doppler physics medical imaging in human body in seminar content ppt.pptx

medical science human body ultrasound doppler imaging in seminar presentation format

1. DOPPLER PHYSICS MODERATOR- Dr. C P NANJARAJ PRESENTER- Dr. SOWMYA S

2. HISTORY DOPPLER EFFECT DOPPLER EQUATION TYPES OF DOPPLER DOPPLER INSTRUMENTATION SPECTRAL ANALYSIS DOPPLER INDICES DOPPLER ARTIFACTS

3. DOPPLER EFFECT Doppler effect is a wave phenomena where a detected wavelength(or frequency) is different from source wavelength(or frequency) due to relative radial motion between source and observer. DISCOVERY – Physicist Christian Johann Doppler in 1842.

4. Main characters of doppler effect ; When source and observer are moving towards one another, the detected wavelength appears shorter than the source wavelength(frequency higher). When the source and observer are moving away from each other the detected wavelength appears longer than the source wavelength(frequency shorter). Greater the shift greater the relative speed between the source and the observer.

5. D O P P L E R S H I F T Doppler shift is the difference between the transmitted and received frequencies. Transmitted and received Frequencies are in the MHz range . Doppler shift frequencies often in audible range .

6. WHEN RECEIVED FREQUENCY = TRANSMITTED FREQUENCY, NO DOPPLER SHIFT

7. DOPPLER EQUATION The Doppler equation describes the relationship of the Doppler frequency shift to target velocity. In its simplest form, it is assumed that the direction of the ultrasound beam is parallel to the direction of movement of the target. This situation is uncommon in clinical practice. Often, the ultrasound impinges on the vessel at angle θ, in this case the Doppler frequency shift detected is reduced in proportion to the cosine of the angle theta.

8. DOPPLER EQUATION

9. DOPPLER ANGLE

10. Five types of diagnostic doppler instrument are usually distinguished: 1. Continuous wave (CW) doppler 2. Pulsed wave (PW) doppler 3. Duplex doppler 4. Colour imaging ( colour velocity imaging, colour flow imaging) 5. Power doppler imaging .

11. Composition of blood causes important differences b/w doppler signals generated by blood and that generated by solid structures. Specular reflection: implies smooth reflection of light(like mirror) which results in nice sharp image on other hand reflection from rough surface produces scattering. Similarly wall of blood vessel is relatively smooth and red cells act as rough surface. Ultrasound encountering blood is not reflected and is scattered due to RBC’s.

12. If the size of the primary ultrasound wave is much larger than the wavelength of ultrasound used, the primary ultrasound will be scattered in all directions. The average diameter of RBC is 7mcm and the wavelength of ultrasound is about 0.44mm at 3.5mHz. Therefore erythrocytes act as scatterers and hence the return echo size from blood is small compared to solid tissue interface. A typical sonogram displays vessels as echo free structure because of scattering from erythrocytes.

13. Three basic levels of US can be performed , with each level adding information to the preceding level.

14. CONTINUOUS WAVE DOPPLER Simplest form, portable, inexpensive. Uses transmit and receive crystals that continuously transmit and receive ultrasound. Doppler signals are obtained from all vessels in the path of the ultrasound beam (until the ultrasound beam becomes sufficiently attenuated due to depth). It can detect the presence and direction of flow, but unable to distinguish signals arising from different depth . Therefore it lacks range resolution. Used in cardiac scanners- to detect high flow velocities in aorta and bed side to ascertain flow in superficial vessels.

15. CONTINUOUS WAVE DOPPLER

16. P U L S E D-W AV E ( P W ) D O PP L E R The electrical excitation signal is applied to the transmitter element at regular intervals as pulses. Uses only one crystal. Advantage of PW Doppler is that since pulsed ultrasound is employed, the range of the moving target may be measured from the echo-return time. The highest velocity that the instrument can measure is directly proportional to its PRF

17. PULSE REPETITION FREQUENCY PRF= NUMBER OF PULSES TRANSMITTED/ SECOND With increasing scanning depth PRF decreases as more time is needed for the echoes to return. The PRF must be at least twice the frequency of the Doppler signal ( nyquist sampling limit ) to construct the signal successfully . This sets upper limit to the flow velocities to be accurately recorded. Use higher PRF setting for high flow velocities, low PRF for slow venous flow.

