Doppler Effect - Ultrasound

An overview of Doppler Effect in Ultrasonography - the medical imaging of the body using Ultrasound.

Includes Colour Doppler, Power Doppler, Spectral Doppler, Continuous Wave Doppler, Pulsed Wave Doppler, and comparisons with other Radiographic imaging modalities.

1. DOPPLER EFFECT Course: Ultrasound Victor Ekpo MSc Medical Physics programme, College of Medicine, University of Lagos, 2017.

2. OUTLINE Introduction Physics of Doppler Effect Conditions for Doppler Effect Classifications Types Modes Applications Artifacts in Doppler Imaging Conclusion 2

3. INTRODUCTION Ultrasound is sound waves with frequency greater than 20kHz (Medical Ultrasound range: >2MHz) Sound waves travelling through a medium can be reflected, refracted, absorbed or transmitted. Reflection occurs at the point of acoustic impedance mismatch. 3

4. MOVING SOUND One or both the sound source and receiver may be stationary or moving . If the sound source moves towards the listener, the sound is perceived to have a higher frequency/pitch, and a lower frequency as it moves away from the listener . 4

5. DOPPLER EFFECT Doppler effect is the change in frequency of sound (or any wave) due to the relative motion* of the source and receiver. Doppler Shift ( Δ f) = Reflected Frequency – Transmitted frequency Doppler angle ( θ ) = Angle between the direction of source – direction of sound 5

6. CONDITIONS FOR DOPPLER SHIFT The Doppler effect will NOT occur: If the source and observer both move in the same direction at the same speed. Where one source/listener is at the centre of a circle while the other is moving on it with uniform speed . If the source/receiver is at 90 to the receiver/source. 6

7. 7 Longer wavelengths Lower frequency Shorter wavelengths Higher frequency TOWARDS AWAY FROM

8. 8 Sound waves reflected from a moving object are compressed (higher frequency) when moving towards the listener/ transducer, …and expanded (lower frequency) when moving away from the listener/transducer compared to the incident sound wave frequency.

9. 9

10. The velocity of sound ( c ) is given by the equation, Speed, c = = Wavelength ( λ ) * frequency (f) λ o = Consider a sound source travelling at a velocity towards a detector. After time t, the new wavelength, λ 1 =   10 PHYSICS OF DOPPLER EFFECT S D   c

11. With a shortened wavelength, the new frequency f 1 is given by: f 1 = = f 1 = f o Δ f = f 1 - f o = f o - f o = f o - 1 = f o Δ f = f o If c >> v s , then Δ f = f o   11

12. A similar equation can be written for a detector moving towards a source, Δ f = f o It can be shown that for a source moving away, λ 1 = Δ f = f o OR f o For a 2-way motion where a transducer emits ultrasound of frequency f, and it gets reflected back by moving blood of velocity v (and c>>v), then: Δ f = 2f   12

13. If direction of motion is oblique, then: Δ f = ( 2vf Cos θ ) / c where: Δ f = Doppler shift f = transducer frequency v = velocity of source/receiver c = velocity of sound θ = Doppler angle 13 PHYSICS OF DOPPLER EFFECT (contd.)

14. Doppler shift, Δ f increases with: increasing source velocity v, and increasing transducer frequency , f Δ f is maximum at θ = 0 o and minimum at θ = 90 o . To experience Doppler effect, θ ≠ 90 o 14 Δ f = ( 2vfCos θ ) /c Doppler Shift = 2 x Velocity of Blood x Transducer Freq x Cos θ Propagation Velocity of Sound FACTORS AFFECTING DOPPLER SHIFT

15. If the object is moving perpendicular ( θ =90 o ) to the ultrasound beam, there is no change in frequency or wavelength. By measuring the frequency shift, Doppler Ultrasonography can be used to determine any or all of these 4: Speed Direction Flow (Laminar or Turbulent) Location 15 Δ f = ( 2vfCos θ ) /c

16. USES OF DOPPLER METHOD With Doppler, US can be used for static and dynamic imaging . The Doppler method can be used for: detection and characterization of blood flow, detection of foetal heartbeat, detection of air emboli, blood pressure monitoring, and localization of blood vessel occlusions. 16

17. 17 CLASSIFICATIONS OF DOPPLER ULTRASONOGRAPHY OUTPUTS/IMAGING MODES TYPES FUNCTIONS PULSED WAVE DOPPLER CONTINUOUS WAVE DOPPLER COLOUR DOPPLER POWER DOPPLER SPECTRAL DOPPLER DUPLEX MODE TRIPLEX MODE TRANSCRANIAL DOPPLER TISSUE DOPPLER OTHERS

