of the part being examined is similar to that of the x-ray tube motion so it is common in skull tomography in circular motion.

contd…

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contd….The film shows the four image of same coin

A=simple radiograph

B,C,& D is the 3 circular tomogram of same coin at slightly different

distances from focal plane.

One edge of coin is marked by a line to identify the side.

•If the coin represent a densely calcified pulmonary granuloma ,then the exactly doubled image would look like a larger, less dense soft tissue nodule in image C and more than double image in fig. D. that mimic a thick wall cavitary lesion .

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Types of Tomographic Movement:

The more a tomographic motion differs from the shape of the object being examined, less likely to produce phantom image.

There are wide variety of tube motion.

These are: – linear – curvilinear– circular– elliptical– figure of 8– hypocycloidal– spiral

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Linear Movement: The tube and image receptor is synchronized to perform linear movement.

Curvilinear: The tube and I.R. is synchronized to perform movement in curve.

Circular Movement: The movement in circular fashion.

Figure of 8: The movement in figure of 8.

Hypocycloidal & Spiral Movement: As shown in figure.

SPIRAL CURVILINEAR

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Choice of Tomographic Movement:

TYPES OF MOVEMENT ADVANTAGES DISADVANTAGES

Pluridirectional: HypocycloidalSpiral, Figure 8(large exposure angle only)

Maximum blurring of images of any shape of structure outside the layer selected. Very thin layers (approx. 1 mm) allow small structures to be isolated. Used specially in imaging of temporal bone.

Many cuts required to examine fully a thick region. e.g. 10 cuts per 1 cm thickness. Long exposure time (not less than 3 seconds) means that immobilization of the patient is essential. The thin cut can give images with less than the required contrast.

Circular (variable exposure angle)

Good blurring of images of most of the structures outside the selected layer. Shorter exposure time and thicker layer available than with the most complex movements. used in imaging of pituitary fossa ,and sternum.

Can produce confusing circular pseudo shadows as well as double images of linear structures just outside the selected layer.

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TYPES OF MOVEMENT ADVANTAGES DISADVANTAGES

Elliptical (variable exposure angle)

Very good blurring of structures outside the chosen layers providing the major axis of the ellipse is at right angles to the general lines of the structure.

Can produce double images of structures which are in the line of the major axis and just outside of the layer to be recorded.

Linear (variable exposure angle)

Very short exposure time available. Wide range of exposure angles and exposure time can be available. Used in imaging of lungs, larynx, and tracheal bifurcation.

Linear streaking can obscure information contained in the image of selected layer. This is a particular disadvantage when small structures are being examined using a large exposure angle.

contd…

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Equipment for Tomography:

Tomography machines may be varied in appearance and function,

but all have some basic requirements in common.

They are:

1. a linkage mechanism.

2. a pivot unit.

3. a mechanical drive.

4. a drive control, usually a separate wall-mounted unit.

5. tomographic table.

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The Linkage Mechanism:

It is long telescopic steel rod which couples together the x-ray tube and the bucky carriage by means of clamps and locking handles.

In linking them the rod must allow the x-ray tube and the bucky carriage to be further apart at the beginning and the end of their excursions.

To achieve this , the link rod may have a telescopic structure.

The physical dimensions of the link prevent completely free selection of the anode film distance.

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The Pivot Unit:

It is a turret-like structure and sometimes it is also called the

fulcrum tower.

Function of Pivot Unit:

a) A pivot for the opposite movements of the X-ray tube and the

Bucky tray;

b) A means to alter the height of the pivot point.

It is fitted to the edge of the X-ray table which is nearer to the

tube stand and linkage arm.

A scale mounted on an adjacent aspect of the tower is

calibrated in cm or inch and position of the pivot is shown on

the scale by means of a suitable indicator.

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The fulcrum tower includes the switch assembly which effects the X-ray exposure. During its excursion from one side of the tower to the other linkage arm operates primarily two sets of contacts, the first of these initiates the X-ray exposure and the second terminates the exposure.

The value of the exposure interval obtained is dependent upon the period of time required by the linkage arm to travel between these two stations and this in turn depends upon the speed and angle of the tube movement.

contd…

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The Mechanical Drive:

Travel of the X-ray tube during the exposure can be achieved by a variety of means.

