Resident Physics LecturesResident Physics Lectures

05:05:Image FormationImage Formation

George David, M.S.Associate Professor of Radiology

Scanner Processing of EchoesScanner Processing of Echoes

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

AmplificationAmplification

• Increases small voltage signals from transducer incoming voltage signal

» 10’s of millivolts

• larger voltage required for processing & storage

Amplifier

CompensationCompensation

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

Your Scanner Knows…Your Scanner Knows…

• Delay time between sound transmission and echo

• Direction sound transmitted

• Intensity of echo

Your Scanner Assumes…

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

Attenuation CorrectionAttenuation Correction

• intensity of dot indicates strength of echo

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

Need for CompensationNeed for Compensation

• equal intensity reflections from different depths return with different intensities different travel distances

» attenuation is function of path length

Display without compensation

time since pulse

echointensity

CompensationCompensation• 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

Equal EchoesEqual Echoes

VoltageAmplification

VoltageAmplitude

afterAmplification

Equal echoes,equal voltages

Later EchoesEarly Echoes

Voltagebefore

Compensation

Time within a pulse

dB CalculationdB Calculation

Power ratio dB

1 0

2 3

10 10

100 20

1000 30

5 cm

4 MHz

Attenuation0.5 dB/cm/MHz X 10 cm X 4 MHz

or20 dB

orFactor of 100 attenuation

Attenuation0.5 dB/cm/MHz X 10 cm X 4 MHz

or20 dB

orFactor of 100 attenuation

Compensation (TGC)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

Compensation (TGC)Compensation (TGC)

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

CompressionCompression

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

ChallengeChallenge

• Design scale that can weigh both

feather & elephant

Challenge Re-StatedChallenge Re-Stated

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

Dynamic RangeDynamic Range

• ratio of largest to smallest power an electronic system can process

• can be expressed in dBdB

Logarithm ReviewLogarithm Review• logarithms are

exponents

• log10x is exponent to which 10 is raised to get x

• log10100 =2 because 102=100

100,00010,0001,000100101

543210

Input Logarithm

Logarithms & Dynamic Range

Logarithms & Dynamic Range

100,00010,0001,000100101

543210

Input Logarithm

Using logarithms the difference between 10,000 & 100,000 is the same as the difference between 10 & 100

90,000

90 1

1

CompressionCompression1,000

1 10 100 1000

3 = log 1000

1 10 100 1000

2 =log 100

1 = log 10

0 = log 10

100,00010,0001,000100101

543210

Input Logarithm

Can’t easily distinguish

between 1 & 10 here

Difference between 1 & 10 the same as between 100 & 1000

Logarithms stretch low end of scale; compress

high end

CompressionCompression

• 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

100,00010,000

1,000100

101

543210

Input Logarithm

DemodulationDemodulation

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

Demodulation & RadioDemodulation & Radio• Any station (frequency) can carry any format

DemodulationDemodulation

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

Exception: moving structures

DemodulationDemodulation• Intensity information carried on

“envelope” of operating frequency’s sine wave varying amplitude of sine wave

• demodulation separates intensity information from sine wave

Demodulation Sub-steps

Demodulation Sub-steps

• rectify turn negative signals

positive

• smooth follow peaks

RejectionRejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

• Amplification

• Compensation

• Compression

• Demodulation

• Rejection

RejectionRejection

• 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

Image Resolution Image Resolution

• Detail Resolution spatial resolution separation required to produce

separate reflections

• Detail Resolution typesAxial

Lateral

Resolution & Reflector SizeResolution & Reflector Size• minimum imaged size of a reflector in each dimension is equal

to resolution• Objects never imaged smaller than system’s resolution

Axial ResolutionAxial Resolution

• minimum reflector separation in direction of sound travel which produces separate reflections

• depends on spatial pulse lengthspatial pulse length Distance in space covered by a pulse

HEYH.......E.......Y

Spatial Pulse Length

Axial ResolutionAxial Resolution

Separationjust greaterthan half thespatialpulse length

Gap;SeparateEchoes

Axial Resolution = Spatial Pulse Length / 2

Axial ResolutionAxial Resolution

Separationjust lessthan half thespatialpulse length

Overlap;No Gap;No SeparateEchoes

Axial Resolution = Spatial Pulse Length / 2

Improve Axial Resolution by Reducing Spatial Pulse LengthImprove Axial Resolution by

Reducing Spatial Pulse Length

• increase frequency Decreases wavelength decreases penetration;

limits imaging depth

• Reduce cycles per pulse requires damping

» reduces intensity» increases bandwidth

Spat. Pulse Length = # cycles per pulse X wavelength

Speed = Wavelength X Frequency

Lateral ResolutionLateral 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

Lateral ResolutionLateral Resolution

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

Lateral ResolutionLateral Resolution

• Complication: beam diameter

varies with distance from transducer

Near zone length varies with

» Frequency

» transducer diameter

Near zone lengthNearzone

Farzone

Lateral ResolutionLateral 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

Contrast ResolutionContrast Resolution

Contrast ResolutionContrast Resolution

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

89

88

Pre-ProcessingPre-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

Digital Image Bit DepthDigital 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

1 2 3 ...8

0, 100, 01, 10, 11000, 001, 010, 011, 100, 101, 110, 111...00000000, 00000001, ... 11111111

2 1 = 22 2 = 42 3 = 8...2 8 = 256

Bits Values # Values

Gray ScaleGray 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

1

2

3

4

5

6

7

1 2 6 6 4 4 5 3 2 3 7 7 6 4 2 5 5 2

1234567

2 4 11 11 7 8 10 6 3 6 14 14 11 6 4 8 12 4

89

1011121314

Display LimitationsDisplay 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 =

65 =

Change window / level

17 =

65 =

Presentation of Brightness LevelsPresentation of Brightness Levels• pixel values assigned brightness levels

pre-processing

• manipulating brightness levels does not affect image data post-processing

» window» level

125 25 311 111 182 222 176

199 192 85 69 133 149 112

77 103 118 139 154 125 120

145 301 256 223 287 256 225

178 322 325 299 353 333 300

Pre-ProcessingPre-Processing

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

Bits per Pixel Shades Decibels per shade % Intensity Difference

4 16 2.5 78 5 32 1.2 32

6 64 0.6 15

7 128 0.3 7 8 256 0.2 5

Contrast Resolution of Digital Memories with 40 dB dynamic range

40/16

40/32

The EndThe End