18. To ensure that samples originate from only a selected depth when using a pulsed wave Doppler system, we should wait for the echo from the area of interest before transmitting the next pulse. This limits the rate with which pulses can be generated, a lower PRF being required for greater depth. Because of the need for lower PRFs to reach deep vessels, signals from deep abdominal arteries are prone to aliasing if high velocities are present. The PRF also determines the maximum depth from which unambiguous data can be obtained. If PRF is less than twice the maximum frequency shift produced by movement of the target (Nyquist limit), aliasing results.

19. ALIASING IN PULSE WAVE DOPPLER   Aliasing  is  production of  artificial  low frequency signals when the sampling rate( i,e . PRF) is less than twice the doppler signal frequency. When the Doppler shifts exceed a value Nyquist frequency, velocites are perceived as  going in the opposite direction. This ambiguity is known as aliasing. A similar effect is seen in films where wagon wheels can appear to be going backwards due to the low frame rate of the film causing misinterpretation of the movement of the wheel spokes.

20. NYQUIST  SAMPLING  LIMIT PRF must be atleast twice the frequency of doppler signal to construct signal successfully (known as Nyquist sampling rate) Alternatively the Maximum Doppler frequency that can sampled is ½ the PRF If sampling rate< nyquist rate-aliasing occurs (production of artefactual low frequency signal)  Example;  if PRF = 8kHz Max Doppler frequency is 4kHz     if PRF = 4kHz Max Doppler frequency is 2 kHz  

21. Manifestation of aliasing In a clinical setting, aliasing appears in Doppler spectrum wraparound of higher frequencies below the baseline.( I,e . display with high velocities being converted to reversed flow)

22. (a) Waveforms with aliasing, with abrupt termination of the peak systolic and display this peaks bellow the base line. (b) Correction: increased the pulse repetition frequency and adjust baseline (move down)

23. ALIASING IN COLOUR DOPPLER Mixing of colours .

24. HOW TO ELIMINATE ALIASING Increase the velocity scale- machine will automatically increases PRF. by increasing the Doppler angle- Thereby decreasing the frequency shift. Use low frequency transducer. Change the display baseline-assign entire spectral display to one direction .

25. Aliasing of color doppler imaging. Color image shows regions of aliased flow (yellow arrows). Reduce color gain and increase pulse repetition frequency .

26. Power Doppler has no aliasing because it has no d i r e c t i o n a l o r v e l o c i t y c o m pon e n t . Advantage : - 1. useful for localizing the highest velocity region. 2. It is used in identifying the abnormal area in TIPSS and in localizing AV fistulae. Disadvantage :- high velocity may not be accurately measured.

27. COLOUR DOPPLER/COLOUR FLOW IMAGING/ COLOUR VELOCITY IMAGING creates an image from Doppler information, i.e. an image of velocities in regions of blood flow. it is normally combined with a conventional real-time B-scan so that both tissue structure and areas of flow are displayed.

28. Positive Doppler shifts are encoded as red and negative shifts are encoded as blue . Velocity of the flow is represented in shades of color(faster–brighter). Central part usually has higher velocity and appear brighter than periphery.

29. Flow velocity is usually indicated by color brightness.

30. COLOUR BOX Defines the area of the image over which color Doppler data are acquired. It can be adjusted for size,position and direction/angulation. More time is needed to acquire echo data for the wide color box than the narrow color box. As small & superficial as possible. Deep color box – slower PRF results aliasing.

