DOPPLER ECHOCARDIOGRAPHY-1

1. DOPPLER ECHOCARDIOGRAPHY-1 BASIC PHYSICS,PULSE WAVE AND CONTINUOS WAVE DOPPLER DR PRADEEP SREEKUMAR

2. Sound is a mechanical vibration transmitted through an elastic medium • Ultrasound-portion of sound spectrum having frequency greater than 20,000 cycles /sec • Use of ultrasound to study the structure and function of heart and great vessels-echocardiography • Advantages of ultrasound Can be directed as a beam and focussed Obeys laws of reflection and refraction Produce longitudnal waves

3. Generation Of An Ultrasound Image

6. Machines • There are 5 basic components of an ultrasound scanner that are required for generation, display and storage of an ultrasound image. • Pulse generator - applies high amplitude voltage to energize the crystals • Transducer - converts electrical energy to mechanical (ultrasound) energy and vice versa • Receiver - detects and amplifies weak signals • Display - displays ultrasound signals in a variety of modes • Memory - stores video display

7. Depicted as sine wave-peaks and troughs • One cylce=one compression + one rarefaction • Distance between 2 similar points represent wavelength • 0.15 to 1.5 mm in soft tissue • Frequency- number of wavelengths per unit time • V=f X λ(v=velocity,f =frequency, λ is wavelength)

8. Velocity of sound=1540 m/sec in soft tissue • Wavelength=1.54/f • Amplitude Measure of strength of the sound wave Indicated by height of sine wave above and below baseline

11. Higher the frequency greater the resolution • Higher frequency,lesser the penetration • Loss of ultrasound as it propogates through a medium is called attenuation

12. PRICIPLES OF PEIZO ELECTRIC CRYSTALS The charges in a piezoelectric crystal are exactly balanced, even if they're not symmetrically arranged. The effects of the charges exactly cancel out, leaving no net charge on the crystal faces the electric dipole moments—vector lines separating opposite charges—exactly cancel one another out. If you squeeze the crystal , you force the charges out of balance.

13. Now the effects of the charges (their dipole moments) no longer cancel one another out and net positive and negative charges appear on opposite crystal faces. • By squeezing the crystal, voltage is produced across its opposite faces- piezoelectricity • The piezoelectric effect was discovered in 1880 by two French physicists, brothers Pierre and Paul-Jacques Curie, in crystals of quartz, tourmaline, and Rochelle salt (potassium sodium tartrate). They took the name from the Greek work piezein, which means "to press."

14. The phenomenon of generation of a voltage under mechanical stress is referred to as the direct piezoelectric effect • mechanical strain produced in the crystal under electric stress is called the converse piezoelectric effect.

15. Ferro electrics,bariumtianate,leadzirconatetitanate are used as peizo electric crystals. • Dampening material-shortens the ringing response Also absorbs backward and laterally transmitted acoustic energy • Frequency emitted by transducer is directly proportional to propagation speed within crystal and inversely related to thickness

16. Important feature of ultrasound is ability to direct or focus the beam as it leaves the transducer • Proximal cylindrical and distally divergent • Proximal zone –Fresnel zone • Divergent field is called Fraunhofer zone • Imaging is optimal in near field • Decreasing wavelength or increasing transducer size increase near field

18. Haemo”dynamics” • Blood flow is a complex phenomenon • Not a uniform liquid • Flow pulsatile • Vessel walls are elastic

19. Properties of Blood • Density-mass of blood per unit volume • Measure of resistance to accelaration • Greater the density,greater the resistance to flow • Viscosity:resistance to flow offered by fluid in motion • 0.035 poise at 37 degree.