It is an advantage of ceiling suspension that the tomographic drive is more direct and therefore more efficient when taken from such suspension than from a floor mounting.

It is customary for the speed of the motor to be variable by means of a remote control unit.

In some instances the exposure can be made during only one direction of tube travel.

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X-ray tube

Linkage mechanism

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The Drive Control:

The control unit for the tube drive often in a separate wall mounted box.

This usually has switches which permit: selection of the tube’s speed of travel selection of the angle of exposure trial runs of the apparatus to be made without X-ray

exposure.

In some cases a warning lamp is included which indicates when the equipment is energized.

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Tomographic Tables:

Tomographic tables are those planned especially for tomography, although they may allow general radiography to be performed.

Tomographic tables can be categorized in three general groups.

They are: Group-I Group-II and Group-III

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Group-I

Tomographic table in this group characteristically is a simple table for radiography, having a floating top and the particular feature of an integrated tube stand suitable for tomography.

Many of the tables in this group provide only a linear tomographic trajectory.

A typical example of a tomographic table in this group possesses the following attributes:

Linear trajectory of the x-ray tube and the film .A choice of three angle of tube swing ,40º, 20º, 8º.A choice of two speed at each angle of exposure ,as follows 40º at 1 s-3 s 20º with 0.5 s-1.5 s 8º with 2 s-0.6 s.

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Group-II

Tomographic tables in this group differ from those in the Ist

mainly in providing a circular or elliptical movement or both,

in addition to a linear trajectory.

These tables may allow conventional ‘on-table’ radiography.

Their characteristics, in respect of variable angles, variable

speeds and motorized adjustments of layer-height are similar

to the group first.

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Group-III

Tomographic tables in this group are highly specialized.

The range of tomographic movements offered by this group of tables are:

Linear , circular, elliptical or both Hypocycloidal, spiral or both.

In addition to the provision of multidirectional excursions of the X-ray tube, the table may have facilities

To tilt or rotate the patient or both Television fluoroscopy.

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Peculiarities of Tomographic Table:

The tomographic table able to: To produce the different types of movement. There is control on speed of movement. Control on angle of exposure. Control on time of exposure. Adjust the fulcrum and the pivot point.

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The Speed Of Movement And Length Of Trajectory

Effect on exposure time:

The speed at which the tube moves during tomography

controls radiographic exposure time, the faster the tube

movement, the shorter is the interval of the exposure.

Another factor in the time obtained is the angle of exposure,

i.e. the angle of swing through which the tube moves.

When the tube describes a wide angle, exposure time will be

also longer than when the angle is small.

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Effect On Detail Perception

In theory with a given selection of milliamperes, faster tube

travel should mean better visibility of detail ,this being more

readily perceived in sharp layer because the shadows of other

structures are underexposed to a greater degree when motion

is rapid.

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Various Types Of Tomography:

These are:

Wide angle tomography

Zonography {narrow angle tomography}

Circular tomography

Pantomography

Multisection tomography

Trans axial tomography

Auto tomography

Skip tomography

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Wide Angle Tomography:

The purpose of wide angle tomography is to extend, the limits of Roentgen visibility to enable us to see objects that are completely obscured by overlying shadow in general radiograph.

In this the exposure angle is wide and hence the slice thickness becomes thin.

The sharpness of all images is decreased by wide-angle techniques including those originating from the focal.

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Most effective in studying tissues that have a great deal of

image contrast because with wide angle tomography we have

very thin section and no thickness difference b/w adjacent part

of the image, e.g. imaging of inner ear ossicles in which there

is enough contrast in very thin section.

contd…

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Zonography

Also known as Narrow Angle Tomography.

Zonography is not efficient with linear tomography it needs multidirectional tube motion .

Zonography is specially useful for when the tissues being examined have little natural contrast.

The lungs offers ideal medium for this techniques where contrast is low and the interfering ribs are usually several centimeters from our plane of interest.

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High Frequency Generator 500 mA/150kVp Automatic Exposure Control

(AEC – Optional) Anatomical Programs

(APR – Optional) 40°, 30°, 20° and 8°

zonography Fulcrum-height varies from 0-

240mm with 1 mm increment.　

