Physics of Ultrasound & Doppler. Sang Jae Rhee. MD., PhD. Division of Cardiovascular Medicine Wonkwang University Hospital

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1 Physics of Ultrasound & Doppler Sang Jae Rhee. MD., PhD. Division of Cardiovascular Medicine Wonkwang University Hospital

2 Classification of Sound Infrasound Audible sound Ultrasound < 20 Hz 20-20,000 Hz > 20,000 Hz Bats Surgical Ultrasound Diagnostic Ultrasound KHz KHz 2-12 MHz

3 History of Ultrasound Spallanzani( ) demonstrated that bats navigated by means of echo reflection using inaudible sound Piezoelectricity discovered by the Curies in 1880 using natural quartz SONAR was first used in 1940 s war-time Diagnostic Medical applications in use since late 1950 s 1954, Inge Edler & Hellmuth Hertz

4 LAZZARO SPALLANZANI ( )

5 초음파검사 : 고주파초음파를인체에쏘아되돌아오는초음파를받아영상을구성하여서내부장기를관찰하는검사 초음파는공기를통과하기어려우므로, 초음파검사시탐촉자와피부의완벽한접촉이요구됨. Cannot travel through Vacuum.

6 희박 Rarefaction 압축 Compression Sound Wave 음파 강도 (amplitude) A Frequency (f) : number of complete cycle per second or hertz Velocity( v ) = f (Frequency) λ ( Wavelength ) 파장 (Wave length), λ

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8 Frequency is the inverse of the period and is defined by a number of events that occur per unit time. The units of frequency is 1/sec or Hertz (Hz). Since f = 1/P, it is also determined by the source and cannot be changed.

9 Velocity of Sound in Various Types of Tissues Tissue type Velocity (m/sec) Air 330 Fat 1450 Water(20 ) 1480 Soft Tissue(Avg.) 1540 Liver 1550 Kidney 1560 Blood 1570 Muscle 1580 Bone

10 Wave equation Transducer V = f = v / f = 1540 (m/s) / f 3 MHz = mm 6 MHz = mm 10 MHz = mm Transducer frequency Transducer wavelength

11 Amplitude and Intensity The strength/intensity of a sound wave at any given time Amplitude : Height of wave Decreases with increasing depth Defines the Brightness of the image Function of source (transducer) Major determinant of power output Intensity is proportional to amplitude 2 intensity = power/area (units = mw/cm 2 ) Theoretical concern of bio-effects of US mediated by amplitude/power

12 How Ultrasound Works How does an ultrasound machine make an image? Piezoelectric Effect ( 압전자효과 ) of Ultrasound Piezoelectric crystal 두께가얇을수록고주파의초음파를발생시킴 Pressure( 압력 ) 과 Electric( 전자 )

13 How Ultrasound Works Ultrasound transducer produces pulses of ultrasound waves These waves travel within the body and interact with various tissues The reflected waves return to the transducer and are processed by the ultrasound machines An image which represents these reflections is formed on the monitor

14 Interactions of Ultrasound with tissue

15 Reflection( 반사 ) vs Attenuation( 약화 ) Reflection( 반사 ) Gives us the image on the screen Occurs at a boundary btw 2 adjacent tissues or media The amount of reflection depends on differences in acoustic impedance(z) btw media The ultrasound image is formed from reflected echoes. 밀도의차가클수록또직각에가깝게입사할수록반사는강함 심장판막, 좌심실후벽, 크고부드러운표면 Scatter ( 산란 ) redirection of sound in several directions. Cause by interaction with small refector or rough surface Only portion of sound wave returns to transducer 좌심실측벽 Attenuation( 약화 ; 감쇄 ) The deeper the wave travels in the body, the weaker it becomes The amplitude of the wave decreases with increasing depth Air (lung)> bone > muscle > soft tissue >blood > water

16 Types of Reflection Specular echoes originate from relatively large, strongly reflective, regularly shaped objects with smooth surfaces. angle dependent Ex) endocardial & epicardial surfaces, valves, pericardium incident reflected Scattered echoes originate from small, weakly reflective, irregularly shaped objects, less angle-dependent and less intense incident backscattered

17 Refraction( 굴절 ), Absorption( 흡수 ) Refraction( 굴절 ) Occur at interfaces with differing propagation speeds, Eliminated by perpendicular incidence Absorption( 흡수 ) Converted to heat(thermal energy)

18 Attenuation = attenuation coefficient x depth Attenuation coefficient proportional to frequency

19 Acoustic Impedance(Z) Z = PV P: density of the medium V; propagation velocity of US amount of reflection depends on differences in acoustic impedance For soft-tissue/air, soft-tissue/bone and bone/air interfaces, almost total reflection occurs