21. Factors determining flow • Flow rate is determined by • Pressure gradient • Resistance • Viscosity of blood • Radius of lumen • Length of vessel

22. Types of flow Laminar flow Shape of parabola Concentric layers,each parallel to vessel wall Velocity of each layer differs Maximal velocity is at centre of vessel Decreasing profile towards peripheries

26. Turbulent flow • Obstruction produce increased velocities, flow vortices • Whirlpools shed off in different directions producing variable velocities- chaos • Predicted by Reynolds number • Reynolds number depends on Re=( ρ x c x D)/v ρ-Density of blood D-Vessel diameter c-Velocity of flow V-viscosity

27. The Reynolds number is dimensionless If Re is less than 1200 the flow will be -laminar 1200-2000 flow is described as -transitional Greater than 2000 -turbulent

28. Doppler Principle

29. First described by Johann Christian Doppler, an Austrian mathematician and scientist who lived in the first half of the19th century. Doppler’s initial descriptions referred to changes in the wavelength of light as applied to astronomical events. In 1842, he presented a paper entitled "On the Coloured Light of Double Stars and Some Other Heavenly Bodies" where he postulated that certain properties of light emitted from stars depend upon the relative motion of the observer and the wave source.

30. Doppler effect describes the frequency shift of the signal in relation to the relative motion of a source and an observer. • The wave generated by a source that moves away from an observer/receiver appears to him to be of lower frequency than the wave generated by a stationary source, or generated by a source moving toward the observer. • The frequency of the signal detected by the receiver moving toward the still source is higher, compared to the frequency detected by the still receiver, or a receiver moving away from the source.

31. Applied in echocardiography to determine flow direction,flowvelocities,flow characteristics • Stationary rbc-zero doppler shift(received frequency= transmitted frequency) • Positive doppler shift-RBCs moving towards transducer ,received frequency >transmitted frequency • Negative dopplershift:RBC’s moving away from transducer- transmitted frequency more than receiving frequency

33. Doppler shift represents difference between received and transmitted frequencies ,which occur due to motion of RBC’s relative to the ultrasound beam • Fd = (2f V cos Ø)/C

34. Why the factor 2? • Double doppler shift 1st shift-transducer stationary source,RBC the moving receiver 2nd shift is when,RBCs are moving source and transducer is the stationary receiver.

35. Doppler equation- rearranged

36. Factors affecting doppler equation • Estimation of blood flow velocity is dependent on incident angle between ultrasound beam and blood flow • When RBCs parallel-maximum velocity • When RBCs perpendicular-no doppler shift • When angle between ultrasound beam and blood flow is less than or equal 20 degree,cosine close to 1 and percent error is less than or equal to 7%

37. The Effect of Angle

38. AngleCosinePercentage error 0 1 0 10 0.98 2 20 0.94 7 30 0.87 13 60 0.5 50 90 0 100

39. Angle correction • It is possible to correct for angle • Not recommended as in most cases its possible to align ultrasound beam parallel by utilising multiple views, serial assessment difficult unless same angle correction used • It is assumed that angle between ultrasound beam and direction of blood flow is parallel

40. By adjusting according to the direction of assumed flow, it changes the angle calculations in the Doppler equation resulting in different estimates of flow velocity. • The use of this control does not actually change the direction of the Doppler beam and its use does not alter the quality of either the audio output or the spectral recording

41. Effect of frequency • Lower the frequency,higher the velocity detected • A 2 MHz transducer detects higher velocity compared to a 5 MHz transducer

42. SPECTRAL DOPPLER DISPLAY Flow velocity • Displayed on y axis • Velocity of RBCs within sampled volume is calculated • Absence of velocity-zero baseline

43. Spectral velocity recordings

44. Direction of flow • Flow direction also displayed on Y axis • Positive doppler shift-flow towards transducer Traditionally displayed above baseline • Negative doppler shift-flow away from transducer Displayed below zero baseline

45. Intensity or amplitude • Blood cells do not move at equal velocities • Produce different frequency shifts • Amplitude or intensity of doppler signal reflects number of blood cells moving within a range of velocities at a particular point of time • Bright signal-strong doppler shift frequency at a particular point of time . • Darker regions-weak doppler shift

46. Timing • Time is displayed along x axis • Displayed along with ECG. • Change in blood velocity,flow direction can be accurately timed in relation to cardiac cycle.

47. Doppler Audio signals • Doppler shift frequencies are in audible range • Guide for localising blood flow and for proper aligning ultrasound beam parallel to flow • Laminar flow-smooth tone • Turbulent flow-harsh sound.

48. Pulsed and Continuous Wave Doppler

50. Continuous Wave Doppler • older and electronically more simple • continuous generation of ultrasound waves • continuous ultrasound reception • two crystal transducer • Blood flow along entire beam is observed