Equipment For Zonography

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Equipment For Multidirectional Tomography

High Frequency Generator 500mA/150kVp Automatic Exposure Control

(AEC – Optional) Anatomical Programs

(APR – Optional) Exposure from knee to skull

(16.5 sec to 64 sec) + 135° to - 35° rotation

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Difference Between Zonography & The Wide Angle Tomography

WIDE ANGLE TOMOGRAPHY NARROW-ANGLE TOMOGRAPHY

1. Tomographic arc of more than 10º (usually 30º to 50º)

1. Tomographic arc are less than 10º

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. 5. Best for tissue with low contrast (lung)

6. Can be done with either linear or circular motion.

6. Usually done with circular tomographic motion.

7. Unlikely to cause phantom images. 7. Frequently causes phantom images.

8. Long exposure times. 8. Short exposure times (with properly designed equipments)

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contd…

NARROW ANGLE TOMOGRAPHY

PHANTOM HAVING LEAD LETTERS

WIDE ANGLE TOMOGRAPHY

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Circular Tomography:

The movement of the x-ray tube, film and grid for circular tomography is shown in fig.

As shown in fig. there is movement of grid also a/c to the movement of tube and always parallel to the base of image , if it is not than there is great grid cutoff.

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Advantages: Can produce a uniform section thickness. With the circular tomography all portion of phantom image are

uniformly blurred ,no matter how they orientation in space while as in linear motion only that image is being blurred that is perpendicular to the tube movement.

Disadvantage: High cost Because of long exposure time the chest tomography is not

suitable. Sharp cutoff of the blur patterns, which is conducive to

phantom image formation.

Difference b/w Circular Tomography & Linear Tomography

Characters LINEAR TOMOGRAPHY

CIRCULAR TOMOGRAPHY

Equipment cost inexpensive. expensive

Section thickness depends upon orientation of body part.

independent of orientation.

Blurring Pattern tapered & indistinct. abrupt and sharply defined.

Parasite strike outside the focal plane absent

Phantom images not produced likely to occur.

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Trans Axial Tomography:

In this the plane of section runs through patient transversely.

The X-ray film lies flat on a rotating horizontal table besides

the patient.

Table is positioned, a little below the desired focal plane.

The X-ray direct obliquely through patient fall on to the film.

Tube remains stationary throughout the exposure.

The patient and the film both rotate in the same direction and

at same velocity.

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Only those points actually on the focal plane remain in short focus throughout a rotation.

Section thickness is determined by angle between X-ray tube and film.

More obliquely the central ray is directed towards the film, thinner is the tomo section.

All points are equally magnified and image is not distorted. A patient sits on a special rotating chair in an upright position

rotating table allows us to take image in coronal as well as in sagital plane.

When the patient is facing towards and opposite, there is coronal cut.

When patient turned sideways, there is sagital cut section.

contd…

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Trans Axial Tomographic Equipment:

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Skip Tomography:

It is a method that stops the exposure through a portion of tube’s motion.

Exposure skips during central portion of tomo when dense vertebral structure are superimposed over point of interest.

It is applicable to wide angle technique in which goal is to broaden the limits of visualization.

Usually 20º of time angle is skipped.

Technique works when there is fairly large distances between object if interest and object to be blurred.

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Auto Tomography:

A method of body-section radiography involving movement of the

patient instead of the x-ray tube.

It used to show midline structure of brain stem, fourth ventricle.

The X-ray tube and film both remain stationary while head is rotated

back and forth through an angle of approx 10º.

The only structures that remain in focus are those located along the

axis of rotation. Other structures are blurred, including dense portion

of temporal bone which tends to obscure delicate architecture of

brainstem.

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Auto Tomography: Breathing Technique

Tube and IR stationary, objects move

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Pantomography: It is a special technique that produces a panoramic

roentgenogram of a curved surface.

The patient remains stationary throughout examination.

An X-ray tube and film holder both rotate during exposure.

Film holder had protective pad and is considerably longer than the film.

Film is exposed through a narrow slit in its holder and moves across it as tube rotates and image is laid out as film passes the slit.

