Doppler Principles in Ultrasound

Doppler Principles

• Objectives include; explaining the Doppler effect in moving blood display with colour, power and spectral Doppler, identifying advantages and disadvantages of Doppler modes, optimizing parameters, understanding M-mode ultrasound, examining artifacts and strategies.
• The overview includes; Doppler, colour, power, spectral, and M-mode principles, plus ALARA.

Recall

• Pulsed waves can be transmitted or attenuated.
• Attenuation includes wave reflection back to the transducer
• Pulsed waves reflect off stationary tissues
• Doppler looks at waves reflected off moving targets primarily in the circulatory system

Frequency and Spatial Pulse Length (SPL)

• Long SPL results in poor axial resolution.
• Short SPL results in improved axial resolution.

Doppler Principles

• The Doppler effect describes a perceived sound frequency change when an object emits sound and moves toward or away from an observer.
• Ultrasound uses the Doppler effect for motion detection and measurement, such as blood flow, direction, and velocity.
• Doppler is also used in weather radar, police radar, and motion sensors.

Doppler Effect Explained

• A video demonstrates the principles of the Doppler effect.
• https://www.youtube.com/watch?v=ffg4TOpXZyg

Doppler in Ultrasound

• The transducer alternates between sending and receiving echoes.
• The depth of returning echoes is calculated by how long it takes to return to the transducer
• Doppler evaluates frequency shifts known as the Doppler shift with:
  • The transducer infers the depth and direction of the echo
  • Velocity is estimated based on the frequency change

Understanding Doppler Shift

• Doppler shift (DS) measures the change in frequency with:

  • DS = Difference between transmitted and received frequencies
  • DS = received frequency – transmitted frequency

• A positive DS means the object moves toward the transducer.

• A negative DS means the object moves away from the transducer.

• Positive Doppler shift occurs when the reflected frequency is higher.

• Negative Doppler shift occurs when the reflected frequency is lower.

Doppler Equation

• V = blood velocity
• C = propagation speed of sound through the medium
• The additional constant factor of 2 accounts for the echo's roundtrip effect in diagnostic ultrasound.

DS and Frequency

• A source frequency increase raises the Doppler shift; the machine compensates when determining blood velocity.
• A trade-off exists between reflectivity and absorption with higher frequencies.
• Lower transmit frequencies should be used for optimal Doppler sensitivity as depth increases due to absorption.

Importance of Angles

• The angle at which you perceive changing frequency due to motion affects the resultant DS value.
• Greatest DS changes are perceived when the angle is at 0 degrees.
• The least or no changes occur when the angle is at 90 degrees

Doppler in Ultrasound

• Doppler shift is most accurately calculated with the transducer at zero degrees to the vessel
• Doppler should be used at an angle less than 60 degrees to the vessel with angle correction.

Pulse Wave Attributes: PRP and PRF

• Each transmit burst represents the beginning of one acoustic line.
• Multiple acoustic lines create one frame or image.
• Pulsed Repetition Period (PRP) is inversely proportional to Pulsed Repetition Frequency (PRF).
• When PRF increases, PRP decreases.

Types of Doppler Applications

• Colour Doppler
• Power Doppler
• Spectral Doppler
• M-Mode

Colour Doppler (CD)

• Assesses vessel patency
• Displays the presence and direction of blood flow
• Provides real-time flow imaging with a grey scale image
• Does not provide velocity measurements

Colour Doppler How it Works

• Calculates Doppler frequency shift at a depth, displaying it as a colour.
• Uses pulsed waves with range specificity for Doppler shifts to be detected and displayed, and adjust scales.
• Colour box size adjusts the sampled area.
• Larger colour box equals lower frame rates.

Temporal Resolution & FOV Size

• Notice how the 2D and CD FOV have been optimized to image the Fetal Heart
• http://obgynkey.com/color-doppler-in-fetal-echocardiography/

Colour Coding

• Arteries typically display as RED, veins as BLUE, but the portal venous system demonstrates in red.
• Set the velocity scale to the range of DS you intend to display.
• The highest detectable mean velocity toward the transducer to be displayed is 63.6 cm/s.

Beam Steering

• Colour Doppler boxes should also be steered to optimize the calculated Doppler shift.

• A positive Doppler shift displays in blue and is inverted.

• Aliasing relates to the scale setting.

• Overall gain can bleed colour outside of the vessel wall.

Colour Box Adjustments

• A colour box appears when CD is initiated

• Position the colour box over the area of interest

• Size the colour box to cover the area, but no more, to avoid slowing the frame rate

• Angle the colour box to improve the Doppler shift calculation

• Cannot beam steer with a curved probe

Colour Doppler Advanced Applications

• Advanced applications and software vary based on budget and intended use.

