Properties of blood Hemodynamics- physical principles of blood flow & circulation Density – mass per unit volume(g/ml) Resistance to acceleration 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
R= 8Lv/ ∏r2 V viscosity of blood R radius of lumen L length of the vessel Q= ∆P/R poiseuille’s law Q= ∆P∏r2/ 8Lv Q= ∆P∏r2
Types of flow
Laminar flow 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
Turbulant flow Valve obstruction, regurgitation,septal defects Increased velocities, flow vortices Predicted by reynold’s number Re= ᵨcd/v
Laminar flow – narrow spectral envelope-most cells travel over a narrow range of velocities – Large spectral window-echo free area under spectral doppler trace Turbulant flow – Spectral widening-direction and range of velocities increase-greater range of doppler shift frequencies – Spectral window diminished
Continuity principle fig
Doppler echocardiography utilizes ultrasound to record -direction,velocity and pattern of blood flow based upon the changes in frequency of the backscatter signal from small moving structures, ie, red blood cells, intercepted by the ultrasound beam
Comparison of 2-D echo and doppler 2-D echocardiography target is tissue Type of information is structural Optimal alignment is perpendicular Preferred transducer frequency is high Doppler imaging Blood is target Obtain information on physiology Parallel alignment between beam and target Low transducer frequency is preferred
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
Doppler shift (F[d]) = F[r] - F[t] F[d] = 2 x F[t] x [(V x 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.
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
Angle of doppler beam in relationship to direction of blood flow
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. By convention, time is displayed on the x axis and frequency shift on the y axis.
Spectral display Intensity/amplitude -proportional to the number of blood cells with that velocity-represented by brightness of the signal.
Pressure determination ∆P pressure difference between 2 points V1 proximal,v2 distal velocity ᵨ density of liquid dvchange in velocity over the time period dsdistance over which pressure decrease R viscous resistance in the vessel v velocity of blood flow Simplified ∆P = 4(V2²-V1²) =4V²
Continous wave doppler Continuous transmission of doppler signal towards the moving RBCs & continuous reception of reflected signals Blood flow along the entire beam is observed Range resolution not possible High velocities can be recorded Aliasing does not occur
Pulse wave doppler Short intermittent bursts of ultrasound emitted Listens only at a fixed and brief time interval to receive signals from a specific distance Select doppler information from a particular location using sample volume PRF-number of pulses transmitted from transducer each second,determines the sampling rate PRF limits the max.velocity detectable-Nyquist limit :PRF/2 PRF/2= ±∆f = 2 f○ v cosø/C V = C PRF/ 4 f○ v cosø
Sample volume 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
main disadvantage of PW - inability to accurately measure high blood flow velocities, limitation technically known as "aliasing" inability of pulsed Doppler to faithfully record velocities above 1.5 to 2 m/sec when the sample volume is located at standard ranges in the heart 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.
Control of aliasing◦ V = C PRF/4f◦cosø Increasing the PRF D = c t /2 ; c propagation speed thro’tissue t time taken for us signal to return to the transducer 2 pulse must travel to the structure & then back again T= 2D/C PRF = 1/T = C/2D Decreasing the transmitted frequency
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
Changing from PW to CW Utilising high PRF mode: transmission of any given pulse occurs before the reception of all the echoes from the previous pulse Use multiple sample gates at various locations Signals received at different depths simultaneously Disadvg- exact location of doppler shift is not known
The spectral outputs from PW and CW appear differently When there is no turbulence, a laminar (narrow band) spectral output- PW CW - all the various velocities encountered by the ultrasound beams are detected by CW
Comparison between CW & PW cwpw Depth resolutionnoyes Sample volumelargesmall Detection of high velocitiesyesno Aliasingnoyes Spectral contentWidenarrow Use in duplex instrumentsyes sensitivitymoreless Transducer powerLowerHigher Control Of Sample Volume Placement PoorGood
Doppler audio signal audio outputs – High pitched sounds -large Doppler shifts -high velocities – low pitched sounds -lesser Doppler shifts Flow direction information – stereophonic audio output Flow – Laminar flow -smooth, pleasant tone –uniform V. – Turbulent flow -high-pitched and whistling or harsh and raspy sound- different velocities
It can usually be said that when an operator wants to know where a specific area of abnormal flow is located that PW Doppler is indicated. When accurate measurement of elevated flow velocity is required, CW Doppler should be used
The Use of the Doppler Controls Gray Scale : the gray scale control provides a means of altering the various ranges of gray (from white to black) on the spectral display and has no effect on the audio output of the Doppler system. Different Doppler instruments have from two to more than eight different ranges of gray scale display. Lighter shades of gray indicate that there are fewer red cells moving at that velocity in comparison to darker shades of gray or black where many red cells are moving at that velocity.