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The resultant roentgenogram

is a flattened out image of

curved surface.

The rounded configuration of

teeth and mandible are taken

and widely used in dentistry.

Pantomography of jaw show

TM joint on either side of film

and teeth laid out between

them.

contd…

68

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Air shadows N= nasal cavityNP= nasopharynxOP= oropharynxGP= glossopharynxE= external auditory canal

Soft tissue shadows SP=soft palateT=dorsum of tongueE= EpiglotisPP= posterior pharyngeal wall

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Multisection Tomography:

The principle is that several layers

of film are exposed simultaneously

during a single tomographic swing.

In this technique, several layers of

the body section can be recorded

using one exposure.

There is one mechanical fulcrum

for the top film and a virtual axis

for every other film.

In this technique top and bottom

layers are equally magnified.

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Multisection Tomographic Cassette

There is special type of cassette used in multisection tomography that can hold 3-7 film with there respective intensifying screens and spacers.

The spacer may be 5 to 10 mm in thickness and made of radiolucent foam.

The section thickness depends upon the space b/w the spacer.

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Multisection Tomography:

Advantages:

A dose to the patient is reduced, since a number of separate

radiographs are obtained at the cost of a single exposure.

Each of these radiographs is taken at exactly the same moment

in the respiratory or other physiological phase.

It is the only way in which rapidly transient phenomena such

as vascular fillings in angiography- can be satisfactorily

tomographed.

Disadvantages:

It has uncontrolled scatter radiation.

Film quality is not so good.

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QA & QC of Tomographic Equipment:

Tomographic equipment is carefully checked and calibrated by the manufacturer’s service representative when it is installed.

The tube film movement must be stable and exactly balanced and there must be synchronization of the travel time and exposure time at each of the exposure angles and tomographic movements . These are checked with a pinhole test device.

The pinhole test device is a lead plate with a very small beveled hole in the middle, which is positioned on the table top directly in line with the central tray.

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Performance criteria and tolerance limits for tomographic

quality control tests vary somewhat with the type of

tomographic unit. The unit's performance at the time of

acceptance testing will set the baseline standard.

For a typical tomographic unit, the following standards are

provided as a guide to evaluate acceptable performance:

contd…

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1. Fulcrum Height Accuracy: -a. Section level:

The agreement expected between the indicated and measured section levels varies somewhat, depending upon the type of tomographic unit. In all cases, however, agreement to within +/- 5 mm should be achieved. In measurements of this characteristic the level setting should always be approached from the same direction.

b. Level Incrementation:

In incrementing from one tomographic section to the next, level position should be reproducible to within +/-2 mm. In measurements of this characteristic, the level setting should always be approached from the same direction.

contd…

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2. Thickness of Cut: -a. Section Thickness:

This characteristic varies with the type of tomographic motion and the exposure angle and uniformity. It is recommended that tolerance limits be established for each particular unit from images compared from one set of quality control measurements to the next. In measurements of this characteristic, the level setting should always be approached from the same direction.

b. Exposure Angle:

Indicated and measured exposure angles should agree to within +/-5 degrees. For units employing symmetric motion at wide angles, the symmetry of exposure angle should be within +/-5 degrees with respect to the midline.

contd…

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3. Mechanical StabilityThe density of the image pattern on the resultant film

from the pinhole test should be nearly uniform and straight. The image should reveal no unexpected overlaps, inconsistencies of exposure, or asymmetries in motion.

4. Spatial ResolutionMost tomographic units should depict a 40 mesh

screen pattern, ie. 40 holes per inch.

5. Patient Entrance ExposureIn making exposure measurements, care should be

taken to ensure that the dosimeter is positioned in the x-ray beam during the entire exposure. Quantitative criteria are unavailable for the values of PEE expected for tomography. Facilities should set their own baseline standards.

contd…

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Equipment: - One rectangular sponge approximately 5 cm in height. One lead aperture plate, 4 x 4 x 1/8 inch with a 1/16 inch hole

in the centre. One 18 cm by 24 cm loaded cassette.

