1. Ultrasound Physics Have no fear Presentation by Alexis Palley MD
Department of Emergency Medicine
Cooper University Hospital

2. “How does it do that?” Lecture Objectives:
Review basic physics vocabulary
Explain the principles of sound waves
Use ultrasound physics to explain how images are produced
Teach how to use these principles to help your diagnostic abilities

3. Basics Sound is energy traveling though matter as a wave
The wave travels by compressing and rarefacting matter
Depending on the matter- the wave will travel at different velocities or directions
U/S probes emit and receive the energy as waves to form pictures

4. Physical Principles

5. Cycle
1 Cycle = 1 repetitive periodic oscillation
Cycle
VOCAB

6. Frequency VOCAB # of cycles per second Measured in Hertz (Hz)
-Human Hearing ,000 Hz
-Ultrasound > 20,000 Hz
-Diagnostic Ultrasound 2.5 to 10 MHz
(this is what we use!)
VOCAB

7. frequency
1 cycle in 1 second = 1Hz
1 second
= 1 Hertz
VOCAB

8. High Frequency High frequency (5-10 MHz) greater resolution
less penetration
Shallow structures
vascular, abscess, t/v gyn, testicular

9. Low Frequency Low frequency (2-3.5 MHz) greater penetration
less resolution
Deep structures
Aorta, t/a gyn, card, gb, renal

10. Wavelength VOCAB The length of one complete cycle
A measurable distance
VOCAB

11. Wavelength
Wavelength
VOCAB

12. Amplitude
The degree of variance from the norm
Amplitude
VOCAB

13. Producing an image
Probe emits a sound wave pulse-measures the time from emission to return of the echo
Wave travels by displacing matter, expanding and compressing adjacent tissues
It generates an ultrasonic wave that is propagated, impeded, reflected, refracted, or attenuated by the tissues it encounters

14. Producing an image Important concepts in production of an U/S image:
Propagation velocity
Acoustic impedance
Reflection
Refraction
Attenuation

15. Propagation Velocity Sound is energy transmitted through a medium-
Each medium has a constant
velocity of sound (c)
Tissue’s resistance to compression
density or stiffness
Product of frequency (f) and wavelength (λ)
c=fλ
Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.

16. Propagation Velocity Propagation velocity
Increased by increasing stiffness
Reduced by increasing density
Bone: 4,080 m/sec
Air: 330 m/sec
Soft Tissue Average: 1,540 m/sec

17. Impedance
Acoustic impedance (z) of a material is the product of its density and propagation velocity
Z= pc
Differences in acoustic impedance create reflective interfaces that echo the u/s waves back at the probe
Impedance mismatch = ΔZ

18. Acoustic Impedance Homogeneous mediums reflect no sound
acoustic interfaces create visual boundaries between different tissues.
Bone/tissue or air/tissue interfaces with large Δz values reflect almost all the sound
Muscle/fat interfaces with smaller Δz values reflect only part of the energy

19. Refraction
A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity
U/S scanners assume that an echo returns along a straight path
Distorts depth reading by the probe
Minimize refraction by scanning perpendicular to the interface that is causing the refraction

20. Reflection
The production of echoes at reflecting interfaces between tissues of differing physical properties.
Specular - large smooth surfaces
Diffuse – small interfaces or nooks and crannies

21. Specular Reflection
Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirror
Only the echoes returning to the machine are displayed
Specular reflectors will return echoes to the machine only if the sound beam is perpendicular to the interface

22. Diffuse Reflector
Most echoes that are imaged arise from small interfaces within solid organs
These interfaces may be smaller than the wavelength of the sound
The echoes produced scatter in all directions
These echoes form the characteristic pattern of solid organs and other tissues

23. Reflectors
Specular Diffuse

24. Attenuation
The intensity of sound waves diminish as they travel through a medium
In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves
In real systems some waves are scattered and others are absorbed, or reflected
This decrease in intensity (loss of amplitude) is called attenuation.

25. The Machine

26. Ultrasound scanners Anatomy of a scanner: Transmitter Transducer
Receiver
Processor
Display
Storage

27. Transmitter
a crystal makes energy into sound waves and then receives sound waves and converts to energy
This is the Piezoelectric effect
u/s machines use time elapsed with a presumed velocity (1,540 m/s) to calculate depth of tissue interface
Image accuracy is therefore dependent on accuracy of the presumed velocity.

