Doppler Instruments and Types

Types of Doppler Instruments

• Doppler instruments include Continuous wave, Spectral, Spectral and color, and Power Doppler

Doppler Ultrasound Instrument Types

• Doppler instruments include:
• Continuous wave Doppler
• Pulsed wave Doppler
• Spectral, Color, and Power Doppler

Spectral Doppler Instruments

• PW and CW instruments both audibly and visually present doppler shift information.
• The two spectral Doppler instruments are:
• Pulse Wave (PW)
• Continuous Wave (CW)

Visual Forms of Doppler

• Spectral displays are presented as graphs.
• The horizontal axis represents time.
• The vertical axis represents velocity.
• Above the baseline shows positive Doppler shift toward the transducer.
• Below the baseline shows negative Doppler shift away from the transducer.

Aliasing

• Is when flow velocity exceeds the maximum measurable value.
• Velocity tracing wraps around on the spectral display.
• Occurs when Doppler shift information exceeds the Nyquist limit.
• The highest velocity amplitudes are displayed on the spectral analysis as heading in the opposite direction of flow.
• The extent of aliasing depends on the Nyquist limit.

Methods to adjust spectral aliasing

• Adjust PRF (Velocity) scale
• Lower the baseline
• Use lower frequency transducer
• Use CW transducer

Methods to avoid aliasing artifacts

• Raise the scale, which raises the PRF
  • Advantage: Raises the Nyquist limit
  • Disadvantages: Decreased sensitivity to low velocities, aliasing artifact may persist when high velocities are present
• Shift the baseline so the entire velocity scale is devoted to on direction
  • Advantage: Displays high velocity flows in the proper direction
  • Disadvantages: With speakers, audio signal will still arise from the incorrect speaker, wraparound can occur if velocity is exceptionally high
• Select a lower frequency transducer
  • Advantages: Doppler shift is directly related to transducer frequency, lower frequency sound produces lower frequency Doppler shifts, less likely to exceed the Nyquist limit
  • Disadvantages: Lower image resolution due to lower frequency
• Reposition the transducer to use a different ultrasonic window with a shallower sample volume depth
  • Advantage: Higher PRF which increases the Nyquist limit
  • Disadvantage: None
• Switch to continuous-wave Doppler
  • Advantage: No aliasing
  • Disadvantage: Range ambiguity

Nyquist Limit

• Also known as the Nyquist Frequency
• A “speed limit” and is the highest Doppler frequency or velocity that can be measured without aliasing.
• Aliasing occurs if the Doppler shift exceeds the Nyquist Limit.
• The operator can adjust the Limit by manipulating the PRF, as it is also equal to one half of PRF.

Doppler

• Measurement of Doppler Frequency depends on the direction of blood flow and the sound wave's propagation.
• Upstream view is above the baseline.
• Downstream view is below the baseline.

Audible Forms of Doppler

• Doppler shifts are in the audible range
• Speakers or headphones are required to hear an audible form

Nondirectional vs Bidirectional Doppler

• Nondirectional Doppler
• Determines the presence of moving blood by detecting Doppler shift
• The system cannot distinguish flow direction
• Flow sounds the same toward or away from the transducer at the same speed
• Examples: analog zero-crossing detector, fetal heart rate assessment during labor and delivery
• Bidirectional Doppler
• Distinguishes flow toward or away from the transducer signal
• Flow toward is positive
• Flow away is negative
• Requires stereo headphones or speakers or a graph

Crossing Detector

• Provides an estimate of Doppler signal frequency vs. time
• Output may be to a chart recorder or spectral display (rare)

Doppler Shift

• Ultrasound systems determine blood flow velocity by measure the Doppler shift.
• Demodulation extracts lower frequency from higher frequency of doppler shift.
• The system computer uses the Doppler shift information to calculate velocity of blood flow.

