ARDMS SPI - ARDMS Sonography Principles and Instrumentation Color Doppler and Artifacts Questions and Answers — Questions and Answers
Question 1: A sonographer performing a color Doppler study of the carotid artery observes a mosaic pattern of colors in the center of the vessel where flow velocity is highest. This artifact is most likely:
- Clutter, which should be corrected by increasing the wall filter.
- Aliasing, which should be corrected by increasing the PRF/Scale. (Correct answer)
- Mirror image, which should be corrected by changing the Doppler angle.
- Color bleed, which should be corrected by decreasing the color gain.
Correct answer: Aliasing, which should be corrected by increasing the PRF/Scale.
The mosaic pattern in the area of highest velocity is the classic appearance of aliasing. This artifact occurs when the Doppler shift frequency exceeds the Nyquist limit (PRF/2). [12] The most direct way to correct this is to increase the Pulse Repetition Frequency (PRF), also known as the velocity scale, which raises the Nyquist limit and allows the system to display higher velocities correctly. [4]
Question 2: Which of the following is a primary advantage of Power Doppler when compared to conventional Color Doppler?
- It provides quantitative velocity and direction information.
- It has a significantly higher temporal resolution.
- It is more sensitive to slow flow and is not subject to aliasing. (Correct answer)
- It is less susceptible to flash artifact from patient motion.
Correct answer: It is more sensitive to slow flow and is not subject to aliasing.
Power Doppler displays the amplitude or strength of the Doppler signal, not the frequency shift (velocity). [10] This makes it more sensitive to the presence of flow, especially slow flow. [2] Because it does not measure velocity, it is not limited by the Nyquist criterion and therefore does not alias. [9] Its main disadvantages are the lack of directional/velocity information and increased susceptibility to motion artifact. [2]
Question 3: While evaluating blood flow, a sonographer notices low-frequency, high-amplitude color signals originating from the vessel walls that obscure the true flow within the lumen. Which control should be adjusted to mitigate this specific artifact?
- Color Gain
- PRF/Scale
- Doppler Angle
- Wall Filter (Correct answer)
Correct answer: Wall Filter
The artifact described is clutter, or ghosting, which is caused by strong, low-frequency Doppler shifts from moving anatomy like vessel walls. [15] The Wall Filter is a high-pass filter specifically designed to eliminate these low-frequency signals, allowing the weaker, higher-frequency signals from blood flow to be displayed more clearly. [16]
Question 4: During a renal ultrasound, a sonographer places the color Doppler box over a suspected kidney stone. A rapidly changing, mosaic pattern of color appears immediately posterior to the hyperechoic structure, despite the absence of true blood flow. This specific finding is known as:
- Twinkle artifact (Correct answer)
- Crosstalk artifact
- Power Doppler bloom
- Reverberation artifact
Correct answer: Twinkle artifact
The twinkle artifact is a color Doppler phenomenon seen as a rapidly changing mixture of red and blue pixels posterior to a strongly reflecting, rough-surfaced object like a kidney stone. [17, 21] It is thought to be caused by intrinsic machine noise or random phase shifts from the rough surface creating a 'noisy' Doppler signal. [20] It is a useful sign that increases confidence in identifying calcifications. [21]
Question 5: A sonographer is optimizing a color Doppler image and notices that the color extends beyond the vessel walls into the surrounding stationary tissue. Which of the following adjustments should be made FIRST to correct this?
- Increase the PRF/Scale
- Decrease the color gain (Correct answer)
- Decrease the wall filter
- Change the Doppler angle
Correct answer: Decrease the color gain
When color appears in stationary tissue outside the vessel lumen, it is known as color bleed, blooming, or flash artifact. [13, 16] This is caused by the color gain being set too high, which over-amplifies the signal and noise. [7] The first and most appropriate action is to incrementally decrease the color gain until the color is confined within the vessel walls. [7, 13]
Question 6: All of the following are established methods for reducing or eliminating aliasing in a color Doppler image EXCEPT:
- Increasing the Pulse Repetition Frequency (PRF)
- Shifting the baseline
- Decreasing the transducer frequency
- Increasing the wall filter (Correct answer)
Correct answer: Increasing the wall filter
Aliasing occurs when the Doppler shift exceeds the Nyquist limit. It can be corrected by increasing the PRF (scale), shifting the baseline to allow more velocity range in one direction, or using a lower frequency transducer to produce a smaller Doppler shift for the same velocity. [12] Increasing the wall filter is used to eliminate low-velocity clutter artifact and has no effect on correcting aliasing, which is a high-velocity phenomenon. [16]
