Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 15
• Geometry of the Radiographic System

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Similar Triangle Review
a b c h---- --- --- --- A
B C H
3
Magnification Defined
FocalSpot

• size of image
• --------------------size of object

Object
Film (image)
4
Using Similar Triangles
FocalSpot
size of image Magnification
-------------------- size of object
h
H
Object
Film (image)

• focus to film
  distance HMagnification
  ---------------------------------- ---
  focus to object distance
  h

5
Using Similar Triangles
FocalSpot
size of image Magnification
-------------------- size of object
h

• focus to film
  distance Hmagnification
  ---------------------------------- ---
  focus to object distance
  h

H
Object
Film (image)
SO
focus to film dist.size of image size of
object X ---------------------------------

focus to object dist
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Optimizing Image Quality

focus to film
distance Hmagnification
---------------------------------- ---
focus to object distance
h
FocalSpot
h
H
Object
Film (image)

• Minimize magnification
• Minimize object-film distance
• Maximize focal-film distance

7
Automatic Artifact

• Occurs whenever we image a 3D object in 2D
• Work-around
• Multiple views

?
?
8
Distortion Types
minimal distortion when object near central beam
close to film
9
Penumbra

• Latin for almost shadow
• also called edge gradient
• region of partial illumination
• caused by finite size of focal spot
• smears edges on film
• zone of unsharpness called
• geometric unsharpness
• penumbra
• edge gradient

Line sourcefocal spot
Film Image
10
Penumbra Calculation

• Minimizing Penumbra
• Minimize object-film distance (OID)
• Maximize source-object distance (SOD)
• Makes focal spot appear smaller
• Minimize focal spot size

F
Line sourcefocal spot
SOD
Object
SID
OIDP F x ------- SOD
OID
P
11
Motion Unsharpness

• Caused by motion during exposure of
• patient
• tube
• film
• Effect
• similar to penumbra
• Minimize by
• immobilizing patient
• short exposure times

12
Absorption Unsharpness

• Cause
• gradual change in x-ray absorption across an
  objects edge or boundary
• thickness of absorber presented to beam changes
• Effect
• produces poorly defined margin of solid objects

X-RayTube
X-RayTube
X-RayTube
13
Inverse Square Law

• intensity of light falling on flat surface from
  point source is inversely proportional to square
  of distance from point source
• if distance 2X, intensity drops by 4X
• Assumptions
• point source
• no attenuation
• Cause
• increase in exposure area with distance

Intensity a 1/d2
d
14
Trade-offGeometry vs. Intensity
F

• maximize SID to minimize geometric
  unsharpness but
• doubling SID increases mAs by X4
• increased tube loading
• longer exposure time
• possible motion
• going from 36 to 40 inch SID requires 23 mAs
  increase

SOD
SID
OID
P
15
Magnification Types

• Geometric Magnification
• assumes point source
• calculated from similar triangles
• True Magnification
• takes into account finite size of focal spot
• focal spot is area (not point) source

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Magnification
m geometric magM true mag
a
b
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True Magnification
m geometric magM true mag

• Function of ratio of focal spot to object size (f
  / d)
• true geometric magnification equal only when
  object very large compared to focal spot

f
a
d
b
Mm (m-1) X (f / d)
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Focal Spot Size Tolerances
Nominal Size Tolerance ------------------------
------------- gt1.5 mm
30 gt0.8 and lt1.5 mm 40 lt0.8 mm
50
Focal spot that measures 1.1 mm can be legally
labeled as a 0.8 mm
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Measuring Focal Spot Size

• Standard
• slit camera (as used for line spread function)
• Orientation and intensity distribution measured
  with
• pinhole camera
• Limit of resolution measured with
• star test pattern
• line bar phantom

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Slit Camera

• .01 mm wide slit cut in metal (tungsten)
• 2 exposures made with slit turned 90o
• Measure slit width in each direction

SlitCamera
Film (no screen)
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Pinhole Camera

• very small hole in metal (tungsten / tantalum)
• high technique (sometimes several exposures)
  required
• image shows
• size
• energy distribution

PinholeCamera
Film (no screen)
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Focal Spot Intensity Distributions

• edge band distribution
• also called bi-modal
• edges have higher intensity than center
• most common
• poorest resolving power for size
• homogeneous distribution
• centrally peaked (Gaussian) distribution
• best resolving power for size
• most like point source