Phase Quadrature Detector

• This is a processing technique utilized with bidirectional Doppler
• Determines whether blood was flowing toward or away from the transducer based on the Doppler shift

Speed vs. Velocity

• Speed
• Magnitude only
• Represented by distance a red blood cell moves in one second
• Represented in units of distance per unit time: m/s, cm/s, ft/s, mph
• Velocity
• Magnitude and direction
• Distance and direction a red blood cell moves in one second
• Represented in units of distance per unit time + the direction the reflector is moving: 50 mph south, 1.7 m/s toward the transducer

Continuous Wave Doppler

• This simplest form, composed of separate transmit and receive crystals, continuously transmits and receives ultrasound.
• It detects the presence and direction of flow but cannot discern signals arising from different depths.
• They are portable and inexpensive.
• Continuous electrical stimulation of piezoelectric elements result in a continuously emitted ultrasound beam.
• The frequency of the emitted ultrasound wave is determined by the frequency of the stimulating electrical current.
• Requires two transducer crystals/elements, one to transmit and one to receive the ultrasound.
• Reflected signals return from the area of overlap of the transmit and receive beams.
• All velocities within the area of overlap are measured.
  • Advantage: Can measure very high velocities with no aliasing
  • Disadvantages: Range ambiguity (no depth localization), lack of TGC.
• Since velocities are measured along entire sound beam, we have range ambiguity which means that determining where signal came from is impossible.
• Continuous Wave (CW) Doppler Transducers
  • Contains only 2 elements/crystals
  • One continuously transmits while the other continuously receives
  • Cannot perform 2D imaging (no visualization of anatomy)
  • Does not have damping/backing material
  • Very sensitive (detect low amplitude reflections)

Pulsed Wave Doppler

• This is when piezoelectric elements are stimulated with a short electrical burst, thus creating an ultrasound pulse consisting of only a few cycles of ultrasound.

• Only one pulse is transmitted into tissue at a time, and during pulse propagation the transmitting elements act as signal receivers for the reflected ultrasound.

• Contains an incorporated range gate which represents the time during which the ultrasound machine “listens” to the returning echoes.

• Echoes from multiple pulses are combined, allowing the signal to be built up sufficiently for accurate determination of Doppler-shift frequencies.

  • Uses brief pulses of ultrasound energy using only one crystal.
  • The echo delay time can be converted into distance and the depth of the echo source can be determined.
  • The sensitive volume from which flow data are sampled can be controlled in terms of shape, depth, and position
  • Advantages: Select exact location of Doppler interrogation, duplex imaging is possible and measures peak velocity
  • Disadvantages: Difficult to measure high velocities and aliasing can occur:

• The sample volume (range gate) is placed at the desired position.

• The system calculates the time-of-flight based on the depth of the sample volume (uses 13 microsecond/cm rule).

• The transducer emits a pulse then waits required time before listening for a reflection (echo).

• If an echo is received during the listening time, it was created at the depth of the gate and is shown on the spectral display.

• With increasing scanning depth the PRF decreases as more time is needed for the echoes to return.

• At a minimum, the PRF must be at least twice the frequency of the Doppler signal to construct the signal successfully.

• Use higher PRF setting for high flow velocities and low PRF setting for slow flow.

Pulsed Wave Doppler Transducers

• Pulsed-wave (PW) Doppler -Only one crystal is necessary -Crystal alternates between sending and receiving pulses

Duplex Ultrasound Scanning

• Composed duplex ultrasound instruments are real-time B-Mode scanners with built-in Doppler capabilities
• B-Mode image: Outline anatomic structure
• Pulse-Doppler: Shows the flow and movement patterns

Angle Cosine

| Angle (degrees) | Cosine | | - - - - - - - - | - - - - - -| | 0 | 1.00 | | 5 | 0.996| | 15 | 0.970| | 45 | 0.710| | 75 | 0.26| | 90 | 0|

Doppler Spectrum Assessment

• Assess the following:
• Presence of flow - Sensitivity
• Direction of flow
• Amplitude
• Window
• Pulsatility

Doppler Spectrum Assessment: Sensitivity Improvement

• Increase power or gain
• Decrease the velocity scale
• Decreasing the reject or filter
• Slowly increasing the SV (sample volume) size

