1. Resident Physics Lectures
Ultrasound Basics Principles
George David, M.S.
Associate Professor of Radiology

2. Ultrasound Transducer
Acts as both speaker & microphone
Emits very short sound pulse
Listens a very long time for returning echoes
Can only do one at a time
Microphone
receives echoes
Speaker
transmits sound pulses

3. Piezoelectric Principle
Voltage generated when certain materials are deformed by pressure
Reverse also true!
Some materials change dimensions when voltage applied
dimensional change causes pressure change
when voltage polarity reversed, so is dimensional change V

4. US Transducer Operation
alternating voltage (AC) applied to piezoelectric element
Causes
alternating dimensional changes
alternating pressure changes
pressure propagates as sound wave

5. I’m a scanner, Jim, not a magician.
Ultrasound Basics
What does your scanner know about the sound echoes it hears?
I’m a scanner, Jim, not a magician.
Sound
Acme
Ultra-
Sound
Co.
Echo

6. What does your scanner know about echoed sound?
How loud is the echo?
inferred from intensity of electrical pulse from transducer

7. What does your scanner know about echoed sound?
What was the time delay between sound broadcast and the echo?

8. What else does your scanner know about echoed sound?
The sound’s pitch or frequency

9. What Does Your Scanner Assume about Echoes (or how the scanner can lie to you)
Sound travels at 1540 m/s everywhere in body
average speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body
(0.5 dB/cm/MHz, soft tissue average)

10. Luckily These Are Close Enough to Truth To Give Us Images
Sound travels at 1540 m/s everywhere in body
average speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body
(0.5 dB/cm/MHz, soft tissue average)

11. ? Dot Placement on Image Dot position ideally indicates source of echo
scanner has no way of knowing exact location
Infers location from echo ?

12. ? Dot Placement on Image Scanner aims sound when transmitting
echo assumed to originate from direction of scanner’s sound transmission
ain’t necessarily so ?

13. ? Positioning Dot Dot positioned along assumed line
Position on assumed line calculated based upon
speed of sound
time delay between sound transmission & echo ?

14. Distance of Echo from Transducer
Time delay accurately measured by scanner
distance = time delay X speed of sound
distance

15. What is the Speed of Sound?
distance = time delay X speed of sound
What is the Speed of Sound?
scanner assumes speed of sound is that of soft tissue
1.54 mm/msec
1540 m/sec
13 usec required for echo object 1 cm from transducer (2 cm round trip)
13 msec
1 cm
Handy rule of thumb

16. So the scanner assumes the wrong speed?
Sometimes
Luckily, the speed of sound is almost the same for most body parts
soft tissue ==> 1.54 mm / msec
fat ==> 1.44 mm / msec
brain ==> 1.51 mm / msec
liver, kidney ==> 1.56 mm / msec
muscle ==> 1.57 mm / msec ?

17. ? ? Gray Shade of Echo Ultrasound is gray shade modality
Gray shade should indicate echogeneity of object ? ?

18. How does scanner know what gray shade to assign an echo?
Based upon intensity (volume, loudness) of echo ? ?

19. Gray Shade
Loud echo = bright dot
Soft echo = dim dot
Loud
Soft

20. Complication
Deep echoes are softer (lower volume) than surface echoes.
Loud
Soft ?

21. Gray Shade of Echo
Correction needed to compensate for sound attenuation with distance
Otherwise dots close to transducer would be brighter

22. Echo’s Gray Shade Gray Shade determined by Measured echo strength
accurate
Calculated attenuation
Who am I?
Charles Lane

23. Attenuation Correction
scanner assumes entire body has attenuation of soft tissue
actual attenuation varies widely in body
Fat 0.6
Brain 0.6
Liver 0.5
Kidney 0.9
Muscle 1.0
Heart 1.1
Tissue Attenuation Coefficient
(dB / cm / MHz)

24. Ultrasound Display One sound pulse produces Multiple pulses
one image scan line
one series of gray shade dots in a line
Multiple pulses
two dimensional image obtained by moving direction in which sound transmitted

25. How Do We Move the Beam?
Electronically
Phased Arrays

26. Sound Wave Definition? Sound is a Wave
Wave is a propagating (traveling) variation in a “wave variable”
“An elephant is big, gray, and looks like an elephant.”

