1. Resident Physics Lectures
05: Image Formation
George David, M.S.
Associate Professor of Radiology

2. Scanner Processing of Echoes
Amplification
Compensation
Compression
Demodulation
Rejection

3. Amplification Increases small voltage signals from transducer
incoming voltage signal
10’s of millivolts
larger voltage required for processing & storage
Amplifier

4. Compensation Amplification Compensation Compression Demodulation
Rejection

5. Your Scanner Knows… Delay time between sound transmission and echo
Direction sound transmitted
Intensity of echo

6. 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

7. 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

8. Need for Compensation
equal intensity reflections from different depths return with different intensities
different travel distances
attenuation is function of path length
Display without
compensation
echo
intensity
time since pulse

9. Compensation Problem Solution
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

10. Equal Echoes Voltage before Compensation Time within a pulse
Later Echoes
Early Echoes
Voltage
before
Compensation
Time within a pulse
Voltage
Amplification
Voltage
Amplitude
after
Amplification
Equal echoes,
equal voltages

11. dB Calculation Power ratio dB 1 0 2 3 10 10 100 20 1000 30 Attenuation
4 MHz
Attenuation
0.5 dB/cm/MHz X 10 cm X 4 MHz or
20 dB
Factor of 100 attenuation
5 cm

12. 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

13. Compensation (TGC)
TGC adjustment affects all echoes at a specific distance range from transducer

14. Compression Amplification Compensation Compression Demodulation
Rejection

15. Design scale that can weigh both feather & elephant
Challenge
Design scale that can weigh both feather & elephant

16. Challenge Re-Stated
Find a scale that can tell which feather weighs more & which elephant weighs more

17. Dynamic Range
ratio of largest to smallest power an electronic system can process
can be expressed in dB

18. Logarithm Review logarithms are exponents
log10x is exponent to which 10 is raised to get x
log10100 =2 because 102=100
100,000
10,000
1,000
100 10 1 5 4 3 2
Input
Logarithm

19. Logarithms & Dynamic Range
100,000
10,000
1,000
100 10 1 5 4 3 2
Input
Logarithm
90,000 1 90 1
Using logarithms the difference between 10,000 & 100,000 is the same as the difference between 10 & 100

20. Compression 100,000 10,000 1,000 100 10 1 5 4 3 2 Input Logarithm
1000
Can’t easily distinguish between 1 & 10 here
100,000
10,000
1,000
100 10 1 5 4 3 2
Input
Logarithm
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

21. 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

22. Demodulation Amplification Compensation Compression Demodulation
Rejection

23. Demodulation & Radio
Any station (frequency) can carry any format

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

25. Demodulation
Intensity information carried on “envelope” of operating frequency’s sine wave
varying amplitude of sine wave
demodulation separates intensity information from sine wave

26. Demodulation Sub-steps
rectify
turn negative signals positive
smooth
follow peaks

27. Rejection Amplification Compensation Compression Demodulation

28. Rejection also known as object reason suppression threshold
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

29. Image Resolution Detail Resolution Detail Resolution types
spatial resolution
separation required to produce separate reflections
Detail Resolution types
Axial
Lateral

30. Resolution & Reflector Size
minimum imaged size of a reflector in each dimension is equal to resolution
Objects never imaged smaller than system’s resolution

31. 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

32. Axial Resolution = Spatial Pulse Length / 2
Gap;
Separate
Echoes
Separation
just greater
than half the
spatial
pulse length

33. Axial Resolution = Spatial Pulse Length / 2
Overlap;
No Gap;
No Separate
Echoes
Separation
just less
than half the
spatial
pulse length

34. 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

35. Lateral Resolution = Beam Diameter
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

36. Lateral Resolution
if separation is greater than beam diameter, objects can be resolved as two reflectors

37. 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

38. 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

39. Contrast Resolution

40. Contrast Resolution
difference in echo intensity between 2 echoes for them to be assigned different digital values 88 89

41. 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

42. 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 1 2 3 . 8
0, 1
00, 01, 10, 11
000, 001, 010, 011, 100, 101, 110, 111
, ,
2 1 = 2
2 2 = 4
2 3 = 8
2 8 = 256
Bits
Values
# Values

43. 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

44. 7 6 5 4 3 2 1 1 2 6 6 4 4 5 3 2 3 7 7 6 4 2 5 5 2 14 13 12 11 10 9 8 7 6 5 4 3 2 1 2 4 11 11 7 8 10 6 3 6 14 14 11 6 4 8 12 4

45. Display Limitations 17 = 17 = 65 65 = =
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 = 17 =
Change window / level 65 65 = =

46. Presentation 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
120
145
301
256
223
287
225
178
322
325
299
353
333
300

47. 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
Bits per Pixel Shades Decibels per shade % Intensity Difference
40/16
40/32

48. The End