Doppler Spectrum Assessment: Direction of Flow

• Pulsed Doppler uses Phase detection to provide bidirectional Doppler information
• Flow can be:
• Monophasic
• Biphasic
• Triphasic
• Bidirectional

Doppler Spectrum Assessment: Amplitude

• The spectrum displays echo amplitude by varying the brightness of the display
• The amplitude of the echoes are determined by: echo intensity, power, gain, dynamic range

Doppler Spectrum Assessment: Window

• Received Doppler shift consists of a range of frequencies
• A narrow range of frequencies will result in a narrow display line
• The clear underneath the spectrum is called the window

Spectral Analysis

• A tool that breaks complex signals into building blocks

• It portrays concentration of RBCs based on shading:

• White = lot of RBCs

• Shades of gray = fewer RBCs

• Black = no RBCs

• Identifies individual velocities making up the reflected Doppler signal

  • For Plug flow: narrow range of velocities
  • For laminar flow: range is greater
  • For disturbed and turbulent flow: range is even greater

Methods of Analyzing Flow

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

Fast Fourier Transform (FFT)

• A digital mathematical technique used to process pulsed and continuous-wave Doppler signals which derives the Doppler spectrum based on the returning echoes.
• Analyzes returning signals in order to produce spectral displays.
• Advantages: Exceedingly accurate, displays all individual velocity components that make up the complex reflected signal
• A spectral display for non-plug flow is composed of several fast Fourier transform (FFT) spectra arranged vertically next to each other across the time axis.
• Laminar flow = window
• Turbulent flow = chaotic

Downstream Conditions

• If a vessel has a high resistance (impedance), there will be forward flow in systole with flow reversal in early diastole and no flow in late diastole.
• If a vessel has a low resistance (impedance), there will be unidirectional flow which equals forward flow in systole and diastole during vasodilation.
• Also displays blood velocities during systole and diastole (the entire cardiac cycle). A factor of vessel elasticity and how hard the blood is being pushed by the heart.

Spectral Broadening

• Vertical thickening of the spectral trace, often indicative of pathology.
• If all RBCs were moving at approximately the same speed, the spectral trace would be a thin line (laminar flow).
• As flow becomes disturbed or turbulent, a greater range of velocities are seen with a thicker trace.
• As spectral broadening increase, the window gets smaller or is completely filled in (diminished or eliminated).
• Spectrum of blood flow velocities broadens with progressive vessel narrowing
• Narrow spectral tracings are generally seen in large diameter vessels
• Broad spectral tracings are seen in small vessels.
• High distal resistance flow is seen in the common femoral artery of the resting lower limb. Low distal resistance flow is seen in the middle cerebral artery.

Pulsatile Doppler Spectrum

• Point A represents maximum Doppler shift
• Point B minimum Doppler shift at peak systole
• Points C and D represent minimum and maximum shifts, respectively, in early diastole
• Point E represents maximum shift in late diastole

Adjusting Spectral Doppler Gain

• Used a spectral wall filter, is also impacted by inversion of spectral flow and the use of spectral baseline

Diagnostic Indices

• Mathematical formulas developed to describe the pulsatile nature of the arterial Doppler waveform and/or the amount of downstream resistance.
• Quantitative measurement of vascular resistance/distal impedance.
• RI and PI are going to be different for different vessels, commonly used for renal insufficiency u/s exams
  • Resistivity Index (RI)
  • Pulsatility Index (PI)

Resistivity Index

• A quantitative Doppler derived measurement of the resistance (impedance) present within a vascular segment
• Useful in diagnosing arterial stenosis
• It is a ratio, no units
• RI = Velocitymax – Velocitymin / Velocitymax
• Velocitymax = systolic velocity
• Velocitymin = diastolic velocity

Pulsatility Index (PI)

• A quantitative, Doppler derived assessment of the pulsatile nature of the Doppler waveform in a vessel segment.
• Mean velocity is calculated automatically by the ultrasound instrument.
• PI = Velocitymax – Velocitymin / Velocitymean