27. Sound Wave Variable
Examples
pressure (force / area)
density (mass / volume)
temperature
Also called acoustic variable
Sound is a propagating (moving) variation in a “wave variable”

28. Energy & Power Power Energy 75 Watt Power Energy = Power X Time
rate of energy use
Units: watts or milliwatts
Energy = Power X Time
Units: kilowatt-hours
Power
Energy
Light Bulbs rated in power!
75 Watt
Electric
Bill
300 KW-hr.
75 Watts for 4 hours or
150 Watts for 2 hours
Electricity billed in energy!

29. Intensity Intensity of Sound Beam
intensity = power / cross sectional area

30. Sound Wave Variation Pressure Density Temperature Freeze time
Measure some acoustic variable as a function of position
Pressure
Density
Temperature
Acoustic
Variable
Value
Position

31. MORE Instant #1 Instant #2
Make multiple measurements of an acoustic variable an instant apart
Results would look the same but appear to move in space 1
Instant #1
Instant #2 2

32. MORE
Track acoustic variable at one position over time

33. Sound Waves Waves transmit energy Waves do not transmit matter
“Crowd wave” at sports event
people’s elevation varies with time
variation in elevation moves around stadium
people do not move around stadium

34. Transverse Waves Particle moves perpendicular to wave travel
Water ripple
surface height varies with time
peak height moves outward
water does not move outward

35. Compression (Longitudinal) Waves
Particle motion parallel to direction of wave travel 1 1
Motion of Individual Coil 2 2
Wave Travel

36. Talk louder! I can’t hear you.
Medium
Material through which wave moves
Medium not required for all wave types
no medium required for electromagnetic waves
radio
x-rays
infrared
ultraviolet
medium is required for sound
sound does not travel through vacuum
Talk louder! I can’t hear you.

37. Sound Waves Information may be encoded in wave energy radio TV
ultrasound
audible sound

38. Sound Frequency
# of complete variations (cycles) of an acoustic variable per unit time
Units
cycles per second
1 Hz = 1 cycle per second
1 kHz = 1000 cycles per second
1 MHz = 1,000,000 cycles per second
Human hearing range
,000 Hz

39. Sound Frequency Ultrasound definition
> 20,000 Hz
not audible to humans
dog whistles are in this range
Clinical ultrasound frequency range
MHz
1,000, ,000,000 Hz

40. Magnitude of acoustic variable
Period
time between a point in one cycle & the same point in the next cycle
time of single cycle
Units
time per cycle (sometimes expressed only as time; cycle implied)
Magnitude of acoustic variable
period
time

41. Period 1 Period = ------------------- Frequency
as frequency increases, period decreases
if frequency in Hz, period in seconds/cycle

42. Period Period = 1 / Frequency
if frequency in kHz, period in msec/cycle
if frequency in MHz, period in msec/cycle
1 kHz frequency ==> 1 msec period
1 MHz frequency ==> 1 msec period

43. Reciprocal Units Frequency Units Period Units Hz (cycles/sec)
seconds/cycle
kHz (thousands of cycles/sec)
msec/cycle
MHz (millions of cycles/sec)

44. Sound Period & Frequency are determined only by the sound source
Sound Period & Frequency are determined only by the sound source. They are independent of medium.
Who am I?
Burt Mustin

45. Propagation Speed Speed only a function of medium
Speed virtually constant with respect to frequency over clinical range
Speed depends on medium’s
Density (mass per unit volume)
more dense ==> lower speed
Stiffness (or bulk modulus; opposite of elasticity or compressibility)
more stiffness ==> higher speed
“same letter, same effect”

46. Wavelength distance in space over which single cycle occurs OR
distance between a given point in a cycle & corresponding point in next cycle
imagine freezing time, measuring between corresponding points in space between adjacent cycles

47. Wavelength Units length per cycle
sometimes just length; cycle implied
usually in millimeters or fractions of a millimeter for clinical ultrasound

48. Wavelength Equation As frequency increases, wavelength decreases
Speed = Wavelength X Frequency [ c = l X n ] (dist./time) (dist./cycle) (cycles/time)
As frequency increases, wavelength decreases
because speed is constant

49. Wavelength
Speed = Wavelength X Frequency c = l X n (dist./time) (dist./cycle) (cycles/time)
mm/msec mm/cycle MHz Calculate Wavelength for 5 MHz sound in soft tissue
Wavelength = 1.54 mm/msec / 5 MHz
5 MHz = 5,000,000 cycles / sec = 5 cycles / msec
Wavelength = 1.54 / 5 = 0.31 mm / cycle

