Doppler Ultrasound Types

Types of Doppler Instruments

• Continuous wave Doppler transmits and receives ultrasound continuously
• Spectral Doppler (duplex) combines ultrasound imaging with spectral Doppler
• Spectral and color Doppler (triplex) combines ultrasound imaging, spectral Doppler, and color Doppler
• Power Doppler provides information about the amplitude, or power, of the Doppler signal

Doppler Ultrasound Types

• Continuous wave Doppler
• Pulsed wave Doppler, including Spectral, Color and Power Doppler

Spectral Doppler Analysis

• Pulse Wave (PW) Doppler
• Continuous Wave (CW) Doppler
• Both PW and CW Doppler provide Doppler shift information in visual (spectral display) and audible forms

Visual Spectral Displays

• Spectral displays are presented as graphs
• Time is represented on the horizontal axis
• Velocity is represented on the vertical axis
• Flow toward the transducer is displayed above the baseline and represents a positive Doppler shift
• Flow away from the transducer is displayed below the baseline and represents a negative Doppler shift

Aliasing Defined

• Aliasing occurs when flow velocity exceeds the maximum measurable value
• On a spectral display, aliasing appears as the velocity tracing wrapping around
• This happens when Doppler shift information goes over the Nyquist limit causing the highest velocity amplitudes display in the opposite flow direction
• The Nyquist limit influences the amount of aliasing

Adjusting Spectral Aliasing

• Adjust the Pulse Repetition Frequency (PRF) or Velocity scale
• Lower the baseline to increase the display of flow in one direction
• Switch to a lower frequency transducer
• Use Continuous Wave Doppler for accurate measurements, which is not subject to the Nyquist limit

Avoiding Aliasing Artifacts

• Raise the scale, increasing the PRF, which raises the Nyquist limit, but this may make it less sensitive to low velocities
• Shift the baseline to devote the entire velocity scale to one direction and will display high-velocity flows correctly, but low-velocity signal may arise from the incorrect speaker
• A lower frequency transducer can be selected, as the Doppler shift is directly related to transducer frequency and is less likely to exceed the Nyquist limit
• A different, shallower ultrasonic window can reposition the transducer so the sample volume is closer to the skin, increasing the PRF and the Nyquist limit
• Switching to continuous-wave Doppler avoids aliasing issues but creates range ambiguity, and the exact location of the signal is unknown

Nyquist Limit Defined

• Aka Nyquist Frequency sets the highest Doppler frequency or velocity measurable without aliasing
• When a Doppler shift surpasses the Nyquist Limit, aliasing occurs
• Is operator-adjustable by changing the PRF
• Equivalent to one-half of the PRF
• Nyquist Limit can be thought of as a “speed limit.”

Factors Affecting Aliasing

• Less Aliasing is associated with slower blood velocity, lower transducer frequency, and shallow sample volume with a high PRF
• More Aliasing is associated with faster blood velocity, higher transducer frequency, and deep sample volume with a low PRF

Doppler Angle Considerations

• Flow above the baseline indicates an upstream view or positive shift
• Flow below the baseline indicates a downstream view or negative shift

Audible Form of Doppler

• Doppler shifts exist in the audible range
• Speakers/headphones are required to hear the audible form

Nondirectional Doppler

• Determines moving blood presence by sensing Doppler shift
• Cannot differentiate flow direction
• Identical sounds for flow moving toward/away from the transducer at the same speed
• E.g. analog zero-crossing detector is used, results are printed on a strip chart
• Utilized for fetal heart rate assessment during labor

Crossing Detector Function

• Establishes Doppler signal frequency in relation to time
• Output may connect to a chart recorder, or spectral display (rare)

Bidirectional Doppler

• Discerns flow direction relative to the transducer
• Flow towards the probe is positive, away is negative
• Necessitates usage of stereo headphones/speakers or a graph

Doppler Detector Internal Components

• Demodulator extracts lower frequency from higher frequency to determine the Doppler shift
• Phase quadrature detector determines flow direction in bidirectional Doppler

Doppler Shift and Demodulation

• Ultrasound systems measure the Doppler shift
• Demodulation is the process that extracts the lower frequency from the higher frequency, determining the Doppler shift
• System computers use programmed Doppler equations and Doppler shift data to calculate blood flow velocity

