Doppler vs. B-mode Echo- complementary roles Primary target is the red blood cell Examine the direction, velocity, and pattern of blood flow through the heart and the great vessels. Primary target are the myocardium and the heart valves Provides information about the shape and movement of cardiac structures.
Christian Doppler Australian mathematician and physicist Published his notable work on the Doppler effect at the age of 39 Was Gregory Mendel’s physics professor in the University of Vienna.
Doppler Effect The pitch of sound was affected by motion toward or away from the listener Sound moves toward the listener, frequency increases, pitch rises. Sound moves away from the listener, frequency decreases, pitch falls.
Doppler effect applied to Echocardiography Transducer emits ultrasound reflected from RBC. If RBC (flow of blood) moves toward transducer, frequency of the reflected sound’s wavelength increases If RBC (flow of blood) moves away from the transducer, frequency of the reflected sound’s wavelength decreases
Mathematical relationship Fd: Doppler shift= F[r] (received frequency)- F[t] (transmitted frequency) F 0 : Transmitted frequency of ultrasound V: velocity of blood. : intercept angle between the interrogation beam and the target Can solve for V=Fd(C)/2f 0 (cos
Why do we care about the velocity of blood flow? Modified Bernoulli’s equation: P= 4v 2 Gives us the ability to estimate pressure differences between two chambers (i.e, TR) Stenotic valves (i.e. AS)
Angle of the Doppler beam cos (0°)= 1 cos (10°)= 0.98 cos (20°)= 0.94 cos (30°)= 0.87 cos (60°)= 0.5 cos (90°)= 0 Fd= 2f 0 (V)(cos )/C Fd V(cos ) Misalignment of the interrogation beam will lead to underestimation of the true velocity Becomes significant when is >20 °
Carrier frequency V=Fd(C)/2f 0 (cos ) If Fd stays the same, the lower the f 0 (carrier frequency), the higher the velocity of the jet that can be resolved. Unlike B-mode imaging where higher frequency transducer gives better resolution, here lower frequency transducers gives better resolution.
Spectral analysis The difference in waveform between the transmitted and backscattered signal is compared. A process called fast Fourier transform (FFT) displays this information into a “spectral analysis” (spectral display of entire range of velocities) Time- x axis Velocity- y axis Toward the transducer is positive, away from transducer negative. Amplitude is displayed as “brightness” of the signal.
Continuous wave doppler Two dedicated crystals- one for transmitting and one for listening Receives a continuous signal along the entire length of the ultrasound beam Disadvantage- don’t know where the signal comes from. Advantage- can measure very high Doppler shift/velocities. Most useful when trying to discern maximal velocity along a certain path (AS, TR…etc).
Clinical example- AS The position of the doppler beam is 2-D guided. In the GE system, it’s indicated by a single line Profile is usually filled in- velocity along the path that is below the maximal velocity also represented.
Problematic cases Don’t know where the maximal velocity comes from Serial stenosis- LVOT obstruction or AS?
Problematic cases AS or MR?
Pulse wave doppler Short intermittent busts of ultrasound are transmitted. Only “listens” at a brief time interval Permits returning signal from one specific distance to be selectively analyzed- “range resolution” Sample volume
Clinical Examples position of doppler beam 2-D guided In GE system, the sample volume is indicated by double lines Spectral envelope not filled in Common use- mitral inflow velocity and LVOT velocity
Aliasing Sampling rate is inadequate to resolve the direction of flow PRF (pulse repetition frequency)- number of pulse transmitted from the transducer/second Nyquist limit= PRF/2 Cannot resolve higher frequency (velocity) sound waves
Aliasing Tends to happen at higher velocity jets Doppler shift is has higher frequency- needs higher PRF to resolve the direction of the wave.
Aliasing Tends to happen in at greater depth Sample volume at a shallow site- can interrogate more frequently (higher PRF) Sample volume at deeper site- cannot interrogate as often (lower PRF)
High PRF imaging Shallower sample volume associated with a higher PRF- less likely to have aliasing Listening window will also sample returning signal from twice that depth Velocity from both sites will be recorded Disadvantage: ambiguity Advantage: Higher velocities can be analyzed without aliasing
Color Doppler pulse wave Doppler with multiple sample volume along multiple raster lines direction, velocity and variance determined for each sample volume
Color Doppler Displayed as color information- Amplitude- intensity Direction- red vs blue (toward or away from transducer) Velocity- brightness (bright blue higher velocity) Variance (turbulence)- coded green to give a mosiac apperance. Overlays this information on 2D images Time consuming (temporal resolution is especially poor with a large sector window) Different vendors have different algorithms for generating color Doppler
Example of Color Doppler Color Doppler jet with aliasing in the center due to high velocity Color Doppler jet encoded with variance
Semiquantitative method Important to remember that color codes velocity and not actual volume! Angiography- contrast is actual regurgitation Color doppler encodes “billard ball effect”- color may encode non- regurgitant blood that is “pushed around” by the regurgitant jet.
Semiquantitative method Measures velocity, not regurgitant orifice area (ROA) Velocity can be inversely proportional to ROA Larger ROA may lead to lower velocity Jet looks smaller than a those with smaller ROA.
Color gain Same jet with different color gain appears different. Color gain is turns up or down the amplitude of the color jet.
Color gain To optimize color gain, turn it up until you see speckles in the tissues- then turn it down slightly
Color scale/ Nyquist limit By changing the color Nyquist limit, the jet appearance and size can appear different Should set the Nyquist limit to the highest a given depth allows (generally >0.6 cm/s)
Color Doppler M-mode imaging Pulse Doppler interrogation done along a single line Doppler velocity shift recorded and color coded Provides high temporal and spatial (but still not velocity) resolution to the assessment of flow
Color Doppler M-mode Small amount of left to right flow during systole
Tissue Doppler Imaging Routine Doppler targets blood flow High velocity Low signal amplitude Tissue Doppler (assessing the movement of the myocardium) targets tissue Low velocity High signal amplitude Different Filters
Example of pulse TDI Velocity of tissue along a particular sample volume
Example of Color TDI Velocity of tissue coded by color superimposed on 2-D image Can derive information such as strain, strain rate, dyssynchrony…etc.