Plain Film

Tomography

George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia

Resident Physics LecturesResident Physics Lectures

Body Section Tomography

Body Section Tomography

• Not CT• Body Section Imaging• Goal

keep plane of interest in focus blur all other plans

» enhances contrast

• Popularity decreasing because of CT MRI

GeorgeDavid

Tomography HistoryTomography History

• Predates CT by decades• Used to be popular for inner ear

studies• Still used at MCG for IVP’s

Tomography BlurringTomography Blurring

• Blurring accomplished by synchronous movement of tube & film tube & bucky physically

connected by rod rod pivots around fulcrum tube moves one direction film moves in other direction

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Conventional Tomography BlurringConventional Tomography Blurring

• Image produced on film

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

Tomography BlurringTomography Blurring

• the further from the fulcrum an object is, the more it blurs!

Tomography BlurringTomography Blurring

• objects shaped & oriented in direction of motion elongate do not blur

Tomographic BlurringTomographic Blurring• Blurring improved by use of complex

motions

circular

tri-spiral

elliptical

Hypocycloidal

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Complex BlurringComplex Blurring

• The more complex the blurring motion the sharper the tomographic image

» better blurring

the more expensive the machine

$

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Tomo Patient DosesTomo Patient Doses

• Tomo can be high dose procedure several rads not unusual

• one image per cut with film cuts at many levels routinely employed to find cut of

interest each cut exposes entire field

• Can do multiple images per cut with digital receptor

Thickness of CutThickness of Cut• Same as thickness of region in focus• Determined by angle tube moves• Larger angles yield thinner cuts

more motion = more blurring

• Smaller angles yield thicker cuts 0o (stationary) yields conventional film

Thickness of CutThickness of Cut

Smaller Angle;Thicker Cut

Larger Angle;Thinner Cut

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Cut ThicknessCut Thickness

• Approximate cut thickness for linear tomo

Tomo Angle Thickness of Cut (mm) (degrees)-------------------------------------------------------

0 Infinite (non-tomo) 2 31 4 16 6 1110 620 335 250 1

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Grids & TomoGrids & Tomo

• Linear grid used

• Grid lines parallel with table for linear tomo

• Grid must change orientation (rotate) in complex motion tomo eliminates grid cutoff