31. COL O U R B O X

32. DUPLEX ULTRASOUND SCANNING Duplex ultrasound instruments are real-time B-mode scanners with built-in doppler capabilities. B-mode imager (Outline anatomic structures) Pulsed wave doppler ( fl o w a n d m o v e m e n t pattern)

33. POWER DOPPLER Also known as energy doppler /Amplitude doppler . It directly measures the intensity of the Doppler signal rather than Doppler shift. A power Doppler image depicts the amount of blood moving in each region, i.e. an image of the detected blood pool. An alternative to the display of frequency information with color doppler imaging is to use a color map that displays the integrated power of the doppler signal instead of its mean frequency shift.

34. It is independent of velocity and direction of flow, so there is no possibility of signal aliasing. It is independent of angle than frequency-based color Doppler display allowing detection of smaller velocities than colour Doppler, facilitating examinations in certain technically challenging clinical setting. It has higher sensitivity than colour Doppler, which makes a trade-off with flash artefacts.

35. In contrast to color Doppler, where noise may appear in the image as any color , power mode Doppler permits noise to be assigned to a homogeneous background color that does not greatly interfere with the image. This permits use of higher effective gain settings and increased sensitivity for flow detection.

36. Spectral analysis of Doppler signal contains both frequency and amplitude information of a small tissue sample. The vertical axis represents frequency shift (related to colour Doppler), the brightness of the pixels represents the amplitude of the signal (related to Power Doppler) and the horizontal axis represents time. MODALITY ASSESMENT Colour doppler Frequency shift Power Doppler Amplitude Spectral Doppler Both frequency and amplitude

37. Clinical use; Power Doppler is particularly useful when examining superficial structures, like thyroid, testis, renal grafts and subcutaneous lesions. It may be used to look for tumour vessels, to evaluate tiny low-flow vessels and detect subtle ischaemic areas.

38. Power Doppler uses a color map to show the distribution of the power or amplitude of the Doppler signal. Flow direction and velocity information are not provided in power mode Doppler display, but noise is reduced, allowing higher gain settings and improved sensitivity for flow detection . uses a color map to display information based on the detection of frequency shifts from moving targets. Noise in this form of display appears across the entire frequency spectrum and limits sensitivity

39. ADVANTAGES OF POWER DOPPLER LIMITATIONS OF POWER DOPPLER Sensitive to low flows(distinguishes preocclusive lesions from occlusive lesions ) . No directional and velocity information . Less s usceptible to noise. Poor temporal resolution (very motion sensitive-less suitable for rapid scan along the vessel ) . Less angle dependent Due to the low PRF, power Doppler is particularly sensitive to flash artefact. No a liasing since frequency shift details are not displayed . Imaging in tortuous vessels

40. SPECTRAL ANALYSIS Blood flow is pulsatile. Vessels are not always straight or uniform size Flow is slower at periphery than centre . Vessel lumen may be distorted by atherosclerosis/other pathology. Hence blood flow produces a mixture of doppler frequency shifts. Spectrum analysis is needed to sort out this.

41. Doppler shift spectrum shows the spectrum and direction usually displayed as velocity rather than frequency. The Doppler shift data are displayed in graphic form as a time-varying plot of the frequency spectrum of the returning signal. A fast fourier transformation is used to perform the frequency analysis. Provides quantitative information.

42. A s s e s s t h e fo l l o w i n g Pr ese n ce o f f l o w Di r e c t io n o f flow A m p l i t ude W indow P ulsa t i l i t y

43. DIRECTION OF FLOW Pulsed doppler use quadrature phase detection to provide bidirectional doppler information FLOW CAN EITHER BE, Mono-phasic Bi-phasic Tri-phasic Bidirectional

44. MONOPHASIC FLOW

45. TARDUS PARVUS WAVEFORM

46. BIPHASIC FLOW

47. TRIPHASIC FLOW

48. BIDIRECTIONAL FLOW

49. PSEUDOANEURYSM Pseudoaneurysm , is when there is a breach in the vessel wall such that blood leaks through the wall but is contained by the adventitia or surrounding perivascular soft tissue. Pseudoaneurysms can develop after penetrating trauma or arterial catheterization . The duplex sonographic finding of a “to and fro” sign is typically detected in the communicating channel of the pseudo aneurysm.

