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Doppler echo & colour doppler -fazil bishara

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Blood is not a uniform liquid blood flow is pulsatile and is a very complex phenomenon!!! Density provides a measure of an objects resistance to acceleration. Viscosity refers to the resistance of flow offered by the fluid in motion. SI unit-Pascal sec = N.sec/m2 =kg/m.s Pressure is the driving force behind all fluid flow.

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Hemodynamics- physical principles of blood flow & circulation Density – mass per unit volume( g/ml) Viscosity – ability of molecules to move past one another by overcoming frictional forces ( poise at 37 ◦ c) Flow occurs from high pressure to low pressure end

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Flow rate is determined by Pressure gradient Resistance Viscosity of blood Radius of lumen Length of vessel

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R= 8Lv/ ∏r4 V viscosity of blood R radius of lumen L length of the vessel Q= ∆P/R

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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

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Acceleration of flow- flat flow profile / plug flow Converging flow- flat profile parabolic profile Diverging flow - multiple flow patterns(uniform high velocity flow, stagnant flow, eddy flow) Vessel curvature – high velocity in the inner part of curve in the ascending limb, outer part of the curve in descending limb

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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

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

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Doppler Principle

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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.

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BASIC PRINCIPLES A moving target will backscatter an ultrasound beam to the transducer the frequency observed when the target is moving toward the transducer is higher the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency

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When a whistle blowing train passes a stationary listener, the sound pitch is higher as the train approaches and lower as the train passes…

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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

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Doppler shift (F[d]) = F[r] - F[t] F[d] = (2f[t] V cos Ø)/C Blood flow velocity (V) speed of sound in blood (C) ø, the intercept angle between the ultrasound beam A factor of 2 is used to correct for the "round-trip" transit time to and from the transducer.

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

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This equation can be solved for V, by substituting (F[r] - F[t]) for F[d]: V = [(F[r] -F[t]) x C] ÷ (2 x F[t] x cos ø) the angle of the ultrasound beam and the direction of blood flow are critically important in the calculation ø of 0º and 180º (parallel with blood flow), cosine ø = 1 ø of 90º (perpendicular to blood flow), cosine ø = 0, the Doppler shift is 0 ø up to 20º, cos ø results in a <10 percent change in the Doppler shift ø of 60º, cosine ø = 0.50

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AngleCosinePercentage error

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It is possible to correct for angle, in clinical practice. However, Not recommended as in most cases it is possible to align ultrasound beam parallel by using multiple echo views. It is assumed that angle between ultrasound beam and direction of blood flow is parallel

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Lower the frequency, higher the velocity detected A 2 MHz transducer detects higher velocity compared to a 5 MHz transducer

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Is a graphic display of blood flow velocities plotted over time. Spectral analysis — the difference between the transmitted and backscattered signal is determined by comparing the two waveforms with the frequency content analyzed by fast Fourier transform (FFT). The display generated by this frequency analysis is termed spectral analysis

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1. Zero-crossing method 2. Fourier analysis

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Sine wave crosses the zero line twice, frequency calculated as no of zero crossings divided by 2 unfortunately the returning signal is not a pure sine wave but is a complex wave, hence the technique not used

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Fourier analysis done using a computer algorithm called FFT that uses a mathematical tool to extract frequency information from signals Many sequential FFTs are performed to generate and display a doppler.

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Information displayed include- 1.flow velocity 2.flow direction 3.signal timing 4.signal intensity

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Displayed on y axis Velocity of RBCs within sampled volume is calculated Absence of velocity-zero baseline

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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

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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.

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Blood cells do not move at equal velocities Produce different frequency shifts Amplitude or intensity of doppler signal reflects the 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

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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.

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Pulsed and Continuous Wave Doppler

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older and electronically more simple continuous generation of ultrasound waves continuous ultrasound reception two crystal transducer Blood flow along entire beam is observed

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ADVANTAGE ability to measure high blood velocities accurately DISADVANTAGE 1)lack of selectivity or depth discrimination 2)no provision for range gating to allow selective placing of a given Doppler sample volume in space

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Ultrasound impulses are sent out in short bursts or pulses transducer that alternates transmission and reception of ultrasound ability to provide Doppler shift data selectively from a small segment along the ultrasound beam- sample volume can be selected.

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The transducer does not emit ultrasound continuously, but rather, emits pulses rapidly (approximately 1,000 pulses per second) & quickly (approximately 1 microsecond for every pulse). Therefore, the transducer is operating as a transmitter for an extremely short time (0.1% of the time).

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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.

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three-dimensional, teardrop shaped portion of the ultrasound beam width is determined by the width of the ultrasound beam at the selected depth. length determines the length of time that the transducer is activated to receive information from sv location

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Sampling rate/frequency- the number of digital points sampled per sec. Nyquist frequency- the highest frequency in a signal Nyquist rate- the minimum sampling rate at which the signal could theoretically be recovered, which is twice the nyquist frequency. Nyquist limit- the max. detectable frequency shift, which is one half the PRF.