Procedure: - Position the cassette in the bucky. Place the lead aperture plate on top of the sponge and position

on tabletop. Using the x-ray tube centering device, centre the x-ray tube over the plate. The hole in the lead aperture plate must coincide with the central ray of the x-ray field.

PINHOLE TEST:(Mechanical Stability and Tube Angle)

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Collimate the x-ray beam to a 10 cm by 10 cm field size.

Select radiographic mode and expose the cassette using

approximately 50 kVp and 5 mAs. Do not remove the

cassette from the tray.

Select the most commonly used tomographic mode and

cut thickness.

Select a cut level of 12 cm. Expose the cassette in tomographic mode for the second

time using approximately 50 kVp and 10 to 20 mAs. Process the film.

contd…

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Information Collected from the Pinhole Test:

Geometric representation of dots shows the pattern of motion

weather it linear or circular or any other motion.

It shows us the symmetry of the exposure angle with a

properly functioning unit, the exposed dotted will be equal in

length on either side of center of tomographic arc.

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For example:

a = 5cm. &

b = 5 cm.

Hence,

Tube angle= θ =45

Tan θ=a/b

Tan θ=1

We can find the true exposure angle as follows:

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Radiation Protection:

The technologist should have well concept of anatomical part ,

so that he can take the image of that level in one shoot without

repeating the procedure.

Wherever possible, the patient is positioned in such a way that

the structure of interest is parallel to the film, so reducing the

number of layer required for a complete record of structure.

In skull tomography, the patient is positioned prone whenever

possible to reduce radiation dose to the eye lens.

A small field size is essential not only for radiation protection

but also to improve radiographic contrast.

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Difference B/W Radiography And Tomography

Differentiating Characteristic

TOMOGRAPHY RADIOGRAPHY

Imaging of cross section layer plane

Blurring image need of tomography detoriates the image quality

Movement synchronous movement there

no any movement

Equipment specification

specialized to perform movement

no such specification

Radiation dose more less

Exposure time more less

Immobilization of patient

long time Comparatively less time

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Advancement in Tomography:

On the basis of blurring principle of the conventional tomography many of the newer developments comes in the imaging world.

Because of introduction of computer, newer imaging techniques and the advanced software, newer development of tomography occurs but the main principle of blurring is same.

85

Computed Tomography:

Among all the newer developments, computed tomography is extensively performed in these days.

With the advent of computer, it has brought a great

revolution in every fields. In radiology the conventional tomography get replaced by computer aided tomography called computed tomography.

Computed tomography is the process of creating a cross-sectional tomographic plane or slice of any part of body in which computer is used to make a mathematical reconstruction of a tomogram.

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Historical Aspects of CT:

Computed tomography is the combined result of various highly developed technologies.

CT was the end product of years of work by numerous scientists. Basis of CT were made by Austrian mathematician J. Radon, according to which 2-dimentional image would be reproduced from infinite set of projections of an object, by getting this idea G.N. Hounsfield along with Dr. Cormack made first prototype model of CT in 1972 named EMI scanner and was placed Atbinson morely hospital in England.

87

Principle & Working of CT:

The basic principle behind CT is that the internal structure of

an object can be reconstructed from the multiple projection of

that object.

WORKING:

First the information is collected in raw form by means of X-

ray source and radiation detectors.

These detectors do not form the image but information about

attenuation is fed into computer where image is processed by

several methods of mathematical reconstruction and displayed

as visual image on TV monitor.

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Advantage of CT over Conventional Tomography

CT can resolve differences in tissue density as low as 0.3% compared to 2% of conventional so it can differentiate soft tissue like cyst, blood clot, sinuses etc easily.

Dose level are low to patient as due to fine beam.

2- or 3-dimentional image with axial cuts are possible in CT only.

Image manipulation can be done according to our choice so less risk of incorrect exposure.

CT image

conventional tomogram

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C O N C L U S I O N

Now a days the conventional tomography

is being fully omitted because of high

radiation dose and the long scanning time

but the principle of tomographic blurring

is still works in works in the form of

computer aided tomography .

90

References:

www.wikipedia.com

Christensen’s Physics of Diagnostic Radiology

X-ray Equipments for Student Radiographers: Chesney

Clark’s Positioning in Radiography

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THANKS

S K Y I S L I M I T