Resident Physics Lectures

1
Resident Physics Lectures

• 05Image Formation

George David, M.S. Associate Professor of
Radiology
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Scanner Processing of Echoes

• Amplification
• Compensation
• Compression
• Demodulation
• Rejection

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Amplification

• Increases small voltage signals from transducer
• incoming voltage signal
• 10s of millivolts
• larger voltage required for processing storage

Amplifier
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Compensation

• Amplification
• Compensation
• Compression
• Demodulation
• Rejection

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Your Scanner Knows

• Delay time between sound transmission and echo
• Direction sound transmitted
• Intensity of echo

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Your Scanner Assumes

• Speed of sound in body
• Sound travels in straight line
• Constant sound attenuation in body
• Scanner corrects echo intensities based on this
  assumption

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Attenuation Correction

• intensity of dot indicates strength of echo
• equal intensity echoes should appear to have
  equal gray shade regardless of depth of echo
  structure

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Need for Compensation

• equal intensity reflections from different depths
  return with different intensities
• different travel distances
• attenuation is function of path length

echo intensity
time since pulse
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Compensation

• Problem
• how to display equal echoes from different depths
  at equal intensities
• Solution
• late echoes need to be amplified more than early
  echoes
• compensates for greater attenuation suffered by
  later echoes

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Equal Echoes
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dB Calculation
Power ratio dB 1
0 2 3 10
10 100 20 1000
30
4 MHz
Attenuation 0.5 dB/cm/MHz X 10 cm X 4 MHz or 20
dB or Factor of 100 attenuation
5 cm
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Compensation (TGC)

• Body attenuation varies from 0.5 dB/cm/MHz
• TGC allows manual fine tuning of compensation vs.
  delay
• TGC curve often displayed graphically

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Compensation (TGC)

• TGC adjustment affects all echoes at a specific
  distance range from transducer

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Compression

• Amplification
• Compensation
• Compression
• Demodulation
• Rejection

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Challenge

• Design scale that can weigh both feather
  elephant

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Challenge Re-Stated

• Find a scale that can tell which feather weighs
  more which elephant weighs more

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Dynamic Range

• ratio of largest to smallest power an electronic
  system can process
• can be expressed in dB

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Logarithm Review

• logarithms are exponents
• log10x is exponent to which 10 is raised to get x
• log10100 2 because 102100

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Logarithms Dynamic Range
90,000
1
90
1
Using logarithms the difference between 10,000
100,000 is the same as the difference between 10
100
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Compression
Cant easily distinguish between 1 10 here
3 log 1000
2 log 100
Difference between 1 10 the same as between 100
1000
1 log 10
0 log 10
Logarithms stretch low end of scale compress
high end
1
10
100
1000
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Compression

• Logarithmic amplifier
• hardware which does compression
• accepts widely varying input
• takes logarithm of input
• amplifies logarithm
• Compressed logarithmic output dynamic range
  matches other system components

Input
Logarithm
100,000
5
10,000
4
1,000
3
100
2
10
1
1
0
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Demodulation

• Amplification
• Compensation
• Compression
• Demodulation
• Rejection

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Demodulation Radio

• Any station (frequency) can carry any format

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Demodulation

• Height or magnitude of received sine wave
  indicates beam intensity
• Frequency of echoed sound beam same as operating
  frequency
• Exception moving structures

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Demodulation

• Intensity information carried on envelope of
  operating frequencys sine wave
• varying amplitude of sine wave
• demodulation separates intensity information from
  sine wave

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Demodulation Sub-steps

• rectify
• turn negative signals positive
• smooth
• follow peaks

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Rejection

• Amplification
• Compensation
• Compression
• Demodulation
• Rejection

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Rejection

• also known as
• suppression
• threshold
• object
• eliminate small amplitude voltage pulses
• reason
• reduce noise
• electronic noise
• acoustic noise
• noise contributes no useful information to image

Amplitudes below dotted line reset to zero
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Image Resolution

• Detail Resolution
• spatial resolution
• separation required to produce separate
  reflections
• Detail Resolution types

Axial
Lateral
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Resolution Reflector Size

• minimum imaged size of a reflector in each
  dimension is equal to resolution
• Objects never imaged smaller than systems
  resolution

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Axial Resolution

• minimum reflector separation in direction of
  sound travel which produces separate reflections
• depends on spatial pulse length
• Distance in space covered by a pulse

H.......E.......Y
HEY
Spatial Pulse Length
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Axial Resolution
Axial Resolution Spatial Pulse Length / 2
Gap Separate Echoes
Separation just greater than half
the spatial pulse length
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Axial Resolution
Axial Resolution Spatial Pulse Length / 2
Overlap No Gap No Separate Echoes
Separation just less than half the spatial pulse
length
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Improve Axial Resolution by Reducing Spatial
Pulse Length
Spat. Pulse Length cycles per pulse X
wavelength
Speed Wavelength X Frequency

• increase frequency
• Decreases wavelength
• decreases penetration limits imaging depth
• Reduce cycles per pulse
• requires damping
• reduces intensity
• increases bandwidth

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Lateral Resolution

• Definition
• minimum separation between reflectors in
  direction perpendicular to beam travel which
  produces separate reflections when the beam is
  scanned across them

Lateral Resolution Beam Diameter
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Lateral Resolution

• if separation is greater than beam diameter,
  objects can be resolved as two reflectors

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Lateral Resolution

• Complication
• beam diameter varies with distance from
  transducer
• Near zone length varies with
• Frequency
• transducer diameter

Near zone length
Near zone
Far zone
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Lateral Resolution

• Improving lateral resolution for unfocused beam
  at one depth hurts resolution elsewhere
• axial resolution constant at all depths
• electronic focusing is primary means of reducing
  beam diameter
• improving lateral resolution
• requires phased array transducers
• most common type
• multiple focal zones can be defined
• Slows imaging

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Contrast Resolution
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Contrast Resolution

• difference in echo intensity between 2 echoes for
  them to be assigned different digital values

88
89
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Pre-Processing

• Assigning of specific values to analog echo
  intensities
• analog to digital (A/D) converter
• converts output signal from receiver (after
  rejection) to a value

89
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Digital Image Bit Depth

• bit depth controls of possible values a pixel
  can have
• increasing bit depth results in
• more possible values for a pixel
• better contrast resolution

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Gray Scale

• the more candidate values for a pixel
• the more shades of gray image can be stored in
  digital image
• The less difference between echo intensity
  required to guarantee different pixel values
• See next slide

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Display Limitations

• not possible to display all shades of gray
  simultaneously
• window level controls determine how pixel
  values are mapped to gray shades
• numbers (pixel values) do not change window
  level only change gray shade mapping

17

Change window / level
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65


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Presentation of Brightness Levels

• pixel values assigned brightness levels
• pre-processing
• manipulating brightness levels does not affect
  image data
• post-processing
• window
• level

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Pre-Processing

• Contrast resolution (dB/gray shade) corresponds
  to minimum intensity difference between pixel
  values

Contrast Resolution of Digital Memories with 40
dB dynamic range
40/16
40/32
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The End