tomography s. guilbaud education director school of radiologic technology

TOMOGRAPHY

S. GuilbaudEducation DirectorSchool of Radiologic Technology

S. Guilbaud, Education

Director

Tomography

1. The use of motion in radiography to blur out superimposing anatomy so that the structures of interest can be seen through them.


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Several names have been used to identify tomography. Laminography Planigraphy Body-section radiography


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Tomography affords the opportunity to obtain two objectives. Analysis Localization


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Analysis Division of complex anatomical structures

into small parts such that each may be examined individually.

Typical tomograms focus in on thin sections of anatomy for evaluation.

E.g... Tomography of the scaphoid, IAC


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Localization/Zonography Involves tomograms of large sections of

anatomy (zones). Allows for parts of the body to be imaged

using fewer or a limited number of exposures.


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Most tomography is performed by adding movement to the X-ray tube and film. Auto-tomography uses the movement

of the anatomy to achieve the blurring of superimposing anatomy.


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The tube and film remain stationary while the anatomical part is permitted to move during the exposure. E.g... Lateral projection of thoracic spine. PA oblique projection of sternum. AP projection of C-Spine using wagging

jaw method.


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Fulcrum PointPoint at which the CR. Or tube pivots such that the area of interest remains centered throughout the exposure. The X-ray tube rotates upon its axis

so that the part of interest remains centered.


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This results in the part of interest remaining in the center of the CR & film thus the absence of blurring at the center of the film.

This results in the blurring of superimposing anatomy & blurring of peripheral anatomy.

The distance from the focal point has a direct relationship to blur with. As the distance of the object from the fulcrum

increases, as does the blurring.


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Focal PlaneThe layer of anatomy with a distinct thickness which will be recorded on the film with minimal blurring.Within this focal plane thickness, no anatomical part is so blurred that the human eye can detect this blur. Thus, the anatomy at the focal plane appears with great detail as compared to the peripheral & superimposing anatomy.


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The focal plane is set by the radiographer when she or he selects the depth at which the anatomical structure is located.


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Tomographic arc or angle or amplitude

The actual distance the x-ray tube travels. Total distance the x-ray tube travels

during the exposure. Exposure amplitude is always greater

than or equal to the tomographic amplitude. The x-ray tube is engaged in an exposure

for the entire exposure angle.


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The relationship b/w section thickness & tomographic amplitude is inverse. The greater the angle, the thinner the

slice thickness.


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Section Thickness/Slice Thickness

Slice thickness is considered the width of the focal plane. Controlled by exposure amplitude or

tomographic amplitude.

Section thickness occurs in a plane that is parallel to the image receptor.Although there is an increase in OID during mvmt., the change in SOD & SID compensates causing the magnification ratio to remain unchanged.


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Type of Tomographic Motion

There are 7 types of tomographic movement. Linear Curvilinear Circular Elliptical Figure eight Trispiral Hypocycloidal


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Linear mvmt. Is most popular

Tube & image receptor mvmt are along a straight line.Very inexpensive to manufacture & maintain.


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Curvilinear movement

Mvmt. Of tube is linear however, the tube travel is along the long axis of the table.During mvmt., OIS & SID remains fixed during exposure.


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Circular Movement

Mvmt. is circular while maintaining focal point.Expensive b/c the machine requires special equipment & exposure times of 3-6 seconds.


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Elliptical Movement

X-ray tube moves in elliptical motion.Seldom used b/c of restrictive cost.


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Figure Eight Movement

X-ray tube moves in a figure 8.Reduces edge phantoms caused by elliptical mvmt.Most phantom images are eliminated.


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Trispiral movement

X-ray tube moves in three spiral mvmts. From inside to outside.Able to produce very thin sections.


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Hypocycloidal movement

X-ray tube moves in three concentric circles.Produces the thinnest possible tomographic sections, by providing maximum tomographic amplitude.


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KV, mA & TimeThe 15% kVp rule is critical. Fine density adjustments must be made

using kVp variations b/c of fixed time settings.

It takes at least a 5% kVp adjustments to produce a visible change in density. Special care must be taken as not to

create a problem w/ contrast scale.


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Scatter must be kept to an absolute minimum.

The devices that may be used to reduce the amt. Of scatter produced are: High grid ratio Very close collimation. Lead masks on the table top. Compression bands where applicable.


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Dedicated Tomography Units

Normally come with generators that are able to achieve mA stations as low as; 10, 15, 20, 30, 50 mA. Units that are not dedicated to

tomography only often achieve mA as low as 50 or 25.

Recall that a change of 30% in mAs is required before a visible difference can be noted.


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Tomographic Exposure

The time set during the tomographic exposure is critical. The time must be long enough for the tube to

complete the tomographic amplitude. If the exposure time is excessive, the

amplitude will increase the density at the final tube position.

If the exposure time is too short, it will not permit enough blurring & will project erratic phantoms.

Complex tomographic motions require long exposure times such as 3 – 6 seconds.


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PhantomsImages that do not correspond to the existing structures.They are produced during complex tomographic motions. They are especially noted during

circular tube motions when the part & tube are parallel to the long axis of the structure.


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Phantoms are decreased by reducing the section thickness and increasing the exposure amplitude. Note: Phantoms are especially

prominent in chest and skull tomography.


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References

Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher.

Bushong, S., Radiologic Science for Technologists, Physics, Biology and Protection, 8th Edition, C.V. Mosby Company.

Carlton et al, Principles of Radiographic Imaging, An Art and Science, Delmar Publishing.

Quinn, B. C., Fuchs’s Radiographic Exposure, Processing & Quality Control, 6th Edition, Charles Thomas Publisher.

Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles C. Thomas Publisher.

tomography s. guilbaud education director school of radiologic technology

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