50. AV FISTULA A fistula may be visualized directly, with an abnormal high-velocity connection between the artery and vein. Even if the fistula cannot be visualized directly, changes in the artery upstream from and the vein downstream from the fistula can establish a diagnosis: Increased diastolic arterial flow due to its connection to the low-resistance vein(monophasic). Arterialization of the vein downstream from the fistula (abnormal arterial pulsatility ). Enlargement of the downstream vein due to the increased volume of flow.

51. ADVANTAGES SPECTRAL DOPPLER COLOUR DOPPLER Depicts quantitative flow at one site . Overall view of flow. Allows calculations of velocity and indices. Directional information about flow. Good temporal resolution. Averaged velocity information about flow.

52. WINDOW   Received doppler shift consist of a range of frequencies I,e velocities. Narrow range of frequencies will result in narrow display line. The clear black area between the spectral line and the baseline is called the window.

53. Spectral  Broadening Loss of the Spectral window is called Spectral Broadening. Occurs usually: • As the blood decelerates in diastole. •  If sample volume is placed to close to the vessel wall •  In small vessels •  Tortuous vessels. •  At branching points. •  Excessive gain/power/dynamic range

54. Causes of Spectral Broadening Artificial Large sample volume High gain Physiologic Normal small vessels (hepatic arteries) Normal turbulence (bifurcations) Pathologic Compressed vessels ( eg , hepatic veins in cirrhosis) Turbulent flow (stenotic flow)

55. SPECTRAL BROADENING IS THE HALLMARK OF TURBULENT FLOW

56. WALL FILTERS Wall echoes have a large amplitude and low frequencies. Cause a loud ”wall thump’’ on the audio doppler output Doppler instruments detect motion not only from blood but also from the adjacent structures. Filters typically eliminate these low frequency high intensity noise that may arise from vessel wall motion and adjacent structures.   Wall filters should be kept at the lowest practical level usually 50 to 100Hz or less. WHY?

57. SAMPLE VOLUME(GATE) The sample volume is the three dimensional space from which the doppler frequency shifts are measured. Ideal sample volume – 1/3rd of the vessel width positioned in the center of the vessel. If sample volume is more: - Spectral broadening If sample volume is less: Measured velocity is too low.

58. SAMPLE VOLUME

59. DOPPLER GAIN As with imaging, proper gain settings are essential to accurate and reproducible Doppler measurements. Excessive Doppler gain results in noise appearing at all frequencies and may result in overestimation of velocity. Conversely, insufficient gain may result in underestimation of peak velocity. After placing the sample volume in the vessel, the Doppler gain should be increased to a level where noise is visible in the image, then gradually reduced to the point at which the noise first disappears completely.

60. DOPPLER GAIN CONTROLS THE AMPLITUDE OF, the color display in color or power Doppler mode and The spectral display in pulse Doppler mode.

61. VELOCITY SCALE Controls the range of frequencies displayed. Too high scale- dynamic range too large and low velocity signals are missed simulating an area of thrombosis. If scale is too low- dynamic range is too small to display the high velocity signals accurately resulting in aliasing .

62. VELOCITY SCALE Velocity scale set at 20cm/s. Hepatic artery is visualised.No flow is visualised in the adjacent portal vein. B.Velocity scale set at 7cm/s. Flow is now visualised in the portal vein.

