The propagation speed of sound in a medium is determined by the medium's elasticity and density. Elasticity refers to the ability of the medium to return to its original shape after deformation, while density is the mass per unit volume of the medium. The speed of sound increases with higher elasticity and decreases with higher density. This relationship is described by the equation=v=PE, where v is the propagation speed, E is the elasticity (or modulus of elasticity), and p is the density.
Reference:
ARDMS Sonography Principles and Instrumentation (SPI) Exam Study Guide
"Diagnostic Ultrasound: Principles and Instruments" by Frederick W. Kremkau

The duty factor is the fraction of time that the ultrasound system is actively transmitting a signal. Continuous wave (CW) Doppler has the highest duty factor because it continuously transmits and receives ultrasound waves. Unlike pulsed wave Doppler, which alternates between sending and receiving signals, CW Doppler does not have a listening period, resulting in a duty factor of nearly 100%. Therefore, CW Doppler has the highest duty factor among the modes listed.
Reference:
ARDMS Sonography Principles & Instrumentation Guidelines
Hedrick WR, Hykes DL, Starchman DE. Ultrasound Physics and Instrumentation. 4th ed. Philadelphia, PA: Elsevier Saunders; 2005.

Enhancement is an artifact that results from decreased attenuation. When an ultrasound wave travels through a medium with lower attenuation compared to surrounding tissues, it loses less energy. Consequently, the structures located deeper than the low-attenuation medium appear brighter on the ultrasound image. This artifact is commonly observed behind fluid-filled structures, such as cysts or the urinary bladder, where the sound waves encounter minimal resistance and thus less attenuation. Reference:
ARDMS Sonography Principles and Instrumentation guidelines
"Diagnostic Ultrasound: Principles and Instruments" by Frederick W. Kremkau

Doppler Scale Too High: This would prevent aliasing but could result in loss of low-velocity signals.
Doppler Scale Too Low: When the scale is set too low, velocities exceed the Nyquist limit, resulting in aliasing where the Doppler signal wraps around the baseline.
Doppler Gain Too High: High gain may result in noise and overamplified signals but does not directly cause aliasing.
Doppler Gain Too Low: Low gain results in weak signal detection but does not cause aliasing.
Reference:
"Diagnostic Ultrasound: Principles and Instruments" by Frederick W. Kremkau ARDMS Sonography Principles and Instrumentation study materials

In a variance mode color map, the upper right side typically indicates higher velocity and turbulent flow with a positive Doppler shift. Variance mode maps are designed to display not only the mean velocity and direction of blood flow but also the presence of turbulence. The color green is often used in the upper right quadrant to represent areas of turbulence with positive Doppler shifts, which occur when the blood flow is moving towards the transducer at higher velocities and with increased chaotic motion. Reference:
ARDMS Sonography Principles and Instrumentation guidelines
"Diagnostic Ultrasound: Physics and Equipment" by Peter Hoskins, Kevin Martin, Abigail Thrush