2 Sound is a mechanical vibration transmitted through an elastic medium Ultrasound-portion of sound spectrum having frequency greater than 20,000 cycles /secUse of ultrasound to study the structure and function of heart and great vessels-echocardiographyAdvantages of ultrasoundCan be directed as a beam and focussedObeys laws of reflection and refractionProduce longitudnal waves
6 MachinesThere 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 crystalsTransducer - converts electrical energy to mechanical (ultrasound) energy and vice versaReceiver - detects and amplifies weak signalsDisplay - displays ultrasound signals in a variety of modesMemory - stores video display
7 Depicted as sine wave-peaks and troughs One cylce=one compression + one rarefactionDistance between 2 similar points represent wavelength0.15 to 1.5 mm in soft tissueFrequency- number of wavelengths per unit timeV=f X λ(v=velocity,f =frequency, λ is wavelength)
8 Velocity of sound=1540 m/sec in soft tissue Wavelength=1.54/fAmplitudeMeasure of strength of the sound waveIndicated by height of sine wave above and below baseline
11 Higher the frequency greater the resolution Higher frequency,lesser the penetrationLoss 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 facesthe 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- piezoelectricityThe 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 effectmechanical strain produced in the crystal under electric stress is called the converse piezoelectric effect.
15 Ferro electrics,barium tianate,lead zirconate titanate are used as peizo electric crystals. Dampening material-shortens the ringing responseAlso absorbs backward and laterally transmitted acoustic energyFrequency 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 divergentProximal zone –Fresnel zoneDivergent field is called Fraunhofer zoneImaging is optimal in near fieldDecreasing wavelength or increasing transducer size increase near field
18 Haemo”dynamics” Blood flow is a complex phenomenon Not a uniform liquidFlow pulsatileVessel walls are elastic
19 Properties of Blood Density-mass of blood per unit volume Measure of resistance to accelarationGreater the density,greater the resistance to flowViscosity:resistance to flow offered by fluid in motion0.035 poise at 37 degree.
21 Factors determining flow Flow rate is determined byPressure gradientResistanceViscosity of bloodRadius of lumenLength of vessel
22 Types of flow Laminar flow Shape of parabola Concentric layers,each parallel to vessel wallVelocity of each layer differsMaximal velocity is at centre of vesselDecreasing profile towards peripheries
26 Turbulent flow Obstruction produce increased velocities, flow vortices Whirlpools shed off in different directions producing variable velocities- chaosPredicted by Reynolds numberReynolds number depends onRe=( ρ x c x D)/vρ-Density of bloodD-Vessel diameterc-Velocity of flowV-viscosity
27 The Reynolds number is dimensionless If Re is less than 1200 the flow will be -laminarflow is described as -transitionalGreater than turbulent
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,flow velocities,flow characteristics Stationary rbc-zero doppler shift(received frequency= transmitted frequency)Positive doppler shift-RBCs moving towards transducer ,received frequency >transmitted frequencyNegative doppler shift:RBC’s moving away from transducer- transmitted frequency more than receiving frequency
36 Factors affecting doppler equation Estimation of blood flow velocity is dependent on incident angle between ultrasound beam and blood flowWhen RBCs parallel-maximum velocityWhen RBCs perpendicular-no doppler shiftWhen 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%
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 usedIt 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 velocityDisplayed on y axisVelocity of RBCs within sampled volume is calculatedAbsence of velocity-zero baseline
44 Direction of flow Flow direction also displayed on Y axis Positive doppler shift-flow towards transducerTraditionally displayed above baselineNegative doppler shift-flow away from transducerDisplayed below zero baseline
45 Intensity or amplitude Blood cells do not move at equal velocitiesProduce different frequency shiftsAmplitude or intensity of doppler signal reflects number of blood cells moving within a range of velocities at a particular point of timeBright 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 flowLaminar flow-smooth toneTurbulent flow-harsh sound.