20 Percentage Reflection at Different Interfaces Material Acoustic Impedance(Rayls) Air Fat 1.38 Castor oil 1.40 Water(50 ) 1.54 Brain 1.58 Blood 1.61 Kidney 1.62 Liver 1.65 Muscle 1.70 Skull 7.80 Quartz 15.2 PZT

21 Attenuation vs Frequency Attenuation = attenuation coefficient x depth Attenuation coefficient proportional to Frequency

22 Resolution and Penetration Resolution Penetration Wave length Frequency

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24 Transducer and Image

25 Types of Resolution Spatial Resolution Axial resolution Lateral resolution Contrast Resolution Temporal Resolution

26 Types of Resolution Axial Resolution ( 거리해상력 ) Specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects Frequency (wave length) affects axial resolution Frequency short pulse length resolution

27 Types of Resolution Lateral Resolution ( 측위해상력 ) The ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects Beam-width -- resolution Gain, depth -- resolution Resolved Unresolved

28 Types of Resolution( 해상력 ) Spatial Resolution Also called Detail Resolution The combination of AXIAL and LATERAL resolution Some customer may use this term 480(SD) 720(HD) 1080(full HD) 2160(UHD)

29 Types of Resolution Contrast Resolution The ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects.

30 Types of Resolution Temporal Resolution Also known as frame rate The ability to accurately locate the position of moving structures at particular instants in time Dependent on pulse repetition frequency (PRF) Very important in Cardiology

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32 What determines how far ultrasound waves can travel? The Frequency of the transducer The HIGHER the frequency, the LESS it can penetrate The Lower the frequency, the Deeper it can penetrate Attenuation is directly related to frequency The frequency of a transducer is labeled in Megahertz(MHz)

33 Frequency vs Resolution The frequency also affects the Quality of the ultrasound image The Higher the frequency, the Better the resolution The Lower the frequency, the Less the resolution

34 Reflected Echoes Strong Reflection = White dots Pericardium, calcified structure, diaphragm Weaker Reflection = Grey dots Myocardium, valve tissue, vessel wall liver No Reflections = Black dots Intra-cardiac cavities, gall bladder

35 Echogenicity (caused by Reflection)

36 Transducer( 탐촉자 )

37 Piezoelectric Property Piezoelectric Crystal Ceramics ; ferroelectrics, lead zirconate titanate Expand and contract when voltage is applied Create voltage when compressed or stretched

38 Simple Beam Shape Near Field Fresnel Zone 탐촉자에가까운부위에있는원주형의초음파음속 Far Field Fraunhofer Zone 확산되어원추형처럼퍼진부위 Side Lobes Imaging is optimal within the near field Maximizing the length of the near field Large diameter transducer Decreasing wave length( high frequency )

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41 Transducers Sector Convex Linear Transvaginal/Transrectal Transesophageal 성인 TTE 2-7 MHz 소아 TTE 3-7 MHz TEE MHz

42 Type of Transducer Field of View (FOV)

43 Phased Array Transducer All the elements work together in phased array (all elements are used for each beam line). Phase array steer the beam by applying different delay on each element, and it requires small acoustic window. Main advantage : providing a very broad imaged field at larger depths widely used in cardiac scanning as the transducer fits easily between the ribs (rib gap is a small acoustic window)

44 Type of Transducer A: amplitude B: brightness M: motion

45 심초음파검사방식 A 형심초음파검사 반사되는초음파가음속선상에 spike 모양으로나타남 Spike 크기는반사되는초음파의강도 ; 거리는반사체와의거리 M 형심초음파검사 기록용지또는화면을움직일때반사초음파는파형으로나타남 시간해상력이뛰어나고, Sampling rate 가높아서심장의국소적움직임을측정하는데유리 ( 심장판막움직임 ) B 형심초음파검사 반사되는초음파가음속선상에점으로표시됨 점의밝기는반사되는초음파의강도 ; 거리는반사체와의거리 각각의점들은이면성심초음파영상을형성하는요소 (Pixel)

46 Harmonic imaging Twice Frequency

47 How Harmonics Are Generated Harmonic signals used in this form of imaging do not come from the ultrasound system itself. Signals are generated in the body as a result of interactions with tissue or contrast agents

48 Potential advantages of the harmonic signal Ultrasound beams formed with the harmonic signals have some interesting properties. One of those properties is that the beam formed using the harmonic signal is narrower and has lower side lobes(improve signal-to noise ratios & reduces artifacts) The improvement in beam width and reduction in side lobe significantly improves grayscale contrast resolution Potential advantages of harmonic imaging include improved axial resolution d/t higher frequencies and better lateral resolution d/t narrower beams Furthermore, since the harmonics are generated inside the body, they only have to pass through the fat layer once (on receive), not twice (transmit and receive) Body fat increases the intensity of harmonic waves, thus lesion visibility is increased in obese patients

49 Fat layer

50 Harmonic imaging increases diagnostic confidence in differentiating cystic from solid hepatic lesions, improves detection of gallbladder and biliary calculi, improves pancreatic definition and allows distinction of simple from complex renal cysts.