18. TYPES Continuous Wave Doppler Pulsed Wave Doppler 18

19. CONTINUOUS WAVE DOPPLER It is a continuous wave of ultrasound being sent into the body . Measures mainly the velocity of moving blood. 19

20. CW: TRANSDUCER PROPERTIES CW uses a relatively narrow-band high-Q transducer, to preserve velocity information. 20

21. CW: TRANSDUCER PROPERTIES The piezoelectric elements of the transducer are divided into 2 : Transmitter : continuously transmitting US, and Receiver : detecting reflected echoes. Doppler Shift = Received Freq – Transmitted Freq 21

22. 22 Transmitter and Receiver are angled against each other . The Transmitter produces sinuosoidal US waves of the form Cos 2 π f o t . The received signal is of form: Cos [ 2 π ( f o + f D ) t ] The Doppler signal is thus of form: Cos 2 π f D t , and can be recovered via frequency demodulation of the received signal. Fig: CW Doppler Scan Geometry (angled Tx and Rx)

23. WALL FILTER: CW DOPPLER The Doppler signal contains very low frequency signals (clutter) from moving specular reflectors, such as blood vessel walls. A " Wall Filter " selectively removes these other low frequencies . 23

24. BEAT: CW DOPPLER (contd.) The Doppler ultrasound signal is amplified to an audible sound level (called Beat). Doppler Beat can be heard through a speaker or a headphone. 24

25. CW DOPPLER (contd.) The audio indicates the spread of velocities involved in a heart beat. It forms the basis of the ultrasound stethoscope for foetal heartbeat monitoring . 25

26. Fig: Foetal Heart Rate (FHR) Doppler Detector Some also display the Heart Rate in Beats Per Minute (BPM) Uses the principle of Continuous Wave Doppler. 26

27. BEATS: CW DOPPLER (contd.) The higher the pitch, the greater the velocity. The harsher the sound, the greater the turbulence. 27 Δ f = ( 2vfCos θ ) /c

28. 28 Fig: Block Diagram of CW Doppler System

29. CW DOPPLER (contd.) Output can also be displayed on a Spectrum analyser , constructing a pixel grayscale. Quadrature Detection (type of signal processing) permits determination of the direction of flow of blood in CWD, if used. 29

30. CW DOPPLER SPECTRAL DISPLAY The pixel greyscale represents the magnitude of the short time Fourier Transform of the Doppler signal. A pixel in a Doppler spectrum represents the proportion of red blood cells (RBCs) in the field of view that was moving at a particular velocity at a particular time . 30

31. LIMITATIONS OF CW DOPPLER Poor Spatial Resolution : resulting from the large area of overlap between the transmitter and receiver beams. Range Ambiguity: Measures velocity, but not location. CW cannot be used to distinguish the flow of overlapping vessels at different depths . Lack of TGC (time gain compensation). 31

32. ADVANTAGES OF CW DOPPLER For measuring fast flow (high velocities) without aliasing. Good for assessing deep-lying vessels . It is inexpensive. 32

33. PULSED WAVE DOPPLER It transmits a sequence of short pulses, rather than a continuous sinusoidal wave. PW allows both velocity and depth information to be obtained. 33

34. PW DOPPLER (contd.) One single group of array elements is used for both receiving and transmitting . PW Doppler allows measurement from a small, specific blood volume (region of interest), which is defined by a sample volume . After the pulse has traveled forth and back (travel time T), an electronic range gate is opened for a short period of time to receive the echoes . 34

35. OPTIMIZING PW Doppler Shift measurement: Achieved using longer spatial pulse length (SPL)/high Q-factor transducer.* Depth Selection: Achieved with an electronic time gate circuit (or range gate) to reject all echo signals, except those falling within the determined gate window. 35

36. SAMPLE AND HOLD OPERATION As the echo from each successive transmission is received, a single sample at the expected arrival time of echoes from the range gate is acquired and held until the echo from the next pulse is received . 36

37. Fig: Sample-and-Hold Operation of PW Doppler 37 If the reflectors are moving, the signal received from the range gate will change with each subsequent pulse, and the sample-and-hold operation will construct a staircase signal (as above).

