Aliasing is a common technical issue encountered during Doppler ultrasound imaging, where the display of blood flow velocity is distorted. This artifact causes the measured flow direction or speed to appear misrepresented, often seen as a “wrap-around” effect on the spectral or color Doppler display where the velocity crosses the baseline to the opposite side of the scale. Understanding how to quickly identify and correct aliasing is fundamental for sonographers and technicians to ensure the diagnostic information is accurate and reliable. This guide provides practical steps for adjusting ultrasound system settings to eliminate this common imaging problem during a study.
The Nyquist Limit and Why Aliasing Happens
Aliasing is a sampling error that occurs in pulsed wave Doppler and color flow imaging, both of which rely on sending out discrete pulses of sound. The physical cause of the artifact is the Pulse Repetition Frequency (PRF), which determines how often the machine samples the blood flow velocity. To accurately measure a frequency shift, the system must sample the signal at a rate at least twice the frequency of the Doppler shift.
The maximum Doppler shift frequency that the machine can accurately measure without aliasing is known as the Nyquist Limit, which is precisely half of the PRF. When the actual Doppler shift from the fast-moving blood exceeds this Nyquist Limit, the system cannot correctly determine the true velocity or direction. The resulting misinterpretation causes the velocity waveform to appear cut off at the edge of the display and then incorrectly displayed on the opposite side of the baseline.
The maximum velocity that can be correctly detected is directly proportional to the PRF. Since the PRF is determined by the time it takes for the sound pulse to travel to the sample volume and return, the depth of the vessel being studied plays a significant role. Deeper structures require a longer waiting period for the returning echo, which forces the system to use a lower PRF, thereby lowering the Nyquist Limit and making aliasing more likely.
Adjusting System Settings to Eliminate Aliasing
The most direct method to eliminate aliasing is to increase the Nyquist Limit, which is controlled by adjusting the velocity scale setting on the Doppler machine. Increasing the scale setting immediately increases the Pulse Repetition Frequency, allowing the system to sample at a higher rate. This action raises the maximum measurable velocity, often resolving the aliasing artifact with a single adjustment.
A second effective strategy involves decreasing the depth of the sample volume. The relationship between depth and PRF is inverse, meaning a shallower sample volume allows the machine to transmit pulses more rapidly. Moving the sample gate closer to the transducer increases the system’s PRF, which in turn raises the Nyquist Limit and reduces the likelihood of aliasing.
Another technique is to select a lower frequency transducer for the examination. The magnitude of the Doppler frequency shift is directly related to the operating frequency. Using a lower frequency transducer generates a smaller frequency shift for the same velocity of blood flow, making it less likely to exceed the Nyquist Limit.
Other Techniques When Primary Fixes Fail
When increasing the velocity scale or reducing the sample depth does not fully resolve aliasing, particularly in situations with extremely high-velocity flows like severe stenoses, sonographers can employ a baseline shift. This technique involves moving the spectral display’s zero-velocity baseline downward, which effectively dedicates more of the available velocity range to flow moving toward the transducer. Shifting the baseline does not change the Nyquist Limit or PRF, but it utilizes the existing limits more efficiently to accommodate high unidirectional flow.
High Pulse Repetition Frequency (HPRF) Mode
For the highest flow velocities, some systems offer High Pulse Repetition Frequency (HPRF) mode. This mode uses multiple sample volumes along the path of the sound beam to increase the effective PRF, allowing the measurement of velocities that would otherwise exceed the standard Nyquist Limit. The trade-off for this extended velocity range is a loss of precise depth localization, known as range ambiguity, because the system receives echoes from multiple locations simultaneously.
Continuous Wave (CW) Doppler
The ultimate solution for measuring any velocity without the risk of aliasing is switching from pulsed wave to Continuous Wave (CW) Doppler. CW Doppler continuously transmits and receives sound waves, meaning it does not rely on pulsed sampling and therefore has no PRF or Nyquist Limit. This allows the machine to accurately measure the highest velocities encountered in clinical practice. The drawback of CW Doppler is the complete loss of range resolution, as the signal is collected along the entire path of the beam, making it impossible to pinpoint the exact location of the velocity measurement.