Physics of Ultrasound Ultrasound Imaging and Artifacts รศ.นพ.เดโช จ กราพาน ชก ล สาขาหท ยว ทยา, ภาคว ชาอาย รศาสตร คณะแพทยศาสตร ศ ร ราชพยาบาล

Transcription

1 Physics of Ultrasound Ultrasound Imaging and Artifacts รศ.นพ.เดโช จ กราพาน ชก ล สาขาหท ยว ทยา, ภาคว ชาอาย รศาสตร คณะแพทยศาสตร ศ ร ราชพยาบาล

2 Diagnosis TTE TEE ICE 3D 4D Evaluation of Cardiac Anatomy Hemodynamic Assessment Multidimensional Echocardiography

3 Ultrasound Physics

4 Wave Motion VS Circular Motion

5 Ultrasound Waves Diagnostic medical ultrasound typically uses transducers with a frequency between 1-20 MHz Humans can hear sound waves with frequencies between 20 Hz and 20 KHz Adult echocardiogram: 2-4 MHz Pediatric echocardiogram: 4-8 MHz

6 Frequency One hertz (Hz) = One cycle per second Frequency (f) time = number of cycles Frequency (f) 1 period = 1 cycle 1 cycle Period ( ) Time Frequency (f) = 1 period

7 Wavelength Wavelength (λ) times frequency (f) equals the propagation velocity (c): c f Propagation velocity in the heart is 1540 m/s, the wavelength for any transducer frequency can be calculated as 1.54 (mm) f (MHz)

8

9 Interaction with Tissue: Speed of Sound Compressed (high pressure) Expanded Compressed (high pressure) Expanded Distance As sound travels through tissue it compresses and expands the tissue. Where the tissue is compressed, the speed of sound is higher.

10 Nonlinear Propagation Compressed (high pressure) Expanded Compressed (hight pressure) Expanded Higher pressure portions of the wave travel faster Distance As the wave passes through tissue, the top of the waveform gets pulled forward to be non sinusoidal shape. Propagation in which speed depends on pressure and the wave shape changes is called nonlinear propagation. Harmonic frequency is generated from this nonlinear propagation.

11 Harmonic Imaging Increasing Depth Near Field No harmonics being generated Signal has not traveled enough to be distorted Near Mid Field Harmonics Increasing Harmonics beginning to be produced as signal travels through tissue Mid Field Harmonics Unchanging Additional harmonics generated and attenuated in equal proportion Far Mid Field Harmonics Decreasing Harmonics being attenuated faster than being produced Far Field Fundamental Frequency Only Maximum Harmonic Effect

12 Constructive Interference Compressed (high pressure) Expanded Compressed (high pressure) Expanded Distance The waves are in phase with each other. The waves reinforce each other, resulting in an intensified sound.

13 Destructive Interference Distance The waves are out of phase with each other. The waves cancel each other, resulting in a diminished sound.

14 Harmonic Signal Fundamental ultrasound signals are generated from the transducer passing the fat layer twice (transmit and receive) Harmonic signals are generated from the tissue and transmitted to transducer passing through the fat layer once (on receive) Fundamental signal Harmonic signal

15 Harmonic Imaging Amplitude Fundamental frequency bandwidth 2 nd Harmonic frequency bandwidth f 0 2f 0 (Velocity)

16 Ultrasound-Tissue Interaction Scatter Reflection Moving RBC Refraction Attenuation Specular reflector Small structures (< 1 wavelength in lateral dimension) result in scattering

17 Biologic Effect of Ultrasound

18 Medical Ultrasound Safety: Adverse Biological Effects Thermal bioeffects Heating of soft tissue and bone Nonthermal bioeffects Cavitation

19 Thermal Index (TI) The ratio of attenuated acoustic power at a specified point to the attenuated acoustic power required to raise the temperature at that point in a specific tissue model by 1 C TIB: Bone thermal index TIC: Cranial-bone thermal index TIS: Soft tissue thermal index

20 Acoustic Output Limits Non-ophthalmic applications I spta mw/cm 2 MI 1.9 TI 6.0 Ophthalmic applications I spta.3 50 mw/cm 2 MI 0.23 TI 1.0

21 Mechanical Index (MI) Mechanical bioeffects that occur when a certain level of output is exceed Mechanical index (MI) = Peak negative pressure Frequency

22 Output Display Thermal index (TI) Soft tissue (TIS) Bone (TIB) Cranial bone (TIC) Mechanical index (MI) Range 0.0 to 1.9

