Resident Physics Lectures
Plain Film
Tomography
George David
Associate Professor
Department of Radiology
Medical College of Georgia
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
Tomography History
• Predates CT by decades
• Used to be popular for inner ear
studies
• Still used at MCG for IVP’s
George
David
Tomography 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
Conventional Tomography Blurring
• Image produced on
film
• Objects above or
below fulcrum plane
change position on
film & thus blur
George
David
Tomography Blurring
• the further from the
fulcrum an object is,
the more it blurs!
Tomography Blurring
• objects shaped &
oriented in direction of
motion
 elongate
 do not blur
Tomographic Blurring
• Blurring improved by use of
complex motions
circular
Hypocycloidal
tri-spiral
elliptical
Complex Blurring
• The more complex the blurring
motion
 the sharper the tomographic image
» better blurring
 the more expensive the machine
$
George
David
Tomo 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
George
David
Thickness 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 Cut
Larger Angle;
Thinner Cut
Smaller Angle;
Thicker Cut
Cut Thickness
• Approximate cut thickness for
linear tomo
Tomo Angle
Thickness of Cut (mm)
(degrees)
------------------------------------------------------0
Infinite (non-tomo)
2
31
4
16
6
11
10
6
20
3
35
2
50
1
George
David
Grids & Tomo
• Linear grid used
• Grid lines parallel with table for
linear tomo
• Grid must change orientation
(rotate) in complex motion tomo
 eliminates grid cutoff
George
David