1. Medical Imaging Ultrasound Edwin L. Dove 1412 SC edwin-dove@uiowa.edu 335-5635

3. 3D Reconstruction

4. Ultrasound Principle When you shout into a well, the sound of your shout travels down the well and is reflected (echoes) off the surface of the water at the bottom of the well. If you measure the time it takes for the echo to return and if you know the speed of sound, you can calculate the depth of the well fairly accurately.

5. Ultrasound Principle Ultrasound is sound having a frequency greater than 20,000 cycles per second, that is, sound above the audible range Medical ultrasound is sound having a frequency greater than 2-100 MHz Medical ultrasound imaging is sound that is converted to an image

6. Medical Ultrasound Advantages of acoustic energy: –can be directed in a beam –obeys the laws of reflection and refraction –reflected off object borders –no known unwanted health effects Disadvantages of acoustic energy: –propagates poorly through a gaseous medium –reflected off of borders of small objects –quickly dissipates (as heat)

7. Why Ultrasound in Cardiology? Portable, relatively cheap Non-ionizing During the echocardiogram, it is possible for the cardiologist to: –Watch the heart’s motion – in 2D real-time –Ascertain if the valves are opening and closing properly, and view any abnormalities –Determine the size of the heart chambers and major vessels –Measure the thickness of the heart walls –Calculate standard metrics of health/disease e.g., Volume, EF, SV, CO –Dynamic evaluation of abnormalities

8. Ultrasound Theory Pressure (ultrasound) wave produced by vibrating source “Listen” for reflection Build image by sending wave in different directions

9. Sinusoidal pressure source

10. Quantitative Description p pressure applied in z-direction  density  viscosity

11. Speed of Sound in Tissue The speed of sound in a human tissue depends on the average density  (kg·m 3 ) and the compressibility K (m 2 ·N -1 ) of the tissue.

12. Sound Velocity for Various Tissues

13. Tissue Characteristics Engineers and scientists working in ultrasound have found that a convenient way of expressing relevant tissue properties is to use characteristic (or acoustic) impedance Z (kg·m -2 ·s -1 )

14. Pressure Generation Piezoelectric crystal ‘piezo’ means pressure, so piezoelectric means –pressure generated when electric field is applied –electric energy generated when pressure is applied

15. Charged Piezoelectric Molecules Highly simplified effect of E field

16. Piezoelectric Effect

17. Piezoelectric Principle

18. Vibrating element

19. Transducer Design

20. Transducer

21. Reflectance and Refraction Snells’ Law (Assumes  i =  r )

22. Reflectivity At normal incidence,  i =  t = 0 and

23. Reflectivity for Various Tissues

24. Echos

26. Specular Reflection The first, specular echoes, originate from relatively large, strongly reflective, regularly shaped objects with smooth surfaces. These reflections are angle dependent, and are described by reflectivity equation. This type of reflection is called specular reflection.

27. Scattered Reflection The second type of echoes are scattered that originate from small, weakly reflective, irregularly shaped objects, and are less angle-dependent and less intense. The mathematical treatment of non-specular reflection (sometimes called “speckle”) involves the Rayleigh probability density function. This type of reflection, however, sometimes dominates medical images, as you will see in the laboratory demonstrations.

28. Circuit for Generating Sharp Pulses

29. Pressure Radiated by Sharp Pulse

30. Ultrasound Principle When you shout into a well, the sound of your shout travels down the well and is reflected (echoes) off the surface of the water at the bottom of the well. If you measure the time it takes for the echo to return and if you know the speed of sound, you can calculate the depth of the well fairly accurately.

31. Ultrasound Principle

32. Echoes from Two Interfaces

33. Echoes from Internal Organ

34. Attenuation Most engineers and scientists working in the ultrasound characterize attenuation as the “half-value layer,” or the “half-power distance.” These terms refer to the distance that ultrasound will travel in a particular tissue before its amplitude or energy is attenuated to half its original value.

35. Attenuation Divergence of the wavefront Elastic reflection of wave energy Elastic scattering of wave energy Absorption of wave energy

36. Ultrasound Attenuation

37. Attenuation in Tissue Ultrasound energy can travel in water 380 cm before its power decreases to half of its original value. Attenuation is greater in soft tissue, and even greater in muscle. Thus, a thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half- power distance for bone is still less than muscle, which explains why bone is such a barrier to ultrasound. Air and lung tissue have extremely short half-power distances and represent severe obstacles to the transmission of acoustic energy.

38. Attenuation As a general rule, the attenuation coefficient is doubled when the frequency is doubled.

39. Pressure Radiated by Sharp Pulse

40. Beam Forming Ultrasound beam can be shaped with lenses Ultrasound transducers (and other antennae) emit energy in three fields –Near field (Fresnel region) –Focused field –Far field (Fraunhofer region)

41. Directing Ultrasound with Lens

44. Beam Focusing A lens will focus the beam to a small spot according to the equation

45. Linear Array

46. Types of Probes

47. Modern Electronic Beam Direction

48. Beam Direction (Listening)

49. Wavefronts Add to Form Acoustic Beam

50. Phased Linear Array

51. A-mode Ultrasound Amplitude of reflected signal vs. time

52. A-mode

53. M-mode Ultrasound

54. M-mode

55. B-mode Ultrasound

56. Fan forming

57. B-mode Example

59. Cardiac Ultrasound

60. Standard Sites for Echocardiograms

61. Conventional Cardiac 2D Ultrasound

62. Short-axis Interrogation

63. B-mode Image of Heart

64. Traditional Ultrasound Images End-diastoleEnd-systole

65. B-mode

67. Ventricles

68. Mitral stenosis

69. Results Possible from Echo

70. Geometric problems

72. New developments of Phase-arrays

73. 2D Probe Elements

74. Recent 2D array 5Mz 2D array from Stephen Smith’s laboratory, Duke University

75. 2D and 3D Ultrasound a. Traditional 2Db, c. New views possible with 3D

76. 3D Pyramid of data

77. 3D Ultrasound 2D ultrasound transmitter 2D phased array architecture Capture 3D volume of heart 30 volumes per second

78. 3D Ultrasound Traditional 2DNew 3D

79. Real-time 3D Ultrasound

82. Velocity of Contraction NormalAbnormal

83. Normal artery

84. Progression of Vascular Disease

85. CAD

86. Severe re-canalization

87. Intravascular Ultrasound (IVUS) Small catheter introduced into artery Catheter transmits and receives acoustic energy Reflected acoustic energy used to build a picture of the inside of the vessel Clinical assessment based on vessel image

88. IVUS Catheter 1 - Rotating shaft 2 - Acoustic window 3 - Ultrasound crystal 4 - Rotating beveled acoustic mirror

89. Slightly Diseased Artery in Cross-section Plaque Catheter

90. An array of Images

91. 3D IVUS

92. Doppler Principle

93. Doppler

94. Doppler measurements

95. Doppler angle

96. Normal flow

97. Diseased flow

98. Blood Flow Measurements