Power Doppler

• Looks at colour angio, energy Doppler, amplitude Doppler
• Sensitive to small Doppler shifts because it looks at the amplitude of power of the wave
• Displays moving RBC density with increased density increasing amplitude
• Detects low-velocity flow in vessels or organs
• Does not display blood flow direction

Power Doppler: Advantages and Disadvantages

• Advantages: -Angle independent using amplitudes -Improved sensitivity detecting lowest velocity flow -There are no scale settings or aliasing
• Disadvantages: -More frame averaging and poor temporal resolution -Flash artifact -Cannot be used on the heart -Only qualitative information is available such as; the presence or absence of flow, and no direction, all with no flow characteristics.

Power Doppler with 3D Volume Imaging

Spectral Doppler (SD)

• Provides quantitative information regarding blood flow.
• Graphs the velocity via the frequency shift, over time, in a spectral trace.
• Requires sampling of vessel at less than 60 degrees
• Steering and angle correcting are required

Spectral Doppler Trace

• Numerous DS Frequencies must process into a graph or trace
• Multiple pulsed echoes convert over time generating a waveform.
• You cannot plot all the echoes because there are too many
• Brightness of wave at that depth/dot is based on average DS

Spectral Doppler Gate

• Multiple velocities can be displayed depending on gate size.
• A larger sample gate displays velocities from different depths.

Spectral Doppler Trace - the Spectrum Narrow vs Wide

• Narrow spectrum of velocities indicates:
  • Laminar flow
  • Smooth, minimal turbulence
  • "Clean window" underneath the trace
• Wide spectrum of velocities indicates:
  • Turbulence in the vessel
  • Poor spectral Doppler settings or sampling
  • Spectral broadening

Spectral Doppler Gains

• Appropriate gain of the signal is when its not over gained
• If the signal is over-gained, it can result in loss or decrease of the spectral window in PW Doppler.

Spectral Doppler

• Important diagnostic measurements can be determined from an accurate spectral Doppler waveform
• These measurements include: -Type of flow/vessel (venous vs arterial) -Peak systolic (PS) velocity -End diastolic (ED) velocity -Pulsatility/resistance in the vessel -Turbulent vs laminar flow -Wave characteristics

Resistance in Spectral Doppler Waveforms: Low vs High

• Low Resistance Waveform: -Flow is continuous throughout systole and diastole -High end diastolic flow (above baseline) -Resistive index can be calculated RI =(Vmax - Vmin)/Vmax
• High Resistance Waveform: -No flow during diastole (at baseline), or reversed flow (below baseline) -Large discrepancy between Vmax and Vmin

Integration of B-mode, Colour, and Spectral Doppler

• The three modes of sonographic display need optimization for an accurate and diagnostic assessment of blood flow.
• It is essential to optimize 2D, colour, and spectral Doppler with: -frequency, depth, gains, and focus on the black and white image -scale, gains, and angle for Colour Doppler and Spectral Doppler.

M-Mode Ultrasound

• M-mode displays changes in echo depth or distance over time
• The vertical axis displays echo depth.
• The horizontal axis represents time in seconds.
• Used frequently in cardiac applications to measure rate and valve motion
• Confirms motion in the diaphragm

Adjusting Doppler Settings

• Optimize Doppler settings in ultrasound.
• Adjust knobology to adjust the ultrasound platform.
• https://www.youtube.com/watch?v=tQn8jKtwk6o

More on Aliasing

• Aliasing may occur in either colour or spectral Doppler images.
• This occurs when the PRF of Doppler settings don't capture the highest shifts in returning echoes
• The display displays frequencies back at the bottom of the scale instead.
• Aliasing occurs when the velocity is set higher than the calculated scale
• The Nyquist limit is the maximum Doppler frequency calculated relative to the PRF, and reduce aliasing, increase the PRF of your Doppler scale.

Colour Aliasing

• Display colours on the wrong side side of the scale

Reversing aliasing

• Fix aliasing by increasing the scale

Spectral Aliasing

• Spectral Doppler display inaccurately displays a high velocity moving away moving toward, or vice versa

• "Increase velocity scale to correct the display

Undoing Spectral Aliasing

Increase the PRF or Scale

Doppler Quality Control

• Baseline should show mostly flow and scale increased.
• Reduce artifacts to improve image and show true colours.

Artifacts in Doppler Imaging

• Mirror images can occur with colour Doppler
• Duplication of a vessel / Doppler shift on the opposite side of a strong reflector
• Mirror vessel demonstrates colour also called Ghost Artifact

Spectral Mirror Image

• Often caused by suboptimal angle of insonation, also called high angle
• Perpendicular angles detect +ve and –ve Doppler shift at the same time