63. INVERSION

64. 81 QUANTITATIVE CHARACTERIZATION OF ARTERIAL RESISTANCE PULSATILITY INDEX RESISTIVE INDEX SYSTOLIC/DIASTOLIC RATIO ACCELERATION TIME ACCELERATION INDEX

65. PI=S-D*/M RI= S –D/S max sys height-min diastolic max systolic height-EDV mean height max systolic height

66. A-Peak systolic velocity or frequency B-Minimum or end diastolic velocity or frequency C-Mean velocity

67. Pulsatility Index (PI) The pulsatility index (PI) is defined as maximum velocity excursion divided by mean of the velocity Measures the difference between the maximum and minimum velocities within the cardiac cycle This ratio is used in vessels where reverse flow may occur, for example in the lower limbs. Also known as Gosling index. Can be measured without knowledge of the doppler angle The indices are unitless

68. Resistance Index (RI) It reflects the vessel wall elasticity as well as peripheral resistance the organ supplied. High resistance in the distal vessels produces low diastolic flow in the supplying artery and results in a high value for this index. A low resistance results in a low value as there is higher diastolic flow. It is also known as the Pourcelot index.

69. DOPPLER ARTIFACTS ARTFACTS DUE TO TECHNICAL LIMITATIONS Blooming artifact( colour bleed) Directional ambiguity Partial volume artifact Absent flow due to high velocity settings. Aliasing. ARTIFACTS CAUSED BY PATIENT ANATOMY Pseudoflow Flash artifact Mirror image artifact ARTIFACTS DUE TO MACHINE FACTORS Edge artifact- along the cortical bone. Twinkling artifact- beneath calculi .

70. DOPPLER ARTIFACTS Blooming artifacts: Caused by abnormally high gain settings. Causes the obscuration of thrombus or plaques in the vessel. Also seen with ultrasound contrast agents.

71. MIRROR IMAGE ARTIFACT displays objects on both sides of a strong reflector. The reflectors (diaphragm, pleural surface and aortic wall) directs some of the echoes to a second reflector before it returns them to the transducer resulting multipath reflection. Eg ; Duplication of sub clavian artery (pleura reflector).

72. FLASH ARTIFACT Flash artifact is a sudden burst of random color that fills the frame, obscuring the gray-scale image. Manifests as a color signal due to transducer or patient motion. It is seen in left lobe of liver due to cardiac pulsation and in hypoechoic areas such as cysts and fluid collections.

73. Power Doppler is more susceptible to flash artifact than color flow Doppler because of the longer time required to build the image.

74. PSEUDOFLOW Pseudoflow is defined as presence of flow of a fluid other than blood. Examples - ascites, amniotic fluid, urine (bladder jets).

75. TWINKLING ARTIFACT Appears as discrete focus of alternating colors behind echogenic object. e.g : Renal Calculi, bladder calcification and cholesterol crystals in the gall bladder.

76. GUIDELINES FOR AN OPTIMAL COLOR FLOW DOPPLER EXAMINATION • Adjust the gain and filter settings to obtain an optimal color signal and minimal color noise. • Adjust the velocity scale (PRF) and baseline according to the flow conditions. A low scale is used for low flows and velocities; however, it may produce aliasing. A high scale reduces aliasing but is less sensitive for slow flows. • Obtain an optimal Doppler angle by adjusting the beam steering and probe position. The angle should be 60° or less if velocity measurements are to be made. • The color flow box should be kept as small as possible to allow better frame rate for better resolution and sensitivity. • Adjust the pulsed Doppler sample volume size appropriately (two thirds of the vessel diameter) to obtain accurate velocities. • Avoid transducer motion.

77. To conclude it should be remembered that doppler ultrasound is a dynamic examination and, even with modern equipment, the best results will be obtained if the system controls are adjusted to optimum settings for the task in hand, rather than relying on the preset profiles alone.

78. REFERENCES ALLAN-CLINICAL DOPPLER ULTRASOUND -3 RD EDITION INTRODUCTION TO VASCULAR ULTRASONOGRAPHY-5 TH EDITION- PELLERITO, ZWIEBEL DIAGNOSTIC ULTRASOUND- CAROL M. RUMACK, STEPHANIE R. WILSON, J. WILLIAM CHARBONEAU, DEBORAH LEVINE

79. Thank You….