Doppler Shift

The Doppler effect, also known as the Doppler shift, is a fundamental principle in ultrasound imaging that arises from the relative motion between the transducer and a moving interface within the body, such as red blood cells. This motion causes a change in the frequency of the sound wave.

Here is what you need to know about the Doppler shift:

1. Definition and Characteristics

• Cause: The Doppler shift occurs due to motion between the ultrasound transducer and a moving interface inside the body.
• Frequency Change: This relative motion results in a frequency change of the sound wave. Specifically, when blood flows towards the transducer, the frequency of the sound wave increases, and when it flows away, the frequency decreases.
• Velocity Relationship: Higher velocities of the moving object lead to greater Doppler shifts. The frequency shift is directly linked to the flow's velocity.
• Units and Range: The unit for Doppler frequency is Hertz (Hz), which represents cycles per second. Clinically, these shifts typically range from 20 to 20,000 Hz, making them audible.
• Demodulation: Although transducers used in Doppler studies operate at much higher frequencies (between 2 to 10 MHz), the resulting Doppler shift is considerably lower (20 to 20,000 Hz). Demodulation is the process used to isolate this lower Doppler frequency from the higher transducer frequency.
• Velocity vs. Speed: It is important to note that Doppler measures velocity, which encompasses both speed and direction, rather than just speed.

2. The Doppler Shift Equation and Angle Dependence

• Formula: The Doppler shift equation is: Doppler shift = 2 * reflector speed * incident frequency * cos(angle) / propagation speed.
• Angle of Incidence: The magnitude of the Doppler frequency shift is dependent on the cosine of the angle between the sound beam and the direction of motion.
  • When the sound beam is parallel to the flow (0° or 180°), the measured velocity equals the true velocity.
  • When the sound beam is perpendicular to the flow (90°), the Doppler shift is zero because the cosine of 90° is zero, meaning no flow will be detected.
  • For intermediate angles, only a portion of the true velocity is measured.
• Optimal Angles: For optimal results in both pulsed and continuous wave Doppler, oblique incidences (0° or 180°) are generally used. Pulsed wave Doppler typically employs a normal incidence (90°) to capture flow towards or away from the transducer.

3. Types of Doppler Techniques

• Continuous Wave (CW) Doppler:
  • Mechanism: This technique uses two crystals: one for constant transmission of sound waves and another for continuous reception of echoes.
  • Advantages: CW Doppler allows for accurate measurement of high velocities. This is attributed to its lack of damping, narrow bandwidth, and high Q-factor, which collectively enhance its sensitivity to small Doppler shifts.
  • Disadvantages: A limitation of CW Doppler is potential range ambiguity, as echoes are received from the entire area where the transmit and receive beams overlap.
• Pulsed Wave (PW) Doppler:
  • Mechanism: Unlike CW Doppler, Pulsed Wave Doppler uses a single crystal that alternates between transmitting and receiving sound pulses.
  • Advantages: This method allows for targeting a specific area, known as the sample volume, and only receives echoes from that area, effectively eliminating range ambiguity.
  • Disadvantages: A significant limitation of Pulsed Wave Doppler is its susceptibility to aliasing. Aliasing occurs when high velocities are incorrectly displayed as negative values due to the Nyquist frequency, which is half the pulse repetition frequency (PRF).
• Color Flow Doppler:
  • Mechanism: This is a type of pulsed ultrasound technology that is adept at visualizing blood flow direction. It encodes different velocities into various colors and then layers these color-coded velocities onto a two-dimensional image. Its processing involves both image and sound processors.
  • Advantages: A key advantage of Color Doppler over traditional Doppler methods is its reduced dependence on the precise angle of the flow, as it focuses more on illustrating the direction of blood flow.
  • Limitations: Similar to other pulsed Doppler methods, Color Flow Doppler is prone to range resolution challenges and aliasing.

4. Doppler Artifacts

• Cause: Doppler systems convert frequency shifts into colors and spectra, typically from moving red blood cells. However, low-velocity motions, such as pulsating vessel walls, can also create minor Doppler shifts, leading to artifacts.
• Wall Filters (High-Pass Filters): These filters are used to help eliminate these low-magnitude shifts, effectively reducing artifacts like color flash or ghosting.
• Gain vs. Artifacts: It is important to understand that reducing color Doppler gain will not resolve ghosting artifacts, because doing so removes reflections from tissues before those from blood cells.