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A series of ultrasound pulses are transmitted at a maximum rate determined by the PRF. Each returning ultrasound pulse will change its "phase" relative to each other. This noted phase shift contains the Doppler shift information. The received pulses are compared to a sine wave "reference", and by a process called coherent demodulation, the resultant signal "wave form" is the Doppler shift frequency. Gated Doppler spectral estimation will track Doppler frequencies at a single spatial location over time by collecting a significant number of pulses—of the order of 256 pulses in clinical instruments, and then perform an FFT on this "wave form" to obtain the spectral plot for a single moment of time.

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Fig.1.24 The aliasing phenomenon occurs when the velocity exceeds the rate at which the pulsed wave system can record it properly Inability to accurately measure high blood flow velocities- aliasing

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The Nyquist limit defines when aliasing will occur using PW Doppler. The Nyquist limit specifies that measurements of frequency shifts (and thus the 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.

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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

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Nyquist limit specifies the maximum velocity that can be recorded without aliasing.

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Increase the Nyquist limit- 1)altering variables in Doppler equation 2)high PRF mode 3 )Change from PW to CW

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V =C × PRF 4 f COS Ø Max velocity can be increased by 1)Increasing PRF 2)Decreasing transmitted frequency 3)Increasing speed of sound in tissue 4)Decreasing cosØ

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Increasing the PRF D = c t /2 ; D =distance to the structure/region of interest c = propagation speed through tissue t = time taken for US signal to return to the transducer 2 because pulse must travel to the structure & then back again

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Decreasing the transmitted frequency Selection of a lower frequency transducer, increases the max.velocity detected at any depth.

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Electronic cut and paste Moves the aliased doppler signal upward or downward(unwrapping) Repositioning baseline effectively increases the maximum velocity at the expense of other direction.

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Baseline shift ("zero shift" or "zero off-set") Repositioning of zero baseline effectively increases the maximum velocity in one direction, at the expense of other direction

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A higher than normal PRF used here using multiple sample gates at various locations. Transmission of any given pulse occurs before the reception of all the echoes from the previous pulse. Drawback- exact location of the doppler shift is not known!

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Aliasing not a problem here as sampling limitations does not occur with CW. Limitation- NO range resolution!

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Typically described by their SPECTRAL BROADENING PATTERNS and by the presence or absence of SPECTRAL WINDOWS.

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SPECTRAL broadening is defined as widening or vertical thickness of the doppler shift spectrum Laminar flow- small range of doppler shift frequencies represented resulting in the display of a narrow band of spectral signals Turbulent flow- greater range of doppler shift frequencies due to greater variations in blood flow velocities producing increased spectral broadening.

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SPECTRAL BROADENING may also be increased by - using excessive doppler gains & - by widening of sample gates so that wider range of velocities are displayed

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SPECTRAL WINDOW refers to the echo free area under the spectral doppler trace. Laminar flow- large spectral window Turbulent flow- diminished/eliminated spectral window

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PW Doppler Trace of a typical laminar flow has minimal spectral broadening with a large spectral window. CW Doppler Trace large spectral broadening and absent spectral window!

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cwpw Depth resolutionnoyes Sample volumelargesmall Detection of high velocities yesno Aliasingnoyes Spectral contentWidenarrow Use in duplex instruments yes sensitivitymoreless Transducer powerLowerHigher Control Of Sample Volume Placement PoorGood

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When a specific area of abnormal flow is to be located - PW Doppler is indicated. When accurate measurement of elevated flow velocity is required- CW Doppler should be used

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1. Angle dependency 2. Sample volume position 3. Velocity scale & baseline 4. Wall filters 5. Gain 6. Sample volume length 7. Electrical versus mechanical events

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Doppler images produced by using multiple sample gaits along multiple scan lines The device that detects doppler shift frequency is the AUTOCORRELATOR Where doppler signals are detected, pixels representing that areas are designated a colour, which is determined by the mean doppler shift detected at that site. Colour coding relative to the transducer is direction sensitive

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Blood flow direction – BART system Blood flow velocity- low velocity flow indicated by colours closest to colour baseline - Appear in deeper colour hues - High velocity flow – towards the end of colour bar, appears brighter - No angle correction -Peak velocity estimations are not possibe -Only mean doppler velocities are depicted

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Frequency aliasing -appears as colour reversal. normal blood flow velocities rarely cause aliasing in PW doppler, but frequently in CFI. Laminar vs turbulant flow – smooth homogenous pattern; RBCs move at about the same velocity & in the same general direction Turbulant flow- disorganised mosaic pattern containing all colours on the colour bar

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Frame rate- no of frames produced per second Depends upon -depth colour sector width line density Velocity scale- adjusts the maximum mean velocity that can be displayed Wall filters Gain

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Figenbaum, H : echocardiography Bonita anderson- ECHO Hand book of echo doppler- kerut Moss and adams Otto clinical echocardiography

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