50 Continuous Wave Doppler older and electronically more simplecontinuous generation of ultrasound wavescontinuous ultrasound receptiontwo crystal transducerBlood flow along entire beam is observedContinuous Wave Doppler
52 ADVANTAGEability to measure high blood velocities accuratelyDISADVANTAGE1)lack of selectivity or depth discrimination2)no provision for range gating to allow selective placing of a given Doppler sample volume in space
53 Pulsed Wave DopplerUltrasound impulses are sent out in short bursts or pulsestransducer that alternates transmission and reception of ultrasoundability to provide Doppler shift data selectively from a small segment along the ultrasound beam- sample volume can be selected.
54 The transducer functions as receiver for a limited time period Time corresponds to the interval required for sound to return from specified area.Another burst of sound waves are not transmitted until previous impulses are received.Pulse repetition frequency (PRF)–frequency at which transducer transmits pulses.PRF determines sampling rate.
59 Aliasing is represented on the spectral trace as a cut-off of a given velocity with placement of the cut section in the opposite channel or reverse flow direction
60 Nyquist LimitThe Nyquist limit defines when aliasing will occur using PW Doppler.The Nyquist limit specifies that 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.
62 The Nyquist Limit The simplest sound wave is an oscillation between two amplitudes. 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
63 Shannon's sampling theorem (Claude E. Shannon, born 1916, American mathematician)Also known as the Nyquist criterion, a general "rule" for measurement of frequencies, stating that the measurement (sampling) frequency must be at least twice the maximum frequency to be measured.Whenever Shannon's sampling theorem is not fulfilled, aliasing occurs
64 Nyquist limit specifies the maximum velocity that can be recorded without aliasing.
65 Avoiding aliasingIncrease the Nyquist limit- 1)altering variables in Doppler equation 2)high PRF mode 3 )Change from PW to CW
66 V = C × PRF4 f COS Ø(PRF= Δf× 2= 2f V cos Ø × 2)CMax velocity can be increased by1)Increasing PRF2)Decreasing transmitted frequency3)Increasing speed of sound in tissue4)Decreasing cosØ
68 It is desirable to use as high a PRF as possible for recording abnormally elevated velocity jets. Maximum PRF is limited by the distance the sample volume is placed into the heart.
69 Range ambiguitysome of the range selectivity used in precisely locating the sample volume is lost.pulsing sequence is carried on over and over, some data is returned to the transducerdata from all these volumes are added together
70 Baseline shift ("zero shift" or "zero off-set" ) Electronic cut and pasteMoves the aliased doppler signal upward or downward(unwrapping)Repositioning baseline effectively increases the maximum velocity at the expense of other direction.
72 Pulse Wave VS Continuous Wave Doppler CW PW Depth resolution no yes Sample volume large small High velocity detection yes no Aliasing no yes Sensitivity more less Control of sample volume placement poor good
73 Continuity equationThe continuity equation states that the amount of blood flow through one cardiac chamber (or valve orifice) is the same as the blood flow through the other chambers and orificesIt is based on the principle of conservation of mass.“Whatever mass flows in must flow out.”
75 VELOCITY TIME INTEGRAL Calculation of volumetric flow is complex as flow velocity is not constantBlood flow is pulsatileHence integrated velocity over time is takenVTI is equal to area under curve cm/sec X sec.Measure of distance that blood moves with each heart beat.VTI is also referred to as “stroke distance”
77 Volumetric flow or stroke volume can be thus calculated as SV=CSA X VTIContinuity equation can be rewritten as-CSA1 X VTI 1=CSA2 X VTI 2CSA2 =(CSA1 x VTI 1) /VTI 2
78 VTI is obtained by tracing the leading edge of velocity spectrum.
79 Clinical Applications Calculation of valve areasCalculation of regurgitant volumes and fractionsCalculation of regurgitant orifice areasCalculation of intra cardiac shunt ratios
80 LIMITATIONS OF CONTINUITY PRINCIPLE Erroneous determination of CSAMeasurement of diameter during wrong phase of cardiac cycle.Inconsistent annulus measurementsErroneous determination of VTIIncorrect placement of pulse wave sample volumeSignificant angle between doppler beam and blood flowIncorrect filter/gain settings
81 Bernoulli's principle is named after Swiss mathematecian Daniel Bernoulli who published his principle in his book Hydrodynamica in 1738
82 Bernoulli equationBernoulli described the conversion of energy in a fluid from one form to another, as occurs when fluid flow in a tube that suddenly either increases or decreases its diameter.Bernoulli's law states that total energy at all points along a tube is the same—conservation of energy.Energy (the ability to do work) is composed of pressure energy (pressure x the volume of fluid),kinetic energy (fluid in motion has kinetic energy ,which is proportional to the mass and the velocity squared; mass is density x volume), and gravitational energy (the product of density, volume, height above a surface, and the gravitational constant).
