Doppler Ultrasound Principles

Introduction

• US uses high-frequency, inaudible sound waves to create images and interacts with tissues through reflection, attenuation, and the Doppler effect to provide both structural and flow assessment.¹
• Doppler US provides a real-time assessment of blood flow and is a vital tool for perioperative care. It aids in enhancing vascular access, guiding regional anesthesia, and supports clinicians with hemodynamic evaluation in operative and critical care environments.²
• Doppler US relies on frequency changes caused by sound waves reflecting off moving red blood cells.³
• The Doppler effect is shown on color scales: red indicates flow toward the probe, while blue indicates flow away.²

Physics of Doppler US

B-Mode vs Doppler⁴

• B-mode creates a two-dimensional structural image using the amplitude of reflected echoes.
• Doppler US utilizes the frequency shift of echoes to measure the motion of objects.

Doppler Effect Concept⁴

• When a reflecting object moves toward the transducer → frequency increases.
• When a reflecting object moves away from the transducer → frequency decreases.
• The shift in frequency is proportional to the speed of the object.

• Stationary blood (V_blood = 0)
  • When blood is not moving, sound waves from the transducer remain confined to concentric circles around the source.
  • No Doppler shift occurs because the distance between the wavefronts and the probe remains constant.
• Blood moving slower than sound (V_blood < V_sound)
  • This is the classic Doppler effect seen with US:
    • When blood moves toward the probe, the wavefronts are compressed, resulting in a higher frequency.
    • When blood moves away, the wavefronts are stretched, leading to a lower frequency.
• Blood moving at sound speed (V_blood = V_sound)
  • When blood and sound waves move in the same direction, the Doppler shift is maximized.
• Blood moving faster than sound (V_blood > V_sound)
  • Shockwave-like effects are observed in this scenario; however, they rarely occur in biological tissues.

Doppler frequency Shift Formula⁴

Where:

• Fd = Doppler frequency shift
• Ft = transmitted frequency
• Fr = received frequency
• V = velocity of moving target
• c = speed of sound in tissue
• θ  = angle between the US beam and the direction of flow

Doppler Modes

Continuous Wave (CW) Doppler

• CW Doppler uses two piezoelectric crystals where one continuously transmits sound, and the other continuously receives it.⁶
• Frequency shifts are measured along the entire beam path, not at a particular depth.⁴
• A “filled-in” waveform is depicted because all velocities are captured.⁴
• Measures very high velocities because it is not limited by pulsed sampling; however, it has no range resolution as it cannot determine the exact depth from which the velocity originates.⁴
• Useful for detecting blood flow, but cannot provide depth, direction, or precise velocity information.⁴

Pulsed Wave (PW) Doppler

• PW Doppler works by using pulsed sound waves and a sample volume to measure flow at a certain depth.^4,6
• PW Doppler is often used alongside B-mode imaging, known as duplex scanning, because PW US doesn’t provide structural visualization.⁶
• Displays a “carved out” waveform because a limited range of velocities is captured.⁴
• High pulse repetition frequency (PRF), correct transducer frequency, and an insonation angle less than 60° are key technical requirements for ensuring accurate and reliable velocity measurements.⁶
• Advantages of PW Doppler include its ability to pinpoint the depth of a Doppler signal and measure velocities from a specific location.^4,6
• Disadvantages include PRF constraints that impose a maximum velocity limit, aliasing artifacts when velocities are too high, echoes returning too late, and distortion when pulses are not sent fast enough to capture very high velocities.⁴

Color Doppler

• Color Doppler overlays flow information onto B-mode images, producing a map of flow direction and velocity.^4,6
• Color indicates direction. Red signifies flow toward the transducer, while blue indicates flow away from the transducer.⁶
• The shades produced hint at speed. Lighter shades indicate faster flow, whereas darker shades suggest slower flow.^4,6
• Color Doppler is an essential tool for quickly assessing blood flow activity, identifying regurgitant and turbulent flow, and screening large areas.⁴

Power Doppler

• Power Doppler displays the amplitude instead of the velocity or direction of the Doppler signal.⁶
• It is more sensitive to low flow states than color Doppler and less angle dependent, which makes it preferable for detecting smaller vessels.^4,6

Spectral Doppler

• CW and PW Doppler are spectral Doppler techniques that plot velocity over time, producing waveforms used for quantification.⁴
• Spectral Doppler is commonly used in vascular imaging with duplex scanning as it is essential for determining stenosis, assessing severity, and detecting flow abnormalities.⁶
• A variety of velocities in the sample contribute to the signal. Spectrum analysis provides flow characteristics, which include⁶
  • Peak systolic and diastolic velocities
  • Systolic/diastolic ratio
  • Resistive index and pulsatility index
  • Acceleration time and index
  • Volume flow
• Fast Fourier Transform transforms Doppler frequency shifts into velocity over time for visual representation.⁴

Key Technical Factors and Artifacts

Angle Correction

• To obtain accurate velocity measurements, the insonation angle should be between 30° and 60°.^5-7
• When the Doppler angle is 90°, the cosine is zero, leading to unreliable velocity measurements.^5,7

Gain and Wall Filter Settings

• Gain controls the overall brightness of an US image. Excessive gain results in unclear vessel borders, while inadequate gain may conceal flow.⁴
• Wall filters reject low-frequency noise signals, yet they may unintentionally remove slow blood flow.^4,5,8

Artifacts

• Mirror artifacts take place when highly reflective surfaces, particularly air-filled structures (trachea, lungs, or intestines) or boundary regions of fluids and soft tissues, create a duplicate image of the true structure.^4,8
• A twinkling artifact is seen on color Doppler as rapid shifts between red and blue when rough, highly reflective structures, such as gallstones and renal stones, are being visualized.⁴
• Blooming artifacts occur when color gain is excessive, causing color signals to appear beyond the vessel walls. This mistakenly magnifies the vessels.⁸