23 Acoustic Impedance (Z) Measure of how ultrasound traverses the medium and depends on Density of the medium (p) Propagation velocity of ultrasound through the medium (v) Z = pv

24 Acoustic Impedance Fluid Soft tissue Fibrous tissue Solid (calcium)

25 Applied Ultrasound

26 Ultrasound Transducer Device that converts a signal in one form of energy to another form of energy Mechanical rotating transducer (single element) Electronic phased array transducer (multiple elements) Linear array Annular array Matrix array Circular array Curved array

27 Piezoelectric Effect Piezoelectric element Piezoelectric element Direct piezoelectric effect, also called generator or sensor effect, converts mechanical energy into electrical energy. Mechanical stress generates an electric charge proportional to that stress. Inverse piezoelectric effect converts electrical energy into mechanical energy. Electrical voltage causes a change in length or vibration of piezoelectric material to generate a sound wave.

28 Transducer With 20 PZT Elements Piezoelectric elements

29 Transducer Mechanical annular array Electronic phased array

30 Ultrasound Beam Side lobe Side lobes Main lobe Main lobe

31 Ultrasound Beam (RadioGraphics 2009; 29: )

32 Side Lobes Secondary and smaller acoustic beams falling outside at predictable angle located around the main lobe Created by a single crystal transducer

33 Ultrasound Images Motion mode Brightness mode Amplitude mode

34 Ultrasound Images Motion mode Brightness mode Amplitude mode

35 Creating a Two-dimensional Image Ultrasound beam is electronically steered through a sector arc of 80 Speed of imaging rate of 25 frames/s.

36 Creating a Two-dimensional Image The time needed to acquire all the data for one image frame is directly related to the number of scan lines There is tradeoff between scan line density and image frame rate

37 Depth VS Time elapsed Timing is proportional to distance from the transducer

38 Resolution The ability to distinguish two objects that are close together Temporal resolution The ability to accurately locate structures or events at a particular instant in time Spatial resolution Axial resolution: the ability to distinguish two objects that are close together along the axis of the ultrasound beam Lateral resolution: in the direction perpendicular to the beam s axis

39 Temporal Resolution Dependent on frame rate Can be improved by Minimizing depth - the maximum distance from the transducer as this affects the PRF Narrowing the sector to the area of interest - narrowing the sector angle Minimize line density (but at the expense of lateral resolution)

40 Axial Resolution The ability to recognize two different objects at slightly different depths from the transducer along the axis of the ultrasound beam 2 where SPL = λ no. of cycles It is improved by Higher frequency (shorter wavelength) transducers but at the expense of penetration Maximize line density (at the expense of frame rate, i.e. temporal resolution)

41 Lateral Resolution Lateral resolution = F D

42 Ultrasound Imaging

43 Medical Image Orientation

44 Gain Control The degree of amplification of the returning ultrasound signal Affects all parts of the image equally Seen as a change in brightness of the images on the entire screen

45 Time Gain Compensation (TGC) Depth Signal Amplitude

46 Reject Filters out low signal decrease noise in the image No reject High reject

47 Dynamic Range (Compression) Determines the number of gray shades used to map the gray scale image on the display Higher more shades of gray (softer looking image) Lower fewer shades of gray (sharp looking image)

48 Wide Dynamic Range

49 Narrow Dynamic Range

50 High Compression

51 Low Compression

52 Harmonic Imaging

53 Type of Doppler Study Pulse wave Doppler Doppler information is generated from a small gate that is interrogated Continuous wave Doppler One crystal constantly sends, another constantly receives Allows evaluation of very high velocity

54

55 Color flow Doppler This is a variety of pulse wave Doppler. Multiple points in the region of interest are analysed and colour coded rapidly.