86 Discrepancies between Catheter derived and Doppler derived Pressure Gradients Pressure gradients derived at cardiac cathetrisation and measured by Doppler may differ.Catheter derived gradient is peak to peak gradient between LV and AortaNon simultaneous measurementDoppler gives peak instantaneous gradient and is greater than peak to peak gradient.
87 PRESSURE RECOVERY PHENOMENON Complex hemodymanic concept- pressure of fluid decreases as velocity increases.Once flow passes through a narrowing pressure drops and increases towards original valueRate and magnitude of pressure recovery is variableIn prosthetic valves,3 effective orifices-2 large orifices by sides and a central small orifice.Maximum velocity and lowest pressure is at narrowest orifice.
88 Immediately distal to orifice, pressure increases (recovers)and velocity decreases. Doppler gradients are measured at narrowest orifice, while catheter gradients are recorded downstream to prosthetic valve where the pressure has already recovered.Hence, Doppler derived pressure gradients are more compared to catheter derived pressure gradients.
90 Determination of Pressure Gradients Pressure gradients are useful for assessment of severity of valvular stenosis and estimation of intracardiac pressures .Pressure gradients commonly determined by Doppler include1)Maximum instantaneous pressure gradient2)Mean pressure gradient
91 Maximum Instantaneous pressure gradient =4v2 (v=peak velocity)Mean pressure gradient cannot be obtained from the mean (or average) velocity, but must be calculated by making multiple instantaneous gradient (and hence pressure calculations) measurements, and then averaging those instantaneous pressures.
93 Estimation of RVSP by TR TR doppler signal represents the pressure difference between RV and RA during SystoleRVSP-RAP=4(V TR 2 )In absence of RVOT obstructionRVSP=Pulmonary artery systolic pressure.
101 Pressure half-time and deceleration time The pressure half-time (PHT) is defined as the time (in milliseconds) required for the peak initial pressure to drop by one half .Doppler signal measures velocityThe modified Bernoulli equation is applied to convert velocity to pressure.PHT is the time (in milliseconds) for the velocity to drop to of the maximum velocity .V2 = (0.707) x V1
103 The DT is the time required for the velocity,beginning with the peak value, when extrapolated, to cross the zero baselineThe PHT is clinically used most frequently in evaluating mitral stenosis and aortic regurgitation
105 MVA = 220/PHTPHT = 0.29 xDTMVA =220/0.29 x DT=759/DT
106 Limitations of PHT in calculation of MVA Non linear early diastolic slopePost balloon mitral valvuloplastySignificant Aortic regurgitationCardiac rhythm disturbances
107 Non linear early diastolic slope Non linear or curvilinear decay –lead to erroneous calculation of PHT.Part of slope considered most representative should be chosen.
108 Post balloon mitral valvuloplasty Accuracy of calculated MVA by PHT declines immediate post BMV.PHT is directly related to chamber compliance and peak transmitral gradientFollowing BMV,abrupt changes in left atrial pressure and compliance occur altering the relationship between PHT and Mitral valve area.This effect on PHT lasts 24 to 48 hours.
109 Significant Aortic regurgitation Severe AR shortens PHT.This is due to markedly elevated LVEDP which reduces diastolic pressure gradient between LA and LV.Mitral valve area is thus overestimated.
110 Cardiac rhythm disturbances Tachycardia-deceleration slope is shortens,PHT shortens and thus valve area is overestimated.
111 Proximal Isovelocity Surface Area (PISA) Proximal Isovelocity Surface Area (PISA) method is based on the continuity equation.When a flow passes through a narrow orifice, as it approaches the narrowest region, there is a flow convergence and flow acceleration.PISA is the surface area of the hemisphere at the aliasing region of the flow convergence.PISA increases with lower aliasing velocity.