50. Wavelength is a function of both the sound source and the medium!
Who am I?
John Fiedler

51. Pulsed Sound For imaging ultrasound, sound is On Cycle (speak)
Not continuous
Pulsed on & off
On Cycle (speak)
Transducer produces short duration sound
Off Cycle (listen)
Transducer receives echoes
Very long duration ON
OFF ON
OFF
(not to scale)

52. Pulse Cycle Consists of same transducer used for
short sound transmission
long silence period or dead time
echoes received during silence
same transducer used for
transmitting sound
receiving echoes
sound
sound
silence

53. Pulsed Sound Example ringing telephone ringing tone switched on & off
Phone rings with a particular pitch
sound frequency
sound
sound
silence

54. Parameters pulse repetition frequency pulse repetition period
Sound
Pulse
pulse repetition frequency
pulse repetition period
pulse duration
duty factor
spatial pulse length
cycles per pulse
frequency
period
wavelength
propagation speed

55. Pulse Repetition Frequency
# of sound pulses per unit time
# of times ultrasound beam turned on & off per unit time
independent of sound frequency
determined by source
clinical range (typical values)
KHz

56. Pulse Repetition Period
time from beginning of one pulse until beginning of next
time between corresponding points of adjacent pulses
Pulse Repetition Period

57. Pulse Repetition Period
Pulse repetition period is reciprocal of pulse repetition frequency
as pulse repetition frequency increases, pulse repetition period decreases
units
time per pulse cycle (sometimes simplified to just time)
pulse repetition period & frequency determined by source
PRF = 1 / PRP

58. Pulsed Sound
Pulse repetition frequency & period independent sound frequency & period
Same Frequency Higher Pulse Repetition Frequency
Higher Frequency Same Pulse Repetition Frequency

59. Pulse Duration Length of time for each sound pulse one pulse cycle =
one sound pulse and one period of silence
Pulse duration independent of duration of silence
Pulse Duration

60. Pulse Duration units equation time per pulse (time/pulse)
pulse duration = Period X # cycles per pulse (time/pulse) (cycles/pulse) (time/cycle)
Pulse Duration
Period

61. Pulse Duration Longer Pulse Duration
Same frequency; pulse repetition frequency,
period, & pulse repetition period
Shorter Pulse Duration

62. Pulse Duration
Pulse duration is a controlled by the sound source, whatever that means.

63. Duty Factor Fraction of time sound generated Determined by source
Units
none (unitless)
Equations
Duty Factor = Pulse Duration / Pulse Repetition Period
Duty Factor = Pulse Duration X Pulse Repetition Freq.
Pulse Duration
Pulse Repetition Period

64. Spatial Pulse Length HEY
distance in space traveled by ultrasound during one pulse
H E Y
HEY
Spatial Pulse Length

65. Spat. Pulse Length = # cycles per pulse X wavelength
Spatial Pulse Length
Equation
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse) (cycles / pulse) (dist. / cycle)
depends on source & medium
as wavelength increases, spatial pulse length increases

66. Spat. Pulse Length = # cycles per pulse X wavelength
Calculate SPL for 5 MHz sound in soft tissue, 5 cycles per pulse (Wavelength=0.31 mm/cycle)
Spat. Pulse Length = # cycles per pulse X wavelength
SPL = 0.31 mm / cycle X 5 cycles / pulse = 1.55 mm / pulse

67. Spat. Pulse Length = # cycles per pulse X wavelength
Spatial Pulse Length
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
as # cycles per pulse increases, spatial pulse length increases
as frequency increases, wavelength decreases & spatial pulse length decreases
speed stays constant

68. Why is Spatial Pulse Length Important
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
Spatial pulse length determines axial resolution

69. Acoustic Impedance Definition increases with higher
Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m3) (m/sec)
increases with higher
Density
Stiffness
propagation speed
independent of frequency

70. Acoustic Impedance of Soft Tissue
Density:
1000 kg/m3
Propagation speed:
1540 m/sec
Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m3) (m/sec)
1000 kg/m3 X 1540 m/sec = 1,540,000 rayls

71. Why is Acoustic Impedance Important?
Definition
Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m3) (m/sec)
Differences in acoustic impedance determine fraction of intensity echoed at an interface