51 Harmonic imaging Harmonic signal is strongest in distance of interest. Noise is mostly at fundamental frequency; Reduces near filed clutter & many of the other sources of imaging artifact signal-to-noise ratio is greatly improved. Improved endocardial border definition Side effect : Strong specular echoes arising from valves

52 Amplification Overall Gain: Amplify all received signals equally TGC (Time gain Compensation(Post Processing) Overall Gain

53 Time-Gain Compensation (TGC) 초음파음속이흉곽내부 Amplitude ratio 1:10 6 = 120 db 로깊이들어갈때 감쇄되는현상보완 선택적으로 Far field echo 를보상적으로 증폭 ( 예. 좌심실후벽 ) Amplitude ratio 1: = db

54 Automatic Manual Present models, however, have automatic TGC Thus the default control setting should be neutral to achieve a balanced picture Using manual setting by old habit will result in a double compensation, with too much gain in the bottom, too little in the top:

55 Dynamic Range vs Compression vs Reject Dynamic Range, Compression and Reject are separate methods of controlling contrast on the ultrasound image Overall goal of using these tools is to produce an image which reveals as much diagnostic information as possible without making it too gray or too dark

56 Dynamic Range make an image look either very gray or very black and white Narrow Dynamic Range Wide

57 Dynamic Range 사전적의미 : 음악에서한음량에서가장조용한소리와가장큰소리의차이를 일컫는말 이라고하며사진에서는센서가구분할수있는밝기 (Brightness) 의 범위입니다. 밝은부분 (255) 과어두운부분 (0) 에얼마나근접한화소가존재하여그차가 얼마나큰가를말하는것 즉센서가기록할수있는가장밝은밝기와가장어두운밝기의 비율 입니다. 예를들어밝은곳과어두운곳의차가크면다이나믹레인지가넓다

58 Reject increases contrast by filtering-out weaker echoes, making the image look darker helps to reduce a foggy appearance Maximal rejection Minimal rejection

59 Compression Alters the display of the range of echo intensities by compressing them into fewer shades of gray.

60 Compression Maximal Compression Minimal Compression

61 Compression Most of the ultrasound machines apply logarithmic compression to the echo signals emerging from the Receiver Amount of compression is under operator control The widest dynamic range shown (60 db) permits the best differentiation of subtle differences in echo intensity and is preferred for most imaging applications

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63 Doppler

64 Doppler Christian Doppler an Austrian physicist

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67 Doppler Shift(ΔF)

68 Zero Shift Positive Shift Negative Shift

69 Higher Doppler frequency obtained if: Velocity is increased Beam is more aligned to flow direction Higher frequency is used

70 Doppler Equation Δf is the difference between transmitted frequency (ft) and received frequency, v is blood velocity, c is the speed of sound in blood (1,540 m/s), θ is the angle of incidence between the ultrasound beam and blood flow.

71 Θ 값이 0 도, 즉 blood stream 방향과평행하면 cos Θ 가 1 이되므로 blood velocity 가정확하게측정됨그러나 30 도를넘어가면 blood flow velocity 가많은측정값의오류가발생한다. 가능하면평행하게.. (20 도이내로 )

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73 Doppler Spectral Doppler Pulsed wave Doppler Continuous wave Doppler Tissue Doppler Color flow Doppler

74 Doppler echocardiography: spectral Doppler Low velocity reject Suppress Tissue echo High velocity reject Suppress noise

75 PW 송수신소자동일 CW 송수신소자별개 특정위치의속도측정 (sample volume) <2m/sec Along the entire US beam High velocity <9m/sec Aliasing No Aliasing phenomenon

76 Doppler Echocardiography: Tissue Doppler image Measurement of moving tissue such as the myocardium Low velocity but very high amplitude High pass filter is bypassed Lower gain amplification Spectral pulsed wave Doppler Mitral annulus velocity Tissue Doppler Doppler Target Tissue(myocardium) Blood(RBC) Velocity Low(low Doppler freq) High Signal(amplitude) High Low

77 Doppler Echocardiography: Tissue Doppler image

78 Doppler Echocardiography: Color Flow Doppler PW Doppler based technique in which the velocities in a region of interest are encoded with colors that represent both mean velocities and directionality of the flow superimposed on a two-dimensional image with a color map

79 Thank you for Your Attention

80 Backup Slide

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87 FIGURE 5.6 Looking for a needle in a haystack. Extracting the low-frequency, low-amplitude Doppler signal for the received composite signal is a technical challenge requiring several procedures, including demodulation and fast Fourier transform. Once isolated, the Doppler frequencies can be analyzed and displayed.

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