28. Transducers Continuous mode Pulsed mode continuous alternating current
doppler or theraputic u/s
2 crystals –1 talks, 1 listens
Pulsed mode
Diagnostic u/s
Crystal talks and then listens

29. Receiver Sound waves hit and make voltage across the crystal-
The receiver detects and amplifies these voltages
Compensates for attenuation

30. Signal Amplification TGC (time gain compensation) Gain Manual control
Selective enhancement or suppression of sectors of the image
enhance deep and suppress superficial
*blinders
Gain
Manual control
Affects all parts of the image equally
Seen as a change in “brightness” of the images on the entire screen
*glasses

31. Changing the TGC

32. Changing the Gain

33. Displays B-mode M-mode Real time gray scale, 2D
Flip book images per second
M-mode
Echo amplitude and position of moving targets
Valves, vessels, chambers

34. “B” Mode

35. “M” Mode

36. Image properties
Echogenicity- amount of energy reflected back from tissue interface
Hyperechoic - greatest intensity - white
Anechoic - no signal - black
Hypoechoic – Intermediate - shades of gray

37. Anechoic
Hyperechoic
Hypoechoic

38. Image Resolution Image quality is dependent on Axial Resolution
Lateral Resolution
Focal Zone
Probe Selection
Frequency Selection
Recognition of Artifacts

39. Axial Resolution
Ability to differentiate two objects along the long axis of the ultrasound beam
Determined by the pulse length
Product of wavelength λ and # of cycles in pulse
Decreases as frequency f increases
Higher frequencies produce better resolution

40. Axial Resolution 5 MHz transducer 10 MHz transducer Wavelength 0.308mm
Pulse of 3 cycles
Pulse length approximately 1mm
Maximum resolution distance of two objects = 1 mm
10 MHz transducer
Wavelength 0.15mm
Pulse of 3 cycles
Pulse length approximately 0.5mm
Maximum resolution distance of two objects = 0.5mm

41. Axial Resolution
screen
body

42. Lateral Resolution
The ultrasound beam is made up of multiple individual beams
The individual beams are fused to appear as one beam
The distances between the single beams determines the lateral resolution

43. Lateral resolution
Ability to differentiate objects along an axis perpendicular to the ultrasound beam
Dependent on the width of the ultrasound beam, which can be controlled by focusing the beam
Dependent on the distance between the objects

44. Lateral Resolution
screen
body

45. Focal Zone Objects within the focal zone Objects outside of focal zone

46. Probe options
Linear Array
Curved Array

47. Ultrasound Artifacts Can be falsely interpreted as real pathology
May obscure pathology
Important to understand and appreciate

48. Ultrasound Artifacts Acoustic enhancement Acoustic shadowing
Lateral cystic shadowing (edge artifact)
Wide beam artifact
Side lobe artifact
Reverberation artifact
Gain artifact
Contact artifact

49. Acoustic Enhancement Opposite of acoustic shadowing
Better ultrasound transmission allows enhancement of the ultrasound signal distal to that region

50. Acoustic Enhancement

51. Acoustic Shadowing
Occurs distal to any highly reflective or highly attenuating surface
Important diagnostic clue seen in a large number of medical conditions
Biliary stones
Renal stones
Tissue calcifications

52. Acoustic Shadowing
Shadow may be more prominent than the object causing it
Failure to visualize the source of a shadow is usually caused by the object being outside the plane of the ultrasound beam

53. Acoustic Shadowing

54. Acoustic Shadowing

55. Lateral Cystic Shadowing
A type of refraction artifact
Can be falsely interpreted as an acoustic shadow (similar to gallstone)

56. Lateral Cystic ShadowingX

57. Beam-Width Artifact
Gas bubbles in the duodenum can simulate a gall stone
Does not assume a dependent posture
Do not conform precisely to the walls of the gallbladder

58. Beam-Width Artifact Gas in the duodenum simulating stones

59. Side Lobe Artifact
More than one ultrasound beam is generated at the transducer head
The beams other than the central axis beam are referred to as side lobes
Side lobes are of low intensity

60. Side Lobe Artifact Occasionally cause artifacts
The artifact by be obviated by alternating the angle of the transducer head

61. Side Lobe Artifact

62. Reverberation Artifacts
Several types
Caused by the echo bouncing back and forth between two or more highly reflective surfaces

63. Reverberation Artifacts
On the monitor parallel bands of reverberation echoes are seen
This causes a “comet-tail” pattern
Common reflective layers
Abdominal wall
Foreign bodies
Gas

64. Reverberation Artifacts

65. Reverberation Artifacts

66. Gain Artifact

67. Contact artifact
Caused by poor probe-patient interface

68. Take homes!

69. review
u/s uses waves echoed off reflective interfaces to display the structures of the body-
works by “talking and listening”
Must be careful when interpreting images
We have more control than we thought!

70. Think before you ultrasound!

71. Think before you ultrasound!
Choose the right frequency probe – frequency = shallow & detailed
Hold the probe perpendicular to the organ wall being studied
Adjust the depth, tgc and gain to help your image
Artifact may be affecting your image
Use knowledge of physical properties of tissues to help with positioning- ie. use bladder for acoustic window to ta u/s.

72. Thank you
Any questions?