38. PW DOPPLER (contd.) The second pulse should be transmitted no sooner than the expected arrival time of the echoes from the range gate that arise from the first pulse*. The pulse travel time T determines the shortest possible time interval between two successive transmit pulses . 38

39. Pulse repetition Frequency (PRF) is the number of pulses that an ultrasound system transmits into the body each second. The PRF of the transmitted pulses is the effective sampling frequency. PRF = 1/T The maximum detectable frequency shift (Nyquist Limit) is determined by the value of one-half PRF. f max = ½ PRF 39 NYQUIST LIMIT

40. CONDITION TO AVOID ALIASING : The PRF must be at least twice the Nyquist Limit. PRF ≥ 2 f max 40 NYQUIST LIMIT f max = ½ PRF PRF = 2 f max Otherwise , at high blood velocities , aliasing will occur.

41. Fig A: Aliasing as shown in spectral Doppler effect 41 Aliased signals wrap around to negative amplitude, masquerading as reversed flow and underestimating velocity. A

42. PW MAX. VELOCITY Recall Doppler frequency is given by: Substituting Maximum PW Doppler Frequency f max for Δ f, we get: f max = ( 2 v max f Cos θ ) / c PRF/2 = (2 v max f Cos θ ) / c 42 Δ f = ( 2vfCos θ ) /c v max = cPRF 4fCos θ

43. PW MAX VELOCITY v max represents the maximum velocity of reflector (e.g. RBCs) that can be measured without aliasing. For a range gate positioned at depth z, z = ½ λ PRF = c/2z 43 v max = ___ c 2 ____ 8 z f Cos θ v max = cPRF 4fCos θ

44. PW MAX. VELOCITY The maximum velocity that can be accurately determined by Pulsed Doppler increases with PRF , lower operating frequency , and increasing angle . 44 v max = ___ c 2 ____ 8 z f Cos θ v max = c PRF 4 f Cos θ

45. DISADVANTAGES OF PW DOPPLER PW cannot accurately measure fast blood velocities due to aliasing. To avoid aliasing: increase the PRF increase Doppler angle reduce the depth reduce the transducer frequency change the baseline use CW instead. 45 v max = ___ c 2 ____ 8 z f Cos θ

46. 46 Fig: Block Diagram of a Pulsed Wave Doppler System

47. MULTIGATED (MG) PULSED WAVE DOPPLER This is a variation of Pulsed Wave Doppler. Whereas PW Doppler system can only provide information from a particular sample, MG PW Doppler obtains information from several depths simultaneously using multiple gates . After demodulation, the received signal is directed to a number of parallel processing chains - each with slightly different range of gate settings. 47

48. MG PW DOPPLER (CONTD.) Velocity distribution profile across the vessel cross-section can be determined. It is a useful diagnostic tool for presence of plaques and stenosis. Orientation and location of a desired vessel still remain a problem, which can be solved by Duplex Scanning . 48

49. MODES OF DOPPLER IMAGING To display Doppler information, one of 3 modes may be chosen: Color Doppler Power Doppler Spectral Doppler 49

50. (a) Power Doppler 50 (b) Colour Doppler (c) Spectral Doppler

51. COLOUR DOPPLER (CD or CCD) Color (flow) scanning involves displaying Colour Doppler data on real time (B-mode) grayscale images. The superimposition is such that tissue volumes containing no detectable flow are displayed in greyscale, while those in motion are in colour (usually red and blue). 51

52. COLOUR DOPPLER Colours are assigned, depending on: DIRECTION : motion towards (Positive Doppler shift) or ( Velocity Mode ) away (Negative Doppler shift) from the transducer. FLOW ( Variance Mode ) : Laminar or Turbulent flow VELOCITY MAGNITUDE: mapped to the colour intensity 52

53. 53

54. LIMITATIONS OF COLOUR DOPPLER Clutter of slow-moving solid structures and noise can overwhelm the smaller echoes from moving blood . Spatial resolution of the colour image is lower than grayscale. 54

55. USES OF COLOUR DOPPLER Imaging the heart and major blood vessels in applications for which mean flow velocity is a diagnostically useful parameter . T o recognize and localize vessels and vascular blockage . 55

56. POWER DOPPLER Power Doppler represents the total power in the Doppler spectrum at each sample volume . Unlike colour Doppler, Power Doppler usually uses a single colour. 56

57. POWER DOPPLER Power Doppler uses the magnitude of return Doppler signal strength / power /intensity / amplitude alone . It ignores the angle and direction of flow (phase). It adopts slower frame rates and thus has greater sensitivity to motion of the patient, tissues, and transducer. 57 Power α Intensity α Amplitude 2

58. Fig: Power Doppler 58

59. 59

60. 60

61. ADVANTAGES OF POWER DOPPLER PW’s greater sensitivity to motion allows detection and interpretation of very subtle and slow blood flow. Power Doppler provides a more continuous display of tortuous vessels. Helps to rule out vascular occlusions, Helps to differentiate between blood carrying vessels and other fluid occurrences It is not prone to aliasing. 61