Speed vs. Velocity

• Speed is magnitude only, measuring the distance an object moves in one second (m/s, cm/s, ft/s, mph).
• Velocity includes both magnitude and direction, measuring distance per unit of time, plus the direction of the motion

Continuous Wave (CW) Doppler Specifics

• It has the simplest design
• It uses separate crystals to continuously transmit and receive ultrasound
• It is able to detect the presence and direction of flow
• It struggles to differentiate signals from varying depths, called range ambiguity
• CW Doppler instruments are portable and inexpensive

CW Doppler Operation

• Continuous electrical stimulation of piezoelectric elements generates a continuous ultrasound beam
• Frequency of the emitted ultrasound wave is determined by the stimulating electrical current frequency

Continuous Wave Requirements

• It requires the inclusion of two transducer crystals/elements
• One crystal transmits ultrasound
• One crystal receives the reflected signal from Red Blood Cells (RBC)
• Reflected signals originate from the area where transmit and receive beams overlap
• Velocities are measured within the overlap area

CW Doppler Systems

• Continuous wave Doppler systems have dual-element transducer assemblies, one for transmitting and one for receiving.
• Doppler sample volume is acquired in the scanned region

CW Doppler Considerations

• CW Doppler can measure very high velocities without aliasing
• CW Doppler has range ambiguity and no depth localization, meaning overlapping signals can be measured
• CW Doppler lacks Time Gain Compensation (TGC)

CW Range Ambiguity

• Measured velocities are collected along the entire sound beam
• The operator cannot know precisely where the signal originates

CW Transducers

• CW Doppler transducers have 2 elements/crystals
• One crystal continuously transmits, while the other continuously receives
• These transducers cannot perform 2D imaging because there is no anatomical visualization possible
• These do not require damping/backing material for increased sensitivity when detecting low-amplitude reflections and are capable of measuring very small shifts in Doppler.

Pulsed Wave (PW) Doppler Specifics

• Interrogation happens at an exact location
• Duplex imaging is possible and peak velocity is measurable
• It is more difficult to measure high velocities, and aliasing can occur
• Only one crystal is needed
• Sample volume (gate) position determines location for sampling

Pulsed-Wave Doppler Technique

• The piezoelectric elements receive short electrical bursts, forming brief ultrasound pulses
• Only one pulse is transmitted at a time; transmitting elements receive the reflected ultrasound during pulse propagation
• A range gate incorporated into the instrument indicates when the ultrasound machine is actively listening for returning echoes
• Echoes are combined to enhance signal determination for Doppler-shift frequencies

PW Doppler Functionality

• Brief pulses of ultrasound energy are used with one crystal
• Time can be translated into distance identifying the depth/source of an echo
• Data from flow is sampled in specific forms, depths and positions

Pulsed Wave Doppler in Practice

• A range gate can be placed in a specific location
• The system uses a 13 microsecond/cm rule to determine the time-of-flight based on the depth
• Transducer pulses and awaits the required time to listen for the return echo
• Once an echo is receives from a certain depth, it is put on the display

Pulsed Wave Transducers

• Only one crystal is required
• The crystal alternates between sending and receiving the information

PRF and Depth

• Increased scanning depth causes the PRF to decrease the time needed for echoes to return
• PRF must be twice the frequency to make a signal
• An upper limit is in place for accurately recording flow velocities
• A higher PRF setting is set for high flow velocities, and a low PRF setting for slow flow velocities

Duplex Ultrasound Instruments

• Real-time B-Mode scanners with built-in Doppler are used
• B-Mode images outline anatomic structures
• Pulse-Doppler shows flow and movement patterns

Angle of Incidence

• The Doppler Frequency measurement is dependent upon direction of blood flow and sound wave propagation

Angle Cosine

• The amount of true velocity measured depends on the Cosine of the angle between the sound beam and the flow direction

Doppler Spectrum Assessment Elements

• Presence of flow = Sensitivity
• Flow direction
• Amplitude
• Window
• Pulsatility

Doppler Spectrum Assessment Tool

• Check flow for: no flow detected, flow detected
• If no flow is detected: check beam flow angle, check SV placement
• If sensitivity is decreased, improve sensitivity by: increase power, decrease velocity scale, decrease reject of filter, increasing SV

