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

3. 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 Speaker transmits sound pulses Microphone receives echoes

4. 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

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

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

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

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

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

10. 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)

11. 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)

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

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

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

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

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

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

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

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

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

21. Complication Deep echoes are softer (lower volume) than surface echoes.

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

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

24. Attenuation Correction scanner assumes entire body has attenuation of soft tissue actual attenuation varies widely in body Fat0.6 Brain0.6 Liver0.5 Kidney0.9 Muscle1.0 Heart1.1 Tissue Attenuation Coefficient (dB / cm / MHz)

25. Ultrasound Display One sound pulse produces 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

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

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

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

29. Energy & Power Power rate of energy use Units: watts or milliwatts Energy = Power X Time Units: kilowatt-hours Electric Bill 300 KW-hr. Electricity billed in energy! Light Bulbs rated in power!

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

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

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

33. MORE Track acoustic variable at one position over time

34. 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

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

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

37. 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.

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

39. Sound Frequency # of complete variations (cycles) of an acoustic variable per unit time Units cycles per second Hz 1 Hz = 1 cycle per second kHz 1 kHz = 1000 cycles per second MHz 1 MHz = 1,000,000 cycles per second Human hearing range 20 - 20,000 Hz

40. Sound Frequency Ultrasound definition > 20,000 Hz not audible to humans dog whistles are in this range Clinical ultrasound frequency range 1 - 10 MHz 1,000,000 - 10,000,000 Hz

41. 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) period Magnitude of acoustic variable time

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

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

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

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

46. 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”

47. 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

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

49. Wavelength Equation Speed = Wavelength X Frequency [ c = X  (dist./time) (dist./cycle) (cycles/time) As frequency increases, wavelength decreases because speed is constant

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

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

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

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

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

55. Parameters frequency period wavelength propagation speed pulse repetition frequency pulse repetition period pulse duration duty factor spatial pulse length cycles per pulse SoundPulse

56. 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) 1 - 10 KHz

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

58. 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

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

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

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

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

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

64. 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

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

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

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

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

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

70. Acoustic Impedance Definition Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m 3 ) (m/sec) increases with higher Density Stiffness propagation speed independent of frequency

71. Acoustic Impedance of Soft Tissue Density: 1000 kg/m 3 Propagation speed: 1540 m/sec Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m 3 ) (m/sec) 1000 kg/m 3 X 1540 m/sec = 1,540,000 rayls

72. Why is Acoustic Impedance Important? Definition Acoustic Impedance = Density X Prop. Speed (rayls) (kg/m 3 ) (m/sec) Differences in acoustic impedance determine fraction of intensity echoed at an interface