62. DISADVANTAGES OF POWER DOPPLER It is susceptible to flash artifacts, which are colour signals due to its greater sensitivity to motion. No information on flow velocity and direction of flow. E valuation of hemodynamic properties such as the pulsation of flow, is limited, since, most often, the image frame rate is too low. 62

63. USES OF POWER DOPPLER Used for tumour imaging, where moving blood volume is a diagnostically useful parameter. 63

64. SPECTRAL DOPPLER The Spectral Doppler uses the Doppler frequency shift of the echo signal as a measure of flow velocity and direction of flow . The Doppler spectrum ascertains the distribution of flow velocities and their frequencies of occurrence at a defined sampling site as a function of time. 64

65. SAMPLING SITE: SPECTRAL DOPPLER The sampling site is determined by using B-mode, Color Doppler or Power Doppler image display ( Duplex imaging ). 65 SPECTRAL DOPPLER

66. The spectra from within or directly behind a stenosis (numbers 2 to 4) show increased velocities, turbulence and reverse flow. These changes in spectrum provide information on flow in the blood vessels. 66

67. FLOW Laminar flow normally exists at the centre of large smooth vessels. Turbulent flow occurs when the vessel is disrupted by plaque and stenosis . 67

68. USES OF SPECTRAL DOPPLER 68 SPECTRAL DOPPLER Spectral Doppler provides a detailed qualitative, semi-quantitative or quantitative evaluation of hemodynamic changes in tissues.

69. DUPLEX SCANNING Duplex scanning combines 2D B-mode imaging and a Doppler type (e.g. Colour Doppler). The duplex system allows estimation of the flow velocity directly from the Doppler shift frequency. 69

70. Duplex mode : Combining B-mode Greyscale + Colour Doppler 70

71. TRIPLEX MODE 71 Combines 3 methods: B-Scale greyscale, Colour Doppler and Spectral Doppler

72. (SOFT) TISSUE DOPPLER IMAGING Used to image soft tissue using a low pass filter. A conventional Doppler system assumes blood flow is concentrated at intermediate and high velocities, while scatterers moving at low velocities correspond to soft tissue. Therefore to eliminate such low velocities, a high pass wall filter is usually used. 72

73. (SOFT) TISSUE DOPPLER IMAGING Tissue Doppler Imaging replaces the high pass wall filter with a low pass filter , thus allowing only low Doppler frequency signal, corresponding to low velocity movements. Can be done in Pulsed Wave Doppler mode or Colour Doppler mode. Used in diagnosing regional abnormalities in ventricular wall motion. 73

74. TRANSCRANIAL DOPPLER ULTRASONOGRAPHY (TCD) TCD is a non-imaging Pulsed Wave Doppler technique measuring local blood flow velocity and direction in the proximal portions of large intracranial arteries. Used for evaluation and management of patients with risk of cerebrovascular disease. 74

75. TCD: Stroke Prevention for Children with SCD In 2017, Dr. M. Adekunle and Prof. O. Akinyanju (of Sickle Cell Foundation Nigeria) recommended annual screening of children with Sickle Cell Disorder (SCD to assess risk of stroke. Children aged 2-16 years with Sickle Cell Disorder (SCD) have a high risk of stroke, esp. children aged 2-8 years. 75

76. TCD: Stroke Prevention for Children with SCD Stroke occurs when part of the brain is damaged by inadequate blood supply . Inadequate blood supply occurs due to cerebral bleeding (bleeding from arteries) or thrombosis (blood clots), which impede flow of blood in the arteries of the brain. 76

77. If the velocity in an artery exceeds 250cm/s, that area is at a higher risk of stroke [Adams]. 77 Fig: Transcranial Doppler Ultrasound of blood vessels in the Circle of Willis of the brain

78. TCD: Stroke Prevention (contd.) Stroke can cause paralysis, seizures, loss of speech or reduced intellectual capacity. Stroke prevention therapy can be carried out for affected children. Adekunle and Akinyanju recommend establishment of more TCD centres across Nigeria. 78

79. 79 KNOBOLOGY Some ultrasound units have a knob for PW, CW, Colour, and Power Doppler modes. (A) (B)

80. ARTIFACTS IN DOPPLER Aliasing Mirror Artifact Vibration Artifact (Bruit) Velocity Artifact Shadowing Artifact 80

81. ALIASING Aliasing is error due to insufficient sampling rate PRF It causes high velocity forward flow to appear as reverse flow. Occurs in Colour Doppler, but not Power Doppler. 81

