Comprehensive and Detailed Explanation From Exact Extract:
The primary purpose of compression is to reduce the dynamic range of received echoes, making the image more visually interpretable by adjusting contrast without losing diagnostic information.
According to sonography instrumentation reference:
"Compression reduces the dynamic range of the signal amplitudes to fit within the display range of the monitor while preserving important differences in tissue echogenicity." Therefore, the correct answer is D: Reduce the dynamic range.
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Power Doppler, unlike color Doppler, is less angle dependent because it detects the strength of the Doppler signal rather than the velocity of the blood flow. This means it is more sensitive to detecting low-velocity flow and flow in smaller vessels, regardless of the angle between the ultrasound beam and the flow direction. Color Doppler provides information on flow direction and velocity but is highly dependent on the angle of insonation, making it less reliable when the angle is suboptimal.
Reference:
ARDMS Sonography Principles and Instrumentation guidelines
Zwiebel, W. J., & Pellerito, J. S. (2017). Introduction to Vascular Ultrasonography. Elsevier.

Aliasing in pulsed-wave Doppler occurs when the sampled Doppler frequency exceeds the Nyquist limit, which is half of the pulse repetition frequency (PRF). This results in an incorrect representation of the blood flow velocities, causing the waveform to wrap around and appear on the opposite side of the baseline. To correct aliasing, the PRF should be increased, which raises the Nyquist limit and allows for accurate measurement of higher velocities without aliasing. Increasing the PRF effectively reduces the likelihood of aliasing artifacts in the Doppler signal.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.

Color Doppler imaging is most effective when the angle between the ultrasound beam and the flow of blood is small.
At an angle of 88 degrees, the flow of blood is nearly perpendicular to the ultrasound beam.
When the angle is close to 90 degrees, the Doppler shift (frequency change) approaches zero, resulting in little to no color being visualized on the Doppler image.
Thus, to obtain a color signal, the angle should be optimized to be as close to 0 degrees as possible, with 60 degrees being the practical limit for accurate Doppler measurements. Reference:
ARDMS Sonography Principles and Instrumentation guidelines on Doppler angle and its effect on Doppler imaging.

Comprehensive and Detailed Explanation From Exact Extract:
Higher frequencies are more attenuated than lower frequencies. When scanning calcified structures, increasing the frequency enhances attenuation, which makes posterior shadowing more pronounced and visible.
According to sonography instrumentation reference:
"Increasing frequency increases attenuation, thereby enhancing acoustic shadowing posterior to highly attenuating structures such as calcifications or bone." Therefore, the correct answer is D: Frequency.
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