56 Doppler Filter Doppler signal from tissue VS blood Blood flow has low amplitude but high frequency Myocardial motion has high amplitude but low frequency High pass filter (Wall filter) Remove low frequency signals from the display Display Doppler signal from blood flow only Low pass filter Remove high frequency signals from the display Display Doppler signal from tissue only

57 Doppler of Blood Amplitude High Pass Filter Tissue Blood Frequency (Velocity)

58 Doppler of Tissue Amplitude Low Pass Filter Tissue Blood Frequency (Velocity)

59 Doppler Filter Amplitude Transitional zone Pass Pass Boost Pass band Pass band Boost Cut Cut Low Pass Filter High Pass Filter (Wall Filter) Frequency

60 High Pass Filter (Wall Filter) Amplitude Transitional zone Pass Boost Cut Pass band Boost Cut Low Pass Filter High Pass Filter (Wall Filter) Frequency

61 Low Pass Filter Amplitude Transitional zone Pass Boost Pass band Cut Low Pass Filter Frequency

62 Doppler Display The perceived returning frequency is lower than the transmitted frequency, it will be plotted below the zero baseline as a negative Doppler shift The spectrum displays echo amplitude by varying the brightness of the display

63 Spectral Display Frequency Low amplitude Time

64 Spectral Display Frequency Mid amplitude Time

65 Spectral Display Frequency High amplitude Time

66 Sampling of Received Doppler Signal

67 Sampling Frequency

68

69 Aliasing

70 Aliasing of Doppler Signal Erroneous display of velocities that have exceeded the Nyquist limit The velocity exceeds the rate at which the pulsed wave system can record it properly Spectral trace is cut-off at a given velocity with placement of the cut section in the opposite channel or reverse flow direction

71 Nyquist Frequency Measurements of frequency shifts (and, thus, velocity) will be appropriately displayed only if the pulse repetition frequency (PRF) is at least twice the maximum velocity (or Doppler shift frequency) encountered in the sample volume

72 The Nyquist Limit A sampled waveform thus needs at least two sample points per cycle. Thus the wave's frequency must not be above half the sampling frequency. This limit is called the Nyquist limit of a given sampling frequency

73 Basic Assumption Used in Imaging Systems 1. Sound travels in a straight line 2. Sound travels directly to a reflector and back 3. Sound travels in soft tissue at exactly 1540 m/s 4. Reflections arise only from structure positioned in the beam s main axis 5. The imaging plane is very thin 6. The strength of a reflection is related to the characteristics of the tissue creating the reflection (Understanding Ultrasound Physics. Sidney K Edelman, 4 th ed 2012)

74 Artifacts Caused by violation of the assumptions Direction Ultrasound path between transducer and reflector is not in straight line Transmit path Receive path Reflection Reflection from side lobe ultrasound beam Speed is not correct Attenuation

75 Artifacts due to Direction of Ultrasound

76 Refraction

77 Mirror Image

78 Reverberation A is the true anatomical structure strongly reflective of ultrasound A1 is the artefact generated by the returning ultrasound beam being re-reflected from the transducer and again reflected from the true anatomical structure at A. As this doubly reflected beam has travelled twice as far it is seen at twice the distance from the transducer. It is weaker but otherwise identical to the real structure at A A A1

79 Reverberation

80 Reverberation

81 Reverberation

82 Reverberation and Shadowing

83 Comet Tail (Ring Down Artifact) Reverberation with the spaces squeezed The reflecting surfaces are located in a medium with a very high propagation speed, such as mechanical heart valve Appearance of solid hyperechoic line directed downward

84 Comet Tail (Ring Down Artifact)

85 Comet Tail (Ring Down Artifact)

86 Ultrasound Beam Side lobes Main lobe (RadioGraphics 2009; 29: )

87 Side Lobe Artifacts Created by the emitted side lobes reflecting to a strong reflector, laterally positioned targets Erroneously displayed and interpreted by the machine as if they originated from the central ultrasound beam The depth of the artifact depends on time taken to and from transducer The intensity of the artefact decreases with distance. Common sources of the artifact are The atrioventricular groove The fibrous skeleton of the heart

88 Side Lobe Artifact Side lobe Main Side lobe

89 Side Lobe Artifact

90 Side Lobe Artifact

91 Side Lobe Artifact

92 Side Lobe Artifact

93

94 Artifacts due to Speed of Ultrasound

95 Range Ambiguity Artifact All reflections are received by the transducer before the next pulse is transmitted. If a reflection is created by a very deep structure and arrives at the transducer after the next pulse has been transmitted This very deep reflection will be interpreted as being created by the second pulse and is placed at a very shallow depth

96 Range Ambiguity Artifact

97 Artifacts due to Attenuation of Ultrasound

98 Acoustic Shadowing

99 Edge Shadow

100 Spit Image If the near shadow is in the center of the probe, the ultrasound beam is splitted in two, resulting in two apparent apertures

101 Spilt image Caused by a near shadow in the middle of the probe footprint

102 Enhancement

103 Enhancement