82. 82 Fig: Increasing PRF can solve Aliasing Artifact

83. Due to reverberations and mirroring at strong reflectors, images in Color or Power Doppler show artifacts, similar to those observed in B-mode . e.g. mirroring of hepatic vessels at the diaphragm . 83

84. Caused by tissue vibrations at the site of an arterial-venous fistula, occlusion or stenosis. pulsating blood pressure is transmitted to adjacent tissue causing tissue vibration. This is due to pulsating blood pressure at occlusions transmitted to adjacent tissue, recognized as a colour mosaic . 84

85. Due to changing Doppler angle and transducer type used. Occurs only in Colour Doppler (Doppler angle does not affect Power Doppler) It can lead to Reverse-Flow and Flow Acceleration artifacts. 85 Convex/Sector Transducer & Linear Vessels : Curved transducer aperture introduces Doppler angles, causing inaccurate depiction of blood vessel velocities. Linear Transducer & Curved Vessels : Produces similar effect as the above.

86. In the display of blood flow in the verterbral column in the Power Doppler mode, due to strong reflectors (bones), a shadow is cast on the blood vessels, making them look interrupted. 86

87. OTHER FORMS OF ANGIOGRAPHY (DSA vs CT vs MR) Digital Subtracted Angiography is the ‘gold standard’ in arterial imaging. It uses X-ray based fluoroscopy. It produces its image by subtracting a pre-contrast image with an image taken after injection of a contrast medium. It is considered invasive, as it involves use of catheter. It requires the patient to be still, and thus not favoured for heart imaging. 87

88. OTHER FORMS OF ANGIOGRAPHY (DSA vs CT vs MR) CT Angiography (CTA) has the advantage of producing 3D images. It is less invasive and stressful for the patient. MR Angiography (MRA) avoids ionizing radiation and nephrotoxic contrast agents . 88

89. CTA vs MRA Advantages of CTA over MRA include: Visualization of calcified plaques and I nsusceptibility to metallic vascular clips. Lower cost : Iodinated contrast material for CT angiography is less expensive than the dose of gadolinium required for MR angiography. 89

90. CTA vs MRA Coronary Artery Disease : CTA offers better sensitivity and specificity better than MRA. Arterial Stenosis of the Aortoiliac and Renal Arteries : No statistically significant difference between CTA and MRA. 90

91. CTA vs MRA Aorto -iliac Arterial Disease : CTA shows higher sensitivity (98.7%) than CCD (96.2 %) in the assessment of aorto -iliac arterial disease [ A. Osama et al, 2012 ]. Agreement between DSA and CCD was 96.1% 91

92. Doppler Ultrasonography has a relatively high sensitivity in comparison with digital subtraction angiography. It is cheaper, less-invasive, non-ionizing and offers minimal risk of bio-effects. 92 CONCLUSION

93. REFERENCES D. R. Dance, et al. Diagnostic Radiology, Physics. IAEA. 2014: Vienna. M. A. Aweda . Principles of Doppler Imaging . Lagos University Teaching Hospital. Lagos. 2012. K. O. Soyebi . An Introduction to Transcranial Doppler Imaging. Lagos University Teaching Hospital. Lagos. 2012. J. T. Bushberg , et al. The Essential Physics of Medical Imaging. 2nd Ed. Philadelphia: Lippincott Williams & Wilkins. 2002. W. Huda, R. Sloan. Review of Radiologic Physics . 3rd ed. Philadelphia: Lippincott Williams & Wilkins. 2009. M. Adekunle , O. Akinyanju . Prevention of Stroke in Children with Sickle Cell Anaemia. Sickle Cell Bulletin. Vol 8. No. 1. Sickle Cell Foundation Nigeria. 2017. Siemens AG. Principles of Ultrasound Imaging . 1999: Med USSE J. M. Adams. Ultrasound’s Transcranial Doppler Imaging Checks for Risk of Stroke. 2016. Cincinnati Children’s Hospital. http://www.blog.cincinnatichildrens.org/radiology / W. R. Hendee , E. R. Ritenour . Medical Imaging Physics . 4th Ed. New York: Wiley- Liss . 2002. A. Osama, et al. Role of Multi-Slice CT Angiography versus Doppler Ultrasonography and Conventional Angiography in assessment of aorto -iliac arterial disease . Egyptian Journal of Radiology and Nuclear Medicine. 2012. R. Herzig , et al. Comparison Of Ultrasonography, CT Angiography, and Digital Subtraction Angiography In Severe Carotid Stenoses . European Journal of Neurology. 2004. 93

94. THANK YOU 94