114 Radius is measured from the orifice to point of colour change. If the flow convergence is not a true hemisphere, the angle subtended by the flow convergence at the orifice has to be measured and divided by 180 to get a correction .Good correlation between angiographic estimates of regurgitant flow and PISA based estimates have been reported.
115 Valve area calculation A0=(2 Л r2x VN)/V0 A0=area of narrowed orifice V0=velocity at narrowed orifice R=radius of shell VN=aliased velocity identified as Niquist limit
116 Angle correctionPISA principle is for flow approaching narrow planar surface In Mitral Stenosis,mitral leaflets may be funnel shaped To account for altered shape,angle corection factor ά /180 is applied. MVA= ( ά /180 ) x (2 Л r2x VN)/VMS
118 Limitations of PISA method Errors in measurement of radiusValve area is proportional to square of radius-even small errors are magnified.Errors in measurement of angleMeasurement is done offline using a protractorAngle measured in one dimension may not be true representation of valve leaflet geometry.
121 Maximum detectable velocity for a 5 MHz probe ,(assuming Doppler shift frequency as 5 KHz ,speed of sound in tissue as 1500m/sec and doppler beam being parallel to blood flow) ,would be a)0.62 m/sec b)0.75 m/sec c)0.92 m/sec d)1.0 m/sec
122 Percentage error when ultrasound beam makes an angle of 20 degrees to blood column a)3 b)7 c)15 d)30
123 When frequency of ultrasound probe is reduced, a)Resolution increases b)Maximum detectable velocity increases c) Attenuation increases d)All of the above
124 Following statements are true about pulse wave doppler compared to continuous wave doppler except a)Better depth resolution b)Larger sample volume c)More problem of aliasing d) Less sensitivity
125 Resistance to flow through a tube is directly proportional to a)Fourth power of radiusb)Second power of radiusc)Length of tubed)None of the above
126 Turbulent flow typically occurs when reynolds number just exceeds
127 Pulse wave is preferred to continuous wave doppler to assess flow in all except a)Pulmonary vein b)Superior venacava c)Pulmonary valve d)Aortic valve
128 Doppler shift is directly proportional to all except a)Speed of sound in tissue b)Velocity of blood flow c)Frequency of transducer d)Cosine of angle between interrogating beam and blood flow
129 True about Mitral valve area assessment by PHT in severe AR is a)PHT increases b)Valve area is underestimated c)MS severity is underestimated d)Increases diastolic gradient between LA and LV.
130 Mean pulmonary artery pressure of a patient with TR jet velocity of 3 m/sec, Peak PR velocity of 2m/sec is (in mm Hg) a)36 b)16 c) 9 d)20
132 Maximum detectable velocity for a 5 MHz probe ,assuming Doppler shift frequency as 5 KHz ,speed of sound in tissue as 1500m/sec and doppler beam being parallel to blood flow, would be a)0.62 m/sec b)0.75 m/sec c)0.92 m/sec d)1.0 m/sec
133 Percentage error when ultrasound beam makes an angle of 20 degrees to blood column a)3 b)7 c)15 d)30
134 When frequency of ultrasound probe is reduced, a)Resolution increases b)Maximum detectable velocity increases c) Attenuation increases d)All of the above
135 Following statements are true about pulse wave doppler compared to continuous wave doppler except a)Better depth resolution b)Larger sample volume c)More problem of aliasing d) Less sensitivity
136 Resistance to flow through a tube is directly proportional to a)Fourth power of radiusb)Second power of radiusc)Length of tubed)None of the above
137 Turbulent flow typically occurs when reynolds number just exceeds
138 Pulse wave is preferred to continuous wave doppler to assess flow in all except a)Pulmonary vein b)Superior venacava c)Pulmonary valve d)Aortic valve
139 Doppler shift is directly proportional to all except a)Speed of sound in tissue b)Velocity of blood flow c)Frequency of transducer d)Cosine of angle between interrogating beam and blood flow
140 True about Mitral valve area assessment by PHT in severe AR is a)PHT increases b)Valve area is underestimated c)MS severity is underestimated d)Increases diastolic gradient between LA and LV.
141 Mean pulmonary artery pressure of a patient with TR jet velocity of 3 m/sec, Peak PR velocity of 2m/sec is (in mm Hg) a)36 b)16 c) 9 d)20