The use of this control at eight different gray scale settings is demonstrated A spectral recording of normal aortic flow with properly set gain and gray scale
The problem with leaving the gray scale control at the maximum setting is that a light level of gray is assigned to low amplitude background noise in the spectral trace. balanced adjustment between the gain control and the gray scale control,so that the cleanest spectral trace with the most shades of gray is displayed.
Uses Doppler echo may offer a valid alternative to invasive cardiac catheterization for hemodynamic assessment of patients with advanced heart failure it may assist in monitoring and optimization of therapy in heart transplant recipients.
Optimisation of doppler signals Angle dependency-parallel alignment of blood flow & ultrasound beam Sample volume position-placed where the blood flow is most parallel to us beam - increasing the depth of the sample volume;reduces PRF; lowers the maximum velocity that can be displayed Velocity scale adjusts the maximum velocity that can be displayed Baseline is a horizontal line with zero doppler shift; velocities towards the transducer above the baseline; away from transducer below the baseline
Wall filters- eliminates the low frequency doppler shifts that typically occurs due to motion of heart valves or heart walls Gain function adjusts the degree of amplification of received doppler signals Should be adjusted to optimally display the entire doppler spectrum without any excessive background noise Sample volume length- 2-5mm, avoid spectral broadening
Doppler examination of mitral valve inflow PW sample volume at the tip of mv leaflets. IVRT (Ao valve closure – m v opening)-can be measured by displaying mv inflow signal & LVOT signal in the same spectral trace E- early rapid filling Deceleration slope Diastasis- equalisation of pressures-low uniform velocities close to the baseline Atrial filling phase- Awave Stroke volume thro’ mv- sv at mitral annulus
Doppler examination of LVOT &Ao Apical 5 chamber view – LVOT, Ao Pw sample vol (3-5mm) just proximal to the aortic valve Ao flow – cw ideal Desc Ao- suprasternal long axis, sv 1cm distal to the origin of left subclavian artery Below the baseline,v shaped, steep acceleration & deceleration
Doppler examination of TV inflow Plax RV inflow tract/apical 4 chamber view Positioned centrally between open leaflet tips on the RV side
Doppler examination of RVOT & PA Sv within the outflow tract, 1cm prox to pulmonary valve leaflets PA – centre of PA 1cm below the pulmonary valve
Doppler examination of hepatic veins subcostal long axis view of ivc Sv placed 1-2cm into the hepatic vein proximal to its jn with IVC Systolic forward flow Diastolic forward flow Atrial flow reversal
Doppler examination of svc Suprasternal notch or rt supraclavicular fossa Sv 5-7 cm into ivc
Doppler artifacts Mirror imaging or crosstalk – Symmetric spectral image on opposite side – Less intense – Decrease power output,optimise alignment Ghosting – Brief displays of colour painted over regions of tissues – Usually solid colour,occur into tissue area of image – Produced by motion of strong reflectors
Basic principles of colour doppler imaging Produced by using multiple sample gaits along multiple scan lines Where doppler signals are detected, pixels representing that areas are designated a colour Colour coding relative to the transducer is direction sensitive
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
Frequency aliasing -appears as colour reversal Timing of the colour doppler signals- achieved by observing CFI in relation to ECG 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
Optimisation of colour flow doppler images Frame rate- no of frames produced per second Depends upon depth,colour sector width,& line density Velocity scale- djusts the maximum velocity that can be displayed Wall filters Gain Angle dependency-best doppler signal when direction of blood parallel to the us beam