Flow Direction

• Phase detection is utilized by pulsed Doppler to help provide bidirectional information
• Flow can be monophasic, biphasic, triphasic or bidirectional

Amplitude Analysis

• Echo amplitude is displayed by variations in brightness
• The amplitude is determined by echo intensity, power, gain and dynamic range

Window

• Received Doppler shift consists of a range of frequencies
• Narrow range frequencies result in narrow display line
• The clear underneath the spectrum

Spectral Analysis Function

• Spectral analysis is a methodology used to analyze the constituent elements of complex signals
• Concentration of RBCs based on display shading can be portrayed, a lot of RBCs in white, shades of gray = fewer RBCs, black = no RBCs
• Individual velocities which reflect Doppler readings can be identified

Spectral Analysis and Flow Patterns

• Plug flow has a narrow range of velocities
• Laminar flows have a range that is greater
• Disturbed and turbulent flows have ranges that are even greater

Analyzing Flow Methods

• Fast Fourier Transform (FFT)
• Autocorrelation (color Doppler)

Fast Fourier Transform (FFT) Details

• A digital mathematical technique that processes pulsed and continuous-wave Doppler signals, deriving the Doppler spectrum through returned echoes
• FFT analyzes the signals to produce spectral displays that are accurate and shows all individual velocity components

Downstream Conditions and Blood Flow

• High distal resistance in a vessel will present forward flow in systole, flow reversal in early diastole and no flow in late diastole
• Low distal resistance in a vessel will have unidirectional flow or constant flow in systole and diastole

Laminar Flow

• Most blood cells within the sample volume travels at similar velocities
• A clear spectral window can be seen with a clear spectral trace

Turbulent Flow

• Chaotic blood flow has velocities (or Doppler shifts) and directions in the sample volume
• The spectral window is filled in, in spectral broadening

Spectral Broadening

• Spectral Broadening is the vertical thickening of a spectral trace
• If all Red Blood Cells (RBCs) move at nearly identical rates, a spectral trace would be a thin line (laminar flow)
• As flow becomes more disturbed/turbulent, spectral broadening happens, creating a thicker velocity range

Broadening of Spectrum Details

• Blood flow velocities spectrum increase as more vessel narrowing occurs
• Narrow spectral tracings are usually found in wide vessels
• Wide tracings are usually found in small vessels
• When sampling the vessels middle, narrow tracings result
• Over larger vessel radius, the spectrum becomes broader

Doppler spectrum measurements

• High Pulsatility/Resistance Waveform has a triphasic waveform, shows sharp systolic peak + reversed diastolic flow in the extremity artery during the resting stage
• Low Pulsatility/Resistance Waveform shows a broad systolic peak + forward flow in diastole, e.g. ICA, renal, vertebral, celiac
• Mixed Pulsatility/Resistance Waveform shows a sharp systolic peak + forward flow in diastole, e.g. ECA & SMA (during fasting)

Calculating vascular resistance

• Both Resistivity Index (RI) and Pulsatility Index (PI) are mathematical formulas for arterial Doppler waveform analysis used to interpret the downstream resistance
• Provides a quantitative measurement of vascular resistance/distal impedance
• Aids in the diagnosis of arterial stenosis

Resistivity Index (RI)

• RI = (Velocitymax – Velocitymin)/Velocitymax

Pulsatility Index (PI)

• PI = (Velocitymax – Velocitymin)/Velocitymean

Modality Roles Summary

• Continuous Wave*
• Used where the highest velocity jets can be identified anywhere in the specified US Beam
• Has Range ambiguity
• Most sensitive
• Very good temporal resolution
• No aliasing
• Peak velocity measurements are used
• Pulsed Wave*
• Accurately identifies the flow location
• Has range resolution
• Moderate sensitivity
• Has great temporal resolution
• Subject to aliasing
• Peak velocity measurements are used
• Color Flow*
• Provides bi-dimensional flow data on the anatomic image utilized
• Has range resolution
• Moderate sensitivity is present, since most color can be affected in different Doppler conditions
• Has reduced temporal resolution
• Subject to aliasing
• Used, mean velo