23
Star Pattern

• Measures resolving power
• Tool
• circular
• alternating lead spokes blanks
• image of spokes blurs at particular diameter
• blur diameter indicates focal spot size

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Bar Phantom

• Measures resolving power
• look for smallest bar set where all three bars
  visible in both directions

25
Bar Phantom Setup
Who is this?
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The Rise Fall of Joe Camel
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Focal Spot Size Varies with Technique

• Electron beam focuses more poorly at high mA or
  low kV
• Blooming
• increase of focal spot size with increasing mA
• more of a problem at low kVs
• more blooming perpendicular to cathode-anode axis
• kilovoltage effects
• size decreases slightly with increasing kVp
• size always measured specified at particular
  technique

28
Off-Axis Variation

• focal spot measurements normally made on central
  ray
• apparent focal spot size changes in anode-cathode
  direction
• smaller toward anode side
• larger toward cathode side
• less effect in cross-axis direction

29
Focal Spot Size

• Trade-off
• heat vs. resolving power
• exposure time vs. resolving power
• Focal Spot Size most critical for
• magnification
• mammography

30
Modulation Transfer Function (MTF)

• How well information reproduced (fraction of
  contrast retained) at various input spatial
  frequencies

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Modulation Transfer Function(MTF)

• Fraction of contrast reproduced as a function of
  frequency

Freq. line pairs / cm
1
MTF
50
Recorded Contrast(reduced by blur)
Contrast provided to film
0
frequency
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MTF

• If MTF 1
• all contrast reproduced at this frequency

Recorded Contrast
Contrast provided to film
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MTF

• If MTF 0.5
• half of contrast reproduced at this frequency

Recorded Contrast
Contrast provided to film
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MTF

• If MTF 0
• no contrast reproduced at this frequency

Recorded Contrast
Contrast provided to film
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Modulation Transfer Function (MTF)

• value between 0 and 1
• MTF 1 indicates all information reproduced at
  this frequency
• MTF 0 indicates no information reproduced at
  this frequency

36
Component MTF

• Each component of imaging system has its own MTF
• each component retains a fraction of contrast as
  function of frequency
• System MTF is product of MTFs for each component.

37
Modulation Transfer Function (MTF)

• Since MTF is between 0 and 1, composite MTF lt
  MTF of poorest component

1/2 1/3 1/4 1/5 ?
? lt 1/5
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Film MTF

• Resolves 10-20 line pairs per mm
• MTF 1 for clinical applications

39
Focal Spot MTF

• Function of magnification
• Deteriorates with increased magnification
• all clinical imaging involves some magnification
• The larger the focal spot, the more deterioration
  of MTF with increased magnification

40
Imaging Screen MTF

• improves with magnification
• Why?
• magnification of an object of given frequency
  reduces frequency seen by screen

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Focal Spot Screen MTF

• focal spot MTF degrades with magnification
• screen MTF improves with magnification
• film MTF unaffected with magnification
• best system resolution
• detail screens
• minimize magnification
• BUT detail screens decrease speed
• increase exposure time
• more potential for motion

42
Imaging Physics Wrap Up
43
Object Motion

• independent of magnification
• depends on
• exposure time
• velocity
• If motion lt 0.1 mm usually doesnt contribute to
  unsharpness
• If motion gt 1 mm, it generally dominates

44
Quantum Mottle

• statistical fluctuation in of x-ray photons
  used by imaging system to form image
• quantum mottle independent of geometric
  (radiographic) magnification
• for a given density the same of photons must
  strike a given area of screen/film

45
Quantum Mottle

• quantum mottle directly affected by photographic
  (optical) magnification
• less photons per unit area of image

46
Quantum Mottle

• influences perception of low-contrast objects
  with poorly defined borders
• not well addressed by MTF
• measures sharp high contrast borders
• geometric (radiographic) magnification may
  improve visibility of low contrast objects
• quantum mottle noise does not increase

47
Skin Exposure Magnification
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Skin Exposure Magnification

• Patient closer to x-ray tube
• Increases exposure
• grids not used with magnification work because of
  air gap
• Decreases exposure
• No bucky factor loss (3-6)

Grid
Film
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Skin Exposure Magnification

• Increase is less than calculated by inverse
  square law
• for 2X mag skin not twice as close
• routine radiography also involves some
  magnification
• smaller volume of patient exposed
• high speed screen/film sometimes used for
  magnification

Grid
Film
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You